Correlated ab initio molecular orbital (MP3, MP4) and density functional (PW91, MPW91) studies on the conformations of 1,2-diphenylethane

Density functional (PW91, MPW91) and high level correlated ab initio molecu lar orbital (MP4SDQ) calculations were carried out to determine he molecula r geometries and the relative energies of the conformations of 1,2-diphenyl ethane. The computed data are indicative of a small difference in the energ ies of the two main conformations: the synclinal (gauche Ph/Ph) and the ant iperiplanar (anti Ph/Ph) ones. The DFT methods with the large basis set (6- 311++G) gave small energy preference (about 0.3 kJ mol(-1)) of the antiperi planar conformation, while the corrections for the electron correlation ene rgies at the MP4SDQ level produced about 1.0 kJ mol(-1) lower energy for th e synclinal conformation. It is proposed that entropy effects play an impor tant role for the observed prevalence of the antiperiplanar conformation. T he results are discussed in relation to their potential utilization as comp uted observables in molecular mechanics parameterization of the 1,2-dipheny lethane skeleton. (C) 2000 Elsevier Science B.V. All rights reserved.