Theoretical Study On Structures And Thermodynamic Properties Of Gd(H₂O)_n³⁺(n =8,9)

Compounds of gadolinium(III) have attracted considerable attentions due to their potential applications as contrast agents for magnetic resonance imaging (MRI) in clinical diagnostics and biomedical research. As the bare Gd3+ ion is toxic, Complexation with polydentate ligand to form compounds of high kinetic and thermodynamic stability in solution is required for in vivo application. These complexes usually have one or more water molecules coordinated to the ion. Therefore, knowledge of interaction between Gd3+ and the ligating water molecules is imperative. In this thesis, structural and thermodynamic properties of gadolinium aqua complexes are investigated using density functional theory. The main contents can be summarized as follows:(1) Density functional theory (DFT) calculations have been performed to study the structures and relative stability of the gadolinium complexes, Gd(H2O)n3+ (n = 8,9), in vacuo and in aqueous solution. The polarizable continuum model (PCM) with various radii for the solute cavity has been used to study the relative stability in aqueous solution. The calculated molecular geometries for n = 8 and 9 obtained in vacuo are consistent with those observed in experiments. It is found that while the nona-aqua complex is favored in the gas phase, in aqueous solution the octa-aqua conformation is preferred. This result, independent of the types of cavities employed, is in agreement with the experimental observation. The reliability of the present calculation is also addressed by comparing the calculated and experimental free energy of hydration, giving that the UA0, UAHF and UAKS cavities are most appropriate when only the first solvation shell is treated explicitly.(2) The structures and relative stability of the gadolinium complexes, Gd(H₂O)n³⁺ (n = 8,9) are investigated using BLYP (fock-35)/MWB28/ aug-cc-pvtz theoretical level, in vacuo and in aqueous solution. The structures and thermodynamic stability at the two levels are compared. It is found that symmetries of the optimized structures remain stable and averaged Gd–O bond distances are nearly identical. However, Gd–O bond lengths yielded at BLYP (fock-35)/MWB28/aug-cc-pvtz level are closer to experimental values. We have studied relative stability of Gd³⁺ aqua complexes using these two levels. Calculations of thermodynamic features show that Gd(H2O)9³⁺ is preferred energetically over Gd(H2O)8³⁺ in the gas phase, while in the solution phase the reverse is true. This is independent of the types of cavities used in the PCM calculations. Finally, the performance of cavity models is evaluated against the experimental free energy of hydration of Gd³⁺. It is found that the cavities generated with the UA0, UAHF/UAKS and Pauling radii are very appropriate at BLYP (fock-35)/MWB28/aug-cc-pvtz level when only the primary solvation shell is taken into account explicitly. However, using B3LYP/ CEP-31G/6-31++g(d,p) level the UA0, UAHF and UAKS cavities are most appropriate when only the first solvation shell is treated explicitly. Finally, the study at these two levels highlights the importance of the solvent effects in the study of thermodynamic properties for the gadolinium complexes.