Phonon Curves and Frequency Distribution for Hydrogen-Bonded Systems: β Modification of Solid HCOOH, HCOOD, and DCOOH

Molecular vibrations of the β form of solid formic acid and of its deuterated isotopes are studied throughout the first Brillouin zone with the use of a Urey–Bradley force field. Phonon dispersion curves and related densities of states are calculated. It is shown that low-energy phonons arising from the hydrogen bond are strongly dispersed. The time of flight neutron spectrum reported by others is discussed and interpreted.