Swift and Connick

In a classical paper. Swift and Connick (34,35) derived solutions of the equation for transverse relaxation, I/T2, and chemical shift, Am, in the case of dilute solutions of paramagnetic ions. Equation (9) gives the increase in transverse relaxation of the bulk water signal, l/72r, due to exchange with water bound to a paramagnetic ion and normalized by the mole fraction of bound water. Pm. 

In most cases only a part of the full curves, as shown in Fig. 6, can be observed experimentally because the temperature range is restricted by the freezing and evaporation of the water solvent. Nevertheless, very often, exchange rate constants can be calculated. In the four temperature regions, the Swift and Connick equations can be approximated by the following expressions ... 

Fig. 6. Temperature variations of observed 0 transverse relaxation rates (a), reduced transverse relaxation rates (b) and reduced chemical shifts (c) calculated from Swift and Connick equations for Bq = 14.1 T (largest effect), 4.7 T, and 1.4 T (smallest effect).

Fig. 9. Variable temperature 0 NMR results (Bq = 14.1 T) for solutions containing Tb(C104)3 (0,DX Mg(C104)2 and Tb(C104)3 ( , ), and Mg(C104)2 and Mn(C104)2 (relaxation agent) (A). Full lines result from non-linear fitting using Kubo-Sack formalism and short-dashed lines were calculated by an approximate 3-site Swift and Connick method [Ref. (40)].

The effect of interconversion between the planar and tetrahedral isomers on the NMR spectrum can be described by extension of the analysis for exchange between two diamagnetic sites (149). It should be noted that the well-known analysis of Swift and Connick (146, 148) is not directly applicable, because the paramagnetic site is not dilute, as it is in the case of solvent exchange on a paramagnetic metal ion. 

A detailed discussion of the contributions from different relaxation mechanisms to the observed relaxation rates in solutions of paramagnetic metal ions was presented by Swift and Connick (104). Pfeifer (92) considered the situation in solutions of paramagnetic hemoproteins, and Koenig... 

The original work of Swift and Connick [82] paved the way for modern applications of this NMR method in paramagnetic contrast agent research. The Lausanne group of Merbach and co-workers have developed elegant applications of the method [83]. Most frequently, the difference between the transverse relaxation rates of 170 in reference buffer (1/T2A) and in solutions containing the BPCA (1/T2) are measured as a function of temperature. The analysis fits the temperature dependence of the reduced transverse relaxation rate, 1/T2R, defined as ... 

It will be apparent that any attempt to elucidate the mechanism of substitution at labile metal ions must involve itself in a consideration of the kinetics of the solvent exchange process. Since the pioneering work of Swift and Connick, the n.m.r. line-broadening and pulse techniques have been used more and more frequently for this purpose. 

Since the pioneering work of Taube and co-workers, shortly followed by Plane and Hunt, and Swift and Connick, one of the most widely studied reactions for bansition-metal complexes has been that of water solvent exchange, using either OHj or... 

The residence time of the coordinated water tm the mechanism of IS relaxation is based on an exchange between bnlk water molecules surrounding the complex and the water molecule(s) coordinated to the lanthanide. Consequently, the exchange rate ( ex = 1 /tm) is an essential parameter for transmitting the relaxing effect to the solvent. Its measurement is based on the works of Swift and Connick for diluted paramagnetic solutions and consists of an analysis of the transverse relaxation rate as a function of temperature. 

The onset of slow exchange may have some rather unexpected effects on the temperature and frequency dependencies of lanthanide induced shifts. The possibility that such effects are present should always be kept in mind particularly when departures from what is believed to be a regular behavior are observed. It can be shown, (Swift and Connick, 1962), that when Tb T2b the induced shift S is given by... 

Swift and Connick first developed a generally applicable equation for the line shape v((w) of resonance A of mole fraction in the presence of a small fraction Pb of paramagnetic species B ... 

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