Abstract
Lattice‐dynamic analysis of solid formic acid based on a planar single hydrogen‐bonded chain has been carried out in order to interpret the observed neutron spectra of HCOOH, HCOOD and DCOOH. We use the direct relationship between the spectral intensities and the vibrational amplitudes of hydrogen atoms to extract information concerning the dynamic modes involving the hydrogen atom motions. Force constants similar to those used by previous investigators were refined (by a least‐squares adjustment routine) to try to achieve agreement with observed zone center vibrational frequencies and with peak positions and intensities in the neutron spectra. Molecular force constants for torsion and out‐of‐plane deformation were varied to achieve agreement between calculated and observed peaks in the 900–1100 cm−1 region. Two calculated phonon branches in the 100–200 cm−1 region were shown to be particularly sensitive to the hydrogen‐bond stretching force constant. The calculation fits the HCOOH spectrum well, but a discrepancy of 30–40 cm−1 for an out‐of‐plane mode in HCOOD and DCOOH remains. A two‐phonon calculation has been carried out which accounts well for observed features around 515 cm−1 in HCOOH and HCOOD.
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