Vibronic magnetic rotational strengths in the B2u state of benzene

The vibronic magnetic circular dichroism (MCD) spectrum of the electronically forbidden 260 nm 1A1g→1B2u transition in benzene is analyzed in terms of e2g vibrational modes. The negative MCD progression is aasigned to the e2g 608 cm−1 and 1596 cm−1 modes and the positive progression to the e2g 1178 cm−1 and 3056 cm−1 modes. Experimental MCD and absorption spectra of benzene and benzene-d6 in solution at room temperature and liquid nitrogen temperature are reported. Magnetic rotational strengths and oscillator strengths associated with each of the four e2g vibrations in benzene and benzene-d6 have been calculated by two different methods: a perturbation treatment involving the π electrons only, and a CNDO calculation for a geometry in which the nuclei are distorted along a normal coordinate. The results of the two calculations agree reasonably well with each other and with experiment. Two terms that dominate the magnetic rotational strengths are identified.