An Ab initio study of the excited states, isomerization energy profiles and conical intersections of a chiral cyclohexylidene derivative

The excited valence and Rydberg states of the chiral (4- methylcyclohexylidene) fluoromethane (4MCF) have been investigated using multiconfigurational CASSCF and CASPT2, and coupled-cluster methods (RI-CC2). A 3s Rydberg state is predicted below the valence ¹ππ* state. To gain insight into the photophysics of the cis - trans isomerization of this olefin, potential energy profiles for the valence ¹ππ * state along the twisting and pyramidalization reaction coordinates have been computed using variational methods (CASSCF and multireference configuration interaction with singles and doubles (MR-CISD)). Starting from geometries with energies close to degeneracy in the valence and ground-state curves, three minima on the crossing seam that can be correlated with the conical intersections known for fluoroethylene, have been found. On the basis of these features, the photochemistry of 4MCF is discussed.