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

Physical Chemistry Chemical Physics Pub Date: 2011-03-11 DOI: 10.1039/C0CP02710C

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.

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