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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.

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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 20 2010, 04:17 AM
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Many ways to fall

Below is a graphic showing an energy diagram for two photochemical methyl neutral cleavage pathways:

Attached Image


A typical energy diagram starts at one energy level, passes through a highest level transition state (the high energy point - indicated by bracket "TS"), then might drop down to an local minimum intermediate state, then climb back up to another transition state before dropping down to a lower energy state for the end product.

Going from the midpoint to the right, a methyl-hydrogen bond cleaves homolytically (one electron goes each way) to form two radical intermediates. This can react with another methyl radical (slight energy increase due to the initial electron repulsion of the orbitals - gotta have enough speed to overcome that) to then join into a formal bonding arrangement and make ethane. In this case the two unpaired electrons jump in together to form a new bond.

To the left, there is another option. Two electrons from the methyl C-H bond can jump over to a neighboring hydrogen to make an H-H bond. Simultaneously, the two electrons in the second C-H bond can jump onto the carbon atom. In this case both electrons move as a pair, and there are two pairs moving simultaneously in a four electron process. The two electrons on the carbon are paired up and this species is referred to as a singlet carbene.

Singlet carbenes have a new reaction pathway to them, they can muscle in and insert directly into a C-H bond. This is a mode of reactivity displayed by many metal species - in fact, C-H activation/insertion has been one of the really hot topics in organic and organometallic chemistry in the last 20 years. I always think of it as a transient 3 center 4 electron bond just before the molecular orbitals switch to the incoming carbon center. This allows a singlet carbene to insert directly into a neutral unactivated methane molecule.

EUV photons have a heckuva lot of energy. A 100 nm photon can deliver a whopping 288 kcal/mol. This is enough to break any bond in the system and overcome a lot of activation energy barriers including either the radical or singlet carbene pathways above.

And it gets uglier.

There are even more possibilities for the neutral methane dissociation, shown below:

Attached Image


Aside from the diradical and singlet carbene pathways, there is also a triplet carbene pathway (think of as a diradical, both electrons are unpaired in different orbitals.) which spits out two hydrogen radicals. There is also the possibility of spitting out three hydrogen atoms one as H2 and one H radical. (thus leaving the electron deficient C-H with two paired electrons in one orbital and one unpaired.) With a Lyman alpha photon (flux responsible for 75 of the neutral methyl cleavage) the different pathways go in different amounts, with the radical pathway #1, and the singlet carbene as #2.

[There is one pathway that doesn't go well at this wavelength, that is the reaction where two hydrogen radicals leave but the carbene is in a singlet state, this shows at some point the triplet carbene went into a singlet state, a classic example of an intersystem crossing, which is technically considered a "forbidden" transition. Not to say it can't ever happen, it's just not cool from a symmetry point of view and is thus disfavored.]

Not mentioned here, but still possible at higher energies, is the "total fraggo" option where only a carbon atom remains with four electrons.

Thus, the high-energy inititiation of a neutral cascade by photodissociation has many different pathways. Similarly, high-energy ion-neutral cascades also have many possible pathways for each intermediate.

In the next post, we'll see how the initial ion-neutral photodissociation of methane is affected by the other atmospheric components, whether nitrogen, argon (both activators for aromatic formation) or hydrogen (inhibitor of aromatic formation.) This represents the fork in the road between Titan atmospheric chemistry (lotsa aromatics), and Jovian/Saturn atmospheric chemistry (not-so-much aromatics).


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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 Itís 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   Titanís 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 Titanís ...   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   Iíve tried to represent Titanís 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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