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Hydration of alkaline earth metal dications: effects of metal ion size determined using infrared action spectroscopy.

Authors
  • Bush, Matthew F
  • O'Brien, Jeremy T
  • Prell, James S
  • Wu, Chih-Che
  • Saykally, Richard J
  • Williams, Evan R
Type
Published Article
Journal
Journal of the American Chemical Society
Publisher
American Chemical Society
Publication Date
Sep 23, 2009
Volume
131
Issue
37
Pages
13270–13277
Identifiers
DOI: 10.1021/ja901011x
PMID: 19754186
Source
Medline
License
Unknown

Abstract

Infrared laser action spectroscopy is used to characterize divalent Mg, Ca, and Ba ions solvated by discrete numbers of water molecules in the gas phase. The spectra of the hexahydrated ions are very similar and indicate that all six water molecules directly solvate the metal ion. The spectra of the heptahydrated ions indicate the presence of populations of structures that have a water molecule in the outer shell for all ions and an average coordination number (CN) for Ba that is higher than that for Ca or Mg. Comparisons between CN values obtained from M06 density functional and local MP2 theory indicate that the B3LYP density functional favors smaller CN values. The spectra of clusters containing at least 12 water molecules indicate that the relative abundance of single-acceptor water molecules for a given cluster size decreases with increasing metal ion size, indicating that tighter water binding to smaller metal ions disrupts the hydrogen bond network and results in fewer net hydrogen bonds. The spectra of the largest clusters (n = 32) are very similar, suggesting that intrinsic water properties are more important than ion-water interactions by that size, but subtle effects of Mg on surface water molecules are observed even for such large cluster sizes.

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