Carbon-13 Chemical Shift Tensors in Aromatic Compounds. 3. Phenanthrene and Triphenylene

Measurements of the principal values of the ¹³C chemical shift tensor are presented for the three carbons in triphenylene and for three different α-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% ¹³C) at low temperatures (∼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 σ33 component relative to the corresponding σ33 values at C1 and C9 in phenanthrene. The upfield shift in σ33 is mainly responsible for the well-known upfield shift of the isotropic chemical shifts of such sterically perturbed carbons. In phenanthrene C9 exhibits a unique σ22 value reflecting the greater localization of π-electrons in the C9-C10 bond. This localization of the π-electrons at the C9-C10 bond in the central ring of phenanthrene also corresponds with the most likely ordering of electrons described by the various Kekulé structures in phenanthrene. The analysis of the ¹³C 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 C9-C10 π-bond with only limited π-electron character in the C8a-C9 and C10-C10a bonds is indicated by both the experimental and theoretical results. © 1992, American Chemical Society. All rights reserved.

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Author Julio Cesar Facelli Biostatistics & Bioinformatics, Division of Translational Bi ...