Solvent effect on the conical intersection of 4-cyclopentadienylidene-1,4-dihydropyridine (CPDHP)

A conical intersection that exists between the S₀ and S₁ surfaces of the title molecule in the gas phase and in non-polar solvents is shown to be eliminated in polar solvents. Potential surfaces are calculated using the high level CASSCF method along the two coordinates predicted to lift the degeneracy according to the Longuet-Higgins loop method. The larger gap between the two surfaces in polar solvents results from the stabilization of a zwitterion transition state.