Neodymium(III) Aqua Ion as a Model System in Ab Initio Crystal Field Analysis Beyond Point Charges and Crystal Field Theory

Correlating the molecular structure with the electronic structure of lanthanide(III) solvates is a challenging task. Neodymium(III) in aqueous solution serves as an appealing and straightforward model to address this issue. Herein, the experimentally determined electronic structure of neodymium(III) in water is compared with ab initio calculated electronic structures based on various models of its molecular structure. This comparison enables the determination of the most reliable molecular structure. The findings reveal that the molecular structure of the neodymium(III) aqua ion that best aligns with its electronic structure corresponds to a nine coordinated neodymium(III) complex, surrounded by 17 water molecules in the second coordination sphere. The role of second-sphere water molecules was investigated by calculating the crystal field splitting of the five Kramers doublets within the ⁴I_9/2 low-energy multiplet for several calculated molecular structures with coordination numbers of eight, nine, and ten. The results demonstrated that the shape of the donor molecular orbitals plays a critical role in the crystal field splitting of the neodymium(III) ion. Furthermore, the findings confirmed that the orientation of the donating orbitals, specifically the orientation of the O-H bonds in water, is essential for accurately describing the electronic structure. Finally, manual alteration of the Nd-O bond lengths revealed that CAS(3,7)CF calculations tend to underestimate the crystal field strength.