13C chemical shielding tensors in polyalkylbenzenes

Abstract

High‐resolution double resonance methods have been employed to study the anisotropic ¹³C chemical shifts in single crystals of hexamethylbenzene, hexaethylbenzene, and pentamethylbenzene. Methyl and methylene shifts have small anisotropies. Shielding of the ring carbons is a maximum when the magnetic field is normal to the ring and a minimum when it lies along the bond to the substituent. The hexaethylbenzene molecule is shown to have D_3d symmetry. The orientations of the two molecules in the unit cell of pentamethylbenzene are determined.