Increasing the Time Resolution of Dynamic Nuclear Magnetic Resonance Spectroscopy through the Use of Lanthanide Shift Reagents

A method for increasing the time resolution of dynamic nmr spectroscopy is described. This technique involves using a lanthanide shift reagent to cause a continuous change of the nmr time scale through the exchange sensitive region, while the rates of the observed processes remain constant. The method is illustrated and tested with the interaction of trimethyl carbamate (TMC, (H₃C)₂NC(O)OCH₃) with tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato)europium(III) in CCl₄ at 27°. The free energy of activation of the isomerization of TMC (which cannot be studied by proton nmr in CCl₄ solution alone) is easily obtained. It is found to be 15.5 ± 0.1 kcal/mol, in excellent agreement with a literature value determined in CDCl₃ solution. In addition, the free energy of activation for isomerization of TMC bound to the metal chelate is found to be considerably increased. This is reasonable in terms of both electronic and steric factors.