Nuclear magnetic resonance exchange rate

Exchange reactions can be sometimes investigated by the techniques of polari-metry, nuclear magnetic resonance and electron spin resonance. The optical activity method requires polarimetric measurements on the rate of racemization in mixtures of d-X (or /-X) and /-Y (or d-Y). 

Richardson, S.J. 1989. Contribution of proton exchange to the oxygen-17 nuclear magnetic resonance transverse relaxation rate in water and starch-water systems. Cereal Chem. 66, 244-246. Richardson, M.J. and Saville, N.G. 1975. Derivation of accurate glass transition temperatures by differential scanning calorimetry. Polymer 16, 753-757. 

This technique has been taken to new heights through the use of 2-D nuclear magnetic resonance " to assign many, if not all, exchangeable protons in terms of the residues to which they are attached. As a result, one can assess individual rate constants defining the kinetics of exchange. 

Fig. 14. Plot of line shape change versus rate of exchange of protons between environments A and B. The intensities of the various line functions are not comparable. Reproduced by permission from "High-Resolution Nuclear Magnetic Resonance, by Pople, Schneider, and Bernstein. McGraw-Hill, New York, 1959.

Exchange Rate Measurements Based on Line-Shape Analysis (DNMR Dynamic Nuclear Magnetic Resonance). Under the measurable exchange rate conditions, two possibilities have been considered ... 

The development of nuclear magnetic resonance spectroscopy for the measurement of the rates of fast reactions (preexchange lifetimes 1-0.001 second) has made it possible to study many alkyl-metal exchange processes which heretofore were experimentally inaccessible. A substantial number of papers dealing with the exchange reactions of Group I, II, and III... 

The techniques for measuring exchange rates by nuclear magnetic resonance spectroscopy have been well documented in the leading texts in this field (37, 71, 118). 

An examination of the structure of lanthanide(III) ion complexes in solution using nuclear magnetic resonance is made in considerable detail. The importance of hydration in both the inner and outer spheres is stressed. The structural data are then used in an attempt to understand the stability constants and the rates of exchange of bound ligands. Fluctional properties are also analysed. Finally, the importance of the structures, the stability constants and the kinetic properties are related to the effects of Ln(III) ions, and through them Ca(II) ions, in biological systems. 

Fig. 10. Calculated nuclear magnetic resonance spectrum of two interacting spin 4 nuclei with a relative chemical shift 28 and a coupling strength J, plotted as a function of the deviation A of the frequency of the driving field from the average frequency of the two nuclei. The spectrum is symmetric around, 4 = 0 and only the positive half is shown. The spectra are for R = <7/8 = 1 and for several values of the exchange rate T = 1/rJ (Alexander, 1962).

Experimentally, water exchange rate constants are mainly determined from nuclear magnetic resonance measurements [6, 7]. Other techniques are restricted to very slow reactions (classical kinetic methods using isotopic substitution) or are indirect methods, such as ultrasound absorption, where the rate constants are estimated from complex-formation reactions with sulfate [3]. The microscopic nature of the mechanism of the exchange reaction is not directly accessible by experimental methods. In general, reaction mechanisms can be deduced by experimentally testing the sensitivity of the reaction rate to a variety of chemical and physical parameters such as temperature, pressure, or concentration. 

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