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]

The radical cationic, carbonium, and carbenium species up in the atmosphere are high energy intermediates. They can exist up there (and in the MS sample chambers) because they are in rarefied environments. They don't bump into other molecules that often.

As these compounds descend in the atmosphere, they will encounter other molecules, bump into things more often, and be able to exchange energies and react much, much better. When you get down to the surface (or in solution) there are a lot of opportunities for molecules to react and find a happy equilibrium. High-energy structures will be just fleeting intermediates on the way to more more stable molecule. They won't be able to last more than a molecular vibration before they bump into something and react or transfer their energy.

'Course at Titan's low temperatures, some of the metastable high-energy compounds might get trapped out in a matrix and not be able to react. One of the best ways to keep things from reacting is to cool it down, putting it in the freezer so to speak and keep it from getting over the energy hump to the next state. If there is a high energy transition state with higher activation energies away from it, you can freeze it out. (IIRC, carbenes [think of it as "methylene diradical" or :CH2] has been studied in a frozen Argon matrix).


I'm not real sure about the Lunine abstract....methane is very non-polar and would not be happy solvating a carbocation. I think it would be EXTREMELY difficult for methane to spontaneously dissociate to a carbocation-hydride. (This doesn't happen in the lab at terrestrial temperatures.) But if a carbocation (or superacid) was plopped or sprinkled into a lake, it would do something.

Definitely not comfortable with referring to normal methane (CH4) as a "protic" solvent. That implies that methane is able to donate a proton. (CH4--> -:CH3 + H+). With a pKa of 45, that won't happen.

BUT (and I think this is where the abstract is going) if a stronger superacid was sprinkled into a lake, the proton affinities might force methane to take on a proton and make CH5+ (again, methane is not happy about this.) Proton affinities show the clear progression CH4-->CH3-CH3->acetylene-->ethylene-->H2O--<ammonia. (Table here: Wikipedia/Proton Affinity (data page)). So if CH5 (or H3+) was dribbled into a lake it would eventually work itself down the chain to protonating water to make hydronium. (Dunno the proton affinity for clathrate, but I assume it's similar or slightly less than water, protonation should break up the clatrate matrix - that would be a neat and "easy" experiment to try.) The ultimate sink on Titan should be ammonia to generate ammonium ion.

[Littlebit]

This is enough to blast an electron out of the molecular (or even atomic) orbital and create a wierd little species called a radical cation. This is a single electron process, so one of the electrons in the molecule is now unpaired, thus a radical. It is also charged positively, since an electron was ripped out of the system. Radical cations are pretty exotic here on Earth. They are usually only found in the vacuum ionization chamber of your local LCMS or GCMS. They do very weird things.

Is methane+ a right wing or left wing radical?


Enjoy your posts, as always Juramike - we are still out here!

[ngunn]

Is methane+ a right wing or left wing radical?

Could be either way, depending on the geometric orientation of the molecule and where you view it from.