Carbon-13 chemical shift tensors in polycyclic aromatic compounds .6. Single-crystal study of perylene

The 40 chemical shift tensors of single-crystal perylene in the alpha crystalline form have been determined with a precision of 0.30 ppm using C-13 chemical shift-chemical shift correlation spectroscopy. The in-plane anisotropy of these tensors describes the delocalization of pi-electrons at the inner alpha positions, which is similar to that found in biaryl linkages rather than typical bridgehead carbons. Molecular distortions, originated in intermolecular interactions and associated with chemical shifts of up to 5 ppm in similar carbons of perylene, have been detected indicating that the accuracy in the shift tensors measured in this study is adequate to probe crystalline effects upon the electronic and molecular structure. The systematic tilt observed in the orientation of the smallest principal components, delta(33), supports the X-ray observation that the molecules bend about their long axis by 1-2 degrees. Chemical shift calculations, using ab initio methods, are in good agreement with the experimental tensors and provide insight into the observed variation of chemical shifts in terms of molecular geometry.