CARBON-13 CHEMICAL SHIELDING TENSORS IN SINGLE-CRYSTAL DURENE

The method of proton‐enhanced nuclear induction spectroscopy [A. Pines, M. G. Gibby, and J. S. Waugh, J. Chem. Phys. 59, 569 (1973)] has been employed to find the eigenvalues and eigenvectors of the chemical shift tensors for the various 13C nuclei in a single crystal of durene (1, 2, 4, 5‐tetramethylbenzene). For each type of carbon the trace of the shielding tensor, Trσ agrees well with the isotropic shift σi found from spectra of liquid solutions. The well‐known tendency of σi to be smaller for aromatic than for aliphatic carbon is found to be ascribable largely to the very small shielding which occurs for the former when the applied field is in the plane of the ring and tangent to it at the C atom in question. The CH3 carbons have much less anisotropic shielding than do the ring carbons. The rather close coincidences between nominal bond directions and the eigenvectors of σ suggest that nonbonded interactions in the crystal play an unimportant role.