Vapor pressure isotope effects in liquid and solid ammonia

The vapor pressures of the isotopic ammonia /sup 14/NH/sub 3/, /sup 14/ND/sub 3/ and /sup 15/NH/sub 3/ have been measured at temperatures between 163K and 240K by differential manometric technique in a precision cryostat. The triple point pressures were found to be 45.49 mmHg for /sup 14/NH/sub 3/, 48.35 mmHg for /sup 14/ND/sub 3/ and 45.83 mmHg for /sup 15/NH/sub 3/. This corresponds to triple point temperatures of 195.41K, 198.96K and 195.58K, respectively. The lattice dynamics of a unit cell with four NH/sub 3/ molecules have been calculated using Wilson's FG matrix method and a modified Schachtschneider-Snyder program to reproduce the Raman, infrared and neutron scattering data and the observed vapor pressure isotope effects. These calculations provide satisfactory agreements with the experimentally determined ln(f/sub c//f/sub g/) values only if the presence of couplings between the N-H stretching and H-N-H bending motions and molecular translation in the direction of the molecular figure axis is assumed. 75 refs., 18 figs., 28 tabs.