Polarized Infrared Spectra of Single Crystals of 9,10-Anthraquinone and 9,10-Anthraquinone-d8

The infrared spectra of 9,10-anthraquinone and 9,10-anthraquinone-d8 of high isotopic purity have been measured in the 4000–33 cm−1 region both in solution and in the solid. Polarized spectra have been taken on thin layers of oriented crystals grown between KBr plates. Group-theoretical analyses were made for the free molecule (Point Group D2h) and the crystal (Site Group Ci and Space Group P21/a-C2h5). The symmetry species of the observed bands were determined on the basis of the polarized spectra of the crystal. A vibrational assignment of the infrared-active fundamental modes is proposed. The vibrational exciton splittings have been analyzed in detail using dilute isotopic single crystals.