Stimulated-echo experiments

This experiment has two limitations (a) nuclei with short T2 are difficult to be detected and (b) multiplets produce a distorted signal phase after n pulse which inverts the coupled spin states. Both limitations are partially overcome by the stimulated echo experiment (STE).64,65 The main difference with the experiment previously described is that the echo attenuation due to the diffusion competes with Tx rather than with T2. The equation analogous to Equation (13) is now ... 

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... 

A simple interpretation of the stimulated echo experiments performed in highly viscous liquids is possible by using models developed by Ivanov [90,91 ] and Anderson [92], in which the molecular reorientation is considered as a... 

RW simulations were performed to demonstrate that the use of suited inter-pulse delays tp in 2H NMR solid- and stimulated-echo experiments allows one to resolve spatially highly restricted reorientations.76 For this purpose, it was assumed that C-2H bonds (t] — 0) show rotational random jumps (cf Fig. 25) where all possible... 

The distributions of correlation times G lnt ) characterizing the /1-process are very broad so that their widths exceed the time window of the stimulated-echo experiment. As a consequence, straightforward determination of the time constants of the dynamics from this experiment is not possible, but elaborate analysis of the... 

Fig. 15. Effects of small-amplitude reorientation on 2H NMR stimulated-echo experiments, as calculated by means of RW simulations. The C-2H bonds perform rotational random jumps on the surface of a cone with a full opening angle % = 6°, which are governed by a broad logarithmic Gaussian distribution of correlation times G(lgr) (a = 2.3). (a) Correlation functions m tp — 30 is) for the indicated mean logarithmic time constants lgr 1. The calculated data are damped by an exponential decay, exp[—(tm/rso)] with rSD = 1 s, so as to mimic effects due to spin diffusion. The dotted lines are fits with Fcos(tm tp) = (1—C) expHtm/t/l + Qexp[—Om/rso)]- (b) Amplitude of the decays, 1-C,p, for various t resulting from these fits. The dotted line is the value of the integral in Eq. (12) as a function of rm. (Adapted from Ref. 76).

Using selective labeling, 2H NMR revealed for the mixed glass chlorobenzene (CB)/decalin (DEC) that both components participate very similarly in the /i-process of this binary glass. This is evident from Fig. 34, where 2H NMR spectra of CB-d5/DEC and CB/DEC-d18 are compared for various temperatures.56 For both components, we see a tp dependence of the line shape, which resembles that found for neat type B systems, implying that the molecular dynamics involved in the [i-process are similar. Specifically, comparison with findings for TOL, suggests that the majority of CB and DEC molecules show reorientation with a typical amplitude of 4-5°. This is confirmed by 2H NMR stimulated-echo experiments (see Fig. 21). 

Pulse Sequence. The stimulated echo experiment (Figure 27) with pi = p2 = p3 = 90°, is recommended by... 

Pulsed-field-gradient stimulated-echo experiments were used to determine the velocity distribution during flowing experiments. An inverse problem was formulated and solved to determine estimates of the permeability distribution. 

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