An ab initio study of the structures and energetics of the planar ground and 90 degrees-twisted excited states of substituted ethylenes

Ab initio molecular orbital calculations are performed on the planar ground states (S-0), the 90 degrees -twisted triplet (T-1), and pyramidalized sin glet (S-1) excited states of ethylene, methaniminium cation (MC), monocyano - (MCE), l,1-dicyano- (DCE), l,1-dihydroxy- (DHE), and l,1-dicyano-2,2-dihy droxy (DCHE) ethylenes. Equilibrium geometries are optimized at the Hartree -Fock (HF) level with the 6-31G* basis set. Electron correlation correction s are estimated by optimizing the HF/6-31G* geometries at the (U)MP2/6-31G* level and then by carrying out single-point calculations at the fourth-ord er Moller-Plesset perturbation theory ((U)MP4/6-311G**//MP2/6-31G*). The ef fects of various types of perturbations on the structures, energetics, dipo le moments, and state ordering of S-0, S-1, and T-1 are carefully investiga ted. "Positive" S-1-T-1 splittings are estimated at the HF level for all st udied molecules, while "negative" S-1-T-1 splittings are obtained at the MP 2 level for MC, DHE, and DCHE. (C) 2001 John Wiley & Sons, Inc.