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 4I9/2 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.