Geometries and energetics in the proton-bound dimers: H +(PH 3) 2, H +(H 2S) 2, and H +(HCl) 2

Abstract

MP2/6-31G* geometrical surveys were performed in the proton-bound dimers of the second-row hydrides, PH 3, H 2S, and HCl. Thermochemical data were determined in the dimer-formation reactions by the use of the fourth-order Møller—Plesset perturbation method with the single, double, and quadruple excitations, where the MP2/6-31G* geometries and Dunning's double-zeta (and/or 6-31++G( d, p)) basis sets were applied. Along the hydrogen-bond direction in the proton-bound dimers, the MP2/6-31G* geometrical surveys increase the Hartree—Fock XH (X = P, S, and Cl) bonds, but decrease the Hartree—Fock XX distances; these geometrical changes result in smaller enthalpy changes (larger binding energies) in the computations with the electron correlation effects. Computed (absolute) proton affinities of the bases, PH 3, H 2S, and HCl, agree well with recent experiments, but the agreement between the computed and experimental enthalpy changes is not so good in the formation reaction of the proton-bound H +(PH 3) 2 dimer. The electron correlation contribution is larger in the HCl system; the proton affinity increases by 6 kcal mol −1 and the enthalpy change decreases by 3.5 kcal mol −1. However, the contribution in the PH 3 system is quite different from the HCl one; proton affinity decreases by 3–5 kcal mol −1 and the enthalpy change also decreases by 1.5 kcal mol −1.