The hydration features of [Mg(HO)] and [Ca(HO)] clusters with n = 3-6, 8, 18, and 27 were studied by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/6-31+G** level of theory. For both ions, it is energetically more favorable to have all water molecules in the first hydration shell when n ≤ 6, but stable lower coordination average structures with one water molecule not directly interacting with the ion were found for Mg at room temperature, showing signatures of proton transfer events for the smaller cation but not for the larger one. A more rigid octahedral-type structure for Mg than for Ca was observed in all simulations, with no exchange of water molecules to the second hydration shell. Significant thermal effects on the average structure of clusters were found: while static optimizations lead to compact, spherically symmetric hydration geometries, the effects introduced by finite-temperature dynamics yield more prolate configurations. The calculated vibrational spectra are in agreement with infrared spectroscopy results. Previous studies proposed an increase in the coordination number (CN) from six to eight water molecules for [Ca(HO)] clusters when n ≥ 12; however, in agreement with recent measurements of binding energies, no transition to a larger CN was found when n > 8. Moreover, the excellent agreement found between the calculated extended X-ray absorption fine structure spectroscopy spectra for the larger cluster and the experimental data of the aqueous solution supports a CN of six for Ca.
In this work, a theoretical investigation was made to assess the coordination properties of Pb(ii) in [Pb(HO)] clusters, with n = 4, 6, 8, 12, and 29, as well as to study proton transfer events, by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/aug-cc-pVDZ-pp/6-311G level of theory, that were calibrated in comparison with B3LYP/aug-cc-pVDZ-PP/aug-cc-pVDZ calculations. Hemidirected configurations were found in all cases; the radial distribution functions (RDFs) produced well defined first hydration shells (FHSs) for n = 4,6,8, and 12, that resulted in a coordination number CN = 4, whereas a clear-cut FHS was not found for n = 29 because the RDF did not have a vacant region after the first maximum; however, three water molecules remained directly interacting with the Pb ion for the whole simulation, while six others stayed at average distances shorter than 4 Å but dynamically getting closer and farther, thus producing a CN ranging from 6 to 9, depending on the criterion used to define the first hydration shell. In agreement with experimental data and previous calculations, proton transfer events were observed for n≤8 but not for n≥12. For an event to occur, a water molecule in the second hydration shell had to make a single hydrogen bond with a water molecule in the first hydration shell.
Coordination-induced desolvation
or ligand displacement by cosolvents
and additives is a key feature responsible for the reactivity of Sm(II)-based
reagent systems. High-affinity proton donor cosolvents such as water
and glycols also demonstrate coordination-induced bond weakening of
the O–H bond, facilitating reduction of a broad range of substrates.
In the present work, the coordination of ammonia to SmI2 was examined using Born–Oppenheimer molecular dynamics simulations
and mechanistic studies, and the SmI2-ammonia system is
compared to the SmI2-water system. The coordination number
and reactivity of the SmI2-ammonia solvent system were
found to be similar to those of SmI2-water but exhibited
an order of magnitude greater rate of arene reduction by SmI2-ammonia than by SmI2-water at the same concentrations
of cosolvent. In addition, upon coordination of ammonia to SmI2, the Sm(II)-ammonia solvate demonstrates one of the largest
degrees of N–H bond weakening reported in the literature compared
to known low-valent transition metal ammonia complexes.
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.
Using both computational and experimental data the SmI2–MeOH system is directly compared to the SmI2–H2O system to uncover the basis for their drastic differences in reactivity.
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