INFRARED-SPECTRA AND CONFORMATION OF METHYL-SUBSTITUTED BENZOIC-ACIDS

Infrared spectra of all isomers of polymethyl-substituted benzoic acid s were recorded in the carbonyl and hydroxyl regions in tetrachloromet hane at various concentrations and interpreted in terms of conformatio n. According to a plot of nu(C=O) of the monomeric form vs Hammett sub stituent constants a, these compounds may be classified into two class es. Derivatives with none or only one methyl group in the ortho positi on are concluded to exist in an equilibrium of two planar conformation s, unless the equilibrium is degenerate. Derivatives with two ortho-si tuated methyl groups are concluded to take up one non-planar conformat ion. These findings are supported by the shape of the hydroxyl and car bonyl bands, which are unsymmetrical in the former class, although the y could not be reliably separated into bands pertinent to the individu al conformers. This conclusion is at variance with the common interpre tation which has invariably ascribed to these and similar ortho deriva tives non-planar conformations with a variable torsional angle. (C) 19 98 John Wiley & Sons, Ltd.