Direct photoisomerization of CH2I2vs. CHBr3 in the gas phase: a joint 50 fs experimental and multireference resonance-theoretical study

Abstract

Femtosecond transient absorption measurements powered by 40 fs laser pulses reveal that ultrafast isomerization takes place upon S₁ excitation of both CH₂I₂ and CHBr₃ in the gas phase. The photochemical conversion process is direct and intramolecular, i.e., it proceeds without caging media that have long been implicated in the photo-induced isomerization of polyhalogenated alkanes in condensed phases. Using multistate complete active space second order perturbation theory (MS-CASPT2) calculations, we investigate the structure of the photochemical reaction paths connecting the photoexcited species to their corresponding isomeric forms. Unconstrained minimum energy paths computed starting from the S₁ Franck–Condon points lead to S₁/S₀ conical intersections, which directly connect the parent CHBr₃ and CH₂I₂ molecules to their isomeric forms. Changes in the chemical bonding picture along the S₁/S₀ isomerization reaction path are described using multireference average coupled pair functional (MRACPF) calculations in conjunction with natural resonance theory (NRT) analysis. These calculations reveal a complex interplay between covalent, radical, ylidic, and ion-pair dominant resonance structures throughout the nonadiabatic photochemical isomerization processes described in this work.