Competing radical and molecular channels in the unimolecular dissociation of methylformate

Abstract

The thermal dissociation of methylformate has been characterized by numerous experimental and theoretical studies that all seem to agree that the lowest energetically accessible process is a 3-center H-atom transfer that leads to the molecular products CH3OH + CO. However, these literature studies seem to be at odds with regards to the role of other competing molecular eliminations and bond-fission processes and therefore a complete and resolved mechanistic picture of methylformate thermal dissociation still eludes us. In this work, we have performed high-level electronic structure theory calculations to characterize the energetics of other overlooked molecular and radical processes that can originate on the complex CH3OCHO potential energy surface (PES). The present calculations do indeed confirm that the lowest energy process accessible on this PES is molecular elimination to form CH3OH + CO. However, unlike prior theoretical studies, the present calculations reveal that the second lowest energy process is a 5-center concerted elimination process that leads to the direct formation of H2 + CH2O + CO. We also note that HCO2 from the lowest-lying bond fission has two energetically comparable electronic states (of 2B2 and 2A1 symmetry). Furthermore, the barrier for H-atom migration of CH3OCHO to form the carbene (CH3OCOH) is similar to that for the direct 4-center elimination process leading to CH2O + CH2O characterized in prior literature studies. Radical and molecular pathways to CH3 + HOCO and CH4 + CO2 can also be facilitated from this carbene. Master equation calculations were performed to characterize the competition between the various molecular and radical processes on this more elaborate CH3OCHO PES. The results of the present theoretical analyses were used to resolve outstanding questions on the role of secondary radical-initiated reactions in characterizing prior literature experimental studies.