Milton D. Johnston

Structure elucidation with lanthanide-induced shifts. 11. Analysis of alkyl-substituted benzonitriles

Abstract The 1H NMR spectra for a series of alkyl-substituted benzonitriles were investigated in the presence of Eu(fod)3. The experimental lanthanide-induced shifts (LIS) were determined for the 1:1 complexes, and the structures of the complexes were evaluated by comparison of the experimental LIS with values predicted with the pseudocontact equation. The contribution to the observed LIS from contact shifts and complex formation shifts is shown to be small, and excellent structure fits are obtained. The correct structure of a compound can be reliably selected from several possibilities, even if only one of the structural alternatives is available for experimental study.

A rigorous statistical analysis of errors incurred in the study of intermolecular fast-exchange equilibria by NMR spectroscopy

Abstract A detailed analysis of the algorithms used in the analysis of NMR fast-exchange equilibria is made. It is shown that, in a system of step-wise equilibria of the type A + B ⇌ AB, AB + B ⇌ AB 2 , …, AB n −1 + B ⇌ AB n , the most efficient way to solve the resulting system of equations is to solve for [B], the amount of uncomplexed B. It is then shown that in analyzing the equilibria, the equilibrium constants must be obtained via nonlinear regression for the greatest reliability. The problem is novel in that the other important parameters, the bound shifts, are obtained by simultaneous linear regression . A detailed analysis of the standard deviations of the various parameters is then made with particular emphasis being placed upon the type of data obtained with lanthanide shift reagents.

The lanthanide-induced-shift-assisted determination of proton spin-lattice relaxation times

Abstract A new method is proposed for the determination of NMR relaxation times in second-order (strongly coupled) spectra. The method exploits the shifting abilities of the lanthanide shift reagent, Eu(fod)3. It is shown that, in favorable cases, the spectral dispersion afforded by a shift reagent can allow one to determine T2s and then extrapolate them back to zero LIS (absence of shift reagent) to get the relaxation times for the protons in the strongly coupled spectra. It is shown that complications due to cross-relaxation are absent. The technique is illustrated for the case of adamantanone interacting with Eu(fod)3 in CDCI3 as solvent. Data derived from 13C NMR spin-lattice relaxation on the same molecule can be used to determine what percentage of the proton relaxation is caused by the dipolar relaxation mechanism.