Many ways to fall
Below is a graphic showing an energy diagram for two photochemical methyl neutral cleavage pathways:
Click to view attachmentA typical energy diagram starts at one energy level, passes through a highest level transition state (the high energy point - indicated by bracket "TS"), then might drop down to an local minimum intermediate state, then climb back up to another transition state before dropping down to a lower energy state for the end product.
Going from the midpoint to the right, a methyl-hydrogen bond cleaves homolytically (one electron goes each way) to form two radical intermediates. This can react with another methyl radical (slight energy increase due to the initial electron repulsion of the orbitals - gotta have enough speed to overcome that) to then join into a formal bonding arrangement and make ethane. In this case the two unpaired electrons jump in together to form a new bond.
To the left, there is another option. Two electrons from the methyl C-H bond can jump over to a neighboring hydrogen to make an H-H bond. Simultaneously, the two electrons in the second C-H bond can jump onto the carbon atom. In this case both electrons move as a pair, and there are two pairs moving simultaneously in a four electron process. The two electrons on the carbon are paired up and this species is referred to as a singlet carbene.
Singlet carbenes have a new reaction pathway to them, they can muscle in and insert directly into a C-H bond. This is a mode of reactivity displayed by many metal species - in fact, C-H activation/insertion has been one of the really hot topics in organic and organometallic chemistry in the last 20 years. I always think of it as a transient 3 center 4 electron bond just before the molecular orbitals switch to the incoming carbon center. This allows a singlet carbene to insert directly into a neutral unactivated methane molecule.
EUV photons have a heckuva lot of energy. A 100 nm photon can deliver a whopping 288 kcal/mol. This is enough to break any bond in the system and overcome a lot of activation energy barriers including either the radical or singlet carbene pathways above.
And it gets uglier.
There are even more possibilities for the neutral methane dissociation, shown below:
Click to view attachment Aside from the diradical and singlet carbene pathways, there is also a triplet carbene pathway (think of as a diradical, both electrons are unpaired in different orbitals.) which spits out two hydrogen radicals. There is also the possibility of spitting out three hydrogen atoms one as H2 and one H radical. (thus leaving the electron deficient C-H with two paired electrons in one orbital and one unpaired.) With a Lyman alpha photon (flux responsible for 75 of the neutral methyl cleavage) the different pathways go in different amounts, with the radical pathway #1, and the singlet carbene as #2.
[There is one pathway that doesn't go well at this wavelength, that is the reaction where two hydrogen radicals leave but the carbene is in a singlet state, this shows at some point the triplet carbene went into a singlet state, a classic example of an intersystem crossing, which is technically considered a "forbidden" transition. Not to say it can't ever happen, it's just not cool from a symmetry point of view and is thus disfavored.]
Not mentioned here, but still possible at higher energies, is the "total fraggo" option where only a carbon atom remains with four electrons.
Thus, the high-energy inititiation of a neutral cascade by photodissociation has many different pathways. Similarly, high-energy ion-neutral cascades also have many possible pathways for each intermediate.
In the next post, we'll see how the initial ion-neutral photodissociation of methane is affected by the other atmospheric components, whether nitrogen, argon (both activators for aromatic formation) or hydrogen (inhibitor of aromatic formation.) This represents the fork in the road between Titan atmospheric chemistry (lotsa aromatics), and Jovian/Saturn atmospheric chemistry (not-so-much aromatics).