13C NMR chemical shift as a probe for estimating the conformation of aromatic groups in the solid state. 1.: Biphenyls

The relationships between C-13 NMR chemical shifts of aromatic carbons and the inter-ring torsion angles (theta) of two phenyl rings in biphenyl and its substituted derivatives were systematically examined. The chemical shifts of C1, C2 and C6 carbons (delta(C1), delta(C2) and delta(C6)) of she biphenyl group in the solid state show systematic correlations with theta when the substituent shielding effects are removed from the solid chemical shifts by subtracting the solution chemical shifts or subtracting the substituent shielding parameters. The substituent parameters deduced from substituted benzenes are utilized for the calculation. An increase in delta(C1), delta(C2) or delta(C6) (high-frequency displacement of the signals) corresponds to an increase in theta. In particular, delta(C6) can be a good measure of theta when no substituents are attached to C6. The isotropic nuclear shielding calculated using ab initio molecular orbital theory with the GIAO-CHF method for the biphenyl agree well with the experimental results. The relationships between the C-13 chemical shifts and the inter-ring torsion angle can be used to estimate the conformation of biphenyl groups incorporated into aromatic and/or liquid crystalline compounds and polymers. Copyright (C) 2000 John Wiley & Sons, Ltd.