Predictive theory for the combination kinetics of two alkyl radicals

Abstract

An ab initio transition state theory based procedure for accurately predicting the combination kinetics of two alkyl radicals is described. This procedure employs direct evaluations of the orientation dependent interaction energies at the CASPT2/cc-pvdz level within variable reaction coordinate transition state theory (VRC-TST). One-dimensional corrections to these energies are obtained from CAS+1+2/aug-cc-pvtz calculations for CH3 + CH3 along its combination reaction path. Direct CAS+1+2/aug-cc-pvtz calculations demonstrate that, at least for the purpose of predicting the kinetics, the corrected CASPT2/cc-pvdz potential energy surface is an accurate approximation to the CAS+1+2/aug-cc-pvtz surface. Furthermore, direct trajectory simulations, performed at the B3LYP/6-31G* level, indicate that there is little local recrossing of the optimal VRC transition state dividing surface. The corrected CASPT2/cc-pvdz potential is employed in obtaining direct VRC-TST kinetic predictions for the self and cross combinations of methyl, ethyl, iso-propyl, and tert-butyl radicals. Comparisons with experiment suggest that the present dynamically corrected VRC-TST approach provides quantitatively accurate predictions for the capture rate. Each additional methyl substituent adjacent to a radical site is found to reduce the rate coefficient by about a factor of two. In each instance, the rate coefficients are predicted to decrease quite substantially with increasing temperature, with the more sterically hindered reactants having a more rapid decrease. The simple geometric mean rule, relating the capture rate for the cross reaction to those for the self-reactions, is in remarkably good agreement with the more detailed predictions. With suitable generalizations the present approach should be applicable to a wide array of radical-radical combination reactions.

Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

“…When a reaction was thought to proceed through addition (as is the case with most of the radical-radical reactions), in the absence of theoretical calculations, we have considered that there was no barrier when the ground state of the adduct arises from pairing up electrons on the two radicals reactants, whereas the surface was likely be repulsive if all the electrons remain unpaired. Therefore, doublet + doublet reactions were considered to have no barrier for the singlet surface but a barrier for the triplet surfaces (which is in good agreement with experimental and theoretical results for H + alkyl or alkyl + alkyl reactions for example, Harding et al 2005;Klippenstein et al 2006). When no energetic barrier was found to be present, the value of the rate constant was estimated using long-range forces, mainly through dispersion interactions (Stoecklin & Clary 1992;, and by taking into account the electronic degeneracy γ el .…”

Rate Constant and Branching Fraction for the NH2 + NO2 Reaction

“…This correction factor accounts for the small degree of recrossing of the variable reaction coordinate transition state dividing surfaces as estimated in prior work for related reactions. 62 One-dimensional correction potentials were employed to approximately account for the limitations in the basis set, in the active space, and for geometry relaxation. The basis set corrections were obtained from CBS extrapolations based on explicit calculations with cc-pVTZ and cc-pVQZ basis sets.…”

Temperature Dependence of Two Key Interstellar Reactions of H3+: O(3P) + H3+ and CO + H3+

“…For the ab initio potential based results, we employ our direct variable reaction coordinate-TST approach. 31,32 We also employ rigid-body trajectory simulations of the capture process for long-range representations of the potential to examine the deviations from statistical predictions for both the overall rate coefficient and the product branching. These trajectory simulations are performed as described in refs 33 and 34.…”