Infrared and Raman Studies of Carbonyl Group Frequencies of p-Substituted Benzoic Acids in the Crystalline State

It was found 1 that Ro ... 0 distances in cyclic dimers of p-substituted benzoic acids in the crystalline state do not follow the Hammett rule. The Ro ... 0 values often serve as an approximate measure of hydrogen bond strength.2 Since substituent constants op and R0 ... 0 express the Bronsted and the Lewis acidity of the carboxylic group in solution and in the crystalline state, respectively, a linear dependence between them was expected. Lack of this dependence was explained1 in terms of packing forces, which are supposed to affect the Ro ... 0 distance much more strongly than may be expected from the substituent effect. Since two molecules of carboxylic acid form a centrosymmetric dimer this leads to coupling of COOR oscillators. In consequence, symmetric and antisymmetric modes are observed in Raman and IR spectra, respectively. Analysis of a vast amount of IR and Raman spectral data as well as the geometry of the H-bond has led Novak~ to the conclusion that spectral characteristics of the carbonyl group vary in a regular way with R0 ... 0 distances of the H-bond at least for a broad meaning of mutual correlation.