COMPARISON OF THEORETICAL METHODS FOR THE DETERMINATION OF THE PROTONATION AND DEPROTONATION ENERGIES OF NH3, H2O, HF, PH3, H2S, HCL, AND HCN

The structures of the bases NH3, H2O, HF, PH3, H2S, HCl, and HCN and the corresponding protonated and deprotonated ions have been optimized by using second-order Møller-Plesset perturbation theory with the 6-31+G(d,p) basis Basis set superposition errors for computed protonation and deprotonation energies of NH3 were evaluated for four different basis set. Single-point calculations on all species were performed with the 6-31+G(2d,2p) basis using the following correlation methods: many-body (Møller-Plesset) perturbation theory at second (MP2), third (MP3), and fourth (MP4) order; the linearized coupled-cluster method (LCCM); the averaged coupled-pair functional (ACPF); configuration interaction with all single and double excitations (CISD); and CISD with the Davidson and the Pople corrections, all relative to a single-reference Hartree-Fock function. The MP2 values for the protonation and deprotonation energies are always the lowest, while the CISD values are often highest. The Møller-Plesset expansion appears to be converging well in general, although there is strong alternation and slow convergence for the deprotonation energies of the first-row bases NH3, H2O, and HF The various correlated results agree with each other to within about 2 kcal/mol for almost all protonation energies and within 2-7 kcal/mol for deprotonation, though the agreement is improved substantially if the MP2 results are excluded. The LCCM, ACPF, and corrected CI results agree with MP3 within about 1 kcal/mol or better for protonation and within 3 kcal/mol or better for deprotonation. The protonation and deprotonation energies computed with the various methods generally agree with experimental values to about 1%. Except for PH4+ and the C-N bond in HCNH+, protonation and deprotonation of these bases increase bond lengths. © 1990 American Chemical Society.