A proton NMR shielding model for the face of a benzene ring

In a strong magnetic field, nuclei located over an aromatic ring experience a diminished magnetic field as a result of the induced magnetic field caus ed by circulating rr electrons. The shielding effect of an aromatic ring ca n be substantial, causing an upheld shift of up to 3 ppm in proton NMR spec tra, yet this effect is not considered in any of the empirical methods for predicting chemical shifts. We have used GIAO, an ab initio subroutine in G aussian 94 to calculate isotropic shielding tensors and to predict the prot on NMR chemical shift for a simple model system: methane held at various po sitions over a benzene ring. The NMR chemical shift shielding increments ex perienced by the protons of methane have been mapped as a function of their position x, y and z in Cartesian coordinates relative to the center of the aromatic ring. A mathematical function was fit to this three-dimensional s hielding increment surface at each of six distances from the face of the be nzene ring. In addition, a single mathematical equation was developed for p redicting the shielding caused by a benzene ring. Chemical shifts predicted by this equation are compared with observed values. Incorporation of this function into a molecular mechanics-chemical shift prediction program shoul d aid structure elucidation of structures having such features. (C) 1998 El sevier Science B.V. All rights reserved.