Knowledge Bank

The $H1$ nuclear resonance spectra of biphenyl and a number of symmetrically bisubstituted, para, para biphenyls---dideutero, dichloro, dibromo, difluoro, diiodo, dinitro---have been studied under a variety of solvent conditions and over a range of temperatures, with a view to determining the conformation of biphenyl itself. Previous $work1$ applied the method of moments to yield several possible sets of chemical shift parameters for biphenyl. We have determined these parameters for biphenyl and the related substances. The observed chemical shifts are in accord with those predicted )on the basis of a ring current $model2$ and of values for substituent effects in mono-substituted $benzenes3$) for a conformation corresponding to an average dihedral angle between the aromatic rings of $45\circ$. The $C13$ resonance spectrum of biphenyl and $F19$ resonance spectrum of para,-para'-difluorobiphenyl indicate that electron density in the ring is not changed appreciably from the corresponding benzene compound. Work is in progress to determine if solvent effects can be extracted from observed temperature variations of the chemical shifts and information obtained about the barrier. The equilibrium conformation will be discussed in terms of the competing interactions giving rise to the barrier.