The Substituent Effect. VI. Inductive and π-Electronic Effects in the Phenol Hydroxyl Chemical Shifts in DMSO

Substituent effects on the NMR chemical shifts of the hydroxyl proton of m- and p-substituted phenols in DMSO have been studied by means of the LSFE equation. The effect of substituents was found to be considerably modified in the DMSO solution as compared with that in aqueous solution through strong substituent-solvent interactions. The LSFE equation was applied to the apparent substituent effects of such solvent-modified systems, and the practical set of parameters for the inductive and the π-electronic effects of the DMSO-modified substituents were estimated. The treatment was successful for +R substituents but not for −R substituents. This may be attributed to the much stronger solvent-substituent interactions. Characteristic features of LSFE treatment were discussed in comparison with Taft and Swain-Lupton treatments. It is suggested that the dual resonance reaction constants for +R and −R para substituents are required for Taft-type correlations.