C-13 CHEMICAL-SHIFT TENSORS IN POLYCYCLIC AROMATIC-COMPOUNDS .2. SINGLE-CRYSTAL STUDY OF NAPHTHALENE

Carbon-13 chemical shift tensors have been determined a single-crystal naphthalene. The high-field component of every tensor is oriented perpendicular to the molecular plane, as in other aromatic compounds. The bridgehead carbon tensor is nearly axially symmetric, with its low-field component approximately perpendicular to the central bridging bond, in agreement with theoretical predictions. Protonated carbon tensors in naphthalene are more asymmetric, with their low-field components directed approximately along the C-H bonds. The Pople model of chemical shielding with MNDO wave functions reproduces the experimental in-plane shielding components. These calculations show that the in-plane components of the carbon shift tensors are governed by the bond orders of the adjacent bonds, and the shift tensors thus provide valuable information relating to aromaticity. The measured tensors deviated significantly from the symmetry of an isolated naphthalene molecule because of the lower symmetry of the molecular site in the crystal.