PNNL

Abstract

The heats of formation of NH3, NH2, NH and the ionization energies of NH3, NH2, NH, and N have been calculated at high levels of ab initio molecular orbital theory at 0 K. Geometries and frequencies were calculated with coupled cluster theory,including a perturbative treatment of the connected triple excitations and with correlation-consistent basis sets up through augmented quadruple zeta in quality. Subsequent extrapolation of the total energies to the complete 1-particle basis set limit was performed to further reduce the basis set truncation error. Additional improvements in the automization energy were achieved by applying corrections for core/valence correlation, scalar relativistic, spin-orbit, and higher order correlation affects. Zero point energies were taken from anharmonic force fields where available or are based on appropriately scaled values. Using the R/UCCSD(T) method, we find the following heats of formation (kcal/mol)at 0K: Hf(NH3)=-9.10 +/- 0.17 (calc.) vs. -9.30 +/- 0.10 (expt.); Hf(NH2) = 45.29 =/- 0.12 (calc.)vs. 45.17 +/- 0.09 (expt.); Hf(NH)=85.92 +/- 0.08 (calc.) vs. 90.0 +/- 4 (expt.); Hf(NH3+)=225.44 +/- 0.23 (calc.) vs. 225.59 +/- 0.08 (expt.); Hf(NH2+)=303.00 +/- 0.20 (calc) vs. 302.60 +/- 0.08 (expt.); and Hf(NH+)=396.56 +/- 0.12 (calc.)