Stimulated echo experiments reorientation

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... 

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). 

To measure the longitudinal relaxation time Ti, an inversion or saturation pulse is applied, followed, after a variable time T, by a two-pulse echo experiment for detection (Fig. 5b). The inversion or saturation pulse induces a large change of the echo amplitude for T < T. With increasing T, the echo amphtude recovers to its equilibriiun value with time constant Ti. The echo amphtude of the stimulated echo (Fig. 5c) decays with time constant T2 when the interpulse delay T is incremented, and with the stimulated-echo decay time constant Tse < T1 when the interpulse delay T is incremented. A faster decay, compared to inversion or saturation recovery experiments, can arise from spectral diffusion, because of a change of the resonance frequency for the observed spins, of the order of Av = 1/t on the time scale of T. Quantitative analysis of spectral diffusion can provide information on the reorientation dynamics of the paramagnetic centers. 

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