Five-dimensional local mode-Fermi resonance model for overtone spectra of ammonia

Abstract

A five-dimensional local mode-Fermi resonance model for overtone spectra of the ammonia molecule has been constructed. The model Hamiltonian is expressed in terms of curvilinear internal valence coordinates and it includes the three stretching vibrations and the doubly degenerate bending vibration. The symmetric bending vibration associated with the inversion motion has been excluded. Thus the model is useful for energy levels with the totally symmetric bending vibration on its ground state. Vibrational energy levels have been calculated using van Vleck perturbation theory. Nonlinear least-squares method has been used to optimize potential energy parameters. Observed vibrational band origins for 14NH3, 14ND3, and 14NT3 have been employed as data. A fit with the standard deviation of 5.5 cm−1 has been obtained using one set of isotope invariant potential energy parameters. The optimised potential energy surface compare well with results of ab initio electronic structure calculations and with results of customary anharmonic force field calculations.