Molecular elimination of methyl formate in photolysis at 234 nm: roaming vs. transition state-type mechanism

Abstract

Ion imaging coupled with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) technique is employed to probe CO(v″ = 0) fragments at different rotational levels following photodissociation of methyl formate (HCOOCH₃) at 234 nm. When the rotational level, J″_CO, is larger than 24, only a broad translational energy distribution extending beyond 70 kcal mol⁻¹ with an average energy of about 23 kcal mol⁻¹ appears. The dissociation process is initiated on the energetic ground state HCOOCH₃ that surpasses a tight transition state along the reaction coordinate prior to breaking into CO + CH₃OH. This molecular dissociation pathway accounts for the CO fragment with larger rotational energy and large translational energy. As J″_CO decreases, a bimodal distribution arises with one broad component and the other sharp component carrying the average energy of only 1–2 kcal mol⁻¹. The branching ratio of the sharp component increases with a decrease of J″_CO; (7.3 ± 0.6)% is reached as the image is probed at J″_CO = 10. The production of a sharp component is ascribed to a roaming mechanism that has the following features: a small total translational energy, a low rotational energy partitioning in CO, but a large internal energy in the CH₃OH co-product. The internal energy deposition in the fragments shows distinct difference from those via the conventional transition state.