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Atmospheric Chemistry of Titan
Bill Harris
post Jun 17 2010, 05:08 AM
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Oh, H-radical as in "H-dot"? They look like awfully small "+" signs, so they are really dithered dots? I'd suggest trying to make them larger. But any sort of small dot in vector graphics is going to do that-- there is only so much you can do with 3pixelsx3pixels.

QUOTE
[Am I sure about this? Yeah, I'm positive!]

Sounds radical. tongue.gif

In the meantime, I'll revise how I view it... now it makes sense.

--Bill


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Juramike
post Jun 19 2010, 03:06 AM
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Alright y'all, I'm real sorry about this, but I can't figure how to go from a Powerpoint slide over to a jpeg, png, or tiff without losing that little piece of resolution that turns a Powerpoint dot into a tiny little plus sign.

When in doubt, most of these equations should be with neutral radicals. (According to Krasnopolsky 2009 model, ion chemistry is important, but the neutral chemistry reactions (radicals, carbenes, radical carbenes) are still the driver.)

So if it looks like a tiny little plus sign, it should be a radical dot (.).

We soldier on....



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Juramike
post Jun 19 2010, 03:21 AM
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Propane (C3H8) [CH3CH2CH3]

Attached Image


The straightforward radical combination of methyl radical with ethyl radical (which originally came from acetylene radical reacting with ethane) gives propane. This happens with some of the atmospheric gas molecules acting as catalytic helpers.

Propane is the fourth most common liquid on Titan. (Methane, nitrogen, ethane>> propane).

[EDIT: Hah! I found a way to do the powerpoint-->jpeg without dither issues! First convert slides to pdf, then select and copy into Irfanview!]


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Juramike
post Jun 19 2010, 12:45 PM
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Went back and fixed all diagrams in the different schemes. Radicals are radicals, and ions are ions. All electrons present and accounted for.


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Bill Harris
post Jun 20 2010, 07:18 PM
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Looks good, Mike. Sorry to have tossed a monkey wrench with my "meatball chemistry", but it would have happened eventually.

Onward...


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Juramike
post Jun 22 2010, 10:59 PM
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Diacetylene (C4H2) [HCC-CCH]
- also known as Butadiyne (locants not necessary)

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Acetylene radical, formed by photodissociation of acetylene, attacks another molecule of acetylene. The transient radical intermediate kicks out a hydrogen radical to regenerate a triple bond.

And there you've got it, 4 carbon atoms joined up! This is one of the key intermediates that can go on to form benzene (C6H6) and even bigger stuff.

Interestingly enough, the formation of 1,3-butadiene goes by a totally different mechanism and different players.


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Juramike
post Jun 25 2010, 04:39 AM
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1,3-Butadiene (C4H6) [H2C=CHCHCH=CH2]

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The formation of 1,3-butadiene starts with the double photodissociation of acetylene to give the acetylene diradical. (As drawn, with two H radicals coming off rather than H2, this would leave the diradical in a spin unpaired triplet state). This is pretty much just C2. One side of this high energy intermediate rips into methane and kicks out hydrogen radical to form a 1-propyne radical (one one radical of the original diradical gets "quenched"). This can then isomerize to allene radical, which is a key intermediate we'll see later (hint: benzene).

Allene radical can react with methyl radical to then form a methylated allene, which isomerizes to the 1,3-butadiene.


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PDP8E
post Jun 25 2010, 04:52 AM
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Mike,
you had me at H2O
pdp8e blink.gif


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Juramike
post Jun 26 2010, 04:57 AM
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1-Butene (C4H8) [H2C=CHCH2CH3]

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1-Butene is 1,3-butadiene's more saturated buddy. It has one double bond removed. To be fair, I'm not sure if this is the 1-butene isomer or the more thermodynamically preferred 2-butene, or the 2-methyl prop-1-ene isomer. (With four carbons, the iso-form is possible, it is the most stabilized double bond, and it makes the most sense mechanistically). Most likely, C4H8 would be a mixture of isomers. The mechanism will assume linear 1-butene is formed.

The sequence starts by the addition of hydrogen radical (atomic hydrogen) to allene to give allyl radical (C3H5). The middle carbon should be the best place to put the unpaired electron, but in order to form 1-butene, you need it on the end. Reaction of allyl radical with methyl radical at the C3 terminus gives 1-butene. The reaction of allyl radical with the unpaired electron at C2 (middle) should give the iso isomer - 2-methyl propene = isobutylene (not shown in the diagram). From 1-butene, a 1,3 H-shift would give more thermodynamically preferred 2-butene. However, 1-butene is perfectly stable at room temperatures on Earth. 2-butene can exist in two forms, cis and trans. In cis the two methyls are on the same side of the double bond, in trans they are opposite. Generally, the trans forms of a double bond is more stable. Both cis and trans forms of 2-butane are stable and separable at Earth room temperatures. Once formed they should also be stable at Titan temperatures. The formation mechanism should "lock in" the isomer. However, there is always the possibility that an excitation, or collision, or protonation/deprotonation with a superacid (like CH5+) can isomerize or switch the double bond geometry.

Next up is saturated n-butane.


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Juramike
post Jun 26 2010, 05:00 AM
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QUOTE (PDP8E @ Jun 25 2010, 12:52 AM) *
Mike,
you had me at H2O
pdp8e blink.gif


I'm still drawing up one of the dominate ion-neutral routes to benzene and friends. It is beautifully complex. smile.gif


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Juramike
post Jun 29 2010, 04:03 AM
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n-Butane (C4H10) [CH3CH2CH2CH3]

Attached Image


Boring butane comes from the straightforward recombination of two ethyl radicals (which comes primarily from acetylene radical ripping a proton off ethane to make ethyl radical). This happens primarily at very low altitudes.

In an atmosphere without a lot of acetylene photochemistry (H2 or methane-rich atmospheres), ethyl radical can form from C-H cleavage of ethane, but it is a lower efficiency process. So very small amounts of butane should be observed on Jupiter and the other H2 rich gas giants. (Recall that acetylene production requires .:CH, which is a 30% product of electronic recombination of CH3+ and an electron, and CH3+ doesn't form in H2 or methane-rich atmospheres, it forms best with Ar or N2 as a diluant gas, like on Titan)


Next up is the Mac Daddy of Titan's organics - benzene.


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Juramike
post Jun 30 2010, 03:56 AM
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Benzene (C6H6)

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At lower altitudes, benzene is formed from the combination of propargyl radical (.C3H3) This is a trimolecular event, it won’t happen very well in rarified environments – that goes by a different process.

The initiation is the formation of acetylene diradical and its reaction with methane to give .C3H3. When two of these (+some gas molecules) get together, they join, undergo 1,5 shift, cyclize, then undergo a final 1,3 H-shift to generate benzene.


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Juramike
post Jul 1 2010, 12:31 AM
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Benzene (C6H6) high altitude ion route

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At higher altitudes, a different process occurs. This model is a little more speculative. The dominant proposed route starts with a large amount of diacetylene (C4H2) which gets photoionized to the radical cation. This then reacts with ethylene (by sucking in ethylene’s pi-electrons). The transient intermediate then kicks out atomic hydrogen (H.) and we are left with a C6 cation. This can cyclize via [3,3]-sigmatropic rearrangement, a concerted set of three two-electron processes. This gives us C6H5+. Reaction of this reactive intermediate with either molecular hydrogen (H2) or ethylene (C2H4) gives us protonated benzene (C6H7+) AKA the benzenium ion. If the reaction is with H2, H2 is sucked up into the system, if the reaction is with C2H4 (as drawn above) acetylene is spit out. The ethylene acts as a molecular hydrogen donor.

On electronic recombination, benzene is liberated. Normally, electron recombination is a pretty harsh process and it can frag up most molecular cations when they recombine. But benzene has many vibrational modes associated with it and so can sometimes get through this OK.
The speculative part is that the proposed Vuitton et al. 2009 literature model used a 10x higher amount of C4H2 than could be accounted for with the formation models for C4H2. Hopefully more measurements and theoretical work will help refine the formation models. But at least the model is consistent with Cassini observations of benzene abundance.




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Juramike
post Jul 2 2010, 12:46 AM
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Structure of Benzenium (C6H7+)

Attached Image


This is one of the predominant ions in Titan’s upper atmosphere. It forms from protonation of neutral benzene, or by the reaction of C6H5 with a “hydrogen molecule” donor – either H2 or ethylene.

Recently, the structure of this cation was elucidated by X-ray crystallography by making a superacid salt. Up until recently, it wasn’t clear what the structure of this cation was, was the proton floating above the pi-system of benzene (a pi-complex) or was it localized to one of the edge carbons (a sigma-complex). Or would benzene have it’s normal pi-system, and enter into a 3c2e sp3 bond with two of it’s hydrogen atoms? The answer is more than academic, since the protonation and downstream reaction of benzene is an important process in the synthesis of many products and pharmaceuticals.

Based on the X-ray analysis, it looks like the proton formally latches on to one of the benzene carbons to make a sp3 hybridized CH2. The remaining pi-system (only 5 carbons now) locks up to a cyclohexadiene structure, with two formal double-bonds (sp2 hybridization), and a carbocation somewhat localized to the distal sp2-hybridized carbon from the CH2. While the benzenium molecule itself is flat, more substituted analogs will allow the CH2 to bend slightly out of plane, as you’d expect if that carbon was no longer part of a conjugated pi-system.



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Juramike
post Jul 4 2010, 01:46 AM
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Beyond Benzene - PAH's and Polyphenyls

Polyaromatic hydrocarbons (fused aromatics)

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Benzene is actually a pretty wussy molecule: it can cleave a C-H bond with “normal” UV light at 248 nm (= 115 kcal/mol). This generates a benzene radical (.C6H6) and a hydrogen radical.
One of the things benzene radical can do is react with acetylene (or diacetylene, or vinyl acetylene) and then close the pendant chain to form a new fused aromatic ring. This process is thought to be an important route to polyaromatic hydrocarbons during soot formation. The benzene radical first attacks the triple bond of acetylene, then places the radical at the terminal end of the alkyne that just got attached. This can repeat the process and react with another acetylene molecule. But this last radical now has a pendant radical that can attack the pi-system of the parent benzene ring to make a new six-membered ring. (Five and six-membered rings form pretty easily in most chemical processes). This places the radical likely at the ring junction (most substituted). This collapses to kick out a hydrogen radical and a fully aromatized fused ring system – naphthalene. Most studies indicate this can happen only at high temperatures – places like interstellar clouds illuminated with high energy photons or in the back of your car’s exhaust pipe.

Polyphenyls (linked aromatics)

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At lower temperatures, another process occurs with benzene radical, it reacts with another molecule of benzene. Attack of the radical into the pi-system gives a transient radical that quickly rearomatizes and kicks out a hydrogen radical. This creates a linked aromatic system (not fused).
Fused ring systems are usually planar. When they get really big, like C30 or so, they can become cup-shaped. Fused ring systems also can have different reactivities. In anthracene (3 benzenes fused in a line) the central carbons are very prone to oxidation. The central double bonds also are prone to reaction. (They do [4+2] cycloadditions easily). But as a general rule, fused ring carbons act more electron-deficient. PAH’s can do some funky chemistry.
In contrast, polyphenyls are about as exciting as linked benzene. The chemistry is almost exactly like that of benzene. Structurally, rings are twisted out of plane in the gas phase, but will flatten out when excited. In solid phase, the rings are planar. (All this is assuming that the hydrogens are at the ortho position. If there is a larger substituent, it will twist. For ortho-terphenyl, the two rings are oriented perpendicular to the central ring to prevent bumping each other.)

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PAH’s require high temperatures to form, while polyphenyls can form at lower temperatures. It is likely that the low temperatures on Titan prefer the formation of polyphenyls such as biphenyl, terphenyl, and higher. There are many recent reports coming out that implicate the formation of biphenyls in Titan’s atmosphere.


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