NMR CHEMICAL SHIFTS IN CARCINOGENIC POLYNUCLEAR HYDROCARBONS

For Polyeyelic aero hydrocarbons displaying a range of carienogenice activites. the relation between molecular orbital calculations and recently measured chemical shifts, derived from analysis of high-field nmr spectra, is examined. The modified and unmodified McWeeny methods of evaluating the dominant ring-current contribution are compared with experimental shielding (relative to benzene) for naphthalene and benz[a]-anthraceue. For dibenz[a,h]anthiracene, benzo[c]phananthrene, and dibeazo[e,g]phenanthrene, shielding calculated by the McWeeny method also correlate with experimental shieldings; large downfield shifts cause bay protons to lie on subsidiary lines (the phenanthrenic proton effect). This agreement enables nmr shifts to provide a test of LCAO calculations underlying theories of carcinogenesis. The significance of small deviations from otherwise satisfactory correlations with LCAO-bused ring-current theory for the K and L regions of carcinogens is discussed in terms of molecular geometry. © 1969, American Chemical Society. All rights reserved.