Shift correlation chemical exchange

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

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) ]. 

The most recent developments in 2D NMR of solids are the heteronuclear chemical shift correlation spectroscopy (421), 2D exchange NMR, which enables very slow molecular reorientations to be monitored (422), and heteronuclear. /-resolved 2D NMR (423). 

Proton chemical shifts are very valuable for the determination of structures, but to use the shifts in this way we must know something about the correlations that exist between chemical shift and structural environment of protons in organic compounds. The most important effects arise from differences in electronegativity, types of carbon bonding, hydrogen bonding, and chemical exchange. 

A 2D CPMAS exchange experiment in which through-space (site) correlation is established via proton spin diffusion was proposed by Wilhelm et al.292 for probing the isotropic chemical shift correlations. This technique has a number of advantages, including site selectivity, multiple correlations and broad spatial correlation range (1-200 nm). It was shown that this technique... 

In the characterization of multidimensional spectroscopy by Ernst et al. (1987), the different classes may be termed separation, correlation and exchange respectively. The DOSY and MOSY experiments represent a type of separation spectroscopy in which the independent chemical shift and mobility information can be independently displayed. In the next section we describe an example involving exchange spectroscopy. 

The chemical exchange processes can be slowed down by HB the N-H groups of the purines to other molecules or by decreasing the temperature of the system. Under these special conditions, the values of the coupling constants can be determined and the scalar interactions used for polarization transfer in chemical-shift correlation experiments (e.g., nucleic acids). 

The two frequency axes may consist of a diverse assortment of pairs of fundamental NMR parameters. Examples might include chemical shift on one axis and a frequency axis for scalar couplings on the second as in the 2D /-resolved NMR experiments. Both axes may be proton chemical shift, in which responses may be correlated by scalar (/) couphng as in the COSY experiment [46—48], by dipolar relaxation pathways as in the NOESY [35, 36, 49—51] and ROESY [35, 36, 52, 53] experiments, or by chemical exchange pathways as in the EXSY experiment [54—59]. Other examples may involve chemical shift on one axis and a multiple quantum frequency on the second axis. Examples here would include proton double [60 62] and zero quantum spectroscopy [63—67], C—INADEQUATE [68, 69], etc. The available axes in a 2D NMR experiment may also be used for hetero-nuclear chemical shift correlation, e.g. H—or H— N, where the respective nucHde pairs are correlated via their one-bond ( /xh) or multiple bond ("/xh) hetero-nuclear couphngs [14, 16, 17, 23—27, 29—31, 70—72]. 

A comparison of a spectrum as a reference with that of an unknown specimen under investigation may be used to confirm the identity of a compound and to detect the presence of impurities that generate extraneous signals. Several special techniques (double resonance, chemical exchange, use of shift reagents, two-dimensional analysis, etc.) are available to simplify some of more complex spectra to identify certain functional groups and to determine coupling correlations. 

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