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

Abstract

AbstractAb initio molecular orbital calculations are performed on the planar ground states (S0), the 90°‐twisted triplet (T1), and pyramidalized singlet (S1) excited states of ethylene, methaniminium cation (MC), monocyano‐ (MCE), 1,1‐dicyano‐ (DCE), 1,1‐dihydroxy‐ (DHE), and 1,1‐dicyano‐2,2‐dihydroxy (DCHE) ethylenes. Equilibrium geometries are optimized at the Hartree–Fock (HF) level with the 6‐31G* basis set. Electron correlation corrections 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‐order Møller–Plesset perturbation theory ((U)MP4/6‐311G**//MP2/6‐31G*). The effects of various types of perturbations on the structures, energetics, dipole moments, and state ordering of S0, S1, and T1 are carefully investigated. “Positive” S1–T1 splittings are estimated at the HF level for all studied molecules, while “negative” S1–T1 splittings are obtained at the MP2 level for MC, DHE, and DCHE. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 242–254, 2001