Spin diffusion, processes

In addition to measuring TCH for the polymorphic system in question, the proton T value must be determined since the repetition rate of a CP experiment is dependent upon the recovery of the proton magnetization. Common convention states that a delay time between successive pulses of 1-5 X T, must be used. Figure 10B outlines the pulse sequence for measuring the proton Tx through the carbon intensity. One advantage to solids NMR work is that a common proton Tx value will be measured, since protons communicate through a spin-diffusion process. An example of spectral results obtained from this pulse sequence is displayed in Fig. 12. 

A new 3D experiment that allows direct probing of proton spin-diffusion process between resolved proton spectra obtained under Lee-Goldburg homo-nuclear decoupling has been reported. It is anticipated that this class of experiments will soon be used to study poorly crystalline hybrid materials as well as finely divided or porous inorganic materials. 

ONP (Fig. 3) in organic molecular crystals can be described as a three-step process (1) optical creation of electronic spin polarization in (transient) excited triplet states (2) polarization transfer to nearby nuclear spins and (3) accumulation of bulk nuclear spin magnetization in the (diamagnetic) ground state via spin-diffusion processes. Highly detailed theoretical descriptions and modeling of ONP processes observed in many systems can be found in the reviews mentioned above thus, only a brief overview is provided here. 

The term spin diffusion has been coined by Bloembergen [1] to characterize the polarization-exchange process in a strongly dipolar-coupled many-spin system. As pointed out by Bloembergen, this process leads to a spatial spread of polarization originating on a given spin that mimics, under certain conditions, a diffusion process. In a true diffusion process, the entropy increases monotonically. In the exact quantum description of the spin-diffusion process, however, the entropy is conserved and the process is, in principle, fully reversible. 

There are two requirements for the use of multiple-pulse sequences to speed up the spin-diffusion process First, the scaling factor of the homonu-clear dipolar coupling should be as large as possible and, second, the effective spin lock has to be larger than the homonuclear dipolar interactions between the S-spins [19] to prevent loss of sum magnetization into dipolar order [15]. 

Fig. 4.8, Schematic representation of the spin-diffusion process by a wave-front in (a) a compound consisting of different domains, e.g., a polymer blend (b) a regular structure with long-range order (e.g., a crystal) and (c) a microscopically disordered compound. The resonance frequency is encoded into the density of the filling pattern and simultaneously into the direction of the long elliptical axis, symbolizing that it can be determined either by the isotropic shift or the orientation of the shift tensor. Quasi-equilibrium is reached in (a), if the wave has extended over a typical domain size in (b) after the spin-diffusion wave has reached the next neighbors and in (c) after the wave has sampled all possible orientations, leading to the typical pattern for amorphous compounds discussed below.

In this section 10.2, we review the various solid-state NMR methods used to investigate interpolymer interactions, molecular motion and the spatial structure of a polymer blend. An interaction between component polymers affects the chemical shifts and lineshapes (see Section 10.2.2.1) and the molecular motions of the component polymers (see Section 10.2.2.2). In Section 10.2.3.1, microheterogeneity from 2 to 50 nm is studied by measuring spin diffusion indirectly from its effects on H spin-lattice relaxation. The spin-diffusion processes can also be monitored by several methods based on the Goldman-Shen experiment [8] (see Section 10.2.3.2). Homonuclear and heteronuclear two-dimensional correlation experiments reveal how and to what extent component polymers interact with each other (see Section... 

Figure 8.18. Schematic illustration of the spin-diffusion process in which the original S-I NOE is efficiently relayed onto neighbouring nuclei and propagated throughout the molecule.

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. 

A number of 2D NMR experiments have been introduced in high-resolu-tion solid-state NMR studies, for example, to investigate chemical exchange processes (20), retrieve chemical shift anisotropies (21) and dipolar couplings (22) and to probe spin-diffusion processes (23). 

The efficiency of spin diffusion processes is reduced, leading to increased T2 and longer FID. This process also manifests as decreased crosspolarization efficiency with increasing temperature. 

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