Wednesday, June 12, 2013

Oxidation of CO

First part of an occasional series

Oxidation of carbon monoxide (CO) to carbon dioxide (CO2) is a vital last step in combustion and the atmospheric degradation of just about any hydrocarbon.  It is slow, taking about two months in the troposphere, but the lifetime is not really slow, like methane (5-10 years) or like forever in the troposphere like the chloroflorocarbons.

As with just about everything, the HO radical, the atmospheric vacuum cleaner, is the species that oxidizes CO.

CO + HO --> CO2 + H
In 2012 Jun Li, Yimin Wang, Bin Jiang, Jianyi Ma, Richard Dawes, Daiqian Xie, Joel M. Bowman, and Hua Guo calculated a new, and much more accurate potential energy surface for this reaction (35000 points).  The paper now has 19 citations.  Recent papers have appeared detailing kinetics (how fast the reaction happens) and the dynamics (how the reaction happens) and the quantum effects (how spooky things happen).  The spooky things turn out to be pretty important.   Not everything is perfect, but their model does advance understanding.. Well it is interesting to Eli and the Bunnies have been doing enough policy stuff and this is his blog so quit complaining.  Of course, there has been a ton of previous experimental and theoretical work

Above is a sketch of the overall reaction energetics.   The solid lines are the calculated energies without zero point energy, more familiarly ZPE.  The wavy ones include ZPE. 

ZPE is one of the first and most convincing evidences of quantum behavior and in particular the Heisenberg Uncertainty Principle.  If the lowest amount of vibrational energy were really zero, then the separation of atoms would be fixed, and there would be no uncertainty in the position of the atoms.  By the Uncertainty Principle, this would require that the kinetic energy involved in the vibration would be unbounded.  Obviously a problem, and the solution is that the vibrational energy is hν (v + 1/2), the lowest simply hν/2, aka the ZPE.

With that out of the way first thing to notice is that the overall reaction, from the reactants at the far left to the products at the far right is really downhill, a net of (29.59-6.97) or  22.62 kcal/mol, or 23.78 kcal/mol with ZPE.  So why is the reaction slow, at the least relatively slow, under atmospheric conditions? 

There are problems both at the beginning (the entrance channel) and the end (the exit channel).

The figure helps, showing the rough shape of the molecules as balls.  Historically (as in it is always done that way but the reasons are hidden in the depths of time) carbon, C, is black, grey in this case, oxygen, O, is red and hydrogen atoms H, are white) 

OH is dipolar, the H end has a more positive charge and the O end more negative.  CO is only slightly dipolar, (0.122 D vs. 1.66 D  for OH), so as the two molecules approach each other the H atom is attracted to the CO. 

In the entrance channel, (looks like a chair) and the cis  configurations, there is a small barrier in the trans(chair like) configuration  (0.88 kcal/mol) and a larger one in the cis(looks like a bowl) configuration (4.35 kcal/mol).  Comparing both to the average kinetic energy at room temperature of 0.89 kcal/mol, it is unlikely that one could get by the cis-TS1 (stands for transition state).  The net result is that collisions where the H atom points to the O atom in CO would not react, simply bouncing back to the reactants while those with the H atom pointing to the C atom might (some would some would simply bounce back to reactants). TS1 involves rotation of the OH and the CO bringing the O atom in OH close to the C atom in CO.

And so begins the dance.

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