The Intensities of the Symmetry-forbidden Electronic Bands of Benzene

The molecular orbital method is used to calculate the intensities of the symmetry-forbidden bands of benzene. Both the bands at 2650 Å and 2000 Å derive intensity by interaction with the upper state of the 1800 Å band. The states of symmetry B1u and B2u are made accessible by e2g vibrations of the benzene framework at 606 cm-1 and 1595 cm-1. The 606 cm-1 vibration is most important in making the transition to the B2u state (at 2650 Å) allowed, whereas that at 1595 cm-1 is most important for the A1g → B1u transition. The alternative assignments B1u and E2g are considered for the 2000 Å band, but the calculated total intensities cannot differentiate between them.