The Substituent Effect. IV. Proton NMR Chemical Shifts of Phenols

Abstract

Abstract NMR chemical shifts of the hydroxyl proton of m- and p-substituted phenols were determined in aprotic DMF and DMSO solutions. The effect of substituents on the hydroxy chemical shift was correlated much more excellently with our LArSR relationship, Δδ=ρ(σ0+rΔ\barσ\barR), than with simple Hammett equation, ρσ0 or ρσ−. The correlation resulted in ρ=1.655 and r=0.639 in DMF with correlation coefficient 0.9992 and ρ=1.530 and r=0.673 in DMSO with a correlation coefficient 0.9990. Anisotropy effect of any substituent was not significant. The deviations of particular substituents were ascribed to the modification of the electronic characters of substituents due to strong substituent-solvent interactions in DMF (or DMSO). The inductive and resonance substituent constants, (σi)DMF and (Δ\barσ\barR)DMF, were determined, on the basis of the assumption that the solvent-modification of a given substituent is practically the same at both meta and para positions.