Dynamics of the water molecule in potassium oxalate monohydrate as studied by single crystal inelastic neutron scattering

We have measured the inelastic neutron scattering spectra for a powder sample and three differently oriented single crystals of K2C2O4·H2O on the TFXA spectrometer at the ISIS facility. The crystal structure consists of oxalate ions hydrogen-bonded by the water molecules into infinite linear chains, which are held together by electrostatic interactions with the K+ ions. The H2O molecules are isolated from each other. The vibrations can then be separated into internal and external vibrations. The single crystals were cut and oriented so as to separate the rocking, twisting and wagging modes as much as possible. The rocking mode appears at 656 cm-1 (overtone at 1278 cm-1). A band at 746 cm-1 with strong overtones at 1448 and 2099 cm-1 is assigned to the overlapping twisting and wagging modes. The translational modes are found at 97, 210 and 215 cm-1.