C-13 CHEMICAL-SHIFT TENSORS IN POLYCYCLIC AROMATIC-COMPOUNDS .5. SINGLE-CRYSTAL STUDY OF ACENAPHTHENE

The 36 experimentally observable C-13 chemical shift tensors in the acenaphthene single crystal are determined with 2D chemical shift-chemical shift correlation spectroscopy. The chemical shift tensors of the two crystallographically unique molecules in the unit cell are measured with an accuracy of 0.52 ppm. A scalar measure of the rms distance between all of the pairs of tensors in the two molecules is 1.82 ppm, indicating that single-crystal NMR measurements clearly detect the differences between the chemical shift tensors in the two molecules. To rationalize these variations, ab initio calculations of the C-13 chemical shift tensors and the derivatives of the various tenser components with respect to the molecular geometry parameters were carried out with the GIAO quantum mechanical method using the molecular structures from neutron diffraction. The calculated and experimental chemical shift tensors correlate within an rms distance of 4.99 ppm. Theoretical arguments are presented that indicate that the differences between the C-13 chemical shift tensors of the two unique molecules likely originate in structural variations that tend to be smaller then those detected by diffraction techniques.