Anomalous intensities and vibrational structure in the absorption spectra of benzene and benzene- d6

• Published in: Chemical physics 1992-12, Vol.168 (2), p.211-223
• Main Authors: Fischer, Gad, Knight, Alan E.W.
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Exact sciences and technology; Intensities and shapes of molecular spectral lines and bands; Molecular properties and interactions with photons; Physics
• ISSN: 0301-0104
• DOI: 10.1016/0301-0104(92)87156-4

The A 1B 2u ← X 1A 1g absorption spectra of benzene are characterized by long progressions in the ring breathing vibration ν 1. For benzene- d 6, in contrast to benzene- h 6, increasing quanta of ν 1 are associated with a proliferation of bands. A model is proposed which incorporates anharmonic coupling between ν 1 and 2ν 10 in the excited state in addition to the anharmonic coupling of different angular momentum components of 6 n 10 2 m ( n, m = 1, 2), as an explanation of this phenomenon. Two parameters, namely the two anharmonic coupling strengths, and the zero-order frequency differences between ν 1 and 2ν 10 for the two benzene isotopes, are used to achieve excellent agreement between the calculated and measured transition intensities and frequencies for the bands groups 1 n 06 1 0, 1 n 06 0 1 and 1 n 06 2 1 ( n = 1, 2). For benzene- d 6 the coupling 6.7 cm −1 for the ν 1…2ν 10, and ν 6[+] + 2 ν 10[−2]…ν 6[−] + 2ν 10[+2] interactions respectively, and the zero-order frequency difference was taken as 20 cm −1. The corresponding couplings for benzene- h 6, derived from those of benzene- d 6, were 17.2 and 9 cm −1 respectively, and the frequency difference was taken as 238 cm −1.