Substituent effects on aromatic proton chemical shifts

Abstract

The proton magnetic resonance spectra of thirty-three ortho-disubstituted benzenes and nine 2-substituted pyridines have been determined in dilute solutions of carbon tetrachloride. Chemical shifts of protons adjacent to the substituents on the ring are well correlated by the parameter Q. Q values for substituents other than the halogens and hydrogen cannot be calculated from first principles; however, experimental methods for determining Q for other functional groups have been developed in this work. It was found that several of these calculated Q values are sensitive to the geometrical configuration of the substituent. with the ring. The behavior of the other ring proton chemical shifts in the 1-X-2-Y-benzenes in which X is held constant and Y varied through a range of substituents may be summarized as follows: 1. the chemical shifts at H5 follow the Hammett sigma_p of Y; 2. the chemical shifts at H6 follow the Hammett sigma_m of Y; and 3. the H4 chemical shifts are linearly related to the meta chemical shifts in the monosubstituted benzenes.