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]

Cyanomethlyene carbene [:CH(CN)]



A very recent article (June 2, 2010 issue of PNAS) by Imanaka and Smith has provided laboratory evidence that high energy photons can ionize nitrogen and cause it to incorporate it into Titan’s organics much more easily than previously thought.

The process is the initial photoionization of molecular nitrogen by EUV photons (wavelenghts < 60 nm) which then makes molecular nitrogen radical cation by blowing an electron out of the molecule. At some point, another electron recombines with the nitrogen molecule (cue dropping bomb sound) which releases a huge amount of energy. The energy released blows apart the dinitrogen triple bond and we are left with a “normal” nitrogen atom radical nitrene (nitryne) and an “excited” nitrogen atom radical nitrene (nitryne). (this is the stepwise sequence of the concerted sequence discussed for HCN – the rates of these steps were deliberately left of the graphic since the Imanaka and Smith work will definitely supercede the Krasnopolsky estimated models.)

The excited nitryne atom is likely in an excited state that accesses D-orbitals. These may be in unpaired spin-coupled state, but well run through the mechanisms assuming it acts like a triradical. (i.e. the actual process may be more concerted). In the case to make cyanomethylene carbene, the excited nitryne reacts with an alkyne. Drawing a stepwise mechanism, the first thing that happens is one of the pi-electrons of the triple bonds forms a new bond with one of the electrons of the nitryne. This now makes a carbon radical nitrene. (still three unpaired electrons in the system). A C-H bond breaks, and hydrogen radical (H.) goes away, and the remaining electron on carbon now forms a carbon-nitrogen double bond with one of the nitrene electrons.

A double-bond equilibration gives cyanomethylene carbene. But what does this molecule look like?
Spectroscopic data suggests that this molecule in the ground state is “close” to a linear molecule, with an H-C-(CN) bond close to 180 degrees That means that the likely hybridization on the “carbene” carbon is sp with the unpaired electrons in two p orbitals in a triplet state. (Ground state suggests it is in an unpaired spin-uncoupled triplet configuration, I’d guess on Titan that these molecules are likely in an excited unpaired but spin-coupled triplet configuration.) Why is this important? It’s probably academic, but it implies that many of the downstream steps could be concerted. This might become important if we deal with molecules (e.g. cis or trans double bonds) with stereochemistry, in this case the stereochemistry would be preserved. In a triplet stepwise reaction, there is always a chance for bond twisting before the second bond snaps shut, causing a loss of stereochemistry.

The :CH(CN) intermediate may be the key intermediate in the formation of tholins and for the incorporation of bonus* nitrogen into Titan’s organics.
It is also an intermediate that gives another route to many of Titan’s organics, such as cyanoacetylene, cyanogens, acrylonitrile, and ethylacetonitrile.

(*more that previously thought)