Electrostatic interactions of MCPA with mineral surfaces can be assumed to be stronger than interactions of MCPA with polar or aromatic organic moieties via hydrogen bonds, charge transfer or lipophilic interactions, which could be supported by molecular modeling simulations. This is intimately connected with the increase in computer power and the recent rapid improvement of PCs and work stations. Although at present seldom used, numerous applications in soil science, especially in pollution research, can be envisaged. Another interesting field for CC are studies of the hypothetical structure of humic substance models. As to be expected, solvation effects play an important role for the calculation of formation energies of the complexes.
Optimized geometries are in good agreement with X-ray data.
Reaction enthalpies and Gibbs free reaction energies have been computed for the substitution of water molecules of the aluminum-hexaaquo complex by citrate molecules. Formation of the tridentate complexes is strongly favored by entropy effects in analogy to previous findings for bidentate acetate and oxalate complexes. Comparison of the stability of acetate, oxalate and citrate complexes shows a pronounced preference for the latter. M ller and H. Lischka Theor. The presence of low-lying Rydberg states interspersed among valence states constitutes a substantial challenge for the accurate quantum chemical calculation of electronically excited states because of the need to treat a relatively large number of states simultaneously.