Kinetics, Products, and Mechanisms Study of the Atmospheric Degradation of (E)-4-Methoxy-3-buten-2-one with Hydroxyl Radicals

Abstract

High-level ab initio calculations have been conducted to investigate the mechanisms and kinetics of the oxidation of (E)-4-methoxy-3-buten-2-one by hydroxyl radicals (OH) in the atmosphere. Potential energy diagrams have been constructed at the CCSD(T)/6-311++G(d,p)//BH&HLYP/6-311++G(d,p) level of theory to characterize the overall reaction mechanism. Transition-state theory, in combination with Eckart tunneling, is employed to calculate the rate coefficients for the title reaction over the 180−360 K temperature range. Rate coefficients can then be fitted to a modified form of the Arrhenius equation: k = 2.00 × 10 −13 × exp(1960.4/T) cm 3 molecule −1 s −1 . The room temperature rate coefficient is calculated to be 1.41 × 10 −10 cm 3 molecule −1 s −1 , which matches well with the experimentally reported ones. The kinetic results imply that the reaction for OH radicals with (E)-4-methoxy-3-buten-2-one mainly leads to the formation of two hydroxyalkyl radicals by the initial addition of OH radicals to the >C�C< double bond. Under NO x -contaminated atmospheric environments, the hydroxyalkyl radicals can undergo further reactions to give various products such as methyl formate, methyl glyoxal, 2-hydroxy-2-methoxyacetaldehyde, peroxyacetyl nitrate (PAN), CO 2 , and a variety of peroxyalkyl nitrates and organic nitrates, which are in qualitative agreement with the experimental observations. A discussion on the atmospheric implications is also presented.