EXSY

ID and 2D magnetisation exchange experiments have found greatest use in the study of inorganic and organometaUic systems, for which a high degree of fluxionahty often exists, and extensive reviews of these areas have been presented in which a wide variety of example applications may be seen [52,54,66,67]. These methods have also found application to the study of supramolecular complexes [68]. 

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The tenn slow in this case means that the exchange rate is much smaller than the frequency differences in the spectrum, so the lines in the spectrum are not significantly broadened. Flowever, the exchange rate is still comparable with the spin-lattice relaxation times in the system. Exchange, which has many mathematical similarities to dipolar relaxation, can be observed in a NOESY-type experiment (sometimes called EXSY). The rates are measured from a series of EXSY spectra, or by perfonning modified spin-lattice relaxation experiments, such as those pioneered by Floflfman and Eorsen [20]. 

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

Figure 6.7 The 3D EXSY-EXSY spectrum of heptamethylbenzenonium sulfate in H.2SO4 representing o i = o)j reflection symmetry. Cross-peaks lying on identical planes (coi, a s) are connected by lines. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al, 14, copyright (1989), with permission from Academic Press, Inc.)...

Fig. 5.3.9 Hp-129Xe 2D EXSY NMR spectra frequency discrimination (states) with 64 scans recorded during combustion for various ex- per spectrum. The average experimental time change times. Adapted from Ref. [2], The per EXSY with 0.5-s recycle delay was 40—50...

D H- H EXSY NMR spectroscopy (EXSY - exchange spectroscopy) was used for the determination of the rotational barrier about the C(2)-Ph bond in 2-phenyl-l-boraadamantane trimethylamine adduct 16 (Figure 5) Eact= 14.0 0.6kcalmoF1, lnd = 28 0.6, AG = 14.5 0.7 kcal mol-1 <2000IZV497>. 

In addition to X-ray crystallographic studies, two-dimensional NMR solution experiments (i.e., COSY, 1D-NOE, and NOESY, discussed in Sections 3.5.9 and 3.5.10) have been carried out on many lanthanide(III), Ln(ffl), chelate complexes to confirm that the structure of the MRI imaging agent, used in aqueous solution, will correspond to the solid-state X-ray crystallographic structure. Two-dimensional exchange spectroscopy (2D-EXSY) has been applied to lanthanide chelates to study the dynamics of conformational equilibria (how acetate arms chelate and how... 

Determination of reaction mechanisms by combining the observed intermediates in a catalytic cycle. To do this, it is often necessary to measure under different conditions - that is, variable temperature NMR. The use of high-pressure NMR cells is crucial in order to measure under the real catalytic conditions. The EXSY experiment helps to unravel exchange pathways, both intra-and intermolecular. 

By using EXSY measurements, it was shown that at 323 K there is only intramolecular exchange, whereas at 348 K intermolecular exchange also occurs via complete dissociation of the complex, producing a free substrate and a solvate complex (Fig. 11.9). 

Fig. 12. A portion of the 500 MHz DEFT-NOESY spectrum of NPl-ImH, recorded at 30°C in 50 mM phosphate buffer, pH 7.0, D2O. Note the strong EXSY cross peak between the 3-Me signal at 24.9 ppm and a minor 3-Me signal at 26.4 ppm. This is not the other heme orientational isomer, as is evident by comparison of spectrum (a) of Fig. 11. This chemical exchange is also observed for many protein side chains, suggesting fluxionality of some loop, probably the A-B loop that passes close to the heme. In the text, the small peak is defined as species Y, and the large as species X. ...

Although the kinetics of imidazole exchange between NP2 and its low-spin imidazole adduct are very slow and no chemical exchange cross peaks in the NOESY/EXSY spectrum of a mixture of the high-spin and low-spin forms of NP2 are observed, the V-methylimidazole complex... 

FIGURE 28. Two-dimensional natural abundance N solid-state EXSY spectmm of the dUithium-naphthalenediide(TMEDA)2 complex at 292... 

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]. 

Perutz and coworkers [180] have used 2D EXSY to study the dynamic behavior of RhCp(PMe3)(q -naphthalene), 109, which is thought to be a model intermediate for the oxidative addition of arenes to a metal center. In this complex, there are two processes taking place. The first involves an equilibrium between the -naphthalene complex, 109, and the naphthyl hydride complex, 110. The second process involves an intramolecular [l,3]-shift which moves the coordination site of the naphthalene ring from one side of the ring to the other (Scheme 1.12). 

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