Spectroscopic Moment: A Parameter of Substituent Groups Determining Aromatic Ultraviolet Intensities

Abstract

By the theory of Sklar and Förster, the added intensity produced in the benzene 2600A transition by monosubstitution is proportional to the square of a transition moment or ``spectroscopic moment'' induced by the substituent. The sign of the moment may be positive or negative. On disubstitution, para intensities are greater than meta and ortho if both signs are the same, less if they are opposite. The theory is used with available data to determine the relative spectroscopic moment for 25 different substituents. This moment is closely related to the ``mesomeric moment'' and to directing power, but the correlation is not perfect. The theory is extended to account for intensity changes in the weak long-wavelength transitions (1A—1Lb) of derivatives of other cata-condensed ring systems, such as naphthalene, phenanthrene, 1,2-benzanthracene, etc. The increases of intensity produced by substituents, and their spectroscopic moments, appear to be almost identical with those found in benzene.