Absorption of Polysubstituted Benzene Derivatives

Abstract

The intensification, by nonchromophoric substitutions, of the forbidden, 1A1g→1B2U, transition in benzene is due, on the one hand, to the unsymmetrical distortion of the ring by the normal vibrations and, on the other hand, to the transition moment produced at the equilibrium position by the migration of charge from the substitution into the ring or vice versa. The latter effect is treated by the method of antisymmetrical molecular orbitals, all calculations being limited to the first order. The rules for combining transition moments due to the various groups in a polysubstituted benzene are calculated. It is shown that, for weak nonchromophoric substitutions, the intensity ratios (except for the less important variation in the normal vibrations) are for C6H5X:oC6H4X2:mC6H4X2:pC6H4X2:1,3,5 C6H3X3:1,2,3 C6H3X3:1,2,4 C6H3X3=1:1:1:4:0:0:3. It is further shown that if one substitutes benzene by a meta-directing group, M, and by an orthopara-directing group, P, of the same intensifying power, the ratios for the mono : ortho : meta : para-derivatives should be 1 : 3 : 3 : 0. These ratios are shown to be in good qualitative agreement with experimental data.