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

We report the structural and energetic features of the Mg and Ca cations in ammonia microsolvation environments. Born-Oppenhemier molecular dynamics studies are carried out for [Mg(NH)] and [Ca(NH)] clusters with = 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 Mg is that the Mg-N distances increase as a function of until the first solvation shell is populated by six ammonia molecules, and then the distances slightly decrease while CN = 6 does not change. For Ca, 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(NH)] hexamine. The evaporation of NH molecules was found at 300 K only for Mg clusters with ≥ 10; this was not the case for Ca 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(NH)(NH)] and [Ca(NH)] 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.