WISE experiment

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

Among several indicators of motional state of organic solids H linewidths has been used from very beginning of solid-state NMR. However, inherently low resolution of H MAS NMR spectra caused by the very strong homonuclear interactions and relatively low MAS speed lead to a very limited application of H MAS NMR for studies of mobility.1 A 2D wideline separation (WISE) experiment correlates carbon chemical shifts recorded under MAS with broad H lines.1,24 As a result a broad ll lines are separated in the components corresponding to the 13C sites in their close proximity. For mobile domains in a sample, the H - ll coupling are relatively weak which leads to relatively narrow 11 static lines. The opposite is observed for the rigid domains. 

While the (heteronuclear and homonuclear) residual dipolar couplings have to be eliminated in such J-resolved experiments, they are the main source of information in the WISE experiment described in the next section. For the investigation of viscoelastic materials the experiment thus must be performed under static conditions or under slow MAS. 

The 2D WISE experiment [68, 71] is based on the simple pulse scheme presented in Figure 14.11. 

For elastomers and rubbery-like materials well above the T, the high molecular mobility reduces the dipolar couplings dramatically. The WISE experiment allows one to investigate site-selectively residual dipolar interactions and thus molecular dynamics by editing the corresponding proton slices of the 2D data set. 

Fig. 10. The pulse sequence for the WISE experiment.21 This two-dimensional experiment separates H wideline spectra according to the isotropic l3C chemical shift of the 13C each H is bonded to. An initial H 90° pulse creates transverse H magnetization that is allowed to evolve in t. A short cross-polarization step then transfers the remaining H magnetization to the nearest 13C spin, i.e. the bonded one. The resulting 13C transverse magnetization is then allowed to evolve in ti under magic-angle spinning, where an FID is recorded.

Studies of dynamics in wet and dry carbohydrates have also been performed. Slow motions of polysaccharide chains in native starch and retrograding starch gels were monitored with two-dimensional 2H exchange and stimulated echo experiments.117 The effects of hydration on the dynamics of /j-cyclodextrin polymers have been examined with 13C T and H T p measurements,118 and WISE experiments were used to probe the effects of hydration on polysaccharide chains in maize starch forms A and B.119... 

Pulse Sequence. The pulse sequence for WISE experiments can be considered as a derivative of the CP pulse sequence (Figure 49). After the initial (tt/2) pulse, the H magnetization is allowed to evolve during t. Subsequently, the remaining H magnetization is transferred via CP to C. The resulting C magnetization is then recorded in 12 The very short CP step (50 150 p,s) is essential to prevent H spin diffusion. 

The pulse sequence for the 2D WISE experiment (Fig. 20a) consists of three... 

Fig. 14. The results of the constant time J-WISE experiment of Fig. 13(b) applied to L-alanine.18 (a) The full two-dimensional spectrum showing 111 lineshapes (with sidebands due to dipolar coupling), (b) Slices through the f dimension...

Fig. 6.6.9. A stacked plot for a proton-fluorine WISE experiment for a eommercial Viton sample, with decoupling during the evolution time ti. Spectrometer operating conditions contact time 50 xs spin rate 10 kHz 128 points in t with 10 ps increments pulse power in both channels equivalent to 3 ps for 90° pulse angle recycle delay 3 s.

In this section, we will concern ourselves mainly with relaxation processes in solid polymer samples, as distinct from high-resolution NMR used for diagnosing the chemical structure of molecules. The latter can be done on solid samples using cross-polarization, magic-angle spinning (CP-MAS), but these techniques can eliminate valuable information on the very relaxation processes of interest to us. An interesting exception, however, is the WISE experiment, which is discussed below. 

Fig. 2. Traces along the proton dimension of 2D-WISE experiments performed on 10% hydrated (left) and 35% hydrated (right) onion cell wall material. The corresponding carbon resonances are given. Proton spectral widths of 150 and 70 kHz were used for the 10 and 35% hydrated samples respectively. Reprinted from Carbohydr. Res., Vol. 322(1-2), S. Hediger, L. Emsley and M. Ficher, Solid-state NMR characterization of hydration effects on polymer mobility in onion cell wall material , pp. 102-112, Copyright 1999, with permission from Elsevier Science.

At this early stage, a small portion of the mixture can be examined in a series of small batch-wise experiments. 

Fig. 9.28 A WISE experiment recorded for a core-shell system comprising mobile...

A series of ID and 2D NMR experiments performed on nanocomposites of hectorite, a smectite clay mineral, showed new opportunities for studying large assemblies of layered minerals with polymeric materials.In particular. rotational-echo double-resonance (REDOR) experiments afforded H- Si distances with precision, and a mobility gradient of intercalated polyethyleneoxide (PEG) segments was demonstrated in H-" Si wideline separation (WISE) experiments. 

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