A Multisite Model for Lanthanide Shift Reagent Coordination to Monofunctional Substrates. Effects of Rotational and Site Averaging on Shifts and Relaxation Rates

The shifts and line broadenings induced by various Ln(fod)a shift reagents in the proton spectrum of pinacolone, (CH₃)CCOCH₃, are analyzed using an approach which combines a two-site model for lanthanide coordination to the carbonyl group with several methods for averaging over internal rotations of the methyl and tert-butyl groups within the substrate. The applications of all possible combinations of three rotational averaging schemes (free rotation, restricted rotation, and averaging along the axis of rotation) to evaluate the geometrical parameters for the dipolar shift and relaxation indicate that combinations containing the latter scheme cannot be used satisfactorily to simulate the data. At present, it is difficult to ascertain which of the other combinations is most satisfactory, since all generate similar results. As might be expected from steric considerations, lanthanide binding to the carbonyl takes place at the site nearer the methyl group. The simultaneous application of both shift and relaxation rate data to derive information concerning the shift reagent-substrate complex removes many of the ambiguities present if only one kind of data is employed. Details are given of the approach to the data analysis and the interpretation of the results.