Atmospheric Chemistry of Titan Options

[Juramike]

Here is a "Benzene-O-Vision" graphic showing the amount of benzene and phenyl radicals at high altitudes on Titan. This is based on detections of benzene and phenyl radical (which recombined in the sample chamber to make benzene) using the INMS instrument during closest approach. The numbers are normalized to constant pressure altitude, roughly 1000 km.



The data was taken from Table 1 in: Vuitton et al, Journal of Geophysical Research 113 (2008) E05007. "Formation and distribution of benzene on Titan". doi: 10.1029/2007JE002997 [EDIT 5/24/10: Article freely available here] and overlaid on a map of Titan.

The authors mentioned that the errors in these measurements are 20%.

These detections are well above the detached haze layer. Most are at the same sun azimuth angle. (T23 observation had the lowest angle.) Assuming that the temporal difference is minimal (each dot is from a different flyby), there doesn't appear to be an obvious correlation with latitude.

This graphic does show that benzene is present even waaaay up in the thermosphere and ionosphere.

[Juramike]

Acetonitrile (CH3CN)



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)



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]