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

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 2 O) n, 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 Møller−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 proton-transfer 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 hydrogen-bonded form. The potential energy curves, along with the geometry data and HX vibrational 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.