The carbonyl stretching frequencies of 1-substituted 2-pyrrolidinones and related compounds were measured in binary mixtures of hexane-CDCl3 and acetonitrile-D2O and plotted against the mole fraction of the less polar co-solvent The v(C=O) vs. mole fraction correlations enabled to recognize and assign all species resulting from solvent-solute interactions in binary mixtures. The slopes of the above dependencies for species with angular hydrogen bonds correlate well with the carbonyl group relative basicities and reveal that the solvent sensitivity of species in organic hexane-CDCl3 mixtures remarkably depends on the steric effects of substituents in the vicinity of the C=O group. On the other hand a similar dependence for aqueous acetonitrile-D2O mixtures indicates that n(C=O) sensitivity is proportional to the alkyl group effects as expected in Taft substituent constants.
