The Raman-active O–H and O–D stretching vibrations and Raman spectra of gaseous formic acid-d1 and -OD

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Abstract
In keeping with current theoretical activity concerning the OH and OD stretching bands of the carboxylic acids, we report the Raman spectra of gaseous formic acid‐OD and formic acid‐d1 for the first time. We emphasize the OH and OD stretching bands, which can be studied cleanly in these isotopomers but not in normal or perdeuterated formic acid. The spectra of the dimers and monomers below 2000 cm−1 are assigned, and current knowledge of the vibrations of the molecules is summarized. The Raman spectra allow the estimation of the energies of the Bu combination levels that may be in Fermi resonance with the infrared active Bu, OH or OD stretching fundamental, as well as those of the Ag overtone and combination levels that may interact with the Raman‐active stretching fundamental. We conclude that the sharp features on the Raman OH and OD stretching bands are due to overtone and combination transitions, that the stretching modes cause the underlying broad scattering, namely three broad bands, centered at 2430, 2270, and 2080 cm−1 for (HCOOD)2 and at 3240, 3074, and 2880 cm−1 for (DCOOH)2. We further conclude that the higher and lower frequency broad bands are due to sum and difference transitions with the hydrogen bond modes, which lie between 60 and 240 cm−1. The infrared OD and OH stretching bands of (HCOOD)2 and (DCOOH)2 are consistent with this interpretation. The Raman OD stretching band of HCOOD ⋅ HCOOH coincides with that of (HCOOD)2 apart from the overtone and combination transitions. The centers of Raman intensity, corrected for instrument, wave number, and temperature dependencies, of the OD stretching bands of (HCOOD)2 and HCOOH ⋅ HCOOD are 2300 ±5 and 2305 ±10 cm−1, respectively, and that of the OH stretching band of (DCOOH)2 is 3035±10 cm−1. This data and the near coincidence of the infrared and Raman bands of (HCOOD)2 show that the vibrational coupling of the two OD bonds in the dimer is not unusually large. Comparison of the infrared and Raman OH stretching bands indicates a larger coupling between OH oscillators. These results agree with earlier result for acetic acid.