The atmospherically important reaction of hydroxyl radicals with methyl nitrate: a theoretical study involving the calculation of reaction mechanisms, enthalpies, activation energies, and rate coefficients

摘要

A theoretical study, involving the calculation of reaction enthalpies and activation energies, mechanisms and rate coefficients, has been made of the reaction of hydroxyl radicals with methyl nitrate, an important process for methyl nitrate removal in the earth's atmosphere. Four reaction channels were considered:- formation of H2O + CH2ONO2, CH3OOH + NO2, CH3OH + NO3, and CH3O + HNO3 . For all channels, geometry optimization and frequency calculations were carried out at the M06-2X/6-31+G** level, while relative energies were improved at the UCCSD(T*)-F12/CBS level. The major channel is found to be the H abstraction channel, to give the products H2O + CH2ONO2. The reaction enthalpy (ΔH298KRX) of this channel is computed as -17.90 kcal.mol-1. Although the other reaction channels are also exothermic, their reaction barriers are high (> 24 kcal.mol-1) and therefore these reactions do not contribute to the overall rate coefficient in the temperature range considered (200-400 K). Pathways via three transition states have been identified for the H abstraction channel. Rate coefficients were calculated for these pathways at various levels of variational transition state theory (VTST) including tunneling. The results obtained are used to distinguish between two sets of experimental rate coefficients, measured in the temperature range 200-400K, one of which is approximately an order of magnitude greater than the other. This comparison, as well as the temperature dependence of the computed rate coefficients, shows that the lower experimental values are favoured. The implications of the results to atmospheric chemistry are discussed.