The anharmonic constants and average structure of ammonia

Abstract

The cubic and quartic potential constants for NH 3 and ND 3 have been calculated by assuming intramolecular potential functions of internal coordinates and by means of the nonlinear transformation of the internal coordinates into the normal coordinates. The ordinary F matrix elements in the internal-symmetry coordinate system and the Morse-like parameters representing the anharmonicity of the NH bond-stretching vibrations are taken into account in the valence force model, and in addition the hydrogen—hydrogen potential terms are assumed in the vander Waals model. A majority of the cubic constants, the vibration—rotation interaction constants α, and the l-type doubling constants q t calculated by these models agree essentially with the experimental values reported by Benedict and Plyler. The comparison has indicated that almost all the third-order potential constants in the internal coordinate system ignored in the valence force model should have small negative values, as observed for several bent XY 2 molecules. Possible orders of magnitude of these constants have been estimated. The vibrational anharmonic constants χ have been estimated in a similar manner from the cubic and quartic constants. The zero-point average structure is derived from the experimental rotational constants to be: r z (NH) = 1·024 0 Å, α z (HNH) = 107·3 2°, and r z (ND) = 1·020 6 Å, α z (DND) = 107·2 2°.