THE INTERNAL ROTATIONAL BARRIER OF BIPHENYL STUDIED WITH MULTICONFIGURATIONAL 2ND-ORDER PERTURBATION-THEORY (CASPT2)

A detailed ab initio study of the molecular structure and rotational b arriers of biphenyl has been performed. First, non-dynamical correlati on effects involving the pi system are taken into account at the CASSC F level. These wave functions are subsequently employed as reference f unctions in a multiconfigurational second-order perturbation treatment (CASPT2). The performance single-reference approaches is in addition analysed. The molecular geometries of biphenyl in twisted, coplanar, a nd perpendicular conformations have been optimized at the CASSCF level . A rotational angle of 44.3 degrees is predicted for the minimum ener gy conformer in agreement with gas-phase electron diffraction data (44 .4 +/- 1.2 degrees). The highest level of theory employed yields the v alues 12.93 (6.0 +/- 2.1) and 6.40 (6.5 +/- 2.5) kJ/mol for the barrie r heights at 0 degrees and 90 degrees, respectively (electron diffract ion data within parentheses). In the light of the present findings, th e reliability of the available experimental data is discussed.