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 1 2010, 03:36 AM
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
In the presence of N2 or Ar, ionization occurs first in the carrier gas to form either N2 radical cation or Ar radical cation. Abstraction of an electron from methane (CH4) also splits out a hydrogen radical, generating CH3+ a carbenium ion.
In the presence of H2, the photoelectron ejection creates a molecular hydrogen radical cation (= two protons with only one lonely electron in the molecular orbital keeping it all together..). This reacts with another H2 molecule to grab a hydrogen atom and also split out a hydrogen radical. This forms H3+, which is another 3-center, 2-electron triangular association. This is a very common molecule in deep space. But it is not happy. It has a very low proton affinity. It wants to give the proton away. (Fun fact: only dioxygen, argon, neon, and helium have a lower proton affinity – that means that H3+ can protonate (transfer it’s proton) to everything else – it is almost the ultimate superacid.) It is so strong a superacid, that it can actually protonate methane to make CH5+, a carbonium ion. This also has a 3-center, 2-electron triangular association. This time it is between the sp3 hybridized orbital on the carbon, and the two 1s orbitals of the hydrogen. The two electrons are shared among these three orbitals. In the presence of pure CH4 (graphic above), electron ejection generates methane radical cation, which steals a hydrogen atom from methane to create CH5+ and a remnant CH3 radical. As we will see much later, the CH3 radical participates in radical chemistry processes that lead primarily to simple saturated organics. To get the really good funky stuff, we need to make ethylene or other unsaturated intermediates. So N2 and Ar atmospheres (with CH4) generate CH3+, but CH4 and H2 atmospheres generate CH5+. That is the key difference. The downstream chemistry of these two cation intermediates determine the product mix. Here's my memory trick: Carbenium ions (CH3+) are fun. Carbonium ions (CH5+) are boring. -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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Jun 1 2010, 07:59 PM
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#3
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Member Group: Members Posts: 153 Joined: 14-August 06 Member No.: 1041 |
Here's my memory trick: Carbenium ions (CH3+) are fun. Carbonium ions (CH5+) are boring. So three's a threesome and five is a crowd? Fun stuff Mike. Ok, fun to a few of us. It seems like, with all the radical chemistry we should find more nitrated compounds. Where are they and/or why not? |
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