Non-adiabatic photoisomerization versus photodissociation dynamics of the chiral fluoroethylene derivative (4-methylcyclohexylidene) fluoromethane

The photoisomerization around the C=C double bond and the competing elimination of hydrogen fluoride (HF) are studied in (4-methylcyclohexylidene) fluoromethane. Both reactions are mediated by twisted conical intersections (CI) around the C=C bond. Potential energy surfaces (PES) for the electronic ground state and first bright excited state of pi pi* character are calculated using the CASSCF method along two reaction coordinates: the torsion around the C=C bond and the distance between the center of masses of the hydrocarbon moiety and the HF fragment. Non-adiabatic couplings between both PES are obtained at the same level of theory. Wavepacket dynamics on the coupled surfaces show that after light irradiation torsion in the pi pi* state dominates over HF dissociation, although the system starts with enough kinetic energy to reach the CI leading to HF-elimination. (c) 2010 Elsevier B.V. All rights reserved.