2D exchange NMR experiments

The molecular motions underlying the dynamic mechanical and dielectric f3 transition in PMMA have been studied in detail [77] by using the 2D exchange NMR experiment. This detects slow reorientations that occur during a mixing time, fm, by measuring the angular-dependent NMR frequencies (expressed in ppm) before and after tm. The 2D frequency spectrum S( >i,... 

The time-consuming 2D experiment can be replaced by ID selective inver-sion-recovery/saturation-transfer experiments [119]. Its application to study the heterogeneity of PS/PVME is mentioned afterwards (see Section 10.3.1.1). Furthermore, Campbell and VanderHart [109] showed that the selective techniques under multiple-pulse homonuclear decoupling are not necessary. They realized that at certain t value in a 2D exchange NMR experiment, polarization gradient necessary for spin diffusion can be achieved by the chemical shift difference. The optimum preparation period for the ID analogue of the 2D exchange NMR experiment was discussed. 

We note that the just described pulse sequence is identical to the 2D exchange NMR experiment of solution NMR, which is an isotropic-shift exchange experiment. The same sequence is also used in 2D nuclear Overhauser effect spectroscopy (NOESY) NMR in solution and used extensively in the elucidation of the stmcture of biomacromolecules in solution. There, however, the exchange is based on incoherent magnetization transfer by incoherent cross-relaxation. ... 

In view of the fact that specific heat measurements40 show the presence of dynamic disorder and possible quantum tunnelling at low temperatures. To check this hypothesis, Blinc et al.41 have performed a site-specific search for this motion. Deuteron intra-H-bond motion seemed to be the most likely candidate for this kind of dynamic disorder. Blinc et al.41 have measured 87Rb, deuteron SLR time at low temperatures, as well as performed a 2D deuteron exchange NMR experiment on the O-D- O deuterons in D-RADP and Rbo.68(ND4)o.32D2As04 (DRADA-32). 

Figure 2 Radio frequency pulse sequence for a 2D deuteron exchange NMR experiment.

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]. 

The basic principle of 2D exchange NMR involves the measurement of the frequency of the same molecular segment at two different times. A slow dynamic process is detected on account of the change, during a mixing time between the two evolution periods, in the NMR frequency caused by a reorientation of the molecular segment In this section, we describe static and MAS 2D exchange experiments [4]. 

The ultraslow motions can be observed by exchange NMR experiments. There are a number of techniques, performed in ID and 2D modes which are based on the manipulation of spinning sidebands under MAS. The first of them, applied to study of solid peptides was ODESSA (one-dimensional exchange spectroscopy by sideband-alternation) [76] and next the time-reversed ODESSA [77] (tr-ODESSA). The interesting modification, which represents this group of sequences is CODEX (centreband-only detection of exchange) introduced by Schmidt-Rohr and coworkers [78]. CODEX can be executed at any MAS speed, while ODESSA and tr-ODESSA... 

Jenkins et al. used high-resolution H MAS NMR in combination with 2D exchange NOESY and PFG NMR diffusion experiments to characterize the methanol swollen polymer anion-exchange membranes. High-resolution MAS NMR observed resonances from water and methanol in both free environment and membrane-associated environment within the anion-exchange membranes. H high-resolution MAS PFG NMR experiments identified different molecular diffusion environments in the solvent, while H 2D NOESY NMR experiments confirmed spatial contacts between membrane-associated species and the membrane [186]. 

Chelating catalyst-substrate complexes interconvert in solution at sufficiently high temperatures. This exchange can be either intermolecular (with complete dissociation of the substrate) or intramolecular (when only double bond dissociates). Most conveniently, these two mechanisms of exchange can be distinguished via 2D EXSY NMR experiments. ... 

The experiment was performed on a glass-forming discotic liquid crystal (2S, 3S)-2-chloro-3-methyl-pentanoyloxi-pentakis (pentyloxi)-triphenylene [9.7]. The triphenylene cores are stacked to form a discotic columnar phase. Several mechanisms can lead to axial motions around the columnar axis and 2D-exchange NMR can be used to distinguish these mechanisms. Figure 9.6... 

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