Analysis of neutron inelastic scattering spectra of normal and deuterated formic acid

Calculations based on a planar single-chain model of solid formic acid have been carried out to interpret the observed neutron inelastic scattering spectra of HCOOH, HCOOD, and DCOOH, and to use the direct relationship between vibrational eigenvectors and neutron-scattering intensities to extract information concerning the dynamic modes of the system. Force constants similar to those used by previous investigators were refined (using a least-squares adjustment routine) to try to achieve agreement with observed zone-centre vibrational frequencies and with peak positions and intensities in the neutron-scattering spectra. The calculated neutron spectra were particularly sensitive to the out-of-plane force constants. The best calculation fits the HCOOH spectrum well, but results in a 30-40cm-1 discrepancy for an out-of-plane mode in HCOOD and DCOOH. A two-phonon calculation has been carried out which accounts well for observed features around 515cm-1 in HCOOH and HCOOD. (author)