Ab initio effective core potential calculations on lanthanide complexes: basis sets and electron correlation effects in the study of [Gd-(H 2O) 9] 3+

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Calculations on [Gd-(H 2O) 9] 3+ at the restricted Hartree-Fock (RHF), density functional theory (DFT) and Møller- Plesset 2 (MP2) levels were performed using an effective core potential (ECP) including 4f electrons in the core for gadolinium. An optimized valence basis set with different contraction schemes for the metal, and the STO-3G, 3-21G, 6-31G ∗, 6-31G ∗∗ and D95 ∗∗ basis sets for water molecules were used. Geometry optimization provided three main conformations: a minimum and two saddle points, each corresponding to a tricapped trigonal prism arrangement of the coordinated water molecules. The minimum energy conformation is of D 3 symmetry and does not correspond to any known experimental structure. The geometries of the two saddle points are of C 3 h and D 3 h symmetry, respectively, and correspond to two different crystallographic structures. At the DFT/D95 ∗∗ (RHF/D95 ∗∗) level these are respectively 4.95 (4.92) and 6.97 (6.26) kcal mol −1 above the global minimum. The results show that the major structural features of the coordination cage observed in the crystallographic structures are mainly due to intramolecular interactions between the coordinated water molecules. Intermolecular interactions, such as crystal packing forces and hydrogen-bond networks, should be responsible for the energy stabilization of the C 3 h and D 3 h structures in the solid state. At the RHF level, the STO-3G basis set is very sensitive to the choice of basis set on the metal center and, even in the best cases, provides unsatisfactory results. On the contrary, the 3-21G basis set provides, independently of the metal basis set, quite reliable geometries and conformational energies in qualitative agreement with those obtained using polarized basis sets. The results from the polarized basis sets themselves are consistent; further, inclusion of the electron correlation causes only a little shortening of the gadolinium-oxygen bond distances with respect to RHF and the conformational energies are substantially unaffected. These results highlight that calculations with the adopted ECP for the metal can provide reliable results in the study of gadolinium complexes also at the RHF level; further, a satisfactory representation of the potential energy surface of these systems can be obtained by 6-31G ∗ single point energy calculations on optimized 3-21G geometries.