High-level ab initio characterization of the OH + CH3NH2 reaction

Abstract

The complex multichannel OH + CH₃NH₂ reaction is investigated using high-level ab initio methods considering not just the abstraction, but the substitution pathways, too. The H-abstraction channels are proven to be exothermic with classical(adiabatic) relative energies of -25.58(-26.23) and -17.93(-19.32) kcal mol^-1 for the methyl- and amino-H-abstraction processes resulting in H₂O + CH₂NH₂ and CH₃NH, respectively. There is another reaction path that is thermodynamically favored: the amino-substitution leading to NH₂ + CH₃OH with a classical(adiabatic) reaction energy of -6.43(-7.76) kcal mol^-1. The H- and methyl-substitution reactions, which provide H + HOCH₂NH₂/CH₃NHOH and CH₃ + NH₂OH, respectively, have higher energies relative to the reactants making them endothermic. The entrance channel of the reaction is studied using one-dimensional energy curves where the reactants are frozen in their equilibrium structures and they approach each other from different directions. The geometry optimization of the stationary points, including the reactants, transition states, post-reaction complexes and products, is carried out by the MP2 and CCSD(T)-F12b methods using the aug-cc-pVDZ and aug-cc-pVTZ basis sets. To reach higher chemical accuracy we perform single-point energy calculations at the CCSD(T)-F12b/aug-cc-pVQZ level of theory that are augmented with different energy corrections accounting for post-CCSD(T) correlation, core correlation, scalar relativistic effect, spin-orbit coupling and zero-point energy. Rate coefficients are computed using transition-state theory with Wigner tunneling correction and compared with previous theoretical and experimental data.