Influence of ions on molecular vibrations and hydrogen bonds in methanol–water mixtures

Mg(II) and Ca(II) Microsolvation by Ammonia: Born-Oppenheimer Molecular Dynamics Studies

We report the structural and energetic features of the Mg²⁺ and Ca²⁺ cations in ammonia microsolvation environments. Born-Oppenhemier molecular dynamics studies are carried out for [Mg(NH₃)_n]²⁺ and [Ca(NH₃)_n]²⁺ clusters with n = 2, 3, 4, 6, 8, 20, and 27 at 300 K based on hybrid density functional theory calculations. We determine binding energies per ammonia molecule and the metal cation solvation patterns as a function of the number of molecules. The general trend for Mg²⁺ is that the Mg-N distances increase as a function of n until the first solvation shell is populated by six ammonia molecules, and then the distances slightly decrease while CN = 6 does not change. For Ca²⁺, the first solvation shell at room temperature is populated by eight ammonia molecules for clusters with more than one solvation shell, leading to a different structure from that of [Ca(NH₃)₆]²⁺ hexamine. The evaporation of NH₃ molecules was found at 300 K only for Mg²⁺ clusters with n ≥ 10; this was not the case for Ca²⁺ clusters. Vibrational spectra are obtained for all of the clusters, and the evolution of the main features is discussed. EXAFS spectra are also presented for the [Mg(NH₃)₂₇(NH₃)₂₇]²⁺ and [Ca(NH₃)₂₇]²⁺ clusters, which yield valuable data to be compared with experimental data in the liquid phase, as previously done for the aqueous solvation of these dications.

Water-Methanol Mixtures: Simulations of Mixing Properties over the Entire Range of Mole Fractions

Numerous experimental and theoretical investigations have been devoted to the hydrogen bond in pure liquids and mixtures. Among the different theoretical approaches, molecular dynamics (MD) simulations are predominant in obtaining detailed information, on the molecular level, simultaneously on the structure and the dynamics. Water and methanol are the two most prominent hydrogen-bonded liquids, and they and their mixtures have consequently been the subject of many studies; we revisit here the problem of the mixtures. An important first step is to check whether a classical potential model, the components of which are deemed to be satisfactory for the pure liquids, is able to reproduce the known thermodynamic excess properties of the mixtures sufficiently well. We have used the available BJH (water) and PHH (methanol) flexible models because they are by construction mutually compatible and also well suited to study, in a second step, some dynamic property characteristic of hydrogen-bonded liquids. In this article we show that these models, after a slight reparametrization for use in NpT simulations, reproduce the essential features of the excess mixing and molar properties of water-methanol mixtures. Furthermore, in the pure liquids, the agreement of the radial distribution functions with experiment remains as satisfactory as before. Similarly, the translation self-diffusion coefficients D are modified by less than 10%. In the mixtures, they evolve nonmonotonously as a function of mole fraction.