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

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.