First principles computation of thermochemical properties beyond the harmonic approximation. II. Application to the amino radical NH2

Abstract

The anharmonic potential surface of NH2 has been computed ab initio using the quadratic configuration interaction method and large basis sets. The all-electron augmented coupled cluster surface with the [5s4p2d1f,3s2p] basis set reproduces the experimental fundamentals to within a few cm−1. For this quantitative agreement with experiment, both f functions and core correlation seem to be prerequisite. From our best computed force field, a set of spectroscopic constants has been derived for all important isotopomers of NH2. Using a hybrid analytic/direct summation method recently developed by the present authors, the thermodynamic functions gef(T), hcf(T), S0, and Cp are computed including exact accounts for anharmonicity and rovibrational coupling, and very good analytical approximations to centrifugal distortion and quantum rotation effects. Thermodynamic tables in JANAF style from 100–3000 K, as well as a full set of rovibrational spectroscopic constants, are presented for NH2 and all important isotopomers. The effect of the A 2A1 state is found to become significant around 2000 K.