Evans NMR method

Magnetic susceptibilities of solutions — These are useful parameters for determining equilibrium constants for reactions involving spin changes. The Evans nmr method utilizes the observed shift in the resonance line (say of a proton of t-BuOH or hexamethyldisilox-ane) in solution when a paramagnetic substance is added. The paramagnetic shift A/ is related to the magnetic moment (jj, of the solution at TK by the approximate expression... 

Some of the polycrystalline spin crossover systems of iron(II) described above retain their spin equilibrium property upon dissolution in appropriate solvents. The Evans NMR method of measuring the change of the paramagnetic shift with temperature is the most common technique to study the magnetic behaviour of such systems. The spin transition characteristics has been observed to depend on various chemical modi-... 

The Evans NMR method requires a coaxial NMR tube where two solutions can be physically separated. One chamber in the tube contains a solution of a reference solute and the other contains a solution of the paramagnetic analyte and the reference solute. The reference solute must be inert toward the analyte. Because the chemical shift(s) for the resonances of the reference solute in the resulting NMR spectrum will be different for that in the solution with the paramagnetic analyte than in the solution without the analyte, resonances are observed for each chamber. The frequency shift of the selected reference resonance (measured in Hz) is proportional to the mass susceptibility of the analyte. Application of high-field NMR spectrometers is ideal for these studies because rather small chemical shift differences can be resolved. 

It is thus evident that, in order to interpret the results obtained by one of the relaxation methods, a thorough investigation of the unperturbed equilibrium properties is required. In general, solution magnetic susceptibilities measured by the NMR method of Evans [83, 84] are used to this end, the equilibrium constant for the equilibrium in Eq. (37) being determined by ... 

Table I. Magnetic Moments of Manganese Gluconate Complexes from the NMR Method of Evans (56)...

The measurement of paramagnetic susceptibilities in solution by the NMR method of Evans (347) has been very extensively employed for many years. More recently the same author (348) has developed the method for magnetic titrations. Such titrations of a metal complex are expressed by the equation ... 

Evans et al have described techniques employing pyrolysis - gas chromatography, (Method 95) proton NMR (Method 94) and carbon analysis for the determination of styrene and methacrylate units in styrene - methylmethacrylate and styrene-ftbutyl meAacrylate copolymers. Agreement between the three independent methods is excellent. The comparison stresses the complementary nature of dl three methods. The pyrolysis gas chromatography method possesses advantages such as simplicity and rapidity. 

The magnetic moment at room temperature for the mixed valence complexes was 2.9 pB dimer as measured by the Evans method. In J,Am.Chem.Soc.99( 9n)6623, the Authors measured the susceptibility in solution over a temperature range and used the NMR method. An earlier method gave a value of 2.5 PB per dimer which was taken as evidence of antiferromagnetic behaviour. The above reference also included results of cyclic voltametiy (Sec.2.5.4.). 

Magnetic susceptibilities were determined with a Gouy balance or using a nmr method originally employed by D.F. Evans (1959). This last method has been used successfully with minor modifications for some organolanthanides (D.F. Evans, 1971). The nmr method, which is very simple to use, is quite applicable in the case of species soluble in an organic medium. 

During the course of the development of our group s alkylation/reductive decyanation strategy, a very reliable method for distinguishing between syn-and anfz-l,3-diols was discovered [17,18]. The acetonide methyl groups reliably display diagnostic C-NMR chemical shifts, allowing for stereochemistry to be determined simply by inspection (Fig. 1). Evans later extended the C-NMR chemical correlation to polypropionate chains [19,20]. 

Useful applications of the Evans proton NMR shift method have permitted the determination of effective magnetic moments peff (see Section 8) and spin multiplicities for several high-spin radical-cation species.This work makes available a simple procedure for such assignments when the spin carriers are stable in solution. 

The Evans method gives excellent results provided adequate care is taken. A most important requirement is that the solution temperature is measured reliably. One effective means of accomplishing this for H NMR is to insert into the NMR tube a capillary or additional coaxial sample of an NMR temperature calibrant solvent, usually methanol (158) or ethylene glycol (88). In this way the temperature measurement is made simultaneously with the susceptibility measurement. A second important factor is the variation of the solvent density with temperature (126). Because the density difference between the solvent and solution depends linearly on the concentration of the solute, it is only... 

Cohen, Y., Avram, L., Evan-Salem, T., Frish, L., Diffusion NMR in supramolecular chemistry , in Analytical Methods in Supramolecular Chemistry, Schalley, C. A., ed. Wiley-VCH Weinheim, 2007, pp. 163-219. 

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