Interpretation for the anomaly of the C=O stretching band in benzoic acid crystal

Molecular dynamics simulation has been used to investigate the infrared spectra of ordered and disordered benzoic acid crystals consisting of various proportions of the A and B configurations, whose C–O and C=O bonds are approximately parallel to the a axis, respectively. The simulation reproduced well the splitting and the temperature dependence in intensity for the C=O stretching bands observed. The absorption coefficient of the lower frequency band was simulated as several times larger than that of the higher frequency band in accord with the observed. To interpret this peculiar result of the molecular dynamics simulation, a simple calculation of the band splitting has been made in terms of the transition dipole–transition dipole interaction. It has been shown that the interaction is an essential factor to produce the large difference in intensity and the splitting for the C=O stretching bands. The higher and lower bands are interpreted as due to the A configuration reduced in intensity by the B form, and due to the B form enhanced in intensity by the A form, respectively. Compression effect in the O⋅⋅⋅O distance of the hydrogen bond is also studied in comparison with the observed value.