Matrix isolation and computational study of isodifluorodibromomethane(F_2CBr-Br): A route to Br_2formation in CF_2Br_2photolysis

摘要

The photolysis products of dibromodifluoromethane (CF_2Br_2) were characterized by matrix isolation infrared and UVNisible spectroscopy, supported by ab initio calculations. Photolysis atwavelengths of 240 and 266 nm of CF_2Br_2:Ar samples (~1: 5000) held at ~5 K yielded iso-CF_2Br_2(F_2CBrBr), a weakly bound isomer of CF_2Br_2, which is characterized here for the firsttime. The observed infrared and UVNisible absorptions of iso-CF_2Br_2are in excellent agreement with computational predictions at the B3LYP/aug-cc-pVTZ level. Single point energy calculationsat the CCSD(T)/aug-cc-pVDZ level on the B3LYP optimized geometries suggest that the isoform is a minimum on the CF2Br2 potential energy surface, lying some 55 kcal/mol above the CF_2Br_2 ground state. The energies of various stationary points on the CF_2Br_2potential energy surface werecharacterized computationally; taken with our experimental results, these show that iso-CF_2Br_2is an intermediate in the Br+ CF_2Br→ CF_2+Br_2reaction. The photochemistry of the isoform was alsoinvestigated; excitation into the intense 359 nm absorption band resulted in isomerization to CF_2Br_2. Our results are discussed in view of the rich literature on the gas-phase photochemistry of CF_2Br_2,particularly with respect to the existence of a roaming atom pathway leading to molecular products