CONFORMATIONAL ANALYSIS .12. CHEMICAL SHIFTS OF METHYLARENES

The chemical shifts of the methyl group of methylarenes have been determined for 29 compounds in dimethyl sulfoxide-d6 as solvent. The compounds include all the isomeric methyl derivatives of benzene, naphthalene, anthracene, phenanthrene, 1,2-benzanthracene, and benzo[c]phenanthrene. A good correlation is obtained relating the chemical shifts of the compounds containing three or fewer rings to the summation of the inverse cubes of the distances (ΣR-3) which separate the methyl group and the centers of the aromatic rings. This correlation indicates the anisotropy of the diamagnetic susceptibility of the aromatic rings as a result of circulating interatomic currents and implicitly assumes that ring currents in each hexagon of polynuclear aromatic hydrocarbons are equal to that of benzene, For those compounds containing three or fewer rings and in which there is no significant steric compression of the methyl group, the ring systems are planar and the correlation line is acceptable with a correlation coefficient, r = 0.986. However, chemical shift contributions as a result of nonplanarity of rings or steric compression of hydrogen decrease the correlation coefficient. In addition, compounds containing more than three rings do not correlate as well with ΣR-3. A significantly better and more general correlation is obtained using previously calculated ring current intensities (I) and evaluating ΣIR-3. This function correlates the data for phenyl, naphthyl, anthryl, and phenanthryl with r = 0.997. For those molecules whose individual ring current intensities are not known, the proper number of experimental chemical shifts permits their evaluation from the established correlation line. The ring current intensities of 1,2-benzanthracene have been calculated from the best experimental fit of four chemical shifts. The resultant correlation of all of the methyl-1,2-benzanthracenes and the other ring systems is excellent with r = 0.995. An extension of the ΣIR-3 function to nonplanar rings has been achieved for the methylbenzo[c]phenanthrenes. © 1969, American Chemical Society. All rights reserved.