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]

Structure of Benzenium (C6H7+)



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.