Semiempirical Molecular Orbital Scheme To Study Lanthanide(III) Complexes:  PM3 Parameters for Europium, Gadolinium,...

Semiempirical parameters for europium, gadolinium, and ytterbium have been developed for use in the PM3 method to allow the structure and energetics of complexes containing lanthanide(III) ions to be accurately modeled. At the semiempirical level, the lanthanide(III) ion is represented by a +3 core and has a minimal basis of 6s5d6p (9 atomic orbitals), the 4f electrons being included within the electronic core. Training sets containing up to 19 lanthanide complexes, with data computed at the density functional theory (DFT) level, have been employed for each lanthanide(III) ion. A gradient-based optimization algorithm has been used, and important modifications of the core repulsion function have been highlighted. The derived parameters lead in general to good predictions of the structure of the complexes and demonstrate improvements in the prediction of water binding energies compared to the AM1/sparkle model. For the 28 Eu(III), 28 Gd(III), and 29 Yb(III) complexes optimized at the DFT level, the PM3 average unsigned mean errors for all interatomic distances between the lanthanide(III) ion and the ligand atoms of the first coordination sphere are 0.04, 0.03, and 0.03 Å, respectively. The derived parameters are shown to be comparable to small-basis set DFT calculations in predicting the experimental structures of various lanthanide(III) complexes. The derived parameter sets provide a starting point should greater accuracy for a more restricted range of compounds be required.