Line broadenings induced by lanthanide shift reagents: Concentration, frequency, and temperature effects

The effects of reagent concentration, resonance frequency, and sample temperature on the line-broadenings induced by the lanthanide shift reagent Yb(fod)₃ in the proton spectrum of pinacolone, (CH₃)₃ CCOCH₃, have been investigated and analyzed in detail. For comparison the effects of other members of the Ln(fod)₃ series have also been examined. It is found that a term containing the modulation by chemical exchange of the chemical shift difference between the coordinated and free substrate molecules makes a significant contribution to the observed line-broadenings leading to their nonlinear dependence on the fraction of complexed substrate. Also the ratio between the line-broadenings of the two different proton-containing groups is a nonlinear function of this fraction. The analysis of the results yields the relaxation rates of the protons in the coordinated state and their mean residence time as well as the electron spin relaxation time of the central lanthanide ion. Details are given of the approach to the data analysis and interpretation.