Heats of formation and ionization energies of NHx, x=0-3

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 sextuple zeta in quality. Subsequent extrapolation of the total energies to the complete one-particle basis set limit was performed to further reduce the basis set truncation error. Additional improvements in the atomization energy were achieved by applying corrections for core/valence correlation, scalar relativistic, spin-orbit, and higher order correlation effects. 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 0 K: DeltaH(f)(NH3)=-9.10+/-0.17 (calc.) versus -9.30+/-0.10 (expt.); DeltaH(f)(NH2)=45.27+/-0.12 (calc.) versus 45.17+/-0.09 (expt.); DeltaH(f)(NH)=85.92+/-0.08 (calc.) versus 90.0+/-4 (expt.); DeltaH(f)(NH3+)=225.44+/-0.23 (calc.) versus 225.59+/-0.08 (expt.); DeltaH(f)(NH2+)=303.00+/-0.20 (calc.) versus 302.60+/-0.08 (expt.); and DeltaH(f)(NH+)=396.56+/-0.12 (calc.). (C) 2001 American Institute of Physics.