Suppression of spin diffusion

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). 

In the preceding section, we discussed methods of suppressing dipolar interactions as a means of eliminating spin diffusion in relaxation measurements, but dipolar interactions are also the principal source of line-broadening in solid-state spectra, and therefore suppression of dipolar interactions also offers the possibility of obtaining high resolution spectra of solids in which chemical shift information is discernible. It is this, rather than the suppression of spin diffusion that has been the motivation in the development of these techniques, particularly the multiple pulse sequences. 

As ultrafast MAS does not completely averages out the effects of multispin dipolar coupHngs in dense H networks, H NOESY-like spectra can be recorded to obtain structural information, using spin diffusion to mediate the magnetization transfer (Fig. 3.10 for an example with the P-L-Asp-L-Ala dipeptide). This is especially useful when the sample contains mobile molecules (e.g., hosted in mesoporous compounds) as spin diffiision and/or NOE transfers are stiU active and provide information about atomic proximities in the absence of recoupling. This has been illustrated at 40 kHz MAS for zeolites and surfactants for enhanced resolution [126]. Moreover, combining deuteration and ultrafast MAS suppresses the spin diffusion between protons and open a new route to observe chemical exchange between H in proteins [127]. 

As mentioned in Sec. II.D.3, the NOE is also a consideration for quantitative analysis. The use of decoupler gating ensures that the NOE will be suppressed, but it is not always necessary, or even desirable, to eliminate the S/N enhancement that the NOE affords. If the molecules in the sample move relatively slowly (as do polymers in solution), and if all or most of the X nuclei are directly bonded to Y s (e.g., if most carbons are protonated), then most NOE s tend to be equivalent. This happens because of spin diffusion, a process by which magnetization migrates to neighboring spins. Correct relative peak areas are maintained for most peaks, with a significant decrease in the experiment time. Some analogous concerns apply to quantitation in solid-state NMR these will be discussed in Sec. II.E.2. 

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. 

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.

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