Direct comparison of 3-centre and 4-centre HBr elimination pathways in methyl-substituted vinyl bromides

Abstract

Elimination of HBr from UV-photoexcited vinyl bromides can occur through both 3-centre and 4-centre transition states (TSs). The competition between these pathways is examined using velocity map imaging of HBr (v = 0-2, J) photofragments. The three vinyl bromides chosen for study have methyl substituents that block either the 3-centre or the 4-centre TS, or leave both pathways open. The kinetic energy distributions extracted from velocity map images of HBr from 193 nm photolysis of the three vinyl bromide compounds are approximately described by a statistical model of energy disposal among the degrees of freedom of the photoproducts, and are attributed to dissociation on the lowest electronic state of the molecule after internal conversion. Dissociation via the 4-centre TS gives greater average kinetic energy release than for the 3-centre TS pathway. The resonance enhanced multi-photon ionization (REMPI) schemes used to detect HBr restrict measurements to J ≤ 7 for v = 2 and J ≤ 15 for v = 0. Within this spectroscopic range, the HBr rotational temperature is colder for the 4-centre than for the 3-centre elimination pathway. Calculations of the intrinsic reaction coordinates and RRKM calculations of HBr elimination rate coefficients provide mechanistic insights into the competition between the pathways.

Collisional Relaxation of Highly Vibrationally Excited Acetylene Mediated by the Vinylidene Isomer

“…Our result is consistent with a combined experimental and theoretical study on methylsubstituted vinyl bromide where >5:1 carbene-to-alkyne ratio is determined. 27 Nevertheless, when observed on a ns time scale, both reaction channels result in the production of acetylene with roughly 23,000 cm −1 of internal energy, due to the fs isomerization of vinylidene to excited acetylene. 25 The excited acetylene can transfer its internal energy through collisions with rare-gas atoms.…”