A recently developed compact effective core potential (ECP) scheme for lanthanides [T. R. Cundari and W. J. Stevens, J. Chem. Phys. 98, 5555 (1993)] is applied to molecular species—the lanthanide trihalides (LnX3, Ln=Ce to Lu; X=F, Cl, Br, I). This research is the first ECP study of lanthanide trihalides in which 4f electrons are explicitly included in the lanthanide valence space. The high-spin 4fn configuration of Ln(III) is described using multiconfiguration (MC) and unrestricted Hartree-Fock (UHF) wave functions. Both methods give excellent agreement with experiment for Ln–X bond lengths, although UHF is faster with no evidence of extensive spin contamination. There is no loss of accuracy from early to late members of the lanthanide series and from the lightest to heaviest halogen for the MC calculations. The root-mean-square difference between MC/ECP Ln–X bond lengths and those estimated from gas-phase experiments is only 0.05 Å (≊2%). The UHF calculations on LnF3 yield only three nonplanar minima (CeF3, PrF3, and PmF3). For the three complexes with pyramidal minima, the planar transition states are very close in energy (ΔEUHF<100 cm−1), suggesting these complexes will probably be fluxional at ambient conditions.

The purpose of this research is to effectively and efficiently predict the geometries of gadolinium complexes that are of a size (~5 0 atoms) and which possess ligand types making them of potential interest as magnetic resonance imaging (MRI) contrast agents. This research extends a standard molecular mechanics (MM) force field for organic compounds to gadolinium complexes. Force field parameters are derived to permit modeling of prototypical hard nitrogen and oxygen ligands commonly found in lanthanide coordination chemistry. Several new ligating atom types are defined-neutral sp3 oxygen (water, alcohols, and ethers), neutral sp3 nitrogen (amines), neutral sp2 oxygen (carbonyls), neutral sp2 nitrogen (imines and pyridines), and negative oxygen (carboxylates). The new force field is generally able to predict the geometries of Gd"'(Schiff base)(H20), and related complexes to within 3% of metric data (i.e., bond lengths and bond angles) as determined by X-ray crystallographic analysis. Torsional angles about individual bonds are also typically predicted to within 5", allowing one to reproduce the three-dimensional (or tertiary) structure of the Gd(II1) complexes. The use of a simple molecular mechanics force field permits the geometry optimization of these complexes to be carried out quickly by using commercially available software on a standard personal computer.

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An X-ray structural analysis of the title compound [2,2'-(dihydroxymethylene)dipyridinium dinitrate, CI1H12NEOE2+.2NO3 -] revealed that it was a doubly protonated derivative of di-2-pyridyl ketone (dpk) which had also undergone a hydration reaction at the ketone to