Theoretical Study of Torsion Potentials in 2,2'-Dihydroxybiphenyl and 2,2'Dihydroxy- 5,5'-Dimethylbiphenyl: Modeling Torsions in Multi-Phenolic Systems.

摘要

Using ab initio (6-31G(*)) and semiempirical (AM1, PM3) electronic structure calculations and force field (MM3) determinations, we have analyzed the molecular structure of 2,2'-dihydroxybiphenyl (BP), 2,2-dihydroxy-5,5'- dimethylbiphenyl (MBP), and 2,5-di(2-hydroxphenyl) phenol (TP). In both BP and MBP systems, there exist three predominant conformations for which the orientations of the of the hydroxyl groups are the distinguishing characteristic. Each method used determined that the optimized geometry of the cis structure was the overall lowest energy conformation. The phenyl groups in the systems show significant twist angles, psi, (42 - 55 degrees), depending upon the hydroxyl group orientation. We have examined the energy of the BP and MBP systems as a function of torsion angle using each of the methods described above. Each of the methods of calculation predicts, for all three conformations, a 10 - 12 kcal/mol barrier to the coplanar torsion angle. The 6-31G(*) calculations predict a total of five energy minima for the three conformations, with no 90 deg barrier in the cis structure. The appearance and size of a 90 deg barrier, as well as the torsion angle for the minimum energy geometry, is dependent upon the theoretical method used. Examination of 2,5-di(2- hydroxyphenyl) phenol (TP) as a model for oligometric systems finds that pairwise interactions as demonstrated in BP and MBP dominate the relative energetic of their conformations.