Computational study of the rate constants and kinetic isotope effects for the CH3+HBr→CH4+Br reaction

Ab initio direct dynamics methods have been used to study the title reaction. The electronic structure information including geometries, gradients and force constants (Hessians) is calculated at the $\mathrm{QCISD}\text{/6-31+}\mathrm{G}\mathrm{(d)}$ level. With the aid of intrinsic reaction coordinate theory, the minimum energy path (MEP) is obtained at the same level, and the energies along the MEP are further refined by performing the single-point calculations at the $\mathrm{PMP}\text{4/6-311++}\mathrm{G}\text{(3df,3pd)}$ level. For this reaction, the theoretical rate constants by using improved canonical variational transition state theory incorporating small-curvature tunneling correction are in excellent agreement with the more recent experimental values. Our calculated results show that for the ${\mathrm{CH}}_3+\mathrm{HBr}$ reaction, the rate constants have a strong nonlinear Arrhenius behavior, i.e., the ${\mathrm{CH}}_3+\mathrm{HBr}$ reaction has negative temperature dependence at $\text{T<536\hspace{0.17em}}\mathrm{K},$ but clearly shows positive temperature dependence at $\text{T>536\hspace{0.17em}}\mathrm{K}.$ The current work predicts that the kinetic isotope effect for the title reaction is normal in the temperature range 200–482 K, i.e., $k_{\mathrm{HBr}}{\mathrm{/k}}_{\mathrm{DBr}}\text{>1.}$ This result is in good agreement with the experimental one.