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 (HCOOCH(3)) 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 HCOOCH(3) that surpasses a tight transition state along the reaction coordinate prior to breaking into CO + CH(3)OH. 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 CH(3)OH co-product. The internal energy deposition in the fragments shows distinct difference from those via the conventional transition state.

Roads leading to roam. Role of triple fragmentation and of conical intersections in photochemical reactions: experiments and theory on methyl formate

“…In current debates 1 such structure has been attributed to either direct versus delayed or transition-state versus roaming-mediated mechanisms. [2][3][4][5][6][7][8][9][10][11][12][13][14] This work contributes to defining the strength and weakness of the concept of transition state, pervasive in chemistry along with the related ones of reaction coordinates and minimum energy paths. Accordingly, chemical reactions are modeled to proceed along a path that the system walks on a potential energy surface, going through the transition state (the ''mountain'' pass along this path), eventually reaching the products' region.…”

Photochemistry of glycolaldehyde in cryogenic matrices

“…Still for methyl formate, the so-called roaming mechanism implying dissociation on vibrationally hot ground state also yielded CO+CH 3 OH but it was only observed in specific conditions and was found to be weak. 25 So, we assume that molecular elimination in the matrix and in the gas phase occurs in a similar manner and with similar efficiency: the system passes through a transition state rather than through a dissociation-recombination process.…”

Reaction Dynamics with Molecular Beams and Oriented Molecular Beams: A Tool for Looking Closer to Chemical Reactions and Photodissociations

“…The radical dissociation channel, HCOOCH 3 ® CH 3 O + HCO, does not suffer from any exit barrier, but requires 97.1 kcal/mol to break the C-O bond. 33 A small portion of methyl formate that undergoes radical dissociation may couple to the S o state via internal conversion. In this manner, the translational energy release is usually small because of no exit barrier.…”