Vibrational band intensities and structural factors

Abstract

Earlier work on the effect of ring substituents on the frequencies and intensity of vibrations of a functional group attached to an aromatic ring has been extended. New data have been obtained for anihnes, benzonitriles, phenols and other classes, including many ortho-substituted compounds. The results have been interpreted in terms of quantitative estimates made by other methods for the inductive and resonance effects of different substituents. There is a regular trend for each functional group between band frequency and Hammett $\sigma $ factor of the substituent. Also, the plot of log (band intensity) against $\sigma $ is essentially linear over a wide range. Ortho-substituents fall regularly into these relations if the Taft $\sigma $ values for ortho-substituent are used, and if no internal hydrogen bonding occurs. The data for meta- and para-derivatives can be analyzed in greater detail using the measures for inductive and resonance effects of different groups obtained by Taft from kinetic data. The results suggest the circumstances under which substituents may be expected to cause large variations of band intensity. Measurements have been made in carbon tetrachloride and chloroform and new data have been obtained about the effect of solvent medium upon spectral characteristics. The possibility of correlating spectral data in aliphatic series with electronic influences of substituents groups has been considered in a preliminary way.