A reinvestigation of the ν2 and ν3 modes of 1,4-benzoquinone on the basis of a quantum mechanical harmonic force field

The assignment of the modes ν2 and ν3 of 1,4-benzoquinone has been the topic of many studies. In a recent investigation Zhao, Imahori, Zhan, Mizutani, Sakata and Kitagawa reported an assignment which differs from the previous assignment of Becker. With small variations of selected force constants of a quantum chemically derived force field, we can achieve good agreement with Beckerʹs data for the ν2 and ν3 modes of unlabeled 1,4-benzoquinone and its isotopomers 18O2 and d4. In contrast, no satisfactory agreement can be found with the frequencies reported by Zhao et al. A systematic investigation of 18O2 and d4 shifts suggests that isotopic shifts of ν3 assigned by Zhao et al. are most likely incompatible with the theoretical force field.