Solvent effects in nuclear magnetic resonance spectroscopy. Part XVI. The influence of steric factors on specific solute–solvent association: a variable temperature study

In the absence of strong specific solute–solvent association, benzene-induced solvent shifts of aromatic protons in 1,3,5-trialkylbenzenes become increasingly negative with an increase in the size of the alkyl groups, i.e. with increasing steric factor. In order to assess the influence of steric factors on specific solute–solvent association; variable temperature studies of various monosubstituted derivatives were carried out with [2H8]toluene as solvent. With electron-withdrawing substituents (NO2 and Br), the principal site of association is over the benzene ring and the solvent shifts of the aromatic protons (relative to the solvent shift in the absence of the polar group) decrease in magnitude with increasing steric factor. With electron-donating substituents (NH2, NMe2, and OMe), the principal site of association is in the region of the substituent group, and appreciable inhibition of association occurs if there is actual steric inhibition of π-electron donation by the substituent group. Frequently the solvent shifts are surprisingly insensitive to steric factors.