Photodissociation mechanism of methyl nitrate. A study with the multistate second-order multiconfigurational perturbation theory

Abstract

The photodissociation reactions of methyl nitrate CH₃ONO₂ starting at the 193 and 248 nm photolytic wavelengths have been studied with the second-order multiconfigurational perturbation theory (CASPT2) by computation of numerical energy gradients for stationary points. In addition, energy profiles of reaction paths and vertical excitations have been investigated with the multistate extension of the multiconfigurational second-order perturbation theory (MS-CASPT2). It is found that excitation at 193 nm yields three reaction paths: (i) the so-called slow channel CH₃ONO₂→ CH₃O + NO₂→ CH₃O + NO + O; (ii) the fast channel CH₃ONO₂→ CH₃O + NO₂; and (iii) CH₃ONO₂→ CH₃ONO + O. The slow channel starts at the S₄ surface, in contrast, the population of the S₃ state can lead to the fast channel or to direct atomic oxygen extrusion. The rather high relative yield of the channel leading to oxygen extrusion from methyl nitrate is explained on the basis of an S₃/S₂ conical intersection that transfers the initial excitation localized in the nπ* S₃ state to the σπ* S₂ state with a consequent weakening of the N–O bond. With respect to photolysis at 248 nm, it was not possible to unambiguously distinguish between S₁ and S₂ as the populated state, however, the S₂ state is suggested as mainly responsible for dissociation at this excitation energy.