The conformational distribution in diphenylmethane determined by NMR spectroscopy of a sample dissolved in nematic liquid crystalline solvent

The deuterium decoupled, proton nuclear magnetic resonance spectrum of a sample of diphenylmethane-d(3) dissolved in a nematic liquid crystalline solvent has been analyzed to yield a set of dipolar couplings, D-ij. These have been used to test models for the conformational distribution generated by rotation about the two ring-CH2 bonds through angles tau(1) and tau(2). Conformational distributions, particularly when obtained from a quantum chemistry calculation, are usually described in terms of the potential energy surface, V(tau(1),tau(2)), which is then used to define a probability density distribution, P(tau(1),tau(2)). It is shown here that when attempting to obtain P(tau(1),tau(2)) from experimental data it can be an advantage to do this directly without going through the intermediate step of trying to characterize V(tau(1,)tau(2)). When applied to diphenylmethane this method shows that the dipolar couplings are consistent with a conformational distribution centered on tau(1)=tau(2)=56.5+/-0.5degrees, which is close to the values calculated for an isolated molecule of 57.0degrees, and significantly different from the asymmetric structure found in the crystalline state