Ab Initio Study of Gas-Phase Proton Transfer in Ammonia-Hydrogen Halides and the Influence of Water Molecules

摘要

The gas-phase proton-transfer reaction between ammonia and the hydrogen halides HF, HCl, and HBr and the influence of one, two, and three water molecules are investigated with high-level ab initio calculations on molecular clusters of NH_3-HX-(H_2O)_m, n = 0, 1, 2, 3 with X = F, Cl, Br. Equilibrium geometries, dissociation energies, HX harmonic frequencies, and potential energy surfaces along the proton-transfer pathway of NH_3-HX are calculated at the second-order moller-Plesset perturbation (MP2) level with the extended basis set 6-311++G(d.p). It is found that although the NH_3-HX dimer exists as a hydrogen-bonded structure for X = F, Cl, and Br, it can be converted to an ion pair in the presence of water molecules. One water molecule is sufficient to promote a proton transfer from HBr to NH_3, resulting in the ion pair NH_4~+...Br~-, whereas at least two water molecules are required to induce a proton transfer from HCl to NH_3. Three water molecules appear to promote a partial proton transfer from HF to NH_3. The potential energy surfaces along the protontransfer pathway demonstrate the energy difference present between two forms of each cluster in which the NH_3-HX system exists either as a hydrogen-bonded unit or as an ion pair NH_4~+...X~-. The successive addition of water molecules to the system gradually increases the stability of the ion pair relative to the hydrogenbonded form. The potential energy curves, along with the geometry data and HX vibration frequencies of the clusters, show how the progressive addition of water molecules affects a particular ammonia-hydrogen halide cluster and also indicate what trends exist between different ammonia-hydrogen halide clusters associated with the same number of water molecules.