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.

A molecular mechanics study of square planar Pt(II) coordination complexes is reported. Three new ligating atom types which are representative of the σ-donor ligands prevalent in Pt(II) coordination chemistryschloride, carboxylate oxygen, and amine nitrogensare developed to extend the MM2 force field. The newly derived MM force field accurately describes the geometry of both the inner and outer coordination sphere of Pt(II) complexes. Quick and efficient modeling is obtained despite simplifications used in constructing the force field, e.g., neglect of anharmonic corrections to the bond-stretching and angle-bending potentials. The simplifications used in construction of the force field allow accurate structural prediction in a timely manner using readily available software and hardware. Additionally, the new MM force field shows the ability to predict structures, in the absence of a starting guess of the geometry from X-ray crystallography, that are subsequently found to be in good agreement with experimental geometries.

Our group has used Ti-promoted aldol additions with an oxazolidineselone as the chiral auxiliary with much success. In these reactions, the Se atom in the auxiliary both promotes stereospecific addition as well as reports on, through the use of 77Se NMR spectroscopy, the ratio of diastereomers produced and the geometry of intermediates as the reaction proceeds. Through stable isotope labeling and NMR spectroscopy, we are able to experimentally observe a Ti enolate in solution and gain insight into its structure and reactivity. Results from molecular modeling calculations are also presented for comparison with NMR data.