Catalytic effect of a single water molecule on the OH + CH2NH reaction

Abstract

In recent work, there has been considerable speculation about the atmospheric reaction of methylenimine (CH₂NH), because this compound is highly reactive, soluble in water, and sticky, thus posing severe experimental challenges. In this work, we have revisited the kinetics of the OH + CH₂NH reaction assisted by a single water molecule. The potential energy surfaces (PESs) for the water-assisted OH + CH₂NH reaction were calculated using the CCSD(T)//BH&HLYP/aug-cc-pVTZ levels of theory. The rate coefficients for the bimolecular reaction pathways CH₂NH⋯H₂O + OH and CH₂NH + H₂O⋯HO were computed using canonical variational transition state theory (CVT) with small curvature tunneling correction. The reaction without water has four elementary reaction pathways, depending on how the hydroxyl radical approaches CH₂NH. In all cases, the reaction begins with the formation of a single pre-reactive complex before producing abstraction and addition products. When water is added, the products of the reaction do not change, and the reaction becomes quite complex, yielding four different pre-reactive complexes and eight reaction pathways. The calculated rate coefficient for the OH + CH₂NH (water-free) reaction at 300 K is 1.7 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ and for OH + CH₂NH (water-assisted), it is 5.1 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. This result is similar to the isoelectronic analogous reaction OH + CH₂O (water-assisted). In general, the effective rate coefficients of the water-assisted reaction are 2∼3 orders of magnitude smaller than water-free. Our results show that the water-assisted OH + CH₂NH reaction cannot accelerate the reaction because the dominated water-assisted process depends parametrically on water concentration. As a result, the overall reaction rate coefficients are smaller.