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Solid-State NMR is a branch of Nuclear Magnetic Resonance which is presently experiencing a phase of strongly increasing popularity. The most striking evidence is the large number of contributions from Solid-State Resonance at NMR meetings, approaching that ofliquid­ state resonance. Important progress can be observed in the areas of methodological developments and applications to organic and inorganic matter. One volume devoted to more or less one of each of these areas has been published in the preceding three issues. This volume can be considered an addendum to this series. Selected methods and applications of Solid-State NMR are featured in three chapters. The first one treats the recoupling of dipolar interactions in solids, which are averaged by fast sample rotation. Following an introduction to effective Hamiltonians and Floquet theory, different types of experiment such as rotary resonance, dipolar chemical shift correlation spectroscopy, rotational resonance and multipulse recoupling are treated in the powerful Floquet formalism. In the second chapter, the different approaches to line narrowing of quadrupolar nuclei are reviewed in a. consistent formulation of double resonance (DaR) and dynamic angle spinning (DAS). Practical aspects of probe design are considered as well as advanced 2D experiments, sensitivity enhancement techniques, and spinning sideband manipulations. The use of such techniques dramatically increases the number of nuclei which can be probed in high resolution NMR spectroscopy. The final chapter describes new experimental approaches and results of structural studies of noncrystalline solids.



Recoupling of Homo- and Heteronuclear Dipolar Interactions in Rotating Solids

The measurement of homo- and heteronuclear dipolar couplings by nuclear magnetic resonance (NMR) techniques is an important tool for the determination of molecular structure in solids. In a static polycrystalline solid, the dipolar coupling between two magnetically dilute spins results in the characteristic “Pake pattern” [1], first observed in the 1H spectrum of gypsum, CaSo4-2H2O, which arises from the interaction between the two protons in the water molecules of hydration. The splitting between the singularities provides a straightforward measurement of the dipolar coupling constant and therefore the internuclear distance between the two spins. Unfortunately, in the more general case, the structural information revealed by internuclear distances cannot be obtained directly from the static 1H NMR spectrum because of the multiplicity of couplings. In situations involving other nuclei, such as 13C, 15N, and 31P, large chemical shift anisotropics, as well as other line-broadening mechanisms, obscure the lineshape perturbations from the through-space dipolar couplings.
Andrew E. Bennett, Robert G. Griffin, Shimon Vega

Solid-State NMR Line Narrowing Methods for Quadrupolar Nuclei: Double Rotation and Dynamic-Angle Spinning

Among the features of nuclear magnetic resonance (NMR) spectroscopy that have made it such a versatile and powerful collection of analytical methods is the freedom to manipulate independently both spin and spatial components of the nuclear spin interaction tensors. This imparts enormous flexibility and selectivity to the type of structural and dynamical information that can be obtained about a chemical system at a molecular level. Depending on the information desired, experiments can be designed to measure, remove, or correlate one or more of the principal spin interactions, including the chemical shift, spin-spin scalar (J) coupling, electric quadrupole coupling, and magnetic dipole-dipole coupling, each of which depends on molecular orientation. Often one of the chief aims of such intervention is to enhance spectral resolution, and thereby augment the information content of the NMR spectrum, through judicious control of time-dependent radiofrequency (B 1) fields and/or manipulation of sample orientation with respect to a large static field (B 0). In low-viscosity liquids, fast isotropic molecular motion averages all anisotropic interactions, so that narrow line widths are readily obtained for each distinguishable site in a molecular system. Narrow spectral lines in these systems reflect, for example, the isotropic value of each site-specific chemical shift tensor (modified potentially by an isotropic.J-coupling contribution as well), which is averaged over all possible orientations by rapid molecular motion on the NMR time scale. Such an isotropic sampling criterion is equivalent to averaging all nuclear spin interactions over a sphere, whereby only those contributions that are independent of molecular orientation remain, namely the isotropic terms.
B. F. Chmelka, J. W. Zwanziger

Structural Studies of Noncrystalline Solids Using Solid State NMR. New Experimental Approaches and Results

The term “glass” describes a state of matter that possesses most of the macroscopic and thermodynamic properties of a crystalline solid, while retaining the structural disorder and isotropic behavior typical of the liquid state. Cooling a liquid quickly below its freezing point under conditions that prevent thermodynamic equilibration results in the glassy state at a well-defined temperature Tg, where collective molecular motion is frozen abruptly. This “glass transition temperature” is a thermodynamic necessity and possesses the phenomenological appearance of a second order phase transition [1].
Hellmut Eckert


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