The Raman spectra of gaseous formic acid -h2 and -d2

The Raman spectra of gaseous formic acid -h2 and -d2 at 21 °C and higher temperatures have been recorded from about 70 to 4000 cm−1. The isotope ratios of the frequencies of the intermonomer vibrations, which deform the hydrogen bonds, have allowed a convincing assignment of these vibrations. All but one of the nine low-frequency infrared and Raman bands of (HCOOH)2, and all seven of (DCOOD)2, have been assigned. It has been thought for 40 years that the symmetric hydrogen-bond stretching vibration is the cause of a strong Raman band of (HCOOH)2 near 230 cm−1, but the new evidence shows clearly that this is the one low-frequency fundamental that scatters too weakly to be detected. The spectra show bands at 1381 and 1307 cm−1 due to monomeric HCOOH which are assigned to ν4 and 2ν8, respectively, as the only feasible assignment. The Raman frequencies of the intramonomer modes of the dimer show that the intermonomer coupling is large for some modes and small for others. The Raman-active hydrogen-bonded O–H and O–D stretching modes, which are subject to a double-minimum potential, scatter very weakly; they could not be distinguished from possible overtone and combination bands in Fermi resonance with the C–H or C–D stretching modes.