Twist angles and rotational energy barriers of biphenyl and substituted biphenyls

Using B3LYP/6-311+G* and other methods, twist angles and torsional energies were obtained for biphenyl (BP), 2-halogen BPs, 2,2'-dihalogen BPs, and 3,3'-dihalogen BPs, the halogens ranging from F to I. The results were compared with available gas phase and X-ray data. 2,2'-difluoro BP has a rotational double minimum, at 57.9 and 128.9degrees, whereas the other 2,2'-dihalogen BPs have a single minimum at dihedral angles ranging from 84.9 to 94.8degrees. All 3,3'-dihalogen BPs have a double minimum at about 45 and 135degrees. Optimized twist angles and energy barriers were also calculated for 2,2'-dimethyl BP and for perfluoro as well as perchloro BP. Most structures are accounted for by steric effects. For 2,2'-dihalogen BPs, however, attractive forces also appear to play a role, as evidenced by the dihedral angle of 2,2'-dichloro BP lying well below 90degrees.