Predictive Theory for Hydrogen Atom−Hydrocarbon Radical Association Kinetics

Abstract

Procedures for accurately predicting the kinetics of hydrogen atom associations with hydrocarbon radicals are described and applied to a series of reactions. The approach is based on CASPT2/cc-pvdz evaluations of the orientation-dependent interaction energies within variable reaction coordinate transition state theory. One-dimensional corrections to the interaction energies are estimated from CAS+1+2/aug-cc-pvtz evaluations for the H + CH3 reaction, and a dynamical correction factor of 0.9 is applied. This corrected CASPT2 approach yields results that are within 10% of those obtained with the full CAS+1+2/aug-cc-pvtz potential for the H + CH3, H + C2H5, H + C2H3, and H + C2H reactions. New predictions are made for the H + iso-C3H7, H + tert-C4H9, H + C6H5, and H + C10H7 reactions. For the H + CH3 and H + C2H3 reactions, where the experimental values appear to be the most well-determined, theory and experiment essentially agree to within their error bars. For the other reactions, the agreement is reasonably satisfactory given the often large dispersion in the experimental results. For the reactions with saturated alkyl radicals, the theory predicts that each additional CH3 group increases the steric factor by approximately a factor of 2. In contrast, for the unsaturated radicals, the H + C6H5 and H + C10H7 high-pressure association rate coefficients are nearly identical to that for H + C2H3.

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 .…”

Kinetic studies of chlorobenzene reactions with hydrogen atoms and phenyl radicals and the thermochemistry of 1-chlorocyclohexadienyl radicals

“…The initial radical concentrations [Cl]0 = [C6H5]0 arising from photolysis of chlorobenzene along with the H-atom precursor are given in Table S1. In principle H + phenyl chemistry, which has a high rate constant [16], could account for 15-30% of H consumption at room temperature, 7-20% at 363 K and less than 5% at 630 K. These are k1 is a factor of 3 higher than the analogous H + C6H6 rate constant at room temperature, and similar to this rate constant at 500 K [18]. The benzene reaction proceeds via addition to make cyclohexadienyl and is at the high-pressure limit at 10 Torr of Ar bath gas.…”

Computed Rate Coefficients and Product Yields for c-C5H5 + CH3 → Products

“…It has been pointed out in an earlier paper by Harding et al 4 that this approximation can cause the transition state position to decrease to very low inter-atomic separation at high E's and J's. In our work we have noticed the same trend and have restricted the loose transition state position to have interfragment distance > 2.3Å .…”

“…For all the calculations the energy transfer probability for each collision assumes the exponential down model, with the energy transfer parameters for each bath gas used in this study provided in Table 2. 4. The details of the formulation of the master equation and methodology of solution used in Variflex is described in detail elsewhere.…”