HYDRATION ENERGIES OF DIVALENT BERYLLIUM AND MAGNESIUM-IONS - AN AB-INITIO MOLECULAR-ORBITAL STUDY

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 magn esium cations. Enthalpy and free energy changes at 298 K have been cal culated at a variety of computational levels for the reactions M(2+) [H2O](p) --> M(2+) . nH(2)O . mH(2)O, where M = Be or Mg, [H2O](p) (p = 2, 4, 6, 8; p = n + m) are water clusters, and M(2+) . nH(2)O . mH( 2)O 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 competit ive effects of ion-water and water-water interactions inherent in the hydration process. At the MP2(FULL)/6-311++G*//RHF/6-31G* computation al level, the values of Delta G(298) for the reactions which complete the first hydration shells, Be2+ + [H2O](4) --> Be2+ . 4H(2)O and Mg2 + [H2O](6) --> Mg2+ . 6H(2)O, are -352.0 and -266.7 kcal/mol, account ing for 61.2% and 60.7% of the experimental free energies of hydration of Be2+ and Mg2+. Reactions that incorporate two additional water mol ecules into a second hydration shell only change Delta G(298) by -43.0 and -24.2 kcal/mol, whereas the values of Delta G(298) for the corres ponding reactions that incorporate the first two water molecules in th e primary hydration shell are -244.6 and -135.2 kcal/mol, respectively . The calculated values of Delta G(298) for the formation of the compl exes Be2+ . 4H(2)O . 4H(2)O and Mg2+ . 6H(2)O . 2H(2)O from eight-wate r clusters account for approximately 73.2% and 66.2% of the overall fr ee energies for Be2+ and Mg2+, respectively, but convergence toward th e experimental hydration energies will be quite slow as additional wat er molecules are added to the outer hydration shells. This is consiste nt with the concept of the importance of long-range interactions to th e hydration energy.