High temperature partition functions and thermodynamic data for ammonia and phosphine

Abstract

The total internal partition function of ammonia (14NH3) and phosphine (31PH3) are calculated as a function of temperature by explicit summation of 153million (for PH3) and 7.5million (for NH3) theoretical rotation-vibrational energy levels. High accuracy estimates are obtained for the specific heat capacity, Cp, the Gibbs enthalpy function, gef, the Helmholtz function, hcf, and the entropy, S, of gas phase molecules as a function of temperature. In order to reduce the computational costs associated with the high rotational excitations, only the A-symmetry energy levels are used above a certain threshold of the total angular momentum number J. With this approach levels are summed up to dissociation energy for values of Jmax=45 and 100 for ammonia (Emax=41 051cm−1) and phosphine (Emax=28 839.7cm−1), respectively. Estimates of the partition function are converged for all temperatures considered for phosphine and below 3000K for ammonia. All other thermodynamic properties are converged to at least 2000K for ammonia and fully converged for phosphine.