Carbon-13 Chemical Shielding Tensors in Polycyclic Aromatic Compounds. 1. Single-Crystal Study of Pyrene

The carbon-13 chemical shielding tensors measured in single-crystal pyrene are reported. The two internal bridgehead carbons have principal values of 197, 191, and -18 ppm and the four peripheral bridgehead carbons 213, 187, and -7 ppm with respect to liquid Me4Si. The high field component of every tensor is determined to be perpendicular to the molecular plane, as in benzene. The protonated carbons have principal values similar to benzene, with the low-field components lying approximately along the C-H bonds. The rotation of the principal axes away from the C-H bond direction is measured experimentally and interpreted by quantum chemical calculations. By use of a MNDO wave function and the Pople model of chemical shielding, the magnitudes of the experimental in-plane components are reproduced very well and the calculated orientations of the principal axes are consistent with the experimental data. The results reported here demonstrate that the measurement of¹³C shielding tensors in polycyclic aromatic compounds can be used as a measurement of the aromatic character of bonds adjacent to ¹³C nuclei. © 1987, American Chemical Society. All rights reserved.

Duke Scholars

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