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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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ngunn
post May 6 2010, 11:20 AM
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Thanks for that very interesting explanation Mike. As a non-chemist it's taken me a while to mull it over, alongside this related EGU abstract: http://meetingorganizer.copernicus.org/EGU...U2010-10730.pdf

I am intrigued by the possible implications of this for the chemical erosion of Titan's solid surface materials. Unfortunately the abstract, not being one of those long LPSC ones which spoil us rotten, doesn't include the actual findings on how the 'bedrock' (whether H2O, H2O/NH3 or solid organics) are or might be affected. Do you have any more info/ideas on this? Could this carbonium be what eats up any exposed ammonia on very short timescales?
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Juramike
post May 6 2010, 01:50 PM
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The radical cationic, carbonium, and carbenium species up in the atmosphere are high energy intermediates. They can exist up there (and in the MS sample chambers) because they are in rarefied environments. They don't bump into other molecules that often.

As these compounds descend in the atmosphere, they will encounter other molecules, bump into things more often, and be able to exchange energies and react much, much better. When you get down to the surface (or in solution) there are a lot of opportunities for molecules to react and find a happy equilibrium. High-energy structures will be just fleeting intermediates on the way to more more stable molecule. They won't be able to last more than a molecular vibration before they bump into something and react or transfer their energy.

'Course at Titan's low temperatures, some of the metastable high-energy compounds might get trapped out in a matrix and not be able to react. One of the best ways to keep things from reacting is to cool it down, putting it in the freezer so to speak and keep it from getting over the energy hump to the next state. If there is a high energy transition state with higher activation energies away from it, you can freeze it out. (IIRC, carbenes [think of it as "methylene diradical" or :CH2] has been studied in a frozen Argon matrix).


I'm not real sure about the Lunine abstract....methane is very non-polar and would not be happy solvating a carbocation. I think it would be EXTREMELY difficult for methane to spontaneously dissociate to a carbocation-hydride. (This doesn't happen in the lab at terrestrial temperatures.) But if a carbocation (or superacid) was plopped or sprinkled into a lake, it would do something.

Definitely not comfortable with referring to normal methane (CH4) as a "protic" solvent. That implies that methane is able to donate a proton. (CH4--> -:CH3 + H+). With a pKa of 45, that won't happen.

BUT (and I think this is where the abstract is going) if a stronger superacid was sprinkled into a lake, the proton affinities might force methane to take on a proton and make CH5+ (again, methane is not happy about this.) Proton affinities show the clear progression CH4-->CH3-CH3->acetylene-->ethylene-->H2O--<ammonia. (Table here: Wikipedia/Proton Affinity (data page)). So if CH5 (or H3+) was dribbled into a lake it would eventually work itself down the chain to protonating water to make hydronium. (Dunno the proton affinity for clathrate, but I assume it's similar or slightly less than water, protonation should break up the clatrate matrix - that would be a neat and "easy" experiment to try.) The ultimate sink on Titan should be ammonia to generate ammonium ion.


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Littlebit
post May 11 2010, 09:22 PM
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QUOTE (Juramike @ May 6 2010, 06:50 AM) *
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.

Is methane+ a right wing or left wing radical?


Enjoy your posts, as always Juramike - we are still out here!
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ngunn
post May 11 2010, 09:56 PM
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QUOTE (Littlebit @ May 11 2010, 10:22 PM) *
Is methane+ a right wing or left wing radical?


Could be either way, depending on the geometric orientation of the molecule and where you view it from.
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Juramike
post May 12 2010, 12:30 AM
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Cute.

To be really, really anal, a radical should show a dot to signify an unpaired electron. So .CH3 or (neutral radical) or .+CH3 for the radical cation.
Differently trisubstituted carbon radicals .CR1R2R3 could be in sp3 orbitals. So technically, they could be chiral (have handedness / are not superimposable on their mirror-image) if the three groups are different. The resulting radicals could be chiral (all sp3 hybridization, or may be achiral and planar (unpaired in p, bonding in sp2 orbitals) and react differentially from the two faces to create chiral products.

A trisubstituted carbenium ion +CR3 will rehybridize to placing the unoccupied orbital in a p atomic orbital, and the remaining (bonding) orbitals in sp2 orbitals. This is a planar configuration, and is thus achiral.

As Professor Barry Sharpless once said while teaching us organic chemistry: "Once you lose your chirality, it is gone forever." smile.gif

Here is a neat basic intro to radical chemistry: http://www2.fiu.edu/~herriott/ch10%20-%20r...20reactions.pdf


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Juramike
post May 12 2010, 11:46 PM
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Purple Haze or Where It’s At

A paper by Lavvas et al. (Lavvas et al. Icarus 201 (2009), 626-633. "The detached haze layer in Titan's mesosphere." doi: 10.1016/j.icarus.2009.01.004) examined the detached haze layer and reported that it is the major driver of Titan chemistry.

The graphic below shows that the detached haze layer (the image and plot are scaled to the same altitude) has a shift in the UV-Visible properties (187 and 338 nm) at about 520 km above Titan. This also corresponds to an inversion layer measured by the Huygens probe (solid line in plot). So what makes up this warm thicker layer?

Attached Image


The authors state that photochemical production occurs very high up in Titan’s atmosphere, near 1000 km. Some of these chemical reactions produce particles which slowly grow radially as they thicken downwards. At about 520 km, these monomeric particles reach their highest optical density forming the visible purple layer, and absorbing solar radiation and causing the local warm region in the upper atmosphere. But at this high density, the particles begin to glom together and stick to form even larger aggregates. Fractal growth begins, and the agglomerated particles fall through the upper atmosphere forming the larger globby particles that make up the lower haze layers. According to the authors, the agglomerated particles have a lower optical density than the individual monomers (many small particles = hi optical density; one biiig particle = lower optical density), so the region immediately below the detached haze layer is “clearer”.

Attached Image


The authors created a model with a photochemical production centered around 900 km altitude and a mass flux of 9E-14 g cm-2 s-1 and an average particle size at 520 km of 40 nm which fits the observations and fit the required influx rate for the Titan’s lower stratospheric haze layers from above.
It is very important to note that the saturation vapor pressure of simple organics (such as benzene) is not high enough to condense out at 500 km. The aerosol particles are made of bigger molecules.

The chemistry that occurs at these high altitudes is driven by solar flux of high-energy electron-stealing EUV photons around 145 nm. The solar flux also matches the modeled production rate.

Here are some numbers:
Overall photochemical mass flux (organics+aerosol particles): 9E-14 g cm-2 s-1
Aerosol production flux at 520 km: 2.7-4.6E-14 g cm-2 s-1 (30-50% aerosol particulate production)
Particulated Flux required to maintain stratospheric haze layer: 0.5-2.0 E-14 g cm-2 s-1.

The high-energy photochemical pathways up at 900 km drive much of Titan’s organic chemistry, making organics (such as benzene, seen at 1000 km) and particulates.


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Juramike
post May 16 2010, 09:52 PM
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Titan's organic chemistry is driven by photochemical reactions that originate high in the atmosphere, near 1000 km, where the pressure is only 1.5E-5 Pa. (This is 1E-10 atmosphere, or 1/10 billionth Earth's pressure, much much weaker pressure than I got in my graduate school "vaccum" line. This is about the same range as a typical high-vacuum molecular turbopump.)

Taking 1.5E-5 Pa altitude as an upper limit for stuff to happen, here is a graphic that shows the "chemically available" atmospheric volume for each of the planets:
Attached Image

(full res here: http://www.flickr.com/photos/31678681@N07/4612160825/)

As expected, Jupiter dominates, but Saturn may actually be more impressive, with a lower gravity, the scale height is larger - it has a fluffier atmosphere. For similar reasons, smaller Neptune has more atmosphere. Values for Saturn, Uranus, and Neptune are based on scale height - which is probably underestimating the 1.5E-5 limit. The Jovian estimate is based on Galileo probe data.

Same graphic but detail for the silicate and icy bodies:
Attached Image

(full res here: http://www.flickr.com/photos/31678681@N07/4613181600/)

Titan has more atmosphere, and it extends farther out, so it's chemically-available volume is larger than Earth's. With more mass and higher density, both Earth and Venus hold their atmosphere in closer. With a low surface pressure, but a much weaker gravity field, Pluto has a large but thin atmospheric volume.

Ganymede, Callisto, the Moon, and Mercury have a surface atmosphere well below the 1.5E-5 Pa cutoff and so are not shown.

The atmosphere chemistry that happens on Titan depends on amount of atmosphere, amount of solar flux, and the exact mix of the methane-containing atmosphere (spoiler alert: H2 is bad, N2 and Ar are good).

For other methane-containing atmospheres, it might be possible to scale Titan's atmospheric chemistry to derive photochemically derived organic fluxes on those surfaces as well. (Hint: Pluto!)


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Juramike
post May 18 2010, 12:30 AM
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Titan's Chaotic Chemistry

Titan is a synthetic chemist's worst nightmare. There are high energy processes that span the spectrum (!) of energies and modes of reactivities, plus the products of one reaction become the reactants for another. One recent model takes into account over 415 simultaneous reactions in the atmosphere. Even with all this complexity (a polite word for chaos), some of the models are beginning to get close to the observed Cassini and Huygens results.

One of the tricky bits is that Titan has different levels of reactivity. High-energy photochemistry (ion-neutral) chemistry dominates the upper atmosphere, while lower down "lower energy" radical reactions come into play. Finally, even pi-system photochemistry kicks in with longer wavelength light (200 nm or so) that is able to push with double bonds and pi-systems into an excited state. Once in the excited state, all sorts of things can happen. Photochemistry with excited states can occur even near 0 K. Deep below the haze layers, all the fun photons are absorbed, and the chemistries rely on ground state thermal chemistry - the day to day stuff we are used to. Here the energy barriers need to get crossed by the kinetic energy of the molecules themselves. And on frigid Titan, the molecules are gonna struggle to get up over that hill and over into the next valley. Here is a graphic that tries to put that all into perspective, with an example transformation for each type of reactivity:

Attached Image



The next graphic compares an ion-neutral route and a radical route to ethylene starting from methane. Both occur in Titan's atmosphere:

Attached Image


Of the two types of chemistry, ion chemistry is the more energetic, during the initiation it rips an electron out of the molecular or even atomic orbital. The radical cation either reacts or self-fragments to generate a cation AND a radical species. (two reactive intermediates!). Further reactions proceed until the last step where an electron eventually collides with the system. It took a lot of energy to rip the electon out, and when the electron is popped back in a huge amount of energy is released. This huge amount can't easily be released just through wimpy vibrational or translation mode changes - instead the molecule may frag up. (Picture driving your car along the road, suddenly two solid rocket boosters you've been carrying along are ignited, and your cars structural frame can't absorb the extra energy....) So even at the end, ion neutral chemistry will generate reactive radical intermediates that enter a radical reaction pathway...

In the scheme, a methyl group loses an electron, then blows out a hydrogen radical. The resulting cation sucks in the electrons from a methane molecule (I drew it as a 2e- exchange, it could be several 1e- exchanges also). This leaves it as a C2H5+ cation and kicks out molecular hydrogen (again, it could kick out two H radicals). [C2H5+ is a real important intermediate in Titan chemistry, we'll see him again, soon.] At some point, an electron drops into the system, with a huge release in energy. The least exciting option is a fragmentation to ethylene and the release of yet another hydrogen radical.

For the radical route, a homolytic cleavage (1e- split) creates a methyl radical (CH3.) and a hydrogen radical (H.). Both the methyl radical and hydrogen radical can go on to react with other species (H abstraction or addition to a double bond, for example.) In this scheme, the Hydrogen radical reacts with acetylene to create an ethylene radical, which can then propagate to do further stuff. To cut this story short, the ethylene radical encounters a hydrogen radical, they combine and make a neutral molecule full of happy paired electrons, ethylene. (In reality, it is an encounter with another ethylene radical that causes a disproportionation to ethylene and acetylene, the more energetic ethylene radical likely loses a hydrogen radical to another ethylene and is converted to acetylene.). Once the electrons are paired up, that terminates the propagation.

The two types of chemistry are very important in planetary atmospheres. It was the realization and inclusion of ion-neutral chemistries at the proper level of importance that has generated the most recent round of models. And these were driven by the discovery of large amounts of benzene in Titan's atmosphere (see post #1).


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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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rlorenz
post May 20 2010, 07:17 PM
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QUOTE (Juramike @ May 12 2010, 06:46 PM) *
Purple Haze or Where It’s At

A paper by Lavvas et al. (Lavvas et al. Icarus 201 (2009), 626-633. "The detached haze layer in Titan's mesosphere." doi: 10.1016/j.icarus.2009.01.004) examined the detached haze layer and reported that it is the major driver of Titan chemistry.


A major thrust of the paper is that the detached haze layer is located by the chemical fluxes alone (i.e. they dismiss the
previously-proposed dynamical origin)

I disagree. The detached haze varies both with season and latitude, so I don' t think things are as simple as
their 1-D perspective claims.
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Juramike
post May 21 2010, 05:42 PM
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I agree that the reality is probably a lot more complicated than the model, with changing solar flux rates (11-year cycle) and seasonal illumination changes, and material (starting materials and products) fluxes and thus concentration values going up and down and then the molecules themselves physically going up and down and back and forth. All of those should also affect the chemical flux rates.

IIRC the mean free path length at 1000 km altitude is a few km long, so only a few ricochets are in the way of a 100 km molecular journey.

(I seem to also remember that Voyager 1 had a different value for the detached haze layer, much lower if I remember correctly.)

What measurements would be required to constrain chemical flux vs. dynamics?
(...and did we just get these on the last flyby?)


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Juramike
post May 31 2010, 04:57 AM
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It turns out that the methane is only part of the story. The other non-carbon containing gases present also make a huge difference in the types of chemistry and the amount of products. An interesting experiment was done by Imanaka an Smith (Imanaka and Smith, J. Physical Chem. A. 113 (2009) 11187-11194) as summarized in the graphic below:

Attached Image


The authors found that when methane was irradiated in the presence of Argon or nitrogen, they got lots of unsaturated complex organics. In pure methane, the yield dropped, which is counterintuitive, since there is more methane in pure methane. And when methane was diluted with hydrogen, the yield dropped and only simple saturated compounds were produced. The best yeilds in their study was when methane was diluted with N2 or Ar. This is strange, because N2 or Ar are not even incorporated in the products, what gives?

A detailed look at the chemical mechanisms involved reveals the answer…


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Juramike
post Jun 1 2010, 03:36 AM
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In the presence of N2 or Ar, ionization occurs first in the carrier gas to form either N2 radical cation or Ar radical cation. Abstraction of an electron from methane (CH4) also splits out a hydrogen radical, generating CH3+ a carbenium ion.

Attached Image


In the presence of H2, the photoelectron ejection creates a molecular hydrogen radical cation (= two protons with only one lonely electron in the molecular orbital keeping it all together..). This reacts with another H2 molecule to grab a hydrogen atom and also split out a hydrogen radical. This forms H3+, which is another 3-center, 2-electron triangular association. This is a very common molecule in deep space. But it is not happy. It has a very low proton affinity. It wants to give the proton away. (Fun fact: only dioxygen, argon, neon, and helium have a lower proton affinity – that means that H3+ can protonate (transfer it’s proton) to everything else – it is almost the ultimate superacid.) It is so strong a superacid, that it can actually protonate methane to make CH5+, a carbonium ion. This also has a 3-center, 2-electron triangular association. This time it is between the sp3 hybridized orbital on the carbon, and the two 1s orbitals of the hydrogen. The two electrons are shared among these three orbitals.

Attached Image


In the presence of pure CH4 (graphic above), electron ejection generates methane radical cation, which steals a hydrogen atom from methane to create CH5+ and a remnant CH3 radical. As we will see much later, the CH3 radical participates in radical chemistry processes that lead primarily to simple saturated organics. To get the really good funky stuff, we need to make ethylene or other unsaturated intermediates.

So N2 and Ar atmospheres (with CH4) generate CH3+, but CH4 and H2 atmospheres generate CH5+. That is the key difference. The downstream chemistry of these two cation intermediates determine the product mix.

Here's my memory trick:
Carbenium ions (CH3+) are fun.
Carbonium ions (CH5+) are boring.


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