Sources of Intensity in the 1U(n, π)←1A Band of Benzophenone

A vibronic perturbation experiment is presented in which the polarized excitation spectra (77°K) and and absorption spectra of benzophenone and 4,4′-dimethylbenzophenone were compared in the region 2500 to 4000 Å. This experiment supports the view that the n→π band in benzophenone is essentially forbidden, even though the probable conformation is C1, and that the intensity comes principally from vibronically induced borrowing from 1(Lb+Lb)←1A and 1K←1A (charge transfer) π→π promotions. A secondary but significant source for vibronic borrowing is the 1(Lb—Lb)←1A π→π transition.