Spin exchange/diffusion, solid-state

Further possibilities are opened up for investigation of fluoropolymers by two-dimensional methods. Thus, solid-state F COSY can be used to study potential spin exchange (see Ref. 17 for a nonpolymeric example), which may arise from chemical exchange (rare in polymeric systems, but conceivable for internal rotation of C—CF3 groups in asymmetric environments) or from spin diffusion. 

The preparation and the crystal structures of single crystals of alkali aromatic ion pairs are discussed. A close relationship was found between the ion-pair structures in solution and in the solid state. The physical properties of the pseudo-two-dimensional magnetic alkali biphenyl crystals are reviewed. The effect of spin diffusion manifests itself clearly in the line width and line shape of the exchange-narrowed electron spin resonance (ESR) line. The monoanions of cycloocta-tetraene, produced by X-ray irradiation, rotate rapidly about their eightfold axes. At 20 K this rotation is frozen, and an alternating spin density distribution is found around the ring. The equilibrium position of the monoanion is rotated by 22.5 compared with the equilibrium position of the dianion. 

In Chapter 1, Fyfe, Mueller, and Kokotailo describe the applications of solid-state NMR to the study of zeolite molecular sieve catalysts and related systems. Zeolites provide an apt arena in which to demonstrate the capabilities of modern techniques such as sample spinning, cross-polarization, and multidimensional correlation spectroscopy. In Chapter 2, Karger, and Pfeifer consider the question of molecular diffusion in catalyst systems. Applications of NMR techniques such as imaging, lineshape analysis, relaxation, pulsed field gradient echo spectroscopy, and NMR tracer exchange are described and compared with other, more traditional techniques such as radioactive tracing. In Chapter 3, Haw discusses the use of NMR to probe catalytic processes, showing how the combination of temperature control with novel NMR probes makes it possible to elucidate reaction mechanisms in situ. 

A number of two-dimensional NMR experiments have been introduced in high-resolution solid-state NMR studies, designed, for example, to investigate chemical exchange processes [64], to retrieve chemical shift anisotropies [65] and dipolar couplings [66], and to probe spin-diffusion proces.ses 67j. Opella has proposed an intemuclear distance-determined, spin-diffusion mechanism in molecular crystals [68], and Benn and coworkers have demonstrated C/ C connectivities using the INADEQUATE sequence for the plastic crystal camphor and have used the COSY sequence for Si/ Si connectivities in the reference molecule Q Ms [691. 

Spectral spin diffusion in the solid state involves simultaneous flipflop transitions of dipolar-coupled spins with different resonance frequencies 11,39,63-76], whereas spatial spin diffusion transports spin polarization between spatially separated equivalent spins. In this review we deal only with the first case. The interaction of spins undergoing spin diffusion with the proton reservoir provides compensation for the energy imbalance (extraneous spins mechanism) [68,70,73,74]. Spin diffusion results in an exchange of magnetization between the nuclei responsible for resolved NMR signals, which can be conveniently detected by observing the relevant cross-peaks in the 2D spin-diffusion spectrum [63-65]. This technique, formally analogous to the NOESY experiment in liquids, is already well established for solids and can also be applied to the study of catalysts. 

Before proceeding further with this section, we will emphasize some important points closely associated with the dynamic processes encountered in solid-state NMR. In the most general sense, relaxation, spin diffusion/exchange and polarization transfer can be regarded as different aspects of coherence transfer. They all involve mutual transformation among... 

Furthermore, with the advent of improved instriunentation and experimental techniques interesting in-situ investigations became possible which were related, for instance, to the synthesis of and heterogeneous catalysis on zeofites, catalyst deactivation, diffusion or solid-state ion exchange as well as other postsynthesis modifications. Combinations of IR spectroscopy with various characterization techniques such as, e.g., temperature-programmed desorption of probe molecules (TPD/IR, cf.[223,224]), electron spin resonance spectroscopy (ESR/IR, cf.[225,226]), UV-Vis spectroscopy [227,228], etc. were developed. 

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). 

As already mentioned in the Introduction, magnetic relaxation in the solid state may be governed not only by dynamic phenomena but there may also exist a contribution from static phenomena such as spin diffusion, which consists of mutual exchanges of spin state, or flip-flops , between strongly coupled nuclei that have the same precession frequency but antiparallel spins. This mechanism is explained in more detail in chapter 7. It does not involve any variation in the energy of the system. When it occurs, magnetic relaxation times cannot be interpreted in terms of local dynamics only, but the two contributions have to be separated. 

Brown has reviewed applications of high-resolution H solid-state NMR. The article emphasizes the recent increase in applications of high-resolution H mas solid-state NMR, in particular two-dimensional heteronuclear and homonuclear (double-quantum and spin-diffusion NOESY-like exchange) experiments. It has been shown that the applications of high-resolution solid-state NMR benefit from faster MAS frequencies (up to 80 kHz),... 

There are also multidimensional solid-state NMR experiments, even though, because of the more stringent instrumental requirements, they are more difficult to perform than solution experiments [49-51]. Correlations can be established between the isotropic chemical shift and chemical-shift anisotropy, and between isotropic shift and dipolar coupling. Solid-state exchange NMR provides information about the geometry of molecular motion in the sample, and spin-diffusion measurements are useful for probing domain size [52,53]. 

Two experiments have been presented which allow to measure the effective T2 decay rate of individual amide proton and the magnetisation build-up rates for a selective transfer from H2O to using spin diffusion as a mixing element. " The experiments were demonstrated for a uniformly H, N labelled sample of a microcrystalline SH3 domain in which exchangeable deuterons were back-substituted with protons. The NMR experimental data was correlated with the dipolar couplings calculated from H20-H distances which were extracted from the X-ray structure of the protein. It was shown that the T2 decay rates and H20-H build-up rates are sensitive to distance and dynamics of the detected water molecules with respect to the protein, and that qualitative information about localisation and dynamics of internal water molecules can be obtained in the solid state by interpretation of the spin dynamics of a reporter amide proton. 

The interaction of a (3-hairpin antimicrobial peptide, protegrin-1, with various lipid membranes has been investigated by solid-state P, and NMR. Mixed lipid bilayers containing anionic lipids and cholesterol were used to mimic the bacterial and mammalian cell membranes, respectively. P and spectra of macroscopically oriented samples showed that protegrin-1 induces the formation of an isotropic phase in anionic bilayers containing phosphatidylglycerol. 2D P exchange, H spin diffusion and relaxation time measurements were undertaken. In a related work the results of solid-state H, C, N and P NMR studies of the selective perturbation of lipid bilayers by the cyclic antimicrobial peptide have been presented. ... 

The structural properties of a second, apparently amorphous phase (all) of the molecular glass former triphenyl phosphite were studied by means of multidimensional solid-state NMR spectroscopy and X-ray diffraction. Phase all was prepared by annealing the supercooled liquid in the temperature range 210 K < T < 230 K. In addition to ID H and P spectra and Ti data, P radio-frequency-driven spin-diffusion exchange spectroscopy were used to analyse the arrangement of neighboring TPP molecules on both a local and intermediate scale. ... 

Since the pioneering demonstration that cross-polarization C NMR spectra can be recorded, solid-state NMR spectroscopy has advanced rapidly and is now being used to study the structure and dynamics of a variety of polymer systems. Much of the success of solid-state NMR spectroscopy is due to the evolution of a variety of techniques for studying internuclear distances, anisotropy, torsion angles, atomic orientations, spin diffusion, molecular dynamics, and exchange processes, while maintaining the high resolution and sensitivity necessary for practically useful NMR experiments in polymers. 

Two-dimensional NMR experiments can also be used in the solid state to study the structure and dynamics of polymers, and 2D solid-state NOESY has been used to provide a molecular-level assignment for the polyethylene a transition observed by dielectric and dynamic mechanical spectroscopy [32]. One proposal is that this transition can be assigned to chain diffusion between the crystalline and amorphous regions [33], Two peaks are observed in the C-CPMAS spectra of polyethylene that can be assigned to chains in crystalline and amorphous environments. Figure 3.27 shows the 2D spin exchange... 

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