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Atmospheric Chemistry of Titan
Juramike
post May 2 2010, 03:38 AM
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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.

Attached Image


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.


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Juramike
post May 5 2010, 02:59 AM
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Into the Wierdness: Ion-neutral chemistry

(A major bummer about Titan chemistry literature is the lack of detailed electron-pushing mechanisms and especially frustrating is the lack of clear electron accounting. Many radical cations and even neutral radical in the figures do not show the unpaired electron on the structure. In my diagrams I'll try to detail the play-by-play.)

Much of Titan's chemistry is driven by upper atmosphere high-energy photochemistry. It is likely that plasma chemistry may play a small part, but the major driver is dayside solar radiation chemistry.

One of the biggest surprises of the Cassini mission is the amount of complex hydrocarbons (especially benzene!) found in the upper atmosphere. The thermo and ionosphere are pretty impressive chemical factories.

In these reaches ion-neutral chemistry plays a big role. The first step is the whacking of a molecule by a high energy photon (were talking Extreme ultraviolet, around 50-100 nm - this is mega-energy and waaay above your sunlamp). 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.

One of the things they like to do is bite into a covalent bond and make a charged species, and generate a neutral radical. (Think: Radical-cation + neutral --> Cation + uncharged radical)

Sometimes, structurally complex radical cations will frag up all by themselves in a similar way (radical-cation --> cation + uncharged radical). This happens in your local LCMS and GCMS, but that's a story for a different day.

Attached Image


Here is an example that shows a nitrogen molecule (N2) getting whacked by a high energy photon, making a radical cation, then homolytically (single electron style) biting into a neutral hydrogen atom. One hydrogen gets stolen and gloms onto the dinitrogen to make a cation, while the remant hydrogen atom (now a radical since the electron is unpaired in the atomic orbital) goes flying off. But the dinitrogen cation is not happy, it has a very weak Proton Affinity and wants to give away the proton. When it finds a neutral methane molecule it can donate the proton to the methane molecule. (Y'all can think of it as an electophilic attack by the proton on the methyl, or a nucleophilic attack by electrons in the C-H orbital to the proton - either way it is the same). Compared to most stuff we are used to, neither methane or the nitrogen really want to be protonated. But in this case the nitrogen want the proton much less than the methane. So the methane molecule gets stuck with it for the moment. (Kinda like regifting an ugly sweater).

So you end up with a CH5+ cation. Which is weird. Drawing a pentavalent carbon is the best way to get ridiculed by your colleagues, or lose 10 points on a test score, depending on your situation. In the rarefied upper atmosphere of Titan, it is sorta tolerated (remember, the methyl isn't super happy about that extra proton). Structurally, two of the hydrogen atoms of the carbonium ion (normal R3C+ is a "carbenium" ion) are in a 3-center 2-electron bond. You can think of it as a hydrogen molecule sidebound to a carbenium ion. So these two hydrogens are in a special relationship. (But they can exchange out with the others in the structure).

Think of the methyl carbonium ion as a super acid. It wants to get rid of that proton (but can't to dinitrogen).

It turns out that CH5+ plays a special role, it actually prevents exciting chemistry from happening.


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Posts in this topic
- Juramike   Atmospheric Chemistry of Titan   May 2 2010, 03:38 AM
- - Juramike   Into the Wierdness: Ion-neutral chemistry (A maj...   May 5 2010, 02:59 AM
- - ngunn   Thanks for that very interesting explanation Mike....   May 6 2010, 11:20 AM
- - Juramike   The radical cationic, carbonium, and carbenium spe...   May 6 2010, 01:50 PM
|- - Littlebit   QUOTE (Juramike @ May 6 2010, 06:50 AM) T...   May 11 2010, 09:22 PM
|- - ngunn   QUOTE (Littlebit @ May 11 2010, 10:22 PM)...   May 11 2010, 09:56 PM
- - Juramike   Cute. To be really, really anal, a radical shoul...   May 12 2010, 12:30 AM
- - Juramike   Purple Haze or Where It’s At A paper by Lavvas et...   May 12 2010, 11:46 PM
|- - rlorenz   QUOTE (Juramike @ May 12 2010, 06:46 PM) ...   May 20 2010, 07:17 PM
- - Juramike   Titan's organic chemistry is driven by photoch...   May 16 2010, 09:52 PM
- - Juramike   Titan's Chaotic Chemistry Titan is a syntheti...   May 18 2010, 12:30 AM
- - Juramike   Many ways to fall Below is a graphic showing an e...   May 20 2010, 04:17 AM
- - Juramike   I agree that the reality is probably a lot more co...   May 21 2010, 05:42 PM
- - Juramike   It turns out that the methane is only part of the ...   May 31 2010, 04:57 AM
- - Juramike   In the presence of N2 or Ar, ionization occurs fir...   Jun 1 2010, 03:36 AM
|- - Littlebit   QUOTE (Juramike @ May 31 2010, 08:36 PM) ...   Jun 1 2010, 07:59 PM
- - Juramike   Nitrogen does get incorporated. It is primarily i...   Jun 1 2010, 08:34 PM
- - Juramike   So what does CH5+ do? Basically(!), not much....   Jun 2 2010, 04:15 AM
- - Juramike   Even on electron quenching of the CH5+ radical, it...   Jun 3 2010, 03:32 AM
- - ngunn   About Titan chemistry, not necessarily atmospheric...   Jun 3 2010, 09:01 PM
- - Juramike   Great catch!! From the press release: ...   Jun 4 2010, 12:38 AM
- - Bill Harris   Mike, This has been a good discussion of the chem...   Jun 4 2010, 10:02 PM
- - Juramike   The low temperatures at the surface and lower atmo...   Jun 4 2010, 10:49 PM
- - Juramike   For the moment, the Clark et al. paper can be foun...   Jun 4 2010, 10:52 PM
- - Juramike   Article in press: Strobel, D.F. Icarus (2010) Art...   Jun 6 2010, 07:58 PM
- - Juramike   H2 is the key reagent that shunts the pathway to t...   Jun 8 2010, 12:43 AM
- - Juramike   Titan’s atmospheric chemistry is driven by sunligh...   Jun 9 2010, 02:16 AM
- - remcook   Starting on Monday: http://fd147.univ-rennes1.fr/a...   Jun 9 2010, 07:04 AM
- - Juramike   Very cool! Here is an EXCEL conditional forma...   Jun 10 2010, 02:58 AM
|- - rlorenz   QUOTE (Juramike @ Jun 9 2010, 09:58 PM) H...   Jun 12 2010, 01:00 AM
|- - Juramike   As requested (twice now) here is a comparison of s...   Aug 25 2010, 01:41 AM
- - Bill Harris   This is mindboggling. The potential scope of the...   Jun 10 2010, 02:31 PM
- - Shaka   Will the final exam be essay or multiple choice?   Jun 10 2010, 11:59 PM
- - Juramike   I'm gonna give away the answers!   Jun 11 2010, 12:30 AM
- - Juramike   Enter the mechanism Over the next several graphic...   Jun 11 2010, 05:51 AM
- - Bill Harris   QUOTE (Juramike @ Jun 10 2010, 07:30 PM) ...   Jun 11 2010, 04:13 PM
- - Juramike   Ethane (C2H6) [CH3CH3] Almost all ethane is for...   Jun 12 2010, 03:40 AM
- - Juramike   Ethylene (C2H4) [H2C=CH2] Ethylene is a key inter...   Jun 13 2010, 03:21 AM
- - Juramike   Acetylene (C2H2) [HCCH] Acetylene has several for...   Jun 14 2010, 02:52 AM
- - Juramike   Methyl acetylene (1-propyne) (C3H4) [CH3CCH] This...   Jun 15 2010, 02:58 AM
- - jekbradbury   QUOTE (Juramike @ Jun 14 2010, 10:58 PM) ...   Jun 15 2010, 07:35 PM
- - Juramike   Whoops. You are right, it is allene not cumene. ...   Jun 15 2010, 10:39 PM
- - Juramike   [Well, whaddya know...cyclopropenone has been dete...   Jun 16 2010, 09:28 PM
- - Juramike   Ethyl radical (.C2H5) [.CH2CH3] - key intermediate...   Jun 17 2010, 01:46 AM
- - Bill Harris   Mike, one thing I notice is that many of the react...   Jun 17 2010, 02:47 AM
- - Juramike   H+?? Dangit. Those should all be H. hydrogen r...   Jun 17 2010, 03:03 AM
- - Bill Harris   Oh, H-radical as in "H-dot"? They look...   Jun 17 2010, 05:08 AM
- - Juramike   Alright y'all, I'm real sorry about this, ...   Jun 19 2010, 03:06 AM
- - Juramike   Propane (C3H8) [CH3CH2CH3] The straightforward ...   Jun 19 2010, 03:21 AM
- - Juramike   Went back and fixed all diagrams in the different ...   Jun 19 2010, 12:45 PM
- - Bill Harris   Looks good, Mike. Sorry to have tossed a monkey w...   Jun 20 2010, 07:18 PM
- - Juramike   Diacetylene (C4H2) [HCC-CCH] - also known as Buta...   Jun 22 2010, 10:59 PM
- - Juramike   1,3-Butadiene (C4H6) [H2C=CHCHCH=CH2] The forma...   Jun 25 2010, 04:39 AM
- - PDP8E   Mike, you had me at H2O pdp8e   Jun 25 2010, 04:52 AM
|- - Juramike   QUOTE (PDP8E @ Jun 25 2010, 12:52 AM) Mik...   Jun 26 2010, 05:00 AM
- - Juramike   1-Butene (C4H8) [H2C=CHCH2CH3] 1-Butene is 1,3-...   Jun 26 2010, 04:57 AM
- - Juramike   n-Butane (C4H10) [CH3CH2CH2CH3] Boring butane c...   Jun 29 2010, 04:03 AM
- - Juramike   Benzene (C6H6) At lower altitudes, benzene is f...   Jun 30 2010, 03:56 AM
- - Juramike   Benzene (C6H6) high altitude ion route At highe...   Jul 1 2010, 12:31 AM
- - Juramike   Structure of Benzenium (C6H7+) This is one of t...   Jul 2 2010, 12:46 AM
- - Juramike   Beyond Benzene - PAH's and Polyphenyls Polyar...   Jul 4 2010, 01:46 AM
- - Bill Harris   The genesis of Titan's atmosphere may be more ...   Jul 8 2010, 12:44 PM
- - Juramike   HCN The incorporation of nitrogen into Titan’s ...   Jul 13 2010, 12:00 AM
- - Juramike   .CN radical - key intermediate The formation of...   Jul 14 2010, 12:11 AM
- - Juramike   Cyanomethlyene carbene [:CH(CN)] A very recent...   Jul 19 2010, 12:03 AM
- - GEmin   QUOTE The first experimental evidence showing how ...   Jul 19 2010, 06:36 AM
- - Juramike   Acetonitrile (CH3CN) There are multiple ways to...   Jul 21 2010, 02:08 AM
- - Juramike   Cyanoacetylene (HC3N) [HCC-CN] The dominant mec...   Jul 22 2010, 01:13 AM
- - Juramike   Cyanogen (C2N2) [NC-CN] According to the Krasno...   Jul 23 2010, 12:17 PM
- - Juramike   Dicyanoacetylene (C4N2) [NC-CC-CN] Two differen...   Jul 25 2010, 04:04 PM
- - Juramike   Acrylonitrile (C2H3CN) [H2C=CHCN] This molecule...   Jul 29 2010, 10:42 PM
- - Juramike   I’ve tried to represent Titan’s chemistry in a sli...   Aug 2 2010, 03:02 AM
- - Juramike   The graphic below shows the pattern of dominant re...   Aug 22 2010, 08:06 PM
- - Juramike   Estimated depths solids vs. liquids from the vario...   Aug 25 2010, 02:25 AM
- - Juramike   A recent DPS abstract discusses some possibilities...   Oct 8 2010, 02:26 PM
- - titanicrivers   Thanks for the discussion and the link Mike. One ...   Oct 8 2010, 03:46 PM
- - Juramike   Most of the formation chemistry seems to be happen...   Oct 8 2010, 05:21 PM
- - Juramike   LPSC 2011 abstract describes continuing efforts to...   Jan 30 2011, 04:07 AM
- - Juramike   Detection of high altitude cirrus clouds in Titan...   Feb 3 2011, 07:01 PM
- - centsworth_II   This gives hope of much greater understanding of T...   Feb 9 2011, 04:32 PM
- - ngunn   Interesting link posted on the Cassini Huygens Yah...   May 10 2011, 10:56 AM
- - Juramike   The Cool Way to PAH's in Titan's Upper Atm...   Jul 3 2011, 12:55 AM
- - scalbers   I saw a talk given at the AGU conference last week...   Dec 11 2011, 08:39 PM
- - Paolo   not sure this is the best topic to post it to, thi...   Jan 5 2012, 10:51 AM


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