Abstract
The effects of a metal ion-bound carboxylate group on the acidity of a water molecule bound to the same cation have been assessed by ab initio molecular orbital calculations. In the hexahydrate Mg[H2O]62+ the free energy required to deprotonate one coordinated water molecule is only 40% of that required to deprotonate a free water molecule, indicating that the presence of the magnesium ion facilitates the ionization of water. However, if one of the water molecules in this hexahydrate Mg[H2O]62+ is replaced by a carboxylate ligand, the energy required to dissociate a proton from a metal ion-bound water molecule is increased by approximately 80 kcal/mol and is intermediate between the energy required to deprotonate one water molecule in Mg[H2O]62+ and that for a free water molecule. This effect of the carboxylate group on the pKa of metal ion-bound water appears to be primarily the result of a reduction of the net positive charge of the overall Mg[H2O]52+−(RCOO-) complex rather than any changes in the elect...
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