Ring Rotation in Biphenyl by William Abbott

Ground State Geometry While  -electron delocalization effects would favor a planar conformation, the steric repulsion between the ortho-hydrogens would favor a staggered position. Torn between these two competing forces, one would expect the equilibrium twist angle to lie somewhere between 0° and 90°.

Various methods were used to calculate ground state energies. Only HF/6-31G* produced a conformational energy minimum which closely corresponds with the gas-phase electron diffraction results of ~44.4° 1,2

Excited State Energy Calculations were performed for the first excited singlet state. The results indicate a near-planar conformational energy minimum, which accords with indirect Raman spectroscopy evidence 3 and with the vibrational structure of the fluorescence spectrum of biphenyl in solution 4.

Possible scheme for Fluorescence in Biphenyl

Ground State HOMO

References (1)Almenningen, A.; Bastiansen, O.; Fernholt, L.; Cyvin, B. N.; Cyvin, S.J.; Samdal, S. J. Mol. Struct. 1985, 128, 59. (2)Bastiansen, O.; Samdal, S. J. Mol. Struct. 1985, 128, 115. (3)Matsunuma, S.; Yamaguchi, S.; Hirosem C.; Maeda, S. J. Phys. Chem. 1988, 92, (4)Lim, E.C.; Li, Y.H. J. Chem. Phys. 1970, 52, 6416