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[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]

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).



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