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

Abstract

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 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 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.

Fate of ammonia in the atmosphere?a review for applicability to hazardous releases

“…Advanced ab initio calculations by Tao and co-workers [161][162][163][164] support the formation and stability of NH4CI in the gas phase. The single NH3-HCI complex is hydrogen bonded only.…”

Proton transfer in gas-phase ammonium dinitramide clusters

“…The acids studied include ammonium, [11][12][13] water, 14 hydrogen halides, 15,16 nitrous acid, 17 phenol, 18 and 7-hydroxyquinoline. 19 Our interests focus on ammonium-based strong oxidizers that can be used in rocket propellants, motivated by the practical need to find more environmentally benign oxidizers than the commonly used ammonium perchlorate.…”

An ab initio study of microsolvation of LiF in water: Structures and properties of LiF–Wn, n = 1–9 complexes

“…Several studies have been carried out on microsolvation in water with alkali halides and hydrogen halides. 26–37 It is known that nine water molecules are needed to solvate a NaCl molecule36, 37 where it acquires the ionic structure Na + Cl − . Jungwirth37 has shown that NaCl acquires the ionic structure even on microsolvation in a cluster of six water molecules.…”

“…Jungwirth37 has shown that NaCl acquires the ionic structure even on microsolvation in a cluster of six water molecules. Tao et al 27 have performed high‐level ab initio calculations on the binding of one, two, and three water molecules with HF, HCl, HBr, and NH 3 . In this study, to form the complexes involving two and three water molecules, they have added each water molecule separately.…”