H-abstraction is more efficient than cis-trans isomerization in (4-methylcyclohexylidene) fluoromethane. An ab initio molecular dynamics study.

Non-adiabatic molecular dynamics simulations have been performed in the fluoro-olefin (4-methylcyclohexylidene) fluoromethane (4MCF) using multiconfigurational CASSCF (complete active space self-consistent field) on-the-fly calculations. As an olefin containing a C[double bond, length as m-dash]C double bond, 4MCF is expected to undergo cis-trans isomerization after light irradiation. However, ab initio molecular dynamics shows that a preferential dissociation of atomic hydrogen is taking place after population transfer to the bright ππ* state. This state is strongly mixed with πσ* states allowing dissociation in the electronic excited state before deactivation to the ground state occurs. A minor amount of trajectories experiences F-dissociation, followed by pyramidalization at the sp(2) carbons and CHF dissociation. In contrast, the amount of trajectories undergoing torsion around the double bond, and therefore cis-trans isomerization, is marginal. The H-abstraction reaction is ultrafast, taking place in less than 60 fs.