Two-dimensional exchange spectroscopy 2D-EXSY

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. 

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.

The effect of exchange processes can be observed in two-dimensional spectrum and be analyzed in a very similar way. The 2D exchange spectroscopy (EXSY) is in principle, identical to the NOESY experiment. Cross peaks in 2D EXSY experiments arise from noncoherent magnetization transfer between sites with different resonances. Noncoherent magnetization transfer takes place either by exchange of nuclei between different sites or by cross-relaxation (NOE). However, the mixing time in EXSY is usually chosen to be shorter,... 

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