Estudo teórico da reação O(3P) + HBr: superfícies de energia potencial, cinética e dinâmica

Abstract

In this work, the O(3 P) + HBr → OH + Br reaction, which is relevant to atmospheric chemistry, specially for the catalytic ozone depletion, was studied using high-level ab initio potential energy surfaces. We constructed surfaces for the 3 A ′′ and 3 A ′ electronic states, based on the many-body expansion, with the two-body potentials adjusted by the switched-MLJ function and the three-body potentials interpolated using the RKHS method. The ab initio points were calculated at the MRCISD+Q/CBS+SO level of theory. The main features of the 3 A ′′ are the presence of two van der Waals minima, at the entrance and exit channels, and a transition state with a barrier height of 5.01 kcal mol −1. The 3 A ′ PES has a linear transition state at 6.45 kcal mol −1. We obtained the enthalpy of reaction, at 0 K, of −15.7 kcal mol −1 , in close agreement with the experimental value of −15.14 kcal mol −1. Rate constants, in the temperature range from 200 to 1000 K, were calculated using the variational transition state theory with contributions of multidimensional tunneling (ICVT/µOMT) and also a quantum reactive scattering approach (QM/JS). Their values are in fair agreement with the experimental data in the literature in the whole temperature range available: from 221 to 554 K for the O + HBr reaction, and from 295 to 419 K for the O + DBr reaction. At 298 K, the calculated rate constants, in cm 3 molecule −1 s −1 , for the O + HBr reaction are 3.62•10 −14 (ICVT/µOMT) and 3.35•10 −14 (QM/JS); and the experimental value is 3.78•10 −14. The quality of these results reinforces our confidence in the procedures and approximations used, leading to the possibility of high-level characterization of a variety of gas phase reactions.