Threshold Collision‐Induced Dissociation of Hydrated Magnesium: Experimental and Theoretical Investigation of the Binding Energies for Mg²⁺(H₂O)_x Complexes (x=2–10)

Abstract

The sequential bond energies of Mg(2+)(H2O)x complexes, in which x=2-10, are measured by threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer. From an electrospray ionization source that produces an initial distribution of Mg(2+)(H2O)x complexes in which x=7-10, complexes down to x=3 are formed by using an in-source fragmentation technique. Complexes smaller than Mg(2+)(H2O)3 cannot be formed in this source because charge separation into MgOH(+)(H2O) and H3O(+) is a lower-energy pathway than simple water loss from Mg(2+)(H2O)3. The kinetic energy dependent cross sections for dissociation of Mg(2+)(H2O)x complexes, in which x=3-10, are examined over a wide energy range to monitor all dissociation products and are modeled to obtain 0 and 298 K binding energies. Analysis of both primary and secondary water molecule losses from each sized complex provides thermochemistry for the sequential hydration energies of Mg(2+) for x=2-10 and the first experimental values for x=2-4. Additionally, the thermodynamic onsets leading to the charge-separation products from Mg(2+)(H2O)3 and Mg(2+)(H2O)4 are determined for the first time. Our experimental results for x=3-7 agree well with quantum chemical calculations performed here and previously calculated binding enthalpies, as well as previous measurements for x=6. The present values for x=7-10 are slightly lower than previous experimental results and theory, but within experimental uncertainties.

Vibrational Signatures of Solvent-Mediated Deformation of the Ternary Core Ion in Size-Selected [MgSO4Mg(H2O)n=4–11]2+ Clusters

“…12,13,65 The water binding energies on the bare Mg 2+ (H 2 O) n=3=10 clusters have been reported to fall rapidly after completion of the first hydration shell at n = 6, converging on a value around 4000 cm −1 by n = 10, which is approaching the photon energy of the free OH stretching bands in question. 56,66−70 Although not definitive because of these experimental limitations, the data are most consistent with the 11th water molecule starting the onset of the second hydration shell around one of the Mg 2+ ions, which is consistent with the expectation that this form corresponds to the global minimum.…”

Coordination structure and charge transfer in microsolvated transition metal hydroxide clusters [MOH]+(H2O)1–4

“…Specifically, coordination numbers and sequential binding energies of water molecules have been previously determined by collision-induced-dissociation mass spectrometry (CID-MS) for the singly charged transition metals Ti-Cu 9 as well as for Ca 2+ , Mg 2+ , and Fe 2+ , and Zn 2+ . [10][11][12][13][14] Additionally, structures of hydrated singly and doubly charged alkali and transition metal clusters have been very well characterized by photodissociation spectroscopy. [15][16][17][18][19][20][21][22][23][24][25][26] Building on these results of solvated metal ions, we extend our study of the [MOH] + species to larger clusters to probe the effect of solvation on the M-OH charge transfer as well as to determine how the coordination sphere is influenced by the charged hydroxide ligand.…”

Assessing the Intrinsic Strengths of Ion–Solvent and Solvent–Solvent Interactions for Hydrated Mg2+ Clusters

“…The more restricted CN of the Mg 2+ has previously been attributed to the smaller ion size [39,107,108], whereas the larger size of the Ca 2+ ion has been linked to weaker inner ion-solvent interactions in retrospect to that of the hydrated Mg 2+ ion [51]. As shown by Tables 3 and 5, the k a cum values of explicitly hydrated systems, based upon inner ionsolvent interactions, are larger for [Mg(H 2 O) n ] 2+ (n = 1-6) (1-6) clusters in comparison to that of their Ca 2+ counterparts by differences of 0.601 to 1.341 mdyn/Å (i.e., 1a-6a in 2).…”