Atmospheric Chemistry of Titan Options

[Bill Harris]

The genesis of Titan's atmosphere may be more complex than we've ever imagined:

ScienceDaily (July 7, 2010) — Complex interactions between Saturn and its satellites have led scientists using NASA's Cassini spacecraft to a comprehensive model that could explain how oxygen may end up on the surface of Saturn's icy moon Titan.

http://www.sciencedaily.com/releases/2010/...00707002535.htm

--Bill

[Juramike]

HCN



The incorporation of nitrogen into Titan’s organics usually results in the introduction of a nitrile group (-CN), where a terminal carbon atom is bound through a triple bond to a nitrogen atom. The nitrogen has a lone pair of electrons on it, although these may or may not be drawn in. (i.e. –CN:) This lone pair would love to donate to a proton (or other atoms desperate for electrons such as Lewis acids.)

The whole cascade starts with the blowing apart of molecular nitrogen by strong UV light. There are lots of ways this can happen, one is shown above. In this case, “lower energy” ultraviolet light in the 80 - 100 nm range (which is still pretty dang powerful) excites the molecular nitrogen to the point that it goes total fraggo and liberates a “naked” nitrogen atom and an “excited naked” nitrogen atom*. (The “excited naked” nitrogen atom will be a big player in tholin formation mechanisms). Other ways to get there include ionization of molecular nitrogen with light below 80 nm, then electron recombination back to molecular nitrogen, which is a pretty violent process, and then a total fraggo reaction that again generates a naked nitrogen atom and an excited naked nitrogen atom.

The “naked” nitrogen atom can react with a methyl radical to form a transient CH3N nitrene complex** (my guess is it would be likely in a triplet or an unpaired excited singlet state) that then blows out hydrogen radical to give H2CN. radical. This can react with another hydrogen radical to then kick out molecular hydrogen (H2) and HCN.
[I’m not sure why this process goes stepwise, I would think it possible for the transient CH3N carbene to kick out two hydrogen radicals (if triplet) or molecular hydrogen (possible if unpaired excited singlet state?) all in one go].

According to Krasnopolsky et al., 2009, this sequence accounts for 72% of all HCN formed in Titan’s atmosphere. But a recent article shows that excited state nitrogen chemistry may be also very important and poorly modeled. The atmospheric nitrogen chemistry of Titan is still poorly constrained, but getting better with recent lab experiments and further modeling. We’ll use the Krasnopolsky results, but these will likely shift on publication of the next model.

*****


*an asterisk [*] is used to designate an excited state atom. This is an atom where one of the electrons has been boosted to a higher energy orbital. Normally the electrons are spin paired in the orbital in the ground state. In an excited state atom the electrons can still be spin paired, but one of the electrons is now in a boosted energy orbital. So it may be in the unpaired excited state singlet state.


**very careful electron counting is important here, there are two lone pairs on nitrogen with a bonus electron, so naked nitrogen is like a triradical with a lone pair [:N...]
For the whole reaction we get: radical + radical nitrene (nitryne) --> radicals pair - nitrene (2 lone pairs on nitrogen) --> nitrene --> radical + radical

[Juramike]

.CN radical - key intermediate



The formation of .CN: radical is very straightforward: Higher energy photon hits HCN, causes dissociation of H-C bond, and you get H. and .CN: radical.

.CN: indicates that the radical electron resides on the carbon atom, that the triple bond between carbon and nitrogen still exists, and that the lone pair is still on nitrogen (in an sp-orbital sticking straight out along the axis of the C-N triple bond.). Likewise the radical electron resides in the sp-orbital on the carbon and sticks straight out as well along the axis of the C-N bond away from the triple bond electrons. This molecule is linear, like acetylene.

.CN: radical can play a key role in the formation of cyanoacetylene [HCCCN], cyanogen [NC-CN], and dicyanoacetylene [NC-CC-CN], although there are alternative mechanisms for these compounds that proceed through cyanomethylene carbene (:CH(CN)).

[Juramike]

Cyanomethlyene carbene [:CH(CN)]



A very recent article (June 2, 2010 issue of PNAS) by Imanaka and Smith has provided laboratory evidence that high energy photons can ionize nitrogen and cause it to incorporate it into Titan’s organics much more easily than previously thought.

The process is the initial photoionization of molecular nitrogen by EUV photons (wavelenghts < 60 nm) which then makes molecular nitrogen radical cation by blowing an electron out of the molecule. At some point, another electron recombines with the nitrogen molecule (cue dropping bomb sound) which releases a huge amount of energy. The energy released blows apart the dinitrogen triple bond and we are left with a “normal” nitrogen atom radical nitrene (nitryne) and an “excited” nitrogen atom radical nitrene (nitryne). (this is the stepwise sequence of the concerted sequence discussed for HCN – the rates of these steps were deliberately left of the graphic since the Imanaka and Smith work will definitely supercede the Krasnopolsky estimated models.)

The excited nitryne atom is likely in an excited state that accesses D-orbitals. These may be in unpaired spin-coupled state, but well run through the mechanisms assuming it acts like a triradical. (i.e. the actual process may be more concerted). In the case to make cyanomethylene carbene, the excited nitryne reacts with an alkyne. Drawing a stepwise mechanism, the first thing that happens is one of the pi-electrons of the triple bonds forms a new bond with one of the electrons of the nitryne. This now makes a carbon radical nitrene. (still three unpaired electrons in the system). A C-H bond breaks, and hydrogen radical (H.) goes away, and the remaining electron on carbon now forms a carbon-nitrogen double bond with one of the nitrene electrons.

A double-bond equilibration gives cyanomethylene carbene. But what does this molecule look like?
Spectroscopic data suggests that this molecule in the ground state is “close” to a linear molecule, with an H-C-(CN) bond close to 180 degrees That means that the likely hybridization on the “carbene” carbon is sp with the unpaired electrons in two p orbitals in a triplet state. (Ground state suggests it is in an unpaired spin-uncoupled triplet configuration, I’d guess on Titan that these molecules are likely in an excited unpaired but spin-coupled triplet configuration.) Why is this important? It’s probably academic, but it implies that many of the downstream steps could be concerted. This might become important if we deal with molecules (e.g. cis or trans double bonds) with stereochemistry, in this case the stereochemistry would be preserved. In a triplet stepwise reaction, there is always a chance for bond twisting before the second bond snaps shut, causing a loss of stereochemistry.

The :CH(CN) intermediate may be the key intermediate in the formation of tholins and for the incorporation of bonus* nitrogen into Titan’s organics.
It is also an intermediate that gives another route to many of Titan’s organics, such as cyanoacetylene, cyanogens, acrylonitrile, and ethylacetonitrile.

(*more that previously thought)

[GEmin]

The first experimental evidence showing how atmospheric nitrogen can be incorporated into organic macromolecules is being reported by a University of Arizona team. In an experiment to simulate what happens when sunlight hits Titan's atmosphere, UA researchers put nitrogen and methane gas into a stainless steel cylinder and zapped it with high-energy UV light.

http://uanews.org/node/32574

[Juramike]

Acetonitrile (CH3CN)



There are multiple ways to form acetonitrile. One way is the addition of hydrogen radical to the triple bond of acetylene to generate the C2H3 radical. (vinyl radical, .CH=CH2). This can react with naked nitrogen (not clear if it needs to be excited or not) to generate a funky nitrene-enamine intermediate/transition state which can quickly kick out a hydrogen radical and the other electron combines with one of the free electrons on the nitrene to create an “iminoketene” radical. A quick tautomerization creates the CN triple bond and places the radical electron on the CH2 carbon. This can then find an Hydrogen radical floating around (maybe the very one that got kicked out a second ago) and then forms acetonitrile. According to the Krasnopolsky model, this mechanism using multiple transient intermediates accounts for 69% of the acetonitrile generation.


another possible way from :CH(CN)



Another way shown above is via hydrogen abstraction from our new best friend cyanomethylene carbene (:CH(CN)). Very low temperature studies(1) with carbenes have shown that triplet carbenes can react with molecular hydrogen, but that singlet carbenes cannot. (I’m not sure what an excited singlet carbene would do). These low temperature studies were done at VERY low temperatures, less than 30 K. This is waaay colder than Titan’s relatively balmy 95 K (or lower atmosphere minimum of 70 K). Interestingly, the authors found that while the overall formation of carbenes adding to H2 is exothermic, that there is a significant energy barrier to cross. Singet carbenes can’t do it. Singlet carbene insertion requires the carbene to concertedly (all-at-once) muscle in between the H-H bond. But triplets react a different way, they react like diradicals, one step at a time: the first step is an abstraction of one hydrogen atom from molecular hydrogen, then a combination of the two resulting radicals (the .CH2CN and the leftover H.). But even those transition state energies (IIRC +5 kcal/mol) are just a tad too high at 30 K to work, so the authors proposed quantum tunneling of the hydrogen radical. This is a bit out of my comfort zone, but the authors did detect the hydrogenated carbene products so this is experimentally valid. Also interestingly, molecular deuterium did NOT react. The energy barrier (and quantum tunneling barrier?) for a deuterium nucleus appears to be just too high in a 30 K matrix. The authors propose that the reaction with H2 can actually be use to as a mechanistic test as to whether a particular carbene is in a singlet or triplet state. If it hydrogenates, then it was in a triplet state.
So if the :CHCN formed photochemically high in Titan’s atmosphere is in a triplet state, it could react with molecular hydrogen to easily form CH3CN in one quick step.
This particular reaction was not modeled in the Krasnopolsky model, but I’d assume it should be in the next iteration.

:CH(CN) formed up in Titan's atmosphere is in a rarified environment and will be in a different environment than stuff in a low temperature inert matrix in a terrestrial laboratory. For one thing, the stuff in a matrix will be bumping around in it's cage and be able to relax to it's desired ground state. Not so for the stuff in Titan's upper atmosphere. That stuff is blasting along in a vaccum, all excited. If the first thing it bumps into and reacts with is H2, it can react as an excited species, which may not be in the ground state configuration. So the state of the :CH(CN) carbene will determine it's reactivity and propensity to form acetonitrile via direct hydrogenation.

Reference: (1) Zuev and Sheridan, Journal of the American Chemical Society 123 (2001) 12434-12435. "Low-Temperature Hydrogenation of Triplet Carbenes and Diradicaloid Biscarbenes - Electronic State Selectivity." doi: 10.1021/ja016826y]

[Juramike]

Cyanoacetylene (HC3N) [HCC-CN]



The dominant mechanism (according to the Krasnopolsky 2009 model) is the reaction of nitrile radical (.CN) with acetylene. After the initial addition into the triple bond, the C-H bond of the secondary carbon breaks homolytically, and the hydrogen radical leaves the system, while the newly unpaired electron on the carbon jumps into an empty molecular pi-orbital with the other unpaired electron on the carbon radical to form a triple bond. According to the mode, 62% of the cyanoacetylene is formed by this route.




However, there is another possible route to this molecule, again using our friend :CH(CN). In this case, photoionization of methane forms a high energy ionized intermediate CH3+ then undergoes electron recombination to violently produce .:CH. The Krasnopolsky model actually makes this occur in one step. Either way, you get methyne (the simplest carbyne) which then can react with methylene carbene. This could go a number of different ways, I’ve drawn the fully stepwise mechanism as if we were dealing with a triradical CH, and a diradical :CHCN. In the first step, a single bond is formed between the two carbon atoms. The resulting carbene-radical then combines to form a double bond to make a transient acrylonitrile radical. This kicks out a hydrogen radical (from the secondary carbon) as the resulting carbon radicals combine to form the triple bond (like the step above).

[Juramike]

Cyanogen (C2N2) [NC-CN]



According to the Krasnopolsky 2009 model, cyanogen forms by two main routes. The first route (53% of cyanogen formation) is from the attack of nitrile (.CN) radical on acetonitrile radical (.CH2CN) which then kicks out methylene carbene and forms a new bond between the two nitrile carbons. [I’m baffled by this mechanism, why break a perfectly good C-C bond? You’d think the two radicals would combine to form malononitrile (propanedintrile (NCCH2CN) – a nice stable molecule and handy organic building block]

The other main route (47% of cyanogen formed on Titan) uses cyanomethylene carbene :CH(CN) and an excited naked nitrogen atom. (The top reaction in the scheme shows the formation of the cyanomethylene carbene from the reaction of excited naked nitrogen and acetylene). In this case, the :CH(CN) carbene reacts with another atom of excited naked nitrogen to form a radical carbene. This is shown as the fully stepwise mechanism, it is possible that some of these steps could be concerted. The next step is the radical carbon electron combining with one of the nitrogen nitrene electrons to form a C-N double bond, leaving an unpaired electron on the nitrogen – a nitrogen radical. The last step is a C-H bond hemolytic cleavage followed by the unpaired electrons, one from the nitrogen radical and one from the new carbon radical, jumping in together to form a C-N triple bond.

[Juramike]

Dicyanoacetylene (C4N2) [NC-CC-CN]



Two different literature models have two different pathways to form dicyanoacetylene (C4H2). The route described in Krasnopolsky 2009 model has nitrile radical (.CN) attack the carbon-carbon triple bond of cyanoacetylene (HCC-CN). This makes an intermediate central double bond with one of the carbons holding an unpaired electron. Next door, the C-H bond cleaves, and hydrogen goes flying away as hydrogen radical (H.) and the resulting unpaired electron dives in with the carbon holding the unpaired electron and makes the central triple bond.

The Wilson and Atreya 2004 model proposes that two molecules of cyanomethylene carbene (:CH(CN)) connect their unpaired electons together to form a middle double bond. The resulting intermediate (1,2-dicyanoethylene), blows out H2 most likely in a concerted 4-electron pathway (2 sets of 2 paired electrons – this is a very typical concerted reaction, benzene resonances and [3,3]-sigmatropic rearrangements are 3 sets of 2 paired electrons (6 electron concerted pathways)) to give the final dicyanoacetylene and molecular hydrogen (H2).

[Juramike]

Acrylonitrile (C2H3CN) [H2C=CHCN]



This molecule also has two routes according to the Krasnopolsky 2009 model. In the first route ethyl radical adds to the CN triple bond of cyanide to make a new C-C single bond and an intermediate radical on the nitrogen. The nitrogen radical electron dives in while a C-H bond on the other side homolytically cleaves. The two electrons reform a CN triple bond and the resultant hydrogen radical goes flying off.

The second route starts with cyanomethylene carbene (:CHCN). Reaction of this carbene with methyl radical (CH3) makes a new double bond as one of the electrons of the carbene (which may be in a triplet state) hooks up with the unpaired electron of the methyl radical. The resulting ethyl nitrile radical has the unpaired electron on the carbon alpha to the nitrile group. A C-H bond on the terminal carbon cleaves and one of the unpaired electrons joins the other unpaired electron to form a double bond. The other unpaired electron flies off with the hydrogen nucleus to be a hydrogen radical.

Acrylonitrile is known to polymerize when a suitable base is added. On Earth, it is a major industrial chemical product (several million tons/year scale) used for making all sorts of plastics and polymers. On Titan's surface, if a large concentration of this were present, and with a suffficient thermal “kick” it could undergo polymerization or further reactions in the presence of a base such as an organic amine. (Polymerization from initial nucleophilic Michael addition (1,4-addition) to the beta-carbon, then the resulting enolate undergoes Michael addition to another molecule of acrylonitirile, etc.)

[Juramike]

I’ve tried to represent Titan’s chemistry in a slightly different way than is normally presented in the literature, the graphic below shows only the key intermediates that react with each other to form major hydrocarbon components in Titan’s atmosphere. Only the dominant pathway is shown for each molecule, so this is a more limited view than the typical “splat diagrams” (example splat diagram in post 27 this thread) shown for Titan chemistry:



On the left side of the matrix are key reactive species. Along the top are some “target” compounds and reactive intermediates. At the cross point are the species formed when these two meet. Note that some of the new species then go back up into the top row or left column. (Everything starts with nitrogen or methane).

In red on the left side is the highly reactive intermediate .:CH radical carbene. This is formed from the electronic recombination of CH3+, which itself was formed from N2 radical cation reacting with methane (the N2 radical cation formed from photoionization of nitrogen using EUV light.) All the compounds formed downstream from .:CH are also boxed in red. Note that almost all the unsaturated intermediates propagate from this compound. As stated before, if there was no CH3+, these couldn’t be formed, so atmospheres with large H2 components (like Jupiter and Saturn) would shunt away from the CH3+ pathways, and only the unboxed (boring saturated aliphatic) compounds would be formed. Think of the boxed compounds as Titan special menu items. This graphic may be valid on exoplanet atmospheres as well, boxed compounds show only those components that could exist where H2 (or other CH3+ absorbing) are significantly absent and allow CH3+ to frag itself up on recombination.

[Juramike]

The graphic below shows the pattern of dominant reactions that give nitrogenated products in Titan's atmosphere. On the left side are key reactive species, and on the top are "target" species, some of them used to derive the reactive species.



Note that almost all products can be obtained using cyanomethylene carbene (:CH(CN)), although this may or may not be the dominant route.

Also note that almost all products ultimately come from unsaturated hydrocarbon chemistry, which themselves can only form in a relative absence of H2 (see above post.)

Only HCN would really be expected to form in a hydrogen-rich atmosphere. All the other pathways would be shut down due to CH5+ formation.

[Juramike]

As requested (twice now) here is a comparison of some of the earlier pre-Cassini models with more recent atmospheric model production rates. Again normalized to Krasnopolsky et al., 2009:





(Done using conditional formatting in EXCEL, most literature models only have 2 significant figures listed. Extra digits displayed after normalization to set conditional formatting.)

[Juramike]

Estimated depths solids vs. liquids from the various literature models:



Toublanc et al had a very large ethane flux (acetylene, too.). The Krasnopolsky estimated solids are higher due to the estimated amount of solid haze "C2H2/HCN copolymer" produced.

[Juramike]

A recent DPS abstract discusses some possibilities for Titan chemistry:

Horst et al. DPS Meeting 42 (2010) Abstract 36.20 "Formation Of Amino Acids And Nucleotide Bases In A Titan Atmosphere Simulation Experiment".
Direct link to abstract here.

One of the elements missing from all the above reactions is oxygen. There is not that much of it freely available in Titan's atmosphere. It is either in the form of H2O, from icy meteor infall, or from CO, or CO2. The 1 Gyr surface flux for CO2 varies between 10-100 cm for CO2 (high value in the Wilson and Atreya model), while the H2O surface flux (from meteors) is between 10 cm and lower (high value in the Raulin 1989 model). [A big Menrva crater splat may have caused actual water rain on Titan for a brief period according to an abstract a few years ago.]

The authors simulated Titan atmospheric conditions using N2, CH4, and CO2 and used a plasma discharge and generated molecules that incorporated oxygen. The abstract states that the following compounds were detected by GCMS (I'm assuming a direct injection of sample without a laboratory hydrolysis step before analysis.):
Amino acids glycine and alanine (presumably both enantiomers) were detected (but not H2NCH2CH2COOH?).
Also detected were the pyrimidine heterocycles cytosine, uracil, and thymine (but not orotic acid?)
As well as the fused heterocyclic imidazo-pyrimidine adenine (but not guanine?)

A slightly hyberbolic space.com article is here.

The key questions to relate this work are:
What was the overall yield of these compounds? Are we talking tiny trace amounts or significant geologically relevant surface deposits?
How doest this fit into the atmospheric models? Are these major pathways or minor chemical pathways?
How well does the PAMPRE experiment simulate Titan atmospheric tholins? (although PAMPRE is probably the best game in town for tholin simulation experiments)
How well does the simulated atmosphere reflect conditions in the upper atmosphere of Titan? At the critical formation zone? (Where is that for these pathways?)
And from a chemistry point of view, what are the electronic step-by-step mechanisms that make these?