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]

Acetylene (C2H2) [HCCH]

Acetylene has several formation pathways. The major route, although still only accounting for 38% of all acetyelene formation, is the photolytic dehydrogenation of ethylene, shown below:



Digging into serious detail, the way I understand this, the photon hits the pi-electron cloud and kicks it up to an excited state antibonding molecular orbital. This would be the LUMO (lowest unoccupied molecular orbital), which is now populated in this new singlet excited state. The electrons are still paired (singlet), they are just having nothing to do with each other (antibonding). While the pi-system molecular orbital is a bilobal fuzzy blob between the two carbon atoms, the antibonding molecular orbital faces away from the space between the two atoms. The spin paring is key to molecular hydrogen kicking out. All at once, the two electrons in both C-H bonds swap partners and you get molecular hydrogen and acetylene. Symmetry is preserved and all is harmonious.

If the two hydrogens came off as two radicals in one step, that would imply an initial singlet (most ground states are spin-paired) going to a triplet (spins-unpaired) state. This is a “forbidden” transition. It can still happen, it is just not normally allowed (and is thus rarer). It is also possible that the singlet excited ethylene could switch to a triplet excited ethylene, then kick out two hydrogen radicals. This switch is called “intersystem crossing” and constitutes a bonus step along the reaction pathway.