Torsional Barriers and Equilibrium Angle of Biphenyl: Reconciling Theory with Experiment

The barriers of internal rotation of the two phenyl groups in biphenyl are investigated using a combination of coupled cluster and density functional theory. The experimental barriers are for the first time accurately reproduced; our best estimates of the barriers are 8.0 and 8.3 kJ/mol around the planar and perpendicular conformations, respectively. The use of flexible basis sets of at least augmented quadruple-ζ quality is shown to be a crucial prerequisite. Further, to finally reconcile theory with experiment, extrapolations of both the basis set toward the basis set limit and electron correlation toward the full configuration-interaction limit are necessary. The minimum of the torsional angle is significantly increased by free energy corrections, which are needed to reach an agreement with experiment. The density functional B3LYP approach is found to perform well compared with the highest level ab initio results.