Exchange spectroscopy, two-dimensional

The performance has been further developed [38] leading to improved pulse sequences for multidimensional solid state exchange NMR. In particular, the use 

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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... 

Two-dimensional exchange spectroscopy (2D EXSY) offers an enormous extension of accessible rates towards slow and very slow exchange processes. Extending the timescale is not the only merit of 2D EXSY methods before discussing some practical examples, there should be a few general remarks. The basic pulse sequence used is shown in scheme 4. 

Two-dimensional exchange spectroscopy might also be a valuable tool in the study of the metal skeleton. No example has been found in the literature. Preferably for nuclei with 100% natural abundance, homonuclear COSY or double-quantmn filtered (DQF)-COSY spectra can give the connectivity pattern of the different metal atoms in a cluster. This holds even, in some cases, for quadnipolar nuclei such as Co [21]. 

Fig. 17. Two-dimensional exchange spectroscopy (2D-EXSY) spectra of an aqueous solution containing TKCNls, T1(CN)4 , and HCN in a concentration ratio 0.021 0.018 0.166 M, enriched to 95% in C. Temperature = 25°C. Reprinted from Batta et al. (168). Copyright 1993 American Chemical Society, (a) C NMR spectrum, recorded at 100 MHz, mixing time = 0.035 s. The spin-spin coupling between C and H is observed because the proton exchange between HCN and bulk water is slow (309a). (b) T1 NMR spectrum, recorded at 231 MHz, mixing time = 0.020 s.

Fig. 8. Two-dimensional exchange spectroscopy (often called 2D-ELDOR) of the spin-labeled 3K-8 peptide with mixing time T = 296 nsec. Both the 2D surface and the contour map are shown. The peaks along the diagonal are related to the absorption spectrum of the spin label. The high-held M,= - line is weak because of experimental dead time artifacts. The cross-peaks, especially those between the outermost hyperfine lines, provide direct evidence of Heisenberg spin exchange. The cross-peak intensity can be used to determine the second-order rate constant for collisions between peptides.

Edwards MW, Daly JW, Myers CW (1988) Akaloids from a Panamanian poison frog, Dendrobates speciosus identification of pumiliotoxin-A and allopumiliotoxin class alkaloids, 3,5-disubstituted indolizidines, 5-substituted 8-methylindolizidines, and a 2-methyl-6-nonyl-4-hydroxypiperidine. J Nat Prod 51 1188-1197 Emsley L, Bodenhausen G (1989) Self-refocusing effect of 270° Gaussian pulses. Applications to selective two-dimensional exchange spectroscopy. J Magn Reson 82 211-221... 

Finally, Mildner and Freude [130] have studied the proton transfer between Bronsted acid sites in H - Y zeolites and adsorbed benzene molecules via two-dimensional exchange spectroscopy. This exchange can be considered to be the first elementary step of a Bronsted acid-catalysed reaction. The proton exchange rate turned out to depend on the strength of acidity of the Bronsted acid sites. In addition, activation energies of ca. 90-100kJ/mol could be observed for the proton transfer reaction by this method. 

Multipulse techniques have been used to identify exchange networks (i.e., routes of exchange in multisite exchange systems) with direct measurements of rate constants in solution, as in two-dimensional exchange spectroscopy (discussed in Chapter 5, Section 8), accordion spectroscopy (involving a third Fourier transformation, thus 3D spectroscopy), and time-dependent NOE. ... 

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