Infrared spectra of benzoic acid and benzoic acidhyphen;dwere obtained in the range of temperatures from liquid nitrogen to liquid helium. The Cdblbnd;O stretching band at 1688 cmminus;1, the strongest absorption band of this compound in the overhyphen;all infrared region at room temperature, decreases in intensity with decreasing temperature and disappears near liquidhyphen;helium temperatures. Near this temperature, also lost was the band at 959 cmminus;1which has been assigned to the OH outhyphen;ofhyphen;plane vibration. For the deuterated species both bands at 1689 and 1679 cmminus;1, which have been interpreted as splitting due to Fermi resonance of the Cdblbnd;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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