Title

Hydration energies of divalent beryllium and magnesium ions: An ab initio molecular orbital study

Document Type

Article

Publication Title

Journal of Physical Chemistry

Publication Date

2-29-1996

Abstract

Ab initio molecular orbital calculations have been used to investigate contributions of water molecules in the first and second coordination shells to the overall hydration energy of divalent beryllium and magnesium cations. Enthalpy and free energy changes at 298 K have been calculated at a variety of computational levels for the reactions M2+ + [H2O]p → M2+·nH2O·mH2O, where M = Be or Mg, [H2O]p (p = 2, 4, 6, 8; p = n + m) are water clusters, and M2+·nH2O·mH2O are ion-water complexes with n and m water molecules in the first and second coordination shells, respectively. These reactions involve the disruption of the water cluster and naturally include the competitive effects of ion-water and water-water interactions inherent in the hydration process. At the MP2(FULL)/6-311++G**//RHF/6-31G* computational level, the values of ΔG298 for the reactions which complete the first hydration shells, Be2+ + [H2O]4 → Be2+·4H2O and Mg2+ + [H2O]6 → Mg2+·6H2O, are -352.0 and -266.7 kcal/mol, accounting for 61.2% and 60.7% of the experimental free energies of hydration of Be2+ and Mg2+. Reactions that incorporate two additional water molecules into a second hydration shell only change ΔG298 by -43.0 and -24.2 kcal/mol, whereas the values of ΔG298 for the corresponding reactions that incorporate the first two water molecules in the primary hydration shell are -244.6 and -135.2 kcal/mol, respectively. The calculated values of ΔG298 for the formation of the complexes Be2+·4H2O·4H2O and Mg2+·6H2O-2H2O from eight-water clusters account for approximately 73.2% and 66.2% of the overall free energies for Be2+ and Mg2+, respectively, but convergence toward the experimental hydration energies will be quite slow as additional water molecules are added to the outer hydration shells. This is consistent with the concept of the importance of long-range interactions to the hydration energy. © 1996 American Chemical Society.

Volume

100

Issue

9

First Page

3488

Last Page

3497

DOI

10.1021/jp952531t

ISSN

00223654

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