Full-Dimensional Quantum Calculations of Vibrational Levels of NH₄⁺ and Isotopomers on An Accurate Ab Initio Potential Energy Surface

Abstract

Vibrational energy levels of the ammonium cation (NH₄⁺) and its deuterated isotopomers are calculated using a numerically exact kinetic energy operator on a recently developed nine-dimensional permutation invariant semiglobal potential energy surface fitted to a large number of high-level ab initio points. Like CH₄, the vibrational levels of NH₄⁺ and ND₄⁺ exhibit a polyad structure, characterized by a collective quantum number <i>P</i> = 2­(<i>v</i>₁ + <i>v</i>₃) + <i>v</i>₂ + <i>v</i>₄. The low-lying vibrational levels of all isotopomers are assigned and the agreement with available experimental data is better than 1 cm^–1