Resonance Raman spectra of some radiolytically prepared halogen derivatives of p a r a-benzosemiquinone radical anion

The resonance Raman spectra have been obtained on radiolytically and chemically prepared halogen derivatives (chloro-, bromo-, 2.5 dichloro-, tetra chloro-, and tetra bromo-) of p-benzosemiquinone radical anion. Excitation is in the moderately intense absorption band at 430–460 nm. All Raman spectra show a strongly resonance enhanced and polarized line corresponding to a vibrational frequency of 1590–1620 cm−1 which is assigned to the Wilson phenyl mode 8a (CC stretch). A number of weaker lines are also observed and their assignment discussed. The electronic transitions in resonance are identified as 2B3g–2B1u (in D2h point group) in view of the resonance Raman band intensities. This supports the assignment by Harada based on ASMO CI calculations which has recently been in dispute.