Photodissociation of the geometric isomers of 1,2-dibromoethylene

Abstract

Resonance-enhanced multiphoton ionization (REMPI) spectra of 1,2-dibromoethylene (C(2)H(2)Br(2)) obtained using ultraviolet fundamental wavelengths between 280 and 312.5 nm and monitoring Br(+) fragments in a time-of-flight mass spectrometer are found to differ dramatically from those reported in the literature by detecting C(2)H(2)(+). Laser power plots suggest that the initial excitation process is (2+1) REMPI, which accesses parent excited states between 156.25 and 140.84 nm. Unlike the spectra obtained by monitoring C(2)H(2)(+), the spectra obtained by monitoring Br(+) appear to be identical regardless of which parent isomer (cis or trans) is excited. Based on energetics, it is proposed that Br(+) ions are formed by excitation and fragmentation of a ground-state 2-bromovinyl radical intermediate (CHBr=CH.) generated by the rapid excited-state dissociation of the parent molecules. Density-functional theory calculations using the hybrid Perdew-Burke-Ernzerhof (PBE1PBE) functional and the 6-311++G(3df,3pd) basis set confirm that the barrier to isomerization for the 2-bromovinyl radicals formed from the cis- and trans-1,2-dibromoethylenes is low, which explains why the resultant REMPI spectra cannot be distinguished based on their isomeric origin. Electronic spectra calculated for the 2-bromovinyl radical using the long-range-corrected hybrid PBE functional (LC-omegaPBE) are in qualitative agreement with experimental results.