Exchange phenomena

Changes in nuclear resonance characteristics due to exchange processes may be described fairly accurately. Consider a hypothetical molecule in which there are two proton environments, each containing equal numbers of hydrogen atoms. The H1 resonance spectrum (assume Ahahb— 0) will then consist of two peaks of equal intensity separated by some chemical shift va°—vi° 

The transition region between slow and fast exchange has been 

From Eq. (28), a relation between the chemical shifts and the lifetimes may be derived  

These equations are identical if the lifetimes in environments A and B are equal. and T are the transverse relaxation times in the absence of exchange and the presence of exchange, respectively, and are inversely related to the line widths. 

Use of Eqs. (32), (33), or (34) for determination of r requires an independent determination of TVs, the transverse relaxation times in the absence of exchange. 

The difference between CNDO and INDO is best understood in relation to the phenomenon of exchange, which we briefly describe here. 

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Dirac, P. A. M., 1930, Note on Exchange Phenomena in the Thomas Atom , Proc. Camb. Phil. Soc., 26, 376. Dirac, P. A. M, 1958, The Principles of Quantum Mechanics, 4th edition, Clarendon Press, Oxford. 

Dirac, P. A. M. 1930. Note on Exchange Phenomena in the Thomas Atom. Proc. Cambridge Philos. Soc. 26, 376. 

The phenomenological equations proposed by Felix Bloch in 19462 have had a profound effect on the development of magnetic resonance, both ESR and NMR, on the ways in which the experiments are described (particularly in NMR), and on the analysis of line widths and saturation behavior. Here we will describe the phenomenological model, derive the Bloch equations and solve them for steady-state conditions. We will also show how the Bloch equations can be extended to treat inter- and intramolecular exchange phenomena and give examples of applications. 

Gallo, C. F., and Lama, W. L., Some charge exchange phenomena explained by a classical model of the work function, J. Electrostatics, 2 145-150 (1976)... 

Most previous adsorption studies employing calorimetry have investigated cation exchange phenomena on zirconium phosphates (17-21), titanium phosphate (22) and clay minerals (23,24). 

O.A.G.J. van der Houwen, R.H.A. Sorel, A. Hulshoff, J. Teeuwsen, and A.W.M. Indemans, Ion-exchange phenomena and concomitant pH shifts on the equilibration of reversed-phase stationary phase materials with ion-pairing reagents , J. Chromatogr., 1981,209, 393. 

While the terminology scalar relaxation of the first kind concerns J modulation (J spin spin or scalar or indirect coupling constant) by exchange phenomena, the usual example of the second kind is a spin 1/2 nucleus (7), J coupled to a fast relaxing quadrupolar nucleus (S) with relaxation times Tj and Tf (and spin number /s)- The relevant interaction is of the form... 

The second role of the chemical exchange phenomena can be seen in Eq. (2) the exchange lifetime competes with the in-complex nuclear spin-lattice relaxation time and can become a limiting factor in the attainable PRE. This aspect of the problem is highly relevant in practical consideration in the case of Gd(III) complexes as a potential contrast agent, because the water exchange in these systems is not too fast. This issue is considered to be outside of the scope of this article and we refer to recent literature on the subject 5,160) and to other contributions in this volume. 

Nmr spectroscopic methods of determination of sites, of protonation may be divided into two types, depending on the kind of phenomena that are observed upon protonation. The first type is concerned with the spectral changes (chemical shifts, coupling constants) in the skeleton of the molecule Z caused by protonation the second is concerned with spectral changes at the protonation sites, X or Y (proton exchange phenomena, coupling to the captured proton, and the observation of the resonance of the captured proton itself). 

Selected entries from Methods in Enzymology [vol, page(s)] Absorption spectroscopy, 24, 3 flash kinetic spectrophotometry, 24, 25 ion transport (H+, K+, exchange phenomena), 24,... 

A number of 2D NMR experiments have been developed which allow the study of slow exchange phenomena. The most common (Exchange Spectroscopy, EXSY), is based on the standard pulse sequence 90°x-fi-90°x-tm-90°x - FID(t2), where is an evolution delay, is the mixing time, and tj is the detection period [175, 176]. 

Numerous experimental and theoretical studies of problems in this field have appeared and these have been reviewed under the title Metal Com-plexing by Polyphosphates by van Wazer (342). The phenomena which have been mentioned may be considered in general as due to complex formation. However, a simple qualitative and quantitative description of the facts may also be obtained if solutions of high-molecular polyphosphates arc considered as micro-heterogeneous systems and if the bonding of polyvalent ions is interpreted as involving ion exchange phenomena on the polyphosphate chains (77, 313, 319, 324). 

For each nuclide studied, the sorption distribution coefficients appeared to result from a minimum of two separate mechanisms. In all cases, one mechanism appears to be an ion-exchange phenomena associated with the silicate phases and appears to have a relatively much larger sorption capacity than the other mechanism. In the case of cesium (and probably rubidium) the second mechanism appears to also be related to the silicate phases and may or may not be an ion-exchange phenomena. However, for the other elements studied, the second mechanism appears to be related to the hydrous iron and manganese oxides and again may or may not be an ion-exchange process. 

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