Diacetylene (C4H2) [HCC-CCH]
- also known as Butadiyne (locants not necessary)
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Acetylene radical, formed by photodissociation of acetylene, attacks another molecule of acetylene. The transient radical intermediate kicks out a hydrogen radical to regenerate a triple bond.
And there you've got it, 4 carbon atoms joined up! This is one of the key intermediates that can go on to form benzene (C6H6) and even bigger stuff.
Interestingly enough, the formation of 1,3-butadiene goes by a totally different mechanism and different players.
Full Version: Atmospheric Chemistry of Titan
1,3-Butadiene (C4H6) [H2C=CHCHCH=CH2]
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The formation of 1,3-butadiene starts with the double photodissociation of acetylene to give the acetylene diradical. (As drawn, with two H radicals coming off rather than H2, this would leave the diradical in a spin unpaired triplet state). This is pretty much just C2. One side of this high energy intermediate rips into methane and kicks out hydrogen radical to form a 1-propyne radical (one one radical of the original diradical gets "quenched"). This can then isomerize to allene radical, which is a key intermediate we'll see later (hint: benzene).
Allene radical can react with methyl radical to then form a methylated allene, which isomerizes to the 1,3-butadiene.
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The formation of 1,3-butadiene starts with the double photodissociation of acetylene to give the acetylene diradical. (As drawn, with two H radicals coming off rather than H2, this would leave the diradical in a spin unpaired triplet state). This is pretty much just C2. One side of this high energy intermediate rips into methane and kicks out hydrogen radical to form a 1-propyne radical (one one radical of the original diradical gets "quenched"). This can then isomerize to allene radical, which is a key intermediate we'll see later (hint: benzene).
Allene radical can react with methyl radical to then form a methylated allene, which isomerizes to the 1,3-butadiene.
Mike,
you had me at H2O
pdp8e
you had me at H2O
pdp8e
1-Butene (C4H8) [H2C=CHCH2CH3]
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1-Butene is 1,3-butadiene's more saturated buddy. It has one double bond removed. To be fair, I'm not sure if this is the 1-butene isomer or the more thermodynamically preferred 2-butene, or the 2-methyl prop-1-ene isomer. (With four carbons, the iso-form is possible, it is the most stabilized double bond, and it makes the most sense mechanistically). Most likely, C4H8 would be a mixture of isomers. The mechanism will assume linear 1-butene is formed.
The sequence starts by the addition of hydrogen radical (atomic hydrogen) to allene to give allyl radical (C3H5). The middle carbon should be the best place to put the unpaired electron, but in order to form 1-butene, you need it on the end. Reaction of allyl radical with methyl radical at the C3 terminus gives 1-butene. The reaction of allyl radical with the unpaired electron at C2 (middle) should give the iso isomer - 2-methyl propene = isobutylene (not shown in the diagram). From 1-butene, a 1,3 H-shift would give more thermodynamically preferred 2-butene. However, 1-butene is perfectly stable at room temperatures on Earth. 2-butene can exist in two forms, cis and trans. In cis the two methyls are on the same side of the double bond, in trans they are opposite. Generally, the trans forms of a double bond is more stable. Both cis and trans forms of 2-butane are stable and separable at Earth room temperatures. Once formed they should also be stable at Titan temperatures. The formation mechanism should "lock in" the isomer. However, there is always the possibility that an excitation, or collision, or protonation/deprotonation with a superacid (like CH5+) can isomerize or switch the double bond geometry.
Next up is saturated n-butane.
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1-Butene is 1,3-butadiene's more saturated buddy. It has one double bond removed. To be fair, I'm not sure if this is the 1-butene isomer or the more thermodynamically preferred 2-butene, or the 2-methyl prop-1-ene isomer. (With four carbons, the iso-form is possible, it is the most stabilized double bond, and it makes the most sense mechanistically). Most likely, C4H8 would be a mixture of isomers. The mechanism will assume linear 1-butene is formed.
The sequence starts by the addition of hydrogen radical (atomic hydrogen) to allene to give allyl radical (C3H5). The middle carbon should be the best place to put the unpaired electron, but in order to form 1-butene, you need it on the end. Reaction of allyl radical with methyl radical at the C3 terminus gives 1-butene. The reaction of allyl radical with the unpaired electron at C2 (middle) should give the iso isomer - 2-methyl propene = isobutylene (not shown in the diagram). From 1-butene, a 1,3 H-shift would give more thermodynamically preferred 2-butene. However, 1-butene is perfectly stable at room temperatures on Earth. 2-butene can exist in two forms, cis and trans. In cis the two methyls are on the same side of the double bond, in trans they are opposite. Generally, the trans forms of a double bond is more stable. Both cis and trans forms of 2-butane are stable and separable at Earth room temperatures. Once formed they should also be stable at Titan temperatures. The formation mechanism should "lock in" the isomer. However, there is always the possibility that an excitation, or collision, or protonation/deprotonation with a superacid (like CH5+) can isomerize or switch the double bond geometry.
Next up is saturated n-butane.
QUOTE (PDP8E @ Jun 25 2010, 12:52 AM) 
Mike,
you had me at H2O
pdp8e
you had me at H2O
pdp8e
I'm still drawing up one of the dominate ion-neutral routes to benzene and friends. It is beautifully complex.
n-Butane (C4H10) [CH3CH2CH2CH3]
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Boring butane comes from the straightforward recombination of two ethyl radicals (which comes primarily from acetylene radical ripping a proton off ethane to make ethyl radical). This happens primarily at very low altitudes.
In an atmosphere without a lot of acetylene photochemistry (H2 or methane-rich atmospheres), ethyl radical can form from C-H cleavage of ethane, but it is a lower efficiency process. So very small amounts of butane should be observed on Jupiter and the other H2 rich gas giants. (Recall that acetylene production requires .:CH, which is a 30% product of electronic recombination of CH3+ and an electron, and CH3+ doesn't form in H2 or methane-rich atmospheres, it forms best with Ar or N2 as a diluant gas, like on Titan)
Next up is the Mac Daddy of Titan's organics - benzene.
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Boring butane comes from the straightforward recombination of two ethyl radicals (which comes primarily from acetylene radical ripping a proton off ethane to make ethyl radical). This happens primarily at very low altitudes.
In an atmosphere without a lot of acetylene photochemistry (H2 or methane-rich atmospheres), ethyl radical can form from C-H cleavage of ethane, but it is a lower efficiency process. So very small amounts of butane should be observed on Jupiter and the other H2 rich gas giants. (Recall that acetylene production requires .:CH, which is a 30% product of electronic recombination of CH3+ and an electron, and CH3+ doesn't form in H2 or methane-rich atmospheres, it forms best with Ar or N2 as a diluant gas, like on Titan)
Next up is the Mac Daddy of Titan's organics - benzene.
Benzene (C6H6)
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At lower altitudes, benzene is formed from the combination of propargyl radical (.C3H3) This is a trimolecular event, it won’t happen very well in rarified environments – that goes by a different process.
The initiation is the formation of acetylene diradical and its reaction with methane to give .C3H3. When two of these (+some gas molecules) get together, they join, undergo 1,5 shift, cyclize, then undergo a final 1,3 H-shift to generate benzene.
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At lower altitudes, benzene is formed from the combination of propargyl radical (.C3H3) This is a trimolecular event, it won’t happen very well in rarified environments – that goes by a different process.
The initiation is the formation of acetylene diradical and its reaction with methane to give .C3H3. When two of these (+some gas molecules) get together, they join, undergo 1,5 shift, cyclize, then undergo a final 1,3 H-shift to generate benzene.
Benzene (C6H6) high altitude ion route
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At higher altitudes, a different process occurs. This model is a little more speculative. The dominant proposed route starts with a large amount of diacetylene (C4H2) which gets photoionized to the radical cation. This then reacts with ethylene (by sucking in ethylene’s pi-electrons). The transient intermediate then kicks out atomic hydrogen (H.) and we are left with a C6 cation. This can cyclize via [3,3]-sigmatropic rearrangement, a concerted set of three two-electron processes. This gives us C6H5+. Reaction of this reactive intermediate with either molecular hydrogen (H2) or ethylene (C2H4) gives us protonated benzene (C6H7+) AKA the benzenium ion. If the reaction is with H2, H2 is sucked up into the system, if the reaction is with C2H4 (as drawn above) acetylene is spit out. The ethylene acts as a molecular hydrogen donor.
On electronic recombination, benzene is liberated. Normally, electron recombination is a pretty harsh process and it can frag up most molecular cations when they recombine. But benzene has many vibrational modes associated with it and so can sometimes get through this OK.
The speculative part is that the proposed Vuitton et al. 2009 literature model used a 10x higher amount of C4H2 than could be accounted for with the formation models for C4H2. Hopefully more measurements and theoretical work will help refine the formation models. But at least the model is consistent with Cassini observations of benzene abundance.
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At higher altitudes, a different process occurs. This model is a little more speculative. The dominant proposed route starts with a large amount of diacetylene (C4H2) which gets photoionized to the radical cation. This then reacts with ethylene (by sucking in ethylene’s pi-electrons). The transient intermediate then kicks out atomic hydrogen (H.) and we are left with a C6 cation. This can cyclize via [3,3]-sigmatropic rearrangement, a concerted set of three two-electron processes. This gives us C6H5+. Reaction of this reactive intermediate with either molecular hydrogen (H2) or ethylene (C2H4) gives us protonated benzene (C6H7+) AKA the benzenium ion. If the reaction is with H2, H2 is sucked up into the system, if the reaction is with C2H4 (as drawn above) acetylene is spit out. The ethylene acts as a molecular hydrogen donor.
On electronic recombination, benzene is liberated. Normally, electron recombination is a pretty harsh process and it can frag up most molecular cations when they recombine. But benzene has many vibrational modes associated with it and so can sometimes get through this OK.
The speculative part is that the proposed Vuitton et al. 2009 literature model used a 10x higher amount of C4H2 than could be accounted for with the formation models for C4H2. Hopefully more measurements and theoretical work will help refine the formation models. But at least the model is consistent with Cassini observations of benzene abundance.
Structure of Benzenium (C6H7+)
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This is one of the predominant ions in Titan’s upper atmosphere. It forms from protonation of neutral benzene, or by the reaction of C6H5 with a “hydrogen molecule” donor – either H2 or ethylene.
Recently, the structure of this cation was elucidated by X-ray crystallography by making a superacid salt. Up until recently, it wasn’t clear what the structure of this cation was, was the proton floating above the pi-system of benzene (a pi-complex) or was it localized to one of the edge carbons (a sigma-complex). Or would benzene have it’s normal pi-system, and enter into a 3c2e sp3 bond with two of it’s hydrogen atoms? The answer is more than academic, since the protonation and downstream reaction of benzene is an important process in the synthesis of many products and pharmaceuticals.
Based on the X-ray analysis, it looks like the proton formally latches on to one of the benzene carbons to make a sp3 hybridized CH2. The remaining pi-system (only 5 carbons now) locks up to a cyclohexadiene structure, with two formal double-bonds (sp2 hybridization), and a carbocation somewhat localized to the distal sp2-hybridized carbon from the CH2. While the benzenium molecule itself is flat, more substituted analogs will allow the CH2 to bend slightly out of plane, as you’d expect if that carbon was no longer part of a conjugated pi-system.
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This is one of the predominant ions in Titan’s upper atmosphere. It forms from protonation of neutral benzene, or by the reaction of C6H5 with a “hydrogen molecule” donor – either H2 or ethylene.
Recently, the structure of this cation was elucidated by X-ray crystallography by making a superacid salt. Up until recently, it wasn’t clear what the structure of this cation was, was the proton floating above the pi-system of benzene (a pi-complex) or was it localized to one of the edge carbons (a sigma-complex). Or would benzene have it’s normal pi-system, and enter into a 3c2e sp3 bond with two of it’s hydrogen atoms? The answer is more than academic, since the protonation and downstream reaction of benzene is an important process in the synthesis of many products and pharmaceuticals.
Based on the X-ray analysis, it looks like the proton formally latches on to one of the benzene carbons to make a sp3 hybridized CH2. The remaining pi-system (only 5 carbons now) locks up to a cyclohexadiene structure, with two formal double-bonds (sp2 hybridization), and a carbocation somewhat localized to the distal sp2-hybridized carbon from the CH2. While the benzenium molecule itself is flat, more substituted analogs will allow the CH2 to bend slightly out of plane, as you’d expect if that carbon was no longer part of a conjugated pi-system.
Beyond Benzene - PAH's and Polyphenyls
Polyaromatic hydrocarbons (fused aromatics)
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Benzene is actually a pretty wussy molecule: it can cleave a C-H bond with “normal” UV light at 248 nm (= 115 kcal/mol). This generates a benzene radical (.C6H6) and a hydrogen radical.
One of the things benzene radical can do is react with acetylene (or diacetylene, or vinyl acetylene) and then close the pendant chain to form a new fused aromatic ring. This process is thought to be an important route to polyaromatic hydrocarbons during soot formation. The benzene radical first attacks the triple bond of acetylene, then places the radical at the terminal end of the alkyne that just got attached. This can repeat the process and react with another acetylene molecule. But this last radical now has a pendant radical that can attack the pi-system of the parent benzene ring to make a new six-membered ring. (Five and six-membered rings form pretty easily in most chemical processes). This places the radical likely at the ring junction (most substituted). This collapses to kick out a hydrogen radical and a fully aromatized fused ring system – naphthalene. Most studies indicate this can happen only at high temperatures – places like interstellar clouds illuminated with high energy photons or in the back of your car’s exhaust pipe.
Polyphenyls (linked aromatics)
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At lower temperatures, another process occurs with benzene radical, it reacts with another molecule of benzene. Attack of the radical into the pi-system gives a transient radical that quickly rearomatizes and kicks out a hydrogen radical. This creates a linked aromatic system (not fused).
Fused ring systems are usually planar. When they get really big, like C30 or so, they can become cup-shaped. Fused ring systems also can have different reactivities. In anthracene (3 benzenes fused in a line) the central carbons are very prone to oxidation. The central double bonds also are prone to reaction. (They do [4+2] cycloadditions easily). But as a general rule, fused ring carbons act more electron-deficient. PAH’s can do some funky chemistry.
In contrast, polyphenyls are about as exciting as linked benzene. The chemistry is almost exactly like that of benzene. Structurally, rings are twisted out of plane in the gas phase, but will flatten out when excited. In solid phase, the rings are planar. (All this is assuming that the hydrogens are at the ortho position. If there is a larger substituent, it will twist. For ortho-terphenyl, the two rings are oriented perpendicular to the central ring to prevent bumping each other.)
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PAH’s require high temperatures to form, while polyphenyls can form at lower temperatures. It is likely that the low temperatures on Titan prefer the formation of polyphenyls such as biphenyl, terphenyl, and higher. There are many recent reports coming out that implicate the formation of biphenyls in Titan’s atmosphere.
Polyaromatic hydrocarbons (fused aromatics)
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Benzene is actually a pretty wussy molecule: it can cleave a C-H bond with “normal” UV light at 248 nm (= 115 kcal/mol). This generates a benzene radical (.C6H6) and a hydrogen radical.
One of the things benzene radical can do is react with acetylene (or diacetylene, or vinyl acetylene) and then close the pendant chain to form a new fused aromatic ring. This process is thought to be an important route to polyaromatic hydrocarbons during soot formation. The benzene radical first attacks the triple bond of acetylene, then places the radical at the terminal end of the alkyne that just got attached. This can repeat the process and react with another acetylene molecule. But this last radical now has a pendant radical that can attack the pi-system of the parent benzene ring to make a new six-membered ring. (Five and six-membered rings form pretty easily in most chemical processes). This places the radical likely at the ring junction (most substituted). This collapses to kick out a hydrogen radical and a fully aromatized fused ring system – naphthalene. Most studies indicate this can happen only at high temperatures – places like interstellar clouds illuminated with high energy photons or in the back of your car’s exhaust pipe.
Polyphenyls (linked aromatics)
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At lower temperatures, another process occurs with benzene radical, it reacts with another molecule of benzene. Attack of the radical into the pi-system gives a transient radical that quickly rearomatizes and kicks out a hydrogen radical. This creates a linked aromatic system (not fused).
Fused ring systems are usually planar. When they get really big, like C30 or so, they can become cup-shaped. Fused ring systems also can have different reactivities. In anthracene (3 benzenes fused in a line) the central carbons are very prone to oxidation. The central double bonds also are prone to reaction. (They do [4+2] cycloadditions easily). But as a general rule, fused ring carbons act more electron-deficient. PAH’s can do some funky chemistry.
In contrast, polyphenyls are about as exciting as linked benzene. The chemistry is almost exactly like that of benzene. Structurally, rings are twisted out of plane in the gas phase, but will flatten out when excited. In solid phase, the rings are planar. (All this is assuming that the hydrogens are at the ortho position. If there is a larger substituent, it will twist. For ortho-terphenyl, the two rings are oriented perpendicular to the central ring to prevent bumping each other.)
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PAH’s require high temperatures to form, while polyphenyls can form at lower temperatures. It is likely that the low temperatures on Titan prefer the formation of polyphenyls such as biphenyl, terphenyl, and higher. There are many recent reports coming out that implicate the formation of biphenyls in Titan’s atmosphere.
The genesis of Titan's atmosphere may be more complex than we've ever imagined:
http://www.sciencedaily.com/releases/2010/...00707002535.htm
--Bill
QUOTE
ScienceDaily (July 7, 2010) — Complex interactions between Saturn and its satellites have led scientists using NASA's Cassini spacecraft to a comprehensive model that could explain how oxygen may end up on the surface of Saturn's icy moon Titan.
http://www.sciencedaily.com/releases/2010/...00707002535.htm
--Bill
HCN
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The incorporation of nitrogen into Titan’s organics usually results in the introduction of a nitrile group (-CN), where a terminal carbon atom is bound through a triple bond to a nitrogen atom. The nitrogen has a lone pair of electrons on it, although these may or may not be drawn in. (i.e. –CN:) This lone pair would love to donate to a proton (or other atoms desperate for electrons such as Lewis acids.)
The whole cascade starts with the blowing apart of molecular nitrogen by strong UV light. There are lots of ways this can happen, one is shown above. In this case, “lower energy” ultraviolet light in the 80 - 100 nm range (which is still pretty dang powerful) excites the molecular nitrogen to the point that it goes total fraggo and liberates a “naked” nitrogen atom and an “excited naked” nitrogen atom*. (The “excited naked” nitrogen atom will be a big player in tholin formation mechanisms). Other ways to get there include ionization of molecular nitrogen with light below 80 nm, then electron recombination back to molecular nitrogen, which is a pretty violent process, and then a total fraggo reaction that again generates a naked nitrogen atom and an excited naked nitrogen atom.
The “naked” nitrogen atom can react with a methyl radical to form a transient CH3N nitrene complex** (my guess is it would be likely in a triplet or an unpaired excited singlet state) that then blows out hydrogen radical to give H2CN. radical. This can react with another hydrogen radical to then kick out molecular hydrogen (H2) and HCN.
[I’m not sure why this process goes stepwise, I would think it possible for the transient CH3N carbene to kick out two hydrogen radicals (if triplet) or molecular hydrogen (possible if unpaired excited singlet state?) all in one go].
According to Krasnopolsky et al., 2009, this sequence accounts for 72% of all HCN formed in Titan’s atmosphere. But a recent article shows that excited state nitrogen chemistry may be also very important and poorly modeled. The atmospheric nitrogen chemistry of Titan is still poorly constrained, but getting better with recent lab experiments and further modeling. We’ll use the Krasnopolsky results, but these will likely shift on publication of the next model.
*****
*an asterisk [*] is used to designate an excited state atom. This is an atom where one of the electrons has been boosted to a higher energy orbital. Normally the electrons are spin paired in the orbital in the ground state. In an excited state atom the electrons can still be spin paired, but one of the electrons is now in a boosted energy orbital. So it may be in the unpaired excited state singlet state.
**very careful electron counting is important here, there are two lone pairs on nitrogen with a bonus electron, so naked nitrogen is like a triradical with a lone pair [:N...]
For the whole reaction we get: radical + radical nitrene (nitryne) --> radicals pair - nitrene (2 lone pairs on nitrogen) --> nitrene --> radical + radical
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The incorporation of nitrogen into Titan’s organics usually results in the introduction of a nitrile group (-CN), where a terminal carbon atom is bound through a triple bond to a nitrogen atom. The nitrogen has a lone pair of electrons on it, although these may or may not be drawn in. (i.e. –CN:) This lone pair would love to donate to a proton (or other atoms desperate for electrons such as Lewis acids.)
The whole cascade starts with the blowing apart of molecular nitrogen by strong UV light. There are lots of ways this can happen, one is shown above. In this case, “lower energy” ultraviolet light in the 80 - 100 nm range (which is still pretty dang powerful) excites the molecular nitrogen to the point that it goes total fraggo and liberates a “naked” nitrogen atom and an “excited naked” nitrogen atom*. (The “excited naked” nitrogen atom will be a big player in tholin formation mechanisms). Other ways to get there include ionization of molecular nitrogen with light below 80 nm, then electron recombination back to molecular nitrogen, which is a pretty violent process, and then a total fraggo reaction that again generates a naked nitrogen atom and an excited naked nitrogen atom.
The “naked” nitrogen atom can react with a methyl radical to form a transient CH3N nitrene complex** (my guess is it would be likely in a triplet or an unpaired excited singlet state) that then blows out hydrogen radical to give H2CN. radical. This can react with another hydrogen radical to then kick out molecular hydrogen (H2) and HCN.
[I’m not sure why this process goes stepwise, I would think it possible for the transient CH3N carbene to kick out two hydrogen radicals (if triplet) or molecular hydrogen (possible if unpaired excited singlet state?) all in one go].
According to Krasnopolsky et al., 2009, this sequence accounts for 72% of all HCN formed in Titan’s atmosphere. But a recent article shows that excited state nitrogen chemistry may be also very important and poorly modeled. The atmospheric nitrogen chemistry of Titan is still poorly constrained, but getting better with recent lab experiments and further modeling. We’ll use the Krasnopolsky results, but these will likely shift on publication of the next model.
*****
*an asterisk [*] is used to designate an excited state atom. This is an atom where one of the electrons has been boosted to a higher energy orbital. Normally the electrons are spin paired in the orbital in the ground state. In an excited state atom the electrons can still be spin paired, but one of the electrons is now in a boosted energy orbital. So it may be in the unpaired excited state singlet state.
**very careful electron counting is important here, there are two lone pairs on nitrogen with a bonus electron, so naked nitrogen is like a triradical with a lone pair [:N...]
For the whole reaction we get: radical + radical nitrene (nitryne) --> radicals pair - nitrene (2 lone pairs on nitrogen) --> nitrene --> radical + radical
.CN radical - key intermediate
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The formation of .CN: radical is very straightforward: Higher energy photon hits HCN, causes dissociation of H-C bond, and you get H. and .CN: radical.
.CN: indicates that the radical electron resides on the carbon atom, that the triple bond between carbon and nitrogen still exists, and that the lone pair is still on nitrogen (in an sp-orbital sticking straight out along the axis of the C-N triple bond.). Likewise the radical electron resides in the sp-orbital on the carbon and sticks straight out as well along the axis of the C-N bond away from the triple bond electrons. This molecule is linear, like acetylene.
.CN: radical can play a key role in the formation of cyanoacetylene [HCCCN], cyanogen [NC-CN], and dicyanoacetylene [NC-CC-CN], although there are alternative mechanisms for these compounds that proceed through cyanomethylene carbene (:CH(CN)).
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The formation of .CN: radical is very straightforward: Higher energy photon hits HCN, causes dissociation of H-C bond, and you get H. and .CN: radical.
.CN: indicates that the radical electron resides on the carbon atom, that the triple bond between carbon and nitrogen still exists, and that the lone pair is still on nitrogen (in an sp-orbital sticking straight out along the axis of the C-N triple bond.). Likewise the radical electron resides in the sp-orbital on the carbon and sticks straight out as well along the axis of the C-N bond away from the triple bond electrons. This molecule is linear, like acetylene.
.CN: radical can play a key role in the formation of cyanoacetylene [HCCCN], cyanogen [NC-CN], and dicyanoacetylene [NC-CC-CN], although there are alternative mechanisms for these compounds that proceed through cyanomethylene carbene (:CH(CN)).
Cyanomethlyene carbene [:CH(CN)]
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A very recent article (June 2, 2010 issue of PNAS) by Imanaka and Smith has provided laboratory evidence that high energy photons can ionize nitrogen and cause it to incorporate it into Titan’s organics much more easily than previously thought.
The process is the initial photoionization of molecular nitrogen by EUV photons (wavelenghts < 60 nm) which then makes molecular nitrogen radical cation by blowing an electron out of the molecule. At some point, another electron recombines with the nitrogen molecule (cue dropping bomb sound) which releases a huge amount of energy. The energy released blows apart the dinitrogen triple bond and we are left with a “normal” nitrogen atom radical nitrene (nitryne) and an “excited” nitrogen atom radical nitrene (nitryne). (this is the stepwise sequence of the concerted sequence discussed for HCN – the rates of these steps were deliberately left of the graphic since the Imanaka and Smith work will definitely supercede the Krasnopolsky estimated models.)
The excited nitryne atom is likely in an excited state that accesses D-orbitals. These may be in unpaired spin-coupled state, but well run through the mechanisms assuming it acts like a triradical. (i.e. the actual process may be more concerted). In the case to make cyanomethylene carbene, the excited nitryne reacts with an alkyne. Drawing a stepwise mechanism, the first thing that happens is one of the pi-electrons of the triple bonds forms a new bond with one of the electrons of the nitryne. This now makes a carbon radical nitrene. (still three unpaired electrons in the system). A C-H bond breaks, and hydrogen radical (H.) goes away, and the remaining electron on carbon now forms a carbon-nitrogen double bond with one of the nitrene electrons.
A double-bond equilibration gives cyanomethylene carbene. But what does this molecule look like?
Spectroscopic data suggests that this molecule in the ground state is “close” to a linear molecule, with an H-C-(CN) bond close to 180 degrees That means that the likely hybridization on the “carbene” carbon is sp with the unpaired electrons in two p orbitals in a triplet state. (Ground state suggests it is in an unpaired spin-uncoupled triplet configuration, I’d guess on Titan that these molecules are likely in an excited unpaired but spin-coupled triplet configuration.) Why is this important? It’s probably academic, but it implies that many of the downstream steps could be concerted. This might become important if we deal with molecules (e.g. cis or trans double bonds) with stereochemistry, in this case the stereochemistry would be preserved. In a triplet stepwise reaction, there is always a chance for bond twisting before the second bond snaps shut, causing a loss of stereochemistry.
The :CH(CN) intermediate may be the key intermediate in the formation of tholins and for the incorporation of bonus* nitrogen into Titan’s organics.
It is also an intermediate that gives another route to many of Titan’s organics, such as cyanoacetylene, cyanogens, acrylonitrile, and ethylacetonitrile.
(*more that previously thought)
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A very recent article (June 2, 2010 issue of PNAS) by Imanaka and Smith has provided laboratory evidence that high energy photons can ionize nitrogen and cause it to incorporate it into Titan’s organics much more easily than previously thought.
The process is the initial photoionization of molecular nitrogen by EUV photons (wavelenghts < 60 nm) which then makes molecular nitrogen radical cation by blowing an electron out of the molecule. At some point, another electron recombines with the nitrogen molecule (cue dropping bomb sound) which releases a huge amount of energy. The energy released blows apart the dinitrogen triple bond and we are left with a “normal” nitrogen atom radical nitrene (nitryne) and an “excited” nitrogen atom radical nitrene (nitryne). (this is the stepwise sequence of the concerted sequence discussed for HCN – the rates of these steps were deliberately left of the graphic since the Imanaka and Smith work will definitely supercede the Krasnopolsky estimated models.)
The excited nitryne atom is likely in an excited state that accesses D-orbitals. These may be in unpaired spin-coupled state, but well run through the mechanisms assuming it acts like a triradical. (i.e. the actual process may be more concerted). In the case to make cyanomethylene carbene, the excited nitryne reacts with an alkyne. Drawing a stepwise mechanism, the first thing that happens is one of the pi-electrons of the triple bonds forms a new bond with one of the electrons of the nitryne. This now makes a carbon radical nitrene. (still three unpaired electrons in the system). A C-H bond breaks, and hydrogen radical (H.) goes away, and the remaining electron on carbon now forms a carbon-nitrogen double bond with one of the nitrene electrons.
A double-bond equilibration gives cyanomethylene carbene. But what does this molecule look like?
Spectroscopic data suggests that this molecule in the ground state is “close” to a linear molecule, with an H-C-(CN) bond close to 180 degrees That means that the likely hybridization on the “carbene” carbon is sp with the unpaired electrons in two p orbitals in a triplet state. (Ground state suggests it is in an unpaired spin-uncoupled triplet configuration, I’d guess on Titan that these molecules are likely in an excited unpaired but spin-coupled triplet configuration.) Why is this important? It’s probably academic, but it implies that many of the downstream steps could be concerted. This might become important if we deal with molecules (e.g. cis or trans double bonds) with stereochemistry, in this case the stereochemistry would be preserved. In a triplet stepwise reaction, there is always a chance for bond twisting before the second bond snaps shut, causing a loss of stereochemistry.
The :CH(CN) intermediate may be the key intermediate in the formation of tholins and for the incorporation of bonus* nitrogen into Titan’s organics.
It is also an intermediate that gives another route to many of Titan’s organics, such as cyanoacetylene, cyanogens, acrylonitrile, and ethylacetonitrile.
(*more that previously thought)
QUOTE
The first experimental evidence showing how atmospheric nitrogen can be incorporated into organic macromolecules is being reported by a University of Arizona team. In an experiment to simulate what happens when sunlight hits Titan's atmosphere, UA researchers put nitrogen and methane gas into a stainless steel cylinder and zapped it with high-energy UV light.
http://uanews.org/node/32574
Acetonitrile (CH3CN)
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There are multiple ways to form acetonitrile. One way is the addition of hydrogen radical to the triple bond of acetylene to generate the C2H3 radical. (vinyl radical, .CH=CH2). This can react with naked nitrogen (not clear if it needs to be excited or not) to generate a funky nitrene-enamine intermediate/transition state which can quickly kick out a hydrogen radical and the other electron combines with one of the free electrons on the nitrene to create an “iminoketene” radical. A quick tautomerization creates the CN triple bond and places the radical electron on the CH2 carbon. This can then find an Hydrogen radical floating around (maybe the very one that got kicked out a second ago) and then forms acetonitrile. According to the Krasnopolsky model, this mechanism using multiple transient intermediates accounts for 69% of the acetonitrile generation.
another possible way from :CH(CN)
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Another way shown above is via hydrogen abstraction from our new best friend cyanomethylene carbene (:CH(CN)). Very low temperature studies(1) with carbenes have shown that triplet carbenes can react with molecular hydrogen, but that singlet carbenes cannot. (I’m not sure what an excited singlet carbene would do). These low temperature studies were done at VERY low temperatures, less than 30 K. This is waaay colder than Titan’s relatively balmy 95 K (or lower atmosphere minimum of 70 K). Interestingly, the authors found that while the overall formation of carbenes adding to H2 is exothermic, that there is a significant energy barrier to cross. Singet carbenes can’t do it. Singlet carbene insertion requires the carbene to concertedly (all-at-once) muscle in between the H-H bond. But triplets react a different way, they react like diradicals, one step at a time: the first step is an abstraction of one hydrogen atom from molecular hydrogen, then a combination of the two resulting radicals (the .CH2CN and the leftover H.). But even those transition state energies (IIRC +5 kcal/mol) are just a tad too high at 30 K to work, so the authors proposed quantum tunneling of the hydrogen radical. This is a bit out of my comfort zone, but the authors did detect the hydrogenated carbene products so this is experimentally valid. Also interestingly, molecular deuterium did NOT react. The energy barrier (and quantum tunneling barrier?) for a deuterium nucleus appears to be just too high in a 30 K matrix. The authors propose that the reaction with H2 can actually be use to as a mechanistic test as to whether a particular carbene is in a singlet or triplet state. If it hydrogenates, then it was in a triplet state.
So if the :CHCN formed photochemically high in Titan’s atmosphere is in a triplet state, it could react with molecular hydrogen to easily form CH3CN in one quick step.
This particular reaction was not modeled in the Krasnopolsky model, but I’d assume it should be in the next iteration.
:CH(CN) formed up in Titan's atmosphere is in a rarified environment and will be in a different environment than stuff in a low temperature inert matrix in a terrestrial laboratory. For one thing, the stuff in a matrix will be bumping around in it's cage and be able to relax to it's desired ground state. Not so for the stuff in Titan's upper atmosphere. That stuff is blasting along in a vaccum, all excited. If the first thing it bumps into and reacts with is H2, it can react as an excited species, which may not be in the ground state configuration. So the state of the :CH(CN) carbene will determine it's reactivity and propensity to form acetonitrile via direct hydrogenation.
Reference: (1) Zuev and Sheridan, Journal of the American Chemical Society 123 (2001) 12434-12435. "Low-Temperature Hydrogenation of Triplet Carbenes and Diradicaloid Biscarbenes - Electronic State Selectivity." doi: 10.1021/ja016826y]
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There are multiple ways to form acetonitrile. One way is the addition of hydrogen radical to the triple bond of acetylene to generate the C2H3 radical. (vinyl radical, .CH=CH2). This can react with naked nitrogen (not clear if it needs to be excited or not) to generate a funky nitrene-enamine intermediate/transition state which can quickly kick out a hydrogen radical and the other electron combines with one of the free electrons on the nitrene to create an “iminoketene” radical. A quick tautomerization creates the CN triple bond and places the radical electron on the CH2 carbon. This can then find an Hydrogen radical floating around (maybe the very one that got kicked out a second ago) and then forms acetonitrile. According to the Krasnopolsky model, this mechanism using multiple transient intermediates accounts for 69% of the acetonitrile generation.
another possible way from :CH(CN)
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Another way shown above is via hydrogen abstraction from our new best friend cyanomethylene carbene (:CH(CN)). Very low temperature studies(1) with carbenes have shown that triplet carbenes can react with molecular hydrogen, but that singlet carbenes cannot. (I’m not sure what an excited singlet carbene would do). These low temperature studies were done at VERY low temperatures, less than 30 K. This is waaay colder than Titan’s relatively balmy 95 K (or lower atmosphere minimum of 70 K). Interestingly, the authors found that while the overall formation of carbenes adding to H2 is exothermic, that there is a significant energy barrier to cross. Singet carbenes can’t do it. Singlet carbene insertion requires the carbene to concertedly (all-at-once) muscle in between the H-H bond. But triplets react a different way, they react like diradicals, one step at a time: the first step is an abstraction of one hydrogen atom from molecular hydrogen, then a combination of the two resulting radicals (the .CH2CN and the leftover H.). But even those transition state energies (IIRC +5 kcal/mol) are just a tad too high at 30 K to work, so the authors proposed quantum tunneling of the hydrogen radical. This is a bit out of my comfort zone, but the authors did detect the hydrogenated carbene products so this is experimentally valid. Also interestingly, molecular deuterium did NOT react. The energy barrier (and quantum tunneling barrier?) for a deuterium nucleus appears to be just too high in a 30 K matrix. The authors propose that the reaction with H2 can actually be use to as a mechanistic test as to whether a particular carbene is in a singlet or triplet state. If it hydrogenates, then it was in a triplet state.
So if the :CHCN formed photochemically high in Titan’s atmosphere is in a triplet state, it could react with molecular hydrogen to easily form CH3CN in one quick step.
This particular reaction was not modeled in the Krasnopolsky model, but I’d assume it should be in the next iteration.
:CH(CN) formed up in Titan's atmosphere is in a rarified environment and will be in a different environment than stuff in a low temperature inert matrix in a terrestrial laboratory. For one thing, the stuff in a matrix will be bumping around in it's cage and be able to relax to it's desired ground state. Not so for the stuff in Titan's upper atmosphere. That stuff is blasting along in a vaccum, all excited. If the first thing it bumps into and reacts with is H2, it can react as an excited species, which may not be in the ground state configuration. So the state of the :CH(CN) carbene will determine it's reactivity and propensity to form acetonitrile via direct hydrogenation.
Reference: (1) Zuev and Sheridan, Journal of the American Chemical Society 123 (2001) 12434-12435. "Low-Temperature Hydrogenation of Triplet Carbenes and Diradicaloid Biscarbenes - Electronic State Selectivity." doi: 10.1021/ja016826y]
Cyanoacetylene (HC3N) [HCC-CN]
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The dominant mechanism (according to the Krasnopolsky 2009 model) is the reaction of nitrile radical (.CN) with acetylene. After the initial addition into the triple bond, the C-H bond of the secondary carbon breaks homolytically, and the hydrogen radical leaves the system, while the newly unpaired electron on the carbon jumps into an empty molecular pi-orbital with the other unpaired electron on the carbon radical to form a triple bond. According to the mode, 62% of the cyanoacetylene is formed by this route.
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However, there is another possible route to this molecule, again using our friend :CH(CN). In this case, photoionization of methane forms a high energy ionized intermediate CH3+ then undergoes electron recombination to violently produce .:CH. The Krasnopolsky model actually makes this occur in one step. Either way, you get methyne (the simplest carbyne) which then can react with methylene carbene. This could go a number of different ways, I’ve drawn the fully stepwise mechanism as if we were dealing with a triradical CH, and a diradical :CHCN. In the first step, a single bond is formed between the two carbon atoms. The resulting carbene-radical then combines to form a double bond to make a transient acrylonitrile radical. This kicks out a hydrogen radical (from the secondary carbon) as the resulting carbon radicals combine to form the triple bond (like the step above).
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The dominant mechanism (according to the Krasnopolsky 2009 model) is the reaction of nitrile radical (.CN) with acetylene. After the initial addition into the triple bond, the C-H bond of the secondary carbon breaks homolytically, and the hydrogen radical leaves the system, while the newly unpaired electron on the carbon jumps into an empty molecular pi-orbital with the other unpaired electron on the carbon radical to form a triple bond. According to the mode, 62% of the cyanoacetylene is formed by this route.
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However, there is another possible route to this molecule, again using our friend :CH(CN). In this case, photoionization of methane forms a high energy ionized intermediate CH3+ then undergoes electron recombination to violently produce .:CH. The Krasnopolsky model actually makes this occur in one step. Either way, you get methyne (the simplest carbyne) which then can react with methylene carbene. This could go a number of different ways, I’ve drawn the fully stepwise mechanism as if we were dealing with a triradical CH, and a diradical :CHCN. In the first step, a single bond is formed between the two carbon atoms. The resulting carbene-radical then combines to form a double bond to make a transient acrylonitrile radical. This kicks out a hydrogen radical (from the secondary carbon) as the resulting carbon radicals combine to form the triple bond (like the step above).
Cyanogen (C2N2) [NC-CN]
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According to the Krasnopolsky 2009 model, cyanogen forms by two main routes. The first route (53% of cyanogen formation) is from the attack of nitrile (.CN) radical on acetonitrile radical (.CH2CN) which then kicks out methylene carbene and forms a new bond between the two nitrile carbons. [I’m baffled by this mechanism, why break a perfectly good C-C bond? You’d think the two radicals would combine to form malononitrile (propanedintrile (NCCH2CN) – a nice stable molecule and handy organic building block]
The other main route (47% of cyanogen formed on Titan) uses cyanomethylene carbene :CH(CN) and an excited naked nitrogen atom. (The top reaction in the scheme shows the formation of the cyanomethylene carbene from the reaction of excited naked nitrogen and acetylene). In this case, the :CH(CN) carbene reacts with another atom of excited naked nitrogen to form a radical carbene. This is shown as the fully stepwise mechanism, it is possible that some of these steps could be concerted. The next step is the radical carbon electron combining with one of the nitrogen nitrene electrons to form a C-N double bond, leaving an unpaired electron on the nitrogen – a nitrogen radical. The last step is a C-H bond hemolytic cleavage followed by the unpaired electrons, one from the nitrogen radical and one from the new carbon radical, jumping in together to form a C-N triple bond.
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According to the Krasnopolsky 2009 model, cyanogen forms by two main routes. The first route (53% of cyanogen formation) is from the attack of nitrile (.CN) radical on acetonitrile radical (.CH2CN) which then kicks out methylene carbene and forms a new bond between the two nitrile carbons. [I’m baffled by this mechanism, why break a perfectly good C-C bond? You’d think the two radicals would combine to form malononitrile (propanedintrile (NCCH2CN) – a nice stable molecule and handy organic building block]
The other main route (47% of cyanogen formed on Titan) uses cyanomethylene carbene :CH(CN) and an excited naked nitrogen atom. (The top reaction in the scheme shows the formation of the cyanomethylene carbene from the reaction of excited naked nitrogen and acetylene). In this case, the :CH(CN) carbene reacts with another atom of excited naked nitrogen to form a radical carbene. This is shown as the fully stepwise mechanism, it is possible that some of these steps could be concerted. The next step is the radical carbon electron combining with one of the nitrogen nitrene electrons to form a C-N double bond, leaving an unpaired electron on the nitrogen – a nitrogen radical. The last step is a C-H bond hemolytic cleavage followed by the unpaired electrons, one from the nitrogen radical and one from the new carbon radical, jumping in together to form a C-N triple bond.
Dicyanoacetylene (C4N2) [NC-CC-CN]
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Two different literature models have two different pathways to form dicyanoacetylene (C4H2). The route described in Krasnopolsky 2009 model has nitrile radical (.CN) attack the carbon-carbon triple bond of cyanoacetylene (HCC-CN). This makes an intermediate central double bond with one of the carbons holding an unpaired electron. Next door, the C-H bond cleaves, and hydrogen goes flying away as hydrogen radical (H.) and the resulting unpaired electron dives in with the carbon holding the unpaired electron and makes the central triple bond.
The Wilson and Atreya 2004 model proposes that two molecules of cyanomethylene carbene (:CH(CN)) connect their unpaired electons together to form a middle double bond. The resulting intermediate (1,2-dicyanoethylene), blows out H2 most likely in a concerted 4-electron pathway (2 sets of 2 paired electrons – this is a very typical concerted reaction, benzene resonances and [3,3]-sigmatropic rearrangements are 3 sets of 2 paired electrons (6 electron concerted pathways)) to give the final dicyanoacetylene and molecular hydrogen (H2).
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Two different literature models have two different pathways to form dicyanoacetylene (C4H2). The route described in Krasnopolsky 2009 model has nitrile radical (.CN) attack the carbon-carbon triple bond of cyanoacetylene (HCC-CN). This makes an intermediate central double bond with one of the carbons holding an unpaired electron. Next door, the C-H bond cleaves, and hydrogen goes flying away as hydrogen radical (H.) and the resulting unpaired electron dives in with the carbon holding the unpaired electron and makes the central triple bond.
The Wilson and Atreya 2004 model proposes that two molecules of cyanomethylene carbene (:CH(CN)) connect their unpaired electons together to form a middle double bond. The resulting intermediate (1,2-dicyanoethylene), blows out H2 most likely in a concerted 4-electron pathway (2 sets of 2 paired electrons – this is a very typical concerted reaction, benzene resonances and [3,3]-sigmatropic rearrangements are 3 sets of 2 paired electrons (6 electron concerted pathways)) to give the final dicyanoacetylene and molecular hydrogen (H2).
Acrylonitrile (C2H3CN) [H2C=CHCN]
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This molecule also has two routes according to the Krasnopolsky 2009 model. In the first route ethyl radical adds to the CN triple bond of cyanide to make a new C-C single bond and an intermediate radical on the nitrogen. The nitrogen radical electron dives in while a C-H bond on the other side homolytically cleaves. The two electrons reform a CN triple bond and the resultant hydrogen radical goes flying off.
The second route starts with cyanomethylene carbene (:CHCN). Reaction of this carbene with methyl radical (CH3) makes a new double bond as one of the electrons of the carbene (which may be in a triplet state) hooks up with the unpaired electron of the methyl radical. The resulting ethyl nitrile radical has the unpaired electron on the carbon alpha to the nitrile group. A C-H bond on the terminal carbon cleaves and one of the unpaired electrons joins the other unpaired electron to form a double bond. The other unpaired electron flies off with the hydrogen nucleus to be a hydrogen radical.
Acrylonitrile is known to polymerize when a suitable base is added. On Earth, it is a major industrial chemical product (several million tons/year scale) used for making all sorts of plastics and polymers. On Titan's surface, if a large concentration of this were present, and with a suffficient thermal “kick” it could undergo polymerization or further reactions in the presence of a base such as an organic amine. (Polymerization from initial nucleophilic Michael addition (1,4-addition) to the beta-carbon, then the resulting enolate undergoes Michael addition to another molecule of acrylonitirile, etc.)
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This molecule also has two routes according to the Krasnopolsky 2009 model. In the first route ethyl radical adds to the CN triple bond of cyanide to make a new C-C single bond and an intermediate radical on the nitrogen. The nitrogen radical electron dives in while a C-H bond on the other side homolytically cleaves. The two electrons reform a CN triple bond and the resultant hydrogen radical goes flying off.
The second route starts with cyanomethylene carbene (:CHCN). Reaction of this carbene with methyl radical (CH3) makes a new double bond as one of the electrons of the carbene (which may be in a triplet state) hooks up with the unpaired electron of the methyl radical. The resulting ethyl nitrile radical has the unpaired electron on the carbon alpha to the nitrile group. A C-H bond on the terminal carbon cleaves and one of the unpaired electrons joins the other unpaired electron to form a double bond. The other unpaired electron flies off with the hydrogen nucleus to be a hydrogen radical.
Acrylonitrile is known to polymerize when a suitable base is added. On Earth, it is a major industrial chemical product (several million tons/year scale) used for making all sorts of plastics and polymers. On Titan's surface, if a large concentration of this were present, and with a suffficient thermal “kick” it could undergo polymerization or further reactions in the presence of a base such as an organic amine. (Polymerization from initial nucleophilic Michael addition (1,4-addition) to the beta-carbon, then the resulting enolate undergoes Michael addition to another molecule of acrylonitirile, etc.)
I’ve tried to represent Titan’s chemistry in a slightly different way than is normally presented in the literature, the graphic below shows only the key intermediates that react with each other to form major hydrocarbon components in Titan’s atmosphere. Only the dominant pathway is shown for each molecule, so this is a more limited view than the typical “splat diagrams” (example splat diagram in post 27 this thread) shown for Titan chemistry:
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On the left side of the matrix are key reactive species. Along the top are some “target” compounds and reactive intermediates. At the cross point are the species formed when these two meet. Note that some of the new species then go back up into the top row or left column. (Everything starts with nitrogen or methane).
In red on the left side is the highly reactive intermediate .:CH radical carbene. This is formed from the electronic recombination of CH3+, which itself was formed from N2 radical cation reacting with methane (the N2 radical cation formed from photoionization of nitrogen using EUV light.) All the compounds formed downstream from .:CH are also boxed in red. Note that almost all the unsaturated intermediates propagate from this compound. As stated before, if there was no CH3+, these couldn’t be formed, so atmospheres with large H2 components (like Jupiter and Saturn) would shunt away from the CH3+ pathways, and only the unboxed (boring saturated aliphatic) compounds would be formed. Think of the boxed compounds as Titan special menu items. This graphic may be valid on exoplanet atmospheres as well, boxed compounds show only those components that could exist where H2 (or other CH3+ absorbing) are significantly absent and allow CH3+ to frag itself up on recombination.
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On the left side of the matrix are key reactive species. Along the top are some “target” compounds and reactive intermediates. At the cross point are the species formed when these two meet. Note that some of the new species then go back up into the top row or left column. (Everything starts with nitrogen or methane).
In red on the left side is the highly reactive intermediate .:CH radical carbene. This is formed from the electronic recombination of CH3+, which itself was formed from N2 radical cation reacting with methane (the N2 radical cation formed from photoionization of nitrogen using EUV light.) All the compounds formed downstream from .:CH are also boxed in red. Note that almost all the unsaturated intermediates propagate from this compound. As stated before, if there was no CH3+, these couldn’t be formed, so atmospheres with large H2 components (like Jupiter and Saturn) would shunt away from the CH3+ pathways, and only the unboxed (boring saturated aliphatic) compounds would be formed. Think of the boxed compounds as Titan special menu items. This graphic may be valid on exoplanet atmospheres as well, boxed compounds show only those components that could exist where H2 (or other CH3+ absorbing) are significantly absent and allow CH3+ to frag itself up on recombination.
The graphic below shows the pattern of dominant reactions that give nitrogenated products in Titan's atmosphere. On the left side are key reactive species, and on the top are "target" species, some of them used to derive the reactive species.
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Note that almost all products can be obtained using cyanomethylene carbene (:CH(CN)), although this may or may not be the dominant route.
Also note that almost all products ultimately come from unsaturated hydrocarbon chemistry, which themselves can only form in a relative absence of H2 (see above post.)
Only HCN would really be expected to form in a hydrogen-rich atmosphere. All the other pathways would be shut down due to CH5+ formation.
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Note that almost all products can be obtained using cyanomethylene carbene (:CH(CN)), although this may or may not be the dominant route.
Also note that almost all products ultimately come from unsaturated hydrocarbon chemistry, which themselves can only form in a relative absence of H2 (see above post.)
Only HCN would really be expected to form in a hydrogen-rich atmosphere. All the other pathways would be shut down due to CH5+ formation.
As requested (twice now) here is a comparison of some of the earlier pre-Cassini models with more recent atmospheric model production rates. Again normalized to Krasnopolsky et al., 2009:
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(Done using conditional formatting in EXCEL, most literature models only have 2 significant figures listed. Extra digits displayed after normalization to set conditional formatting.)
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(Done using conditional formatting in EXCEL, most literature models only have 2 significant figures listed. Extra digits displayed after normalization to set conditional formatting.)
Estimated depths solids vs. liquids from the various literature models:
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Toublanc et al had a very large ethane flux (acetylene, too.). The Krasnopolsky estimated solids are higher due to the estimated amount of solid haze "C2H2/HCN copolymer" produced.
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Toublanc et al had a very large ethane flux (acetylene, too.). The Krasnopolsky estimated solids are higher due to the estimated amount of solid haze "C2H2/HCN copolymer" produced.
A recent DPS abstract discusses some possibilities for Titan chemistry:
Horst et al. DPS Meeting 42 (2010) Abstract 36.20 "Formation Of Amino Acids And Nucleotide Bases In A Titan Atmosphere Simulation Experiment".
Direct link to abstract here.
One of the elements missing from all the above reactions is oxygen. There is not that much of it freely available in Titan's atmosphere. It is either in the form of H2O, from icy meteor infall, or from CO, or CO2. The 1 Gyr surface flux for CO2 varies between 10-100 cm for CO2 (high value in the Wilson and Atreya model), while the H2O surface flux (from meteors) is between 10 cm and lower (high value in the Raulin 1989 model). [A big Menrva crater splat may have caused actual water rain on Titan for a brief period according to an abstract a few years ago.]
The authors simulated Titan atmospheric conditions using N2, CH4, and CO2 and used a plasma discharge and generated molecules that incorporated oxygen. The abstract states that the following compounds were detected by GCMS (I'm assuming a direct injection of sample without a laboratory hydrolysis step before analysis.):
Amino acids glycine and alanine (presumably both enantiomers) were detected (but not H2NCH2CH2COOH?).
Also detected were the pyrimidine heterocycles cytosine, uracil, and thymine (but not orotic acid?)
As well as the fused heterocyclic imidazo-pyrimidine adenine (but not guanine?)
A slightly hyberbolic space.com article is here.
The key questions to relate this work are:
What was the overall yield of these compounds? Are we talking tiny trace amounts or significant geologically relevant surface deposits?
How doest this fit into the atmospheric models? Are these major pathways or minor chemical pathways?
How well does the PAMPRE experiment simulate Titan atmospheric tholins? (although PAMPRE is probably the best game in town for tholin simulation experiments)
How well does the simulated atmosphere reflect conditions in the upper atmosphere of Titan? At the critical formation zone? (Where is that for these pathways?)
And from a chemistry point of view, what are the electronic step-by-step mechanisms that make these?
Horst et al. DPS Meeting 42 (2010) Abstract 36.20 "Formation Of Amino Acids And Nucleotide Bases In A Titan Atmosphere Simulation Experiment".
Direct link to abstract here.
One of the elements missing from all the above reactions is oxygen. There is not that much of it freely available in Titan's atmosphere. It is either in the form of H2O, from icy meteor infall, or from CO, or CO2. The 1 Gyr surface flux for CO2 varies between 10-100 cm for CO2 (high value in the Wilson and Atreya model), while the H2O surface flux (from meteors) is between 10 cm and lower (high value in the Raulin 1989 model). [A big Menrva crater splat may have caused actual water rain on Titan for a brief period according to an abstract a few years ago.]
The authors simulated Titan atmospheric conditions using N2, CH4, and CO2 and used a plasma discharge and generated molecules that incorporated oxygen. The abstract states that the following compounds were detected by GCMS (I'm assuming a direct injection of sample without a laboratory hydrolysis step before analysis.):
Amino acids glycine and alanine (presumably both enantiomers) were detected (but not H2NCH2CH2COOH?).
Also detected were the pyrimidine heterocycles cytosine, uracil, and thymine (but not orotic acid?)
As well as the fused heterocyclic imidazo-pyrimidine adenine (but not guanine?)
A slightly hyberbolic space.com article is here.
The key questions to relate this work are:
What was the overall yield of these compounds? Are we talking tiny trace amounts or significant geologically relevant surface deposits?
How doest this fit into the atmospheric models? Are these major pathways or minor chemical pathways?
How well does the PAMPRE experiment simulate Titan atmospheric tholins? (although PAMPRE is probably the best game in town for tholin simulation experiments)
How well does the simulated atmosphere reflect conditions in the upper atmosphere of Titan? At the critical formation zone? (Where is that for these pathways?)
And from a chemistry point of view, what are the electronic step-by-step mechanisms that make these?
Thanks for the discussion and the link Mike. One point that troubles a bit regarding Titan atmospheric simulation experiments is the lack of evidence so far for lightning on Titan. Is it safe to conclude that the sun's effect on the upper atmosphere of Titan is ionizing enough that lightning is not required to produce active species that can form into organic compounds. Do the plasma discharges and rf zaps of these experiments simulate the sun's effects or is there a presumption of lightning going on in Titan's troposphere that Cassini has just not detected as yet. (Hmm ... one wonders with the T72 storm whether the RPWS instrument lightning sensors were positioned to pick up a discharge?)
Most of the formation chemistry seems to be happening at very high altitudes, likely above any cloud-based lightning. (sprites?)
I think the laboratory experiments are trying to get molecular nitrogen to ionize, so either Extreme UltraViolet radiation (got beam source?) or plasma discharge are the best ways to do it. My simplistic mind views a plasma discharge experiment as a scaled up version of a mass spectrometer antechamber.
I'd suppose the major ionizing sources in Titan's upper atmosphere would be sunlight (very shortwave radiation). Not sure of the role that cosmic rays, energized particles (solar and trapped in Saturn orbit) or other electronic discharges would play.
I think the laboratory experiments are trying to get molecular nitrogen to ionize, so either Extreme UltraViolet radiation (got beam source?) or plasma discharge are the best ways to do it. My simplistic mind views a plasma discharge experiment as a scaled up version of a mass spectrometer antechamber.
I'd suppose the major ionizing sources in Titan's upper atmosphere would be sunlight (very shortwave radiation). Not sure of the role that cosmic rays, energized particles (solar and trapped in Saturn orbit) or other electronic discharges would play.
LPSC 2011 abstract describes continuing efforts to identify heavier molecules from the Huygens GCMS using the flight spare at GSFC.
Trainer et al. LPSC 2011, Abstract 1399. "Laboratory Simulations of the Titan Surface to Elucidate the Huygens Probe GCMS Observations."
Why is this cool? Identifying heavier ions, and their relative amounts, can help constrain the atmospheric models and give a clue what's really down there on the surface.
A recent paper many of the same authors (Niemann et al. JGR (2010) E12006), showed that C2H6, C2H2, C2N2, and CO2 were detected at the surface after Huygens landed and volatilized materials in the muds. (Benzene (C6H6) was detected but couldn't be ruled out as atmospheric origin, maybe with more simulation analysis it can be confirmed?)
But even with mole fractions of these four components, if you normalize this and the atmospheric models to ethane, it can be seen that the Krasnopolsky model fits best for C2H2 and C2N2 (consistently 3x too low). The Cordier/Lavvas and Wilson and Atreya model have the predicted flux rate of C2N2 too low. See chart below:
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(also attached as a spreadsheet):
Trainer et al. LPSC 2011, Abstract 1399. "Laboratory Simulations of the Titan Surface to Elucidate the Huygens Probe GCMS Observations."
Why is this cool? Identifying heavier ions, and their relative amounts, can help constrain the atmospheric models and give a clue what's really down there on the surface.
A recent paper many of the same authors (Niemann et al. JGR (2010) E12006), showed that C2H6, C2H2, C2N2, and CO2 were detected at the surface after Huygens landed and volatilized materials in the muds. (Benzene (C6H6) was detected but couldn't be ruled out as atmospheric origin, maybe with more simulation analysis it can be confirmed?)
But even with mole fractions of these four components, if you normalize this and the atmospheric models to ethane, it can be seen that the Krasnopolsky model fits best for C2H2 and C2N2 (consistently 3x too low). The Cordier/Lavvas and Wilson and Atreya model have the predicted flux rate of C2N2 too low. See chart below:
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(also attached as a spreadsheet):
Detection of high altitude cirrus clouds in Titan's atmosphere: http://www.nasa.gov/mission_pages/cassini/...tan-clouds.html
QUOTE
"One of those is cyanoacetylene, a member of the nitrile family. The compound's distinctive signature made it the first to be picked up in the northern ice clouds by Voyager 1 and by Anderson and Samuelson"
This gives hope of much greater understanding of Titan's atmosphere in the years to come.
NASA’s “COSmIC” Simulator Helps Fingerprint Unknown Matter in Space
Located at NASA’s Ames Research Center, Moffett Field, Calif., this specialized facility, called the Cosmic Simulation Chamber (COSmIC)...
The chamber is the heart of the system. It recreates the extreme conditions that reign in space...
...Ames scientists delivered their first major milestone by coupling COSmIC with a cavity ringdown spectrometer, an extremely sensitive device that can detect the spectral fingerprint of matter at the molecular level.
Now, another major milestone has been achieved by coupling COSmIC with a time-of-flight mass spectrometer, an ultra-sensitive device that detects the mass of matter at the molecular level....
To understand Cassini’s data, scientists need this very powerful, very sensitive new tool. They will begin their analysis by forming molecules and species in the lab, measuring them in situ (inside their environment without disturbing them), and then trying to match their identity to Titan’s unknown aerosol molecules.
“Titan’s upper atmosphere data shows a rich spectrum. We will recreate those data in the lab and compare them to Cassini’s data. If they fit, great. If not, we will try something else. We will know when we are coming close to understanding them. We now have the right tool to do this,” said Salama.
Click to view attachment
NASA’s “COSmIC” Simulator Helps Fingerprint Unknown Matter in Space
Located at NASA’s Ames Research Center, Moffett Field, Calif., this specialized facility, called the Cosmic Simulation Chamber (COSmIC)...
The chamber is the heart of the system. It recreates the extreme conditions that reign in space...
...Ames scientists delivered their first major milestone by coupling COSmIC with a cavity ringdown spectrometer, an extremely sensitive device that can detect the spectral fingerprint of matter at the molecular level.
Now, another major milestone has been achieved by coupling COSmIC with a time-of-flight mass spectrometer, an ultra-sensitive device that detects the mass of matter at the molecular level....
To understand Cassini’s data, scientists need this very powerful, very sensitive new tool. They will begin their analysis by forming molecules and species in the lab, measuring them in situ (inside their environment without disturbing them), and then trying to match their identity to Titan’s unknown aerosol molecules.
“Titan’s upper atmosphere data shows a rich spectrum. We will recreate those data in the lab and compare them to Cassini’s data. If they fit, great. If not, we will try something else. We will know when we are coming close to understanding them. We now have the right tool to do this,” said Salama.
Click to view attachment
Interesting link posted on the Cassini Huygens Yahoo group:
http://dx.doi.org/10.1038/ngeo1147
http://dx.doi.org/10.1038/ngeo1147
The Cool Way to PAH's in Titan's Upper Atmosphere
A recent article provides a new mechanism to get to fused ring systems at Titan-like temperatures and low pressures in the upper atmosphere. It is called EAM, or ethynyl addition mechanism. (The previously described HACA mechanism was adapted from high-temperature combustion studies).
Click to view attachment
The sequence starts with styrene, which can be formed by phenyl radical reacting with ethylene. This is known at high temperatures, but may or may not be valid at Titan upper atmosphere conditions. There may be an ion neutral chemistry route to styrene as well, but a a quick glance at the previous Titan atmospheric models, I didn't see styrene specifically mentioned.
Ethynyl radical (generated from EUV photodissociation of acetylene) attacks styrene at the ortho position to the vinyl substituent. The double bond opens, then the radical closes back in to kick out hydrogen radical, and you get vinylacetylenebenzene. This is almost barrierless and is pure downhill, so it can happen at very low temperatures.
[The authors calculated the collision rate at a simulated Titan upper atmosphere conditions, and found that the radiacal lifetime is shorter than any expected collision rates. It was estimated at 70 ms between molecular collisions at 90 K and 1E-6 mbar. So there is little chance ('bout 10%) another molecule can bump into the vinylacetylenebenzene and help remove all that excess energy. No relaxation, so the compound kicks out hydrogen radical. For those lucky molecules that do hit something, the benzene radical could become the final product of reactions and can then go on to do other things with other molecules...]
Another attack of acetylene radical, this time at the beta carbon (interior) of the alkyne substituent, generates a new radical that undergoes cyclization to form a bicyclic napthalene, but with a bonus alkyne substituent hanging off the end. It turns out this sequence is pretty much barrierless and downhill as well.
Why doesn't it form napthalene in the first sequence? It turns out that there is a pretty big activation energy barrier. So while that end product would be thermodynamically more downhill, the transition state energy mountain you need to go over is too big. So it shunts to the vinylacetylene benzene.
Here is a free link to the full paper. It is a pretty hardcore computational mechanism paper, but it does a great job of showing the snapshot-by-snapshot molecular movements and gyrations, including calculated energetics of the EAM process and alternatives that don't happen.
http://www.chem.hawaii.edu/Bil301/Kaiser%20Paper/p244.pdf
A recent article provides a new mechanism to get to fused ring systems at Titan-like temperatures and low pressures in the upper atmosphere. It is called EAM, or ethynyl addition mechanism. (The previously described HACA mechanism was adapted from high-temperature combustion studies).
Click to view attachment
The sequence starts with styrene, which can be formed by phenyl radical reacting with ethylene. This is known at high temperatures, but may or may not be valid at Titan upper atmosphere conditions. There may be an ion neutral chemistry route to styrene as well, but a a quick glance at the previous Titan atmospheric models, I didn't see styrene specifically mentioned.
Ethynyl radical (generated from EUV photodissociation of acetylene) attacks styrene at the ortho position to the vinyl substituent. The double bond opens, then the radical closes back in to kick out hydrogen radical, and you get vinylacetylenebenzene. This is almost barrierless and is pure downhill, so it can happen at very low temperatures.
[The authors calculated the collision rate at a simulated Titan upper atmosphere conditions, and found that the radiacal lifetime is shorter than any expected collision rates. It was estimated at 70 ms between molecular collisions at 90 K and 1E-6 mbar. So there is little chance ('bout 10%) another molecule can bump into the vinylacetylenebenzene and help remove all that excess energy. No relaxation, so the compound kicks out hydrogen radical. For those lucky molecules that do hit something, the benzene radical could become the final product of reactions and can then go on to do other things with other molecules...]
Another attack of acetylene radical, this time at the beta carbon (interior) of the alkyne substituent, generates a new radical that undergoes cyclization to form a bicyclic napthalene, but with a bonus alkyne substituent hanging off the end. It turns out this sequence is pretty much barrierless and downhill as well.
Why doesn't it form napthalene in the first sequence? It turns out that there is a pretty big activation energy barrier. So while that end product would be thermodynamically more downhill, the transition state energy mountain you need to go over is too big. So it shunts to the vinylacetylene benzene.
Here is a free link to the full paper. It is a pretty hardcore computational mechanism paper, but it does a great job of showing the snapshot-by-snapshot molecular movements and gyrations, including calculated energetics of the EAM process and alternatives that don't happen.
http://www.chem.hawaii.edu/Bil301/Kaiser%20Paper/p244.pdf
I saw a talk given at the AGU conference last week mentioning that far infra-red spectral observations suggest that the atmospheric hazes probably have different composition than the traditional Titan tholins. More lab work will be helpful in comparing various compositions to the observations. This is related to post #79 I believe. Here is the abstract...
ABSTRACT FINAL ID: P33F-0
TITLE: Spectral and vertical distribution properties of Titan’s particulates from thermal-IR CIRS data: Physical Implications
SESSION TYPE: Oral
SESSION TITLE: P33F. Titan: An Earth-Like World II
AUTHORS (FIRST NAME, LAST NAME): Carrie M Anderson1, Robert Samuelson2, 1, Sandrine Vinatier3
INSTITUTIONS (ALL): 1. Solar System Exploration Divis, NASA---GSFC, Greenbelt, MD, United States.
2. Astronomy, University of Maryland, College Park, MD, United States.
3. LESIA , Observatoire de Paris-Meudon, Meudon , France.
Title of Team:
ABSTRACT BODY: Analyses of far-IR spectra between 20 and 560 cm-1 (500 and 18 μm) recorded by the Cassini Composite Infrared Spectrometer (CIRS) yield the spectral dependence and the vertical distribution of Titan’s photochemical aerosol and stratospheric ice clouds. Below the stratopause (~300 km) the aerosol appears to be incompletely mixed for the following reasons: 1) the altitude dependence of the aerosol mass absorption coefficient is larger at higher altitudes than at lower altitudes, 2) the aerosol scale height varies with altitude, which implies some kind of layering effect, and 3) the aerosol abundance varies with latitude.
The spectral shape of the aerosol opacity appears to be independent in altitude and latitude below the stratopause, even though inhomogeneities in the abundance appear to be prevalent throughout this altitude region. This implies that aerosol chemistry is restricted to altitude regions above the stratopause, where pressures are less than ~0.1 mbar. The aerosol exhibits an extremely broad emission feature with a spectral peak at 140 cm-1 (71 μm), which is not evident in laboratory simulated Titan aerosols (tholin) that are created at pressures greater than 0.1 mbar.
A strong broad emission feature centered roughly around 160 cm-1 corresponds very closely to those found in nitrile ice spectra. This feature is pervasive throughout the region from high northern to high southern latitudes. The inference of nitrile ices is consistent with the highly restricted altitude ranges over which these features are observed, and appear to be dominated by HCN and HC3N. At low and moderate latitudes these clouds are observed to be located between 60 and 100 km, whereas at high northern latitudes during northern winter these clouds are observed at altitudes between 150 and 165 km. The ubiquitous nature of these nitrile ice clouds is inconsistent with a simple meridional circulation concept, suggesting that the true dynamical situation is more complex.
KEYWORDS: [6281] PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Titan, [5405] PLANETARY SCIENCES: SOLID SURFACE PLANETS / Atmospheres, [5422] PLANETARY SCIENCES: SOLID SURFACE PLANETS / Ices.
(No Image Selected)
(No Table Selected)
SPONSOR NAME: Carrie Anderson
Additional Details
Previously Presented Material: Most of the material was published in Icarus 212, 762-778 in 2011.
Contact Details
CONTACT (NAME ONLY): Carrie Anderson
CONTACT (E-MAIL ONLY): carrie.m.anderson@nasa.gov
ABSTRACT FINAL ID: P33F-0
TITLE: Spectral and vertical distribution properties of Titan’s particulates from thermal-IR CIRS data: Physical Implications
SESSION TYPE: Oral
SESSION TITLE: P33F. Titan: An Earth-Like World II
AUTHORS (FIRST NAME, LAST NAME): Carrie M Anderson1, Robert Samuelson2, 1, Sandrine Vinatier3
INSTITUTIONS (ALL): 1. Solar System Exploration Divis, NASA---GSFC, Greenbelt, MD, United States.
2. Astronomy, University of Maryland, College Park, MD, United States.
3. LESIA , Observatoire de Paris-Meudon, Meudon , France.
Title of Team:
ABSTRACT BODY: Analyses of far-IR spectra between 20 and 560 cm-1 (500 and 18 μm) recorded by the Cassini Composite Infrared Spectrometer (CIRS) yield the spectral dependence and the vertical distribution of Titan’s photochemical aerosol and stratospheric ice clouds. Below the stratopause (~300 km) the aerosol appears to be incompletely mixed for the following reasons: 1) the altitude dependence of the aerosol mass absorption coefficient is larger at higher altitudes than at lower altitudes, 2) the aerosol scale height varies with altitude, which implies some kind of layering effect, and 3) the aerosol abundance varies with latitude.
The spectral shape of the aerosol opacity appears to be independent in altitude and latitude below the stratopause, even though inhomogeneities in the abundance appear to be prevalent throughout this altitude region. This implies that aerosol chemistry is restricted to altitude regions above the stratopause, where pressures are less than ~0.1 mbar. The aerosol exhibits an extremely broad emission feature with a spectral peak at 140 cm-1 (71 μm), which is not evident in laboratory simulated Titan aerosols (tholin) that are created at pressures greater than 0.1 mbar.
A strong broad emission feature centered roughly around 160 cm-1 corresponds very closely to those found in nitrile ice spectra. This feature is pervasive throughout the region from high northern to high southern latitudes. The inference of nitrile ices is consistent with the highly restricted altitude ranges over which these features are observed, and appear to be dominated by HCN and HC3N. At low and moderate latitudes these clouds are observed to be located between 60 and 100 km, whereas at high northern latitudes during northern winter these clouds are observed at altitudes between 150 and 165 km. The ubiquitous nature of these nitrile ice clouds is inconsistent with a simple meridional circulation concept, suggesting that the true dynamical situation is more complex.
KEYWORDS: [6281] PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Titan, [5405] PLANETARY SCIENCES: SOLID SURFACE PLANETS / Atmospheres, [5422] PLANETARY SCIENCES: SOLID SURFACE PLANETS / Ices.
(No Image Selected)
(No Table Selected)
SPONSOR NAME: Carrie Anderson
Additional Details
Previously Presented Material: Most of the material was published in Icarus 212, 762-778 in 2011.
Contact Details
CONTACT (NAME ONLY): Carrie Anderson
CONTACT (E-MAIL ONLY): carrie.m.anderson@nasa.gov
not sure this is the best topic to post it to, this interesting paper was published on today's Nature:
Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle
Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle
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