Direct photoisomerization of CH 2 I 2 vs. CHBr 3 in the gas phase: a joint 50 fs experimental and multireference resonance-theoretical study.

Femtosecond transient absorption measurements powered by 40 fs laser pulses reveal that ultrafast isomerization takes place upon S1 excitation of both CH2 I2 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 CH2 I2 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.