Spin diffusion suppression

ROESY Rotating frame NOESY, detection of NOE in the HH COSY format with suppressed spin-diffusion, detects closely spaced protons also in smaller molecules... 

The proton T relaxation time was determined as a function of temperature for samples with varying content of hydrophilic Aerosil (300 m g ) [7]. Diie to H spin-diffusion, only a single h relaxation time is usually measured in heterogeneous polymers [23]. The presence of Aerosil in PDMS suppresses the T minimum at 195 K, ascribed to the chain motion (a-relaxation) in unfilled PDMS, and leads to the appearance of a minimum at higher temperature, in the vicinity of 280 K as shown in Fig. 6. 

For an evaluation of these methods, it is important to analyze the effects of the delays on cross-relaxation and on the efficiency of Hartmann-Hahn transfer. In order to avoid spin diffusion effects, the effective crossrelaxation rate should be averaged to zero on a time scale that is short compared to the inverse cross-relaxation rates. In practice, this implies that the delays must be separated by less than about 10 ms (Bearden et al., 1988). This condition is excellently fulfilled by Methods C and D, and can also be fulfilled by Method B if the delays are introduced after only a few repetitions of the basis sequence. However, in most cases spin diffusion effects cannot be suppressed using Method A. 

We have found that the dependence of the linewidth on rotor speed for the aminoacid alanine is almost linear (Fig. 3). Although the residual homogeneous dipolar line broadening is the dominant resolution-limiting factor even at 70 kHz, a much more detailed study of the individual lines and spin relaxation properties becomes feasible. For example, a rotation speed dependent relaxation has been observed and discussed by Gil and Alberti [5]. A site selective study will be available due to suppression of spin diffusion. Figure 3 also shows that the width of the lines from the amine protons converges to a quite different value than that from the aliphatic and methyl protons (see below). 

CODEX experiments can be applied at very high spinning speeds, at which undesirable dipolar spin exchange may be suppressed, so that it does not interfere with the detection of motions on the second timescale. An up to 10-fold slow-down of the l3C spin diffusion has been demonstrated by CODEX at 28 kHz for unprotonated equivalent sites in several organic solids.27... 

Fig. 10.3.7 Pulse sequence for spin-diffusion imaging with ID spatial resolution [Wei8] and effect of mobility filters, (a) The magnetization source is selected by the dipolar filter which suppresses the magnetization in the sink. During the spin-diffiision time the magnetization dif ses from the source to the sink, (b) The dipolar filter selects magnetization from chain segments which are highly mobile and intermediately mobile. By use of a lineshape filter the signal loss is analysed only for the mobile components. IP(Tc) is the probability for a particular correlation time to arise in the sample. It is essentially the spectral density of motion.

As mentioned earlier, magic-angle sample spinning averages out the dipolar coupling and, therefore, slows down or suppresses spin diffusion (except for the special case of the rotational-resonance condition). However, in many applications it is crucial to use MAS in order to increase the resolution of the spectrum and to obtain well-resolved lines. The homonuclear dipolar... 

The frequency dependence of NMR spin-lattice relaxation is also a powerful tool in the study of spin dynamics in tlie conducting polymers, to reveal the microscopic dynamics of the charge carrier, polaron and/or conduction electron spins. However, one has to pay attention to one important point the localized paramagnetic spins and molecular motion which substantially contribute to the NMR 77]]. Since the relaxation caused by the spins diffusing is suppressed by a factor of D, the contribution due to the localized spins and the molecular motions could become dominant [6]. Therefore, interpretation of the NMR relaxation rate is generally useful but difficult. 

In most samples the H spin-lattice relaxation rates are exponential and identical for both hydrogen sites that are observed in the free induction decay. This result is probably a consequence of the rapid nuclear spin diffusion process of the H nuclear magnetization to the neighborhood of the Hj sites. Reimer et al. (l9Slc) have measured spin - lattice relaxation rates by a technique that suppresses the nuclear spin diffusion processes (the so-called T iy technique). In these measurements one observes a nonexponential decay, as would be expected because those hydrogen atoms closest to an Hj molecule will relax faster in the absence of rapid spin diffusion. These Tly experiments also indicate that the narrow H NMR line has a relatively faster relaxation rate, which indicates the presence of a greater density of H2 molecules in this phase than in the clustered (broad NMR line) phase. 

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