Effects of hydration on the molecular structure of magnesium-fulvic acid complexes: a MOPAC (PM3) study

A semi-empirical model of Suwannee River fulvic acid, based on spectroscopic and electrochemical evidence, has been used as a basis for determining the structural characteristics and computed enthalpies and entropies of both anhydrous and hydrated mixed metal ion-fulvic acid complexes containing magnesium and either zinc, lead or cadmium. Replacement of Mg2+ With Cd2+, Pb2+, or Zn2+ at alternate binding sites results in changes in binding of Mg2+ from cis to trans at the phthalate binding site in fulvic acid. Hydration of Mg2+, Cd2+, Pb2+ and Zn2+ changes the relative enthalpies of formation for the di-substituted metal-fulvic acid complexes while leaving the type of binding at the phthalate binding site unaffected. The entropies of complex formation for both the anhydrous and hydrated metal ion-fulvic acid complexes are similar. Consequently, standard enthalpies of formation are an indicator of the relative strength of metal ion binding for both metal ion-phthalic acid and metal ion-fulvic acid complexes. Hydrogen bonding contributes to the overall structure and to the co-ordination of waters of hydration in the metal-fulvic acid complexes. (C) 1999 Elsevier Science B.V. All rights reserved.