Mechanism of the CH3NH2-HNO2 reaction: Ab initio DFT/TST study

The methyl amine-nitrous acid reaction in gas phase has been studied by ab initio MO and DFT methods at the B3LYP and MP2 levels using 6-31++G(d,p) basis set. Minima on the potential energy surface (PES) were characterized by absence of any imaginary frequency and the transition states by single imaginary frequencies. Intrinsic reaction coordinate method was used to find the minimum energy paths along which reactants and products are connected to the transition states. The reaction involves a number of steps: bimolecular decomposition of nitrous acid to form the ion-pair ON+…−ONO (as revealed by NPA), is the first step and is theoretically found to be the rate determining step in compliance with the experimental findings. Subsequently, a proton shuttle mechanism operates resulting finally in the formation of methanol and diazomethane as substitution and elimination product, respectively; the former is identified as the major product on the basis of computed rate constants using the transition state theory. Cleavage of old bonds and formation of new bonds during the different steps have been monitored by the study of the electronic population arrangement and the calculated IR spectra of the species occurring at the minima of the PES. A stepwise mechanism of the title reaction is thus theoretically established.