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Atmospheric Chemistry of Titan
Shaka
post Jun 10 2010, 11:59 PM
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Will the final exam be essay or multiple choice? cool.gif


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Juramike
post Jun 11 2010, 12:30 AM
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I'm gonna give away the answers! smile.gif


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Juramike
post Jun 11 2010, 05:51 AM
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Enter the mechanism

Over the next several graphics, I will detail the formation mechanism for many of the predicted Titan organic species. I will show the dominant pathway based on rate information found in Krasnopolsy, V.A. Icarus 201 (2009) 226-256. "A photochemical model of Titan's atmosphere and ionsosphere". doi: 10.1016/j/icarus.2008.12.038. Happily, this article is publicly available and can be downloaded freely here:
http://pagesperso.lcp.u-psud.fr/pernot/ISS...snopolsky09.pdf

I will be referencing the reaction Id number in the Krasnopolsky model (detailed in Tables 3-5) as well as the reaction rate. For the numbers, E9 is huge amounts, E8 is much, E7 is some, E6 is a little, E5 and below is “not so much”. The average altitude of greatest rate for the reaction is also shown. Remember that other reactions can be using up this intermediate as well, so this doesn’t necessarily mean that this is where the greatest production occurs. Generally the hardcore ion-neutral chemisty is way up high, while the gentle haze-surface-catalyzed radical association reactions occur down low.

These schemes and mechanisms will start with the hydrocarbons first, and the nitriles next. This will be somewhat organized from most prevalent, to least prevalent. But key intermediates will be introduced before their products. So if you want to see how an intermediate got made, scroll upwards. The ultimate start point for everything will be UV photons + CH4 or UV photons + N2.

First up is ethane (C2H6)...


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Bill Harris
post Jun 11 2010, 04:13 PM
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QUOTE (Juramike @ Jun 10 2010, 07:30 PM) *
I'm gonna give away the answers! smile.gif
Or, rather, leads us to the answers after weaving a background tapestry.

Following each installment...


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rlorenz
post Jun 12 2010, 01:00 AM
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QUOTE (Juramike @ Jun 9 2010, 09:58 PM) *
Here is an EXCEL conditional formatted graphic that shows how the predicted fluxes have varied between one published model and the next.


Nice presentation.
It'd be interesting to add in some of the older models (Yung, Lara etc.)

btw it is possible that the number of significant digits displayed in your spreadsheet
exceeds the level of fidelity of the models, if not the attention span of the reader
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Juramike
post Jun 12 2010, 03:40 AM
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Ethane (C2H6) [CH3CH3]

Attached Image


Almost all ethane is formed from the combination of two methyl radicals. The methyl radicals are initially generated in a whole bunch of ways previously described, a few are shown in the graphic above. The actual radical combination mechanism is not-so-straightforward, it is actually a three body reaction, with “M” in the reaction scheme designating an atom (or molecule) of a carrier gas.

What does this “M” do? I’m not exactly sure….clearly it comes out of the reaction as it enters it. It thus acts as a catalytic group or surface. My wild speculation is that it may help absorb (or impart) some of the kinetic energy required to make the reaction happen. The Krasnopolsky, 2009 Titan atmospheric model got this reaction rate and sequence from the Lavvas et al., 2008 Titan atmospheric model, who in turn modified it from Cody et al., 2003 (?) which was for Saturn/Neptune atmospheric modeling and had “M” = H2/He.

[In this case “M” is an inert gas. Confusingly, earlier ground state solution-phase chemistry literature (1961) showed that certain metal species (also labeled “M” in the literature reaction schemes) can help stabilize free radicals presumably by making an interaction between the metal and the radical. The additional stability helped the recombination reaction pathway, the yields were higher in the methyl radical combination to form ethane.]

Since this is a three-body gas-phase reaction, it is going to work much better in a more crowded environment lower in the atmosphere. It is very hard to get three molecules together in a rarified environment. This is one reason why many of the radical recombination reactions (most require a diluant gas “M” atom/molecule) will at lower altitudes.

Oddly enough, photodissociation of ethane to make ethyl radical is NOT the main way to form ethyl radical (C2H5.). Most ethyl radical will actually come from acetylene radical (HCC.) attacking ethane (C2H6). But we’ve got more mechanisms to describe before we get there..

Next up, the key intermediate ethylene (C2H4)...


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Juramike
post Jun 13 2010, 03:21 AM
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Ethylene (C2H4) [H2C=CH2]

Ethylene is a key intermediate that is a starting point for many pathways. If it's "inbound" is more than it's "outbound" there might be some that actually makes it down to the surface. But the Krasnopolsky 2009 model has a flux of zero - in that model it all gets used up.

Attached Image


The starting point is the generation of an extremely reactive intermediate - a radical carbene. This results from a high energy photon doing an almost total fraggo number on methane. The force of that photon blows out atomic hydrogen and a hydrogen radical. The molecular hydrogen gives a clue that this part was a concerted 2-electron process and so the carbene is spin paired and in a singlet state (as drawn in the diagram). This is in contrast to the other possible case where three hydrogen radicals get blown out and the carbene is spin unpaired and is in a triplet state.

The carbene can undergo a C-H insertion with a methane molecule (just wedging it's way between the C and the H) to make an ethyl radical transition state that probably only lasts a molecular vibration before kicking out a hydrogen radical and having both unpaired electrons dive into a double bond molecular pi-orbital. Et voila, ethylene is born!

While this process seems really funky, according to the Krasnopolsky 2009 model, 75% of all ethylene production follows this pathway.

Now that you've got ethylene, acetylene is only a UV photon away...


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Juramike
post Jun 14 2010, 02:52 AM
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Acetylene (C2H2) [HCCH]

Acetylene has several formation pathways. The major route, although still only accounting for 38% of all acetyelene formation, is the photolytic dehydrogenation of ethylene, shown below:

Attached Image


Digging into serious detail, the way I understand this, the photon hits the pi-electron cloud and kicks it up to an excited state antibonding molecular orbital. This would be the LUMO (lowest unoccupied molecular orbital), which is now populated in this new singlet excited state. The electrons are still paired (singlet), they are just having nothing to do with each other (antibonding). While the pi-system molecular orbital is a bilobal fuzzy blob between the two carbon atoms, the antibonding molecular orbital faces away from the space between the two atoms. The spin paring is key to molecular hydrogen kicking out. All at once, the two electrons in both C-H bonds swap partners and you get molecular hydrogen and acetylene. Symmetry is preserved and all is harmonious.

If the two hydrogens came off as two radicals in one step, that would imply an initial singlet (most ground states are spin-paired) going to a triplet (spins-unpaired) state. This is a “forbidden” transition. It can still happen, it is just not normally allowed (and is thus rarer). It is also possible that the singlet excited ethylene could switch to a triplet excited ethylene, then kick out two hydrogen radicals. This switch is called “intersystem crossing” and constitutes a bonus step along the reaction pathway.


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Juramike
post Jun 15 2010, 02:58 AM
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Methyl acetylene (1-propyne) (C3H4) [CH3CCH]

This compound is isomeric with allene (C3H4) [H2C=C=CH2].

Attached Image


It is created by initial formation of methyl radical carbene (same intermediate that reacts with methane to make ethylene), but this time, the radical carbene reacts with ethylene. I drew this as a carbene-style C-H insertion reaction where the inserted carbon atoms is also a radical. This can now kick out H radical and form a allene. This is just a 1,3-Hydrogen shift away from methylacetylene.


[EDIT: 6/16/2010: redid mechanism based on a C-H insertion route]


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jekbradbury
post Jun 15 2010, 07:35 PM
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QUOTE (Juramike @ Jun 14 2010, 10:58 PM) *
This compound is isomeric with cumene (C3H4) [H2C=C=CH2].

Isn't that allene, not cumene?
Also, I'm curious to know what mechanistic reason makes the cyclopropene ring preferentially decyclize to propyne instead of allene.
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Juramike
post Jun 15 2010, 10:39 PM
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Whoops. You are right, it is allene not cumene. "Cumulene" (different spelling) is a generic term for compounds with two or more consecultive double bonds. Allene is the simplest of these.

Both allene and methylacetylene can exist in equilibrium. (equilibrium data here: http://en.wikipedia.org/wiki/Methylacetylene)

When I whipped this mechanism out, I woulda thought that the ring strain of the cyclopropene ring would be such that it would love to open up. However, now looking at "Carbenes, Nitrenes and Arenes" by T.L. Gilchrist and C.W. Rees, Appleton Century and Crofts, New York, 1969, I see that 1,1-dihalocyclopropenes are stable intermediates, and can be hydrolyzed to the corresponding cyclopropenone, at least if there are alkyl substituents on the cyclopropane double bond. (This results from a low yeild reaction of dihalocarbenes on a disubsituted acetylene).

Now with that little piece of information, I'd redo my proposed mechanism to have the radical carbene doing a C-H insertion, followed by radical jumping in and H. kicking out to initially form allene. Which then could equilibrates to methyl acetylene. Either way, you've got C3H4 - three carbon building block for further use....


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Juramike
post Jun 16 2010, 09:28 PM
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[Well, whaddya know...cyclopropenone has been detected in interstellar space. It is especially stable because of the stability of cyclopropenyl cation. Presumably the oxygen in this molecule holds a partial negative charge to offset a partial positive charge in the cyclopropenyl part. See (freely available): Hollis et al., The Astrophysical Journal 642 (2006) 933-939 ]



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Juramike
post Jun 17 2010, 01:46 AM
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Ethyl radical (.C2H5) [.CH2CH3] - key intermediate

The direct photodissociation of a C-H bond in ethane is NOT the major process to ethyl radical. Instead, it is this:

Attached Image


The dominant process to ethyl radical is C-H photodissociation to make acetylene radical (.CCH), followed by it plucking a hydrogen radical from ethane to then generate ethyl radical. So in a sense it is an overall photodissociation of the ethane C-H bond, but it goes through an acetylene middleman.

Ethyl radical is important in the generation of propane, the fourth most abundant liquid on Titan. (after methane, ethane, and molecular nitrogen).


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Bill Harris
post Jun 17 2010, 02:47 AM
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Mike, one thing I notice is that many of the reactions produce a hydrogen ion, an H+. A proton?? What eventually happens to this H+? If this will be answered in a later "installment", I'll curb my curiosity and wait...

--Bill


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Juramike
post Jun 17 2010, 03:03 AM
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H+??

Dangit. Those should all be H. hydrogen radical.

I suspect this is a vector graphic-->jpeg issue. They look fine (i.e. little dots) in ACD (drawing package I'm using) and in Powerpoint. But now checking the Irfan view version they go from little dots to little plus signs. (Not cool.)

[Am I sure about this? Yeah, I'm positive! smile.gif ]

Sorry about this, anyone got a fix?


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