Disappearances of COOH infrared bands of benzoic acid

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Abstract
Infrared spectra of benzoic acid and benzoic acid-d were obtained in the range of temperatures from liquid nitrogen to liquid helium. The C=O stretching band at 1688 cm−1, the strongest absorption band of this compound in the over-all infrared region at room temperature, decreases in intensity with decreasing temperature and disappears near liquid-helium temperatures. Near this temperature, also lost was the band at 959 cm−1 which has been assigned to the OH out-of-plane vibration. For the deuterated species both bands at 1689 and 1679 cm−1, which have been interpreted as splitting due to Fermi resonance of the C=O stretching vibration, were lost. Close to the positions of these lost bands, alternative bands, which are assignable to the same modes and increase in intensity with decreasing temperature, are found. The results support the postulate proposed in our earlier work that two distinct equilibrium configurations of benzoic acid dimer coexist in the crystal.

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