Selectivity and Mechanisms Driven by Reaction Dynamics: The Case of the Gas-Phase OH^– + CH₃ONO₂ Reaction

Well-established statistical approaches such as transition-state theory based on high-level calculated potential energy profiles are unable to account for the selectivity observed in the gas-phase OH^– + CH₃ONO₂ reaction. This reaction can undergo bimolecular nucleophilic displacement at either the carbon center (S_N2@C) or the nitrogen center (S_N2@N) as well as a proton abstraction followed by dissociation (E_CO2) pathway. Direct dynamics simulations yield an S_N2:E_CO2 product ratio in close agreement with experiment and show that the lack of reactivity at the nitrogen atom is due to the highly negative electrostatic potential generated by the oxygen atoms in the ONO₂ group that scatters the incoming OH^–. In addition to these dynamical effects, the nonstatistical behavior of these reactions is attributed to the absence of equilibrated reactant complexes and to the large number of recrossings, which might be present in several ion–molecule gas-phase reactions