Relaxation chemical exchange effects

Aksnes, D.W., Hutchison, S.M. and Packer, K.J. (1968) Nuclear spin relaxation and chemical exchange effects in the 19F nuclear magnetic resonance spectrum of the hexafluoroniobate ion. Mol. Phys., 14, 301-309. 

Kroenke CD, Loria JP, Lee LK, Ranee M, Palmer AG (1998) Longitudinal and transverse H-l-N-15 dipolar N-15 chemical shift anisotropy relaxation interference unambiguous determination of rotational diffusion tensors and chemical exchange effects in biological macromolecules. J Am Chem Soc 120 7905-7915... 

See also Chemical Exchange Effects in NMR Fourier Transformation and Sampling Theory NMR Relaxation Rates NMR Spectrometers Nuclear Overhauser Effect Parameters in NMR Spectroscopy, Theory of. 

See also Chemical Exchange Effects in NMR Mac-romolecule-Ligand Interactions Studied By NMR Magnetic Resonance, Historical Perspective NMR Pulse Sequences NMR Relaxation Rates Nucleic... 

See also Chemical Exchange Effects in NMR Diffusion Studied Using NMR Spectroscopy Gas Phase Applications of NMR Spectroscopy Liquid Crystals and Liquid Crystai Solutions Studied By NMR MRI Applications, Clinicai NMR in Anisotropic Systems, Theory NMR Microscopy NMR Relaxation Rates Nuclear Overhauser Effect Polymer Applications of IR and Raman Spectroscopy. 

Woessner D E 1996 Relaxation effects of chemical exchange Encyclopedia of Nuclear Magnetic Resonance ed D M Grant and R K Harris (Chichester Wiley) pp 4018-28... 

We are interested in the effect of chemical exchange on line width, its usual manifestation. The total relaxation frequency contributing to line width is... 

Multidimensional and heteronuclear NMR techniques have revolutionised the use of NMR spectroscopy for the structure determination of organic molecules from small to complex. Multidimensional NMR also allows observation of forbidden multiple-quantum transitions and probing of slow dynamic processes, such as chemical exchange, cross-relaxation, transient Over-hauser effects, and spin-diffusion in solids. 

In the context of NMR, chemical exchange refers to any process in which a nucleus exchanges between two or more environments in which its NMR parameters (chemical shift, scalar or dipolar coupling, relaxation) differ. The effect of this exchange process on the appearance of the NMR spectrum depends on the rate of exchange relative to the mag-... 

Alternatively, the much more common situation in trNOE studies involves fast exchange on the chemical shift time scale where the observed resonance shifts are weighted averages of the corresponding shift in the free and bound state [13]. A full account of the complete relaxation matrix and conformational exchange effects for n spins has been performed by London et al. [13], and a similar treatment was later incorporated into the programme CORCEMA [14]. 

In many of these descriptions of lineshapes, chemical exchange line-shapes are treated as a unique phenomenon, rather than simply another example of relaxation effects on lineshapes. This is especially true for line-shapes in the intermediate time scale, where severe broadening or overlapping of lines may occur. The complete picture of exchange lineshapes can be somewhat simplified, following Reeves and Shaw [13], who showed that for two sites, the lineshape at coalescence can always be described by two NMR lines. This fact can be exploited to produce a clarified picture of exchange effects on lineshapes and to formulate a new method for the calculation of exchange lineshapes [16, 23]. This method makes use of the fact that lineshapes, even near coalescence, retain Lorentzian characteristics [13] (fig. 3). These lines, or coherences, are each defined by an intensity, phase, position, and linewidth, and for each line in the spectrum, the contribution of that particular line to the overall free induction decay (FID) or spectrum can be calculated. 

Various theoretical formalisms have been used to describe chemical exchange lineshapes. The earliest descriptions involved an extension of the Bloch equations to include the effects of exchange [1, 2, 12]. The Bloch equations formalism can be modified to include multi-site cases, and the effects of first-order scalar coupling [3, 13, 24]. As chemical exchange is merely a special case of general relaxation theories, it may be compre-... 

Selective inversion experiments for the determination of slow exchange rates are analogous to the saturation-transfer method in that they involve selective manipulation of one of the exchanging sites, while observing the subsequent effect on the second site as a function of time [48, 69, 70]. Chemical exchange, if present, will provide an alternative route to normal spin relaxation processes which a spin system undergoes, if perturbed at the start of an experiment. The rate of relaxation will depend on both the exchange rate k and the spin-lattice relaxation rate (Ti) (fig. 5). 

In the laboratory frame of reference, the net effects of the chemical exchange and the cross-relaxation are indistinguishable in the spin-diffusion regime. The individual contributions of each cannot be determined from a single exchange experiment. 

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