Potential Function for the Inversion of Ammonia

Abstract

The data on the inversion of NH3 and ND3, summarized by Benedict and Plyler, have been fit in terms of the angular coordinate with a potential function of the form of a harmonic oscillator with a Gaussian barrier, with and without quartic anharmonicity. With the exception of one energy level of NH3, which seems obviously in error experimentally, the fit is to within a mean of 2 cm−1 for the first seven energy levels of NH3 and of ND3, far better than that previously achieved. The barrier height is found to be 2020±12 cm−1 and the distance of the nitrogen atom from the plane of the hydrogen atoms is found to be 0.382±0.004 A, in excellent agreement with the value of 0.3810 A derived by Benedict and Plyler experimentally. The improvement in agreement between observed and calculated energy levels upon addition to the potential function of a quartic anharmonicity term is not sufficient to support unequivocally such an addition. The derived value of the ratio of the reduced mass of NH3 to that of ND3 is consistent with an inversion motion involving an angular, rather than a distance, coordinate.