The potential energy surface and chemical kinetics for the reaction of HO with CO, which is an important process in both combustion and atmospheric chemistry, were computed using high-level ab initio quantum chemistry in conjunction with semiclassical transition state theory under the limiting cases of high and zero pressure. The reaction rate constants calculated from first principles agree extremely well with all available experimental data, which range in temperature over a domain that covers both combustion and terrestrial atmospheric chemistry. The role of quantum tunneling is confirmed to be extremely important, which supports recent work by Continetti and collaborators regarding the loss of hydrogen atoms from vibrationally excited states of HOCO. A sensitivity analysis has been carried out and serves as the basis for a plausible estimate of uncertainty in the calculations.

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Collisional energy transfer remains an important area of uncertainty in master equation simulations. Quasi-classical trajectory (QCT) calculations were used to examine the energy transfer probability density distribution (energy transfer kernel), which depends on translational temperature, on the nature of the collision partners, and on the initial and final total internal energies and angular momenta: P(E, J; E', J'). For this purpose, model potential energy functions were taken from the literature or were formulated for pyrazine + Ar and for ethane + Ar collisions. For each collision pair, batches of 10(5) trajectories were computed with three selected initial vibrational energies and five selected values for initial total angular momentum. Most trajectories were carried out with relative translational energy distributions at 300 K, but some were carried out at 1000 or 1200 K. In addition, some trajectories were computed for artificially "heavy" ethane, in which the H-atoms were assigned masses of 20 amu. The results were binned according to (ΔE, ΔJ), and a least-squares analysis was carried out by omitting the quasi-elastic trajectories from consideration. By trial-and-error, an empirical function was identified that fitted all 45 batches of trajectories with moderate accuracy. The results reveal significant correlations between initial and final energies and angular momenta. In particular, a strong correlation between ΔE and ΔJ depends on the smallest rotational constant in the excited polyatomic. These results show that the final rotational energy distribution is not independent of the initial distribution, showing that the plausible simplifying assumption described by Smith and Gilbert [Int. J. Chem. Kinet. 1988, 20, 307-329] and extended by Miller, Klippenstein, and Raffy [J. Phys. Chem. A 2002, 106, 4904-4913] is invalid for the systems studied.

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