Investigation of the structural conformation of biphenyl by solid state C-13 NMR and quantum chemical NMR shift calculations

The principal values of the C-13 chemical-shift tensor (CST) for biphenyl h ave been determined with the FIREMAT experiment. The internal dihedral angl e between the benzene rings in biphenyl is estimated to fall between 10 and 20 degrees on the basis of quantum mechanical calculations of the CST prin cipal values. A composite model of motion in the system, with contributions both from internal jumping between enantiomeric structures and from overal l molecular librations, yields the smallest variance between predicted and measured values for an internal twist angle of 15 degrees between the rings and a mean libration angle of +/- 12 degrees from the most favored molecul ar orientation. The composite model is clearly preferred to a motionless mo del (with > 98% probability) and is also preferred over either of the isola ted contributing dynamics, i.e., only libration or only internal jumping.