Exchange NMR

It is also possible to measme the rate of enolization by isotopic exchange. NMR spectroscopy provides a very convenient method for following hydrogen-deuterium exchange, and this is now the preferred method. Data for several ketones are given in... 

Fig. 3a and b. Fast exchange-slow exchange NMR transition for the conformational interconversion of octamethyltetrasiloxane. a, MAS 13C-NMR solid state spectra on the left side in comparison to solution spectra in propane-di on the right side (at 75.47 MHz), b. MAS 29Si-NMR spectra at 59.63 MHz. Temperatures are indicated in K, shift positions refer to TMS = 0 ppm and correspond to the scale at the bottom. (Ref. I0))... 

Ribonuclease H is a small, single domain protein that eleaves RNA from RNA-DNA hybrids. RNase H from E. coli T = 66 °C) has been structurally studied by NMR spectroscopy.Hydrogen exchange NMR experiments have been used to examine the structural distribution of stability in RNase H from T. thermophiliis = 86 °C) and to compare its stability with that of the homologous RNase H from the mesophilic E. The general distribution... 

For PIB the apparent activation energy found for the structural relaxation time in the NSE window is almost twice that determined by NMR [136] (see Fig. 4.9 [125]). For aPP, the temperature dependence of NMR results [138] seems, however, to be quite compatible with that of the NSE data nevertheless, 2D exchange NMR studies on this polymer [139] reveal a steeper dependence. This can be seen in Fig. 4.11 [ 126]. 

Physically, dv appears as a broadening of the spectral lines. It has been found that the study of NMR band shapes provides a powerful tool for the measurement of proton-transfer rates through the determination of state lifetimes, related to St. Nuclei such as H and have been studied. This allows the simultaneous determination of different rates of proton exchange. NMR methods are particularly valuable for degenerate equilibria, for which AWJ. and AVj are useless. 

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). 

In fact, two-dimensional solid-state NMR is far superior to one-dimensional techniques for studying structure and dynamics123. In particular, the two-dimensional exchange NMR spectrum124 of a static sample is identical with a two-time distribution function125. Thus a two-dimensional NMR spectrum, which is detected for a fixed mixing time, is an image of the state of the dynamic process under study at that time. 

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,... 

Fig.117 Stacked plots of 13C exchange NMR spectra taken with tm = 50 ms. a T= - 40 °C. b T = 60 °C. c Simulated diagonal powder spectrum corresponding to a. b Simulated spectrum of b obtained by considering rotation around the 033 direction with a root-mean-square amplitude of 12° (from [77])...

Fig. 7. The mechanism of the Cope rearrangement in cyanobullvalene as deduced from two-dimensional MAS 13C exchange NMR spectra in reference 13.

Four-dimensional exchange NMR was introduced some time ago for examining the correlations between different molecular motions.53 The... 

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). 

The X-bond O-D—O, 2D deuteron exchange NMR spectrum in DRADA with x = 0 32 at 40 K is shown in Figure 3 at mixing times fmiX = 10 and 30 s. Cross-peaks... 

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

Figure 3 2D O-D- O deuteron exchange NMR spectra of DRADA-32 at T= 40 K for fmix = 10 and 30 s. In addition to the diagonal peaks, the O-D- 0<->0- D-O exchange cross-peaks are clearly visible.

D deuteron exchange NMR and 87Rb SLR time measurements in (DR ADA-32) PGs show that the 0-H---0 dipoles are not completely frozen out at low temperatures but show dynamic features characteristic of incoherent tunnelling. RADP and D-RADP are thus quantum glasses. A comparison of W 1 SLR rates show that minima occur at 90 K for D-RADP-42, whereas at 25 K for RADP-50, thus demonstrating the tremendous isotope effect in the intra-H-bond dynamics. 

It is not clear, whether the experimentally observed random local freezing of the deuterons in the O-D—O bonds in deuteron glasses corresponds to a true thermodynamic phase transition or whether one deals with a dynamic phenomenon which only seems static because of the finite observation time of the experimental techniques. The recently observed42 splitting between the field-cooled and zero-field dielectric susceptibilities below an instability temperature Tf seems to speak for the occurrence of an Almeida-Thouless-like thermodynamic phase transition in deuteron glasses. It is well known that ID NMR and EPR allow a direct measurement of the Edwards-Anderson order parameter qEA only on time scales of 10 3-10 8 s and 2D exchange NMR possibly seems to be a better technique for such slow motions. 

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