Conformational analysis. Part 12. A theoretical and lanthanide-induced shift (LIS) investigation of the conformation of benzophenone and 3,4′-dichlorobenzophenone

Abstract

Conformational analysis of benzophenone and 3,4′-dichlorobenzophenone has been studied by theoretical (semiempirical and ab initio) calculations and by the Lanthanide-induced shift (LIS) technique. The theoretical calculations were based on ab initio(STO-3G) energies of MNDO optimised geometries. The minimum-energy conformer is a helical conformer with a dihedral angle (θ) between the phenyl and carbonyl group planes of 36°. Analysis of the theoretical curves gave a two-fold potential barrier about this bond of 20.4 kJ mol^–1 and further showed that the most likely rotation of the phenyl rings was via a disrotatory mechanism with a high-energy perpendicular intermediate (θ₁= 0°, θ₂= 90°) 5.4 kJ mol^–1 above the ground state. The LIS investigation of benzophenone was inconclusive due to the molecular symmetry, all torsional angles giving good solutions. For the 3,4′-dichlorobenzophenone the more extensive data set allowed a precise analysis. The best agreement factor was found for 56% of the trans conformer with a torsional angle (θ) of 26°. The lanthanide was also unevenly populated about the carbonyl group, 55% being on the side of the meta-chloro ring, with values of r, φ, and ψ for the Ln–O geometry found to be 2.75 Å, 55–60° and 160–165°, respectively. These results can be explained on the basis of steric effects between the lanthanide and the phenyl rings together with different π densities in the two rings.