A MOLECULAR ORBITAL MODEL FOR INFRARED BAND INTENSITIES: FUNCTIONAL GROUP INTENSITIES IN AROMATIC COMPOUNDS

A simple molecular orbital model has been developed for infrared absorption intensities in aromatic compounds of the form XC(6)H(4)S where S contains the absorbing functional group and X is a meta or para substituent. For a given group S the intensity is related to the pi electronic energy levels in the substituted benzene, C(6)H(5)X. On the basis of a simple electrostatic theory of substituent effects in chemical reactivities, it is shown that the appropriate set of substituent constants should be linearly related to the square root of the observed absorption intensity. This relationship is in fact found for a number of series, including substituted benzonitrile, phenols, anilines and N-methylanilines. The fact that correlations of this type are obtained is indication that the changes in electron distribution occurring in the molecule as a result of vibrational distortions are closely parallel to those occurring in the formation of the transition state during chemical reaction.