Atmospheric Chemistry of Titan |
Atmospheric Chemistry of Titan |
May 2 2010, 03:38 AM
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
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. -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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Jun 2 2010, 04:15 AM
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
So what does CH5+ do? Basically(!), not much. It is just a transporter for a proton that it really doesn’t want. Once it finds a better proton acceptor (HCN used as an example in the graphic below), it gives the proton away and reverts back to a happy neutral CH4 molecule. All done.
CH3+ on the other hand, is an electron-deficient intermediate. It wants electrons - badly. It is willing to steal them from another molecule. For instance, in the graphic above, CH3+ steals electrons from a methane (CH4) molecule, kicking out a hydrogen molecule in the process. These are all 2-electron processes and are shown with a full double-headed arrow. AS the two electrons from a C-H bond goes to form a new C-C bond, another two electrons from another C-H bond jump to make a bond between the two H atoms, thus making a dihydrogen molecule. The end result is an ethyl cation. This is more stable since the electron-deficient carbon’s empty orbital has partial overlap with one of the C-H orbitals. This is called hyperconjugation. Carbenium ion stability is tertiary>secondary>primar>>methyl. The ethyl cation can be thought of as an electron-deficient ethane cation. But it can also be thought of as protonated ethylene. If a suitable base is around, it can pass off the proton (methane wants it even less, but HCN will accept it) and generate ethylene. Ethylene is unsaturated and has a lot of new modes of reactivity due to the double bond – we’ll be seeing it much more. One can build a lot of neat things up from ethylene, and thus CH3+ is the grandaddy of all the neat stuff. The game of proton “hot potato” cascades down due to the energies of proton affinity. Protons really, really hate to be alone. Even protonated helium is better than a helium atom + a lonely proton. The following graphic shows a big proton cascade for many Titan-relevant molecules: It starts up high then goes down to the left. This chart and energy numbers are only relevant for gas phase species. Energies in the liquid (or even solid) phase will be different due to better solvation of the proton and protonated species (which is also solvent dependent – protonated water in hydrocarbon solvent will be less happy than if it was protonated water in water). Still, it should be no surprise that ammonia or other organic amines should be the ultimate proton sink on Titan’s surface. -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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