Gas-phase kinetics study of reaction of OH radical with CH₃NHNH₂by second-order multireference perturbation theory

The gas-phase kinetics of H-abstraction reactions of monomethylhydrazine (MMH) by OH radical was investigated by second-order multireference perturbation theory and two-transition-state kinetic model. It was found that the abstractions of the central and terminal amine H atoms by the OH radical proceed through the formation of two hydrogen bonded preactivated complexes with energies of 6.16 and 5.90 kcal mol-1lower than that of the reactants, whereas the abstraction of methyl H atom is direct. Due to the multireference characters of the transition states, the geometries and ro-vibrational frequencies of the reactant, transition states, reactant complexes, and product complexes were optimized by the multireference CASPT2/aug-cc-pVTZ method, and the energies of the stationary points of the potential energy surface were refined at the QCISD(T)/CBS level via extrapolation of the QCISD(T)/cc-pVTZ and QCISD(T)/cc-pVQZ energies. It was found that the abstraction reactions of the central and two terminal amine H atoms of MMH have the submerged energy barriers with energies of 2.95, 2.12, and 1.24 kcal mol-1lower than that that of the reactants respectively, and the abstraction of methyl H atom has a real energy barrier of 3.09 kcal mol-1. Furthermore, four MMH radical-H2O complexes were found to connect with product channels and the corresponding transition states. Consequently, the rate coefficients of MMH + OH for the H-abstraction of the amine H atoms were determined on the basis of a two-transition-state model, with the total energy E and angular momentum J conserved between the two transition-state regions. In units of cm3molecule-1s-1, the rate coefficient was found to be k1= 3.37 × 10-16T1.295exp(1126.17/T) for the abstraction of the central amine H to form the CH3N•NH2radical, k2= 2.34 × 10-17T1.907exp(1052.26/T) for the abstraction of the terminal amine H to form the trans-CH3NHN•H radical, k3= 7.41 × 10-20T2.428exp(1343.20/T) for the abstraction of the terminal amine H to form the cis-CH3NHN•H radical, and k4= 9.13 × 10-21T2.964exp(-114.09/T) for the abstraction of the methyl H atom to form the C•H2NHNH2radical, respectively. Assuming that the rate coefficients are additive, the total rate coefficient of these theoretical predictions quantitatively agrees with the measured rate constant at temperatures of 200-650 K, with no adjustable parameters.