C-13 CHEMICAL-SHIFT TENSORS IN AROMATIC-COMPOUNDS .3. PHENANTHRENE AND TRIPHENYLENE

Measurements of the principal values of the C-13 chemical shift tensor are presented for the three carbons in triphenylene and for three different alpha-carbons in phenanthrene. The measurements in triphenylene were made in natural abundance samples at room temperature, while the phenanthrene tensors were obtained from selectively labeled compounds (99% C-13) at low temperatures (approximately 25 K). The principal values of the shift tensors were oriented in the molecular frame using ab initio LORG calculations. The steric compression at C4 in phenanthrene and in corresponding positions in triphenylene is manifested in a sizable upfield shift in the sigma-33 component relative to the corresponding sigma-33 values at C1 and C-9 in phenanthrene. The upfield shift in sigma-33 is mainly responsible for the well-known upfield shift of the isotropic chemical shifts of such sterically perturbed carbons. In phenanthrene C-9 exhibits a unique sigma-22 value reflecting the greater localization of pi-electrons in the C-9-C-10 bond. This localization of the pi-electrons at the C-9-C-10 bond in the central ring of phenanthrene also corresponds with the most likely ordering of electrons described by the various Kekule structures in phenanthrene. The analysis of the C-13 chemical shieldings of the bridgehead carbons in triphenylene provides significant experimental information on bonding between rings in polycyclic aromatic compounds. The results confirm that the electronic structure of triphenylene is best described by three fairly isolated benzene rings linked by C-C bonds of essentially single bond character. Similarly in phenanthrene, the bonding structure which correlates the shielding information may be characterized by the dominance of two benzene rings comprising the biphenyl moiety. A strong C-9-C-10 pi-bond with only limited pi-electron character in the C-8a-C-9 and C-10-C-10a bonds is indicated by both the experimental and theoretical results.