Solids molecular motions

Although we live immersed in the mixture of gases that make UP Earth s atmosphere, we are more familiar with the behavior OF liquids and solids because they are more visible. Every day WE use water and other liquids for drinking, bathing, cleaning, and cooking, and we handle, sit upon, and wear solids. Molecular motion is more restricted in liquids than in... 

The principal dilTerence from liquid-state NMR is that the interactions which are averaged by molecular motion on the NMR timescale in liquids lead, because of their anisotropic nature, to much wider lines in solids. Extra infonnation is, in principle, available but is often masked by the lower resolution. Thus, many of the teclmiques developed for liquid-state NMR are not currently feasible in the solid state. Furthemiore, the increased linewidth and the methods used to achieve high resolution put more demands on the spectrometer. Nevertheless, the field of solid-state NMR is advancing rapidly, with a steady stream of new experiments forthcoming. 

Heidemann, A., A. Magerl, M. Prager, D. Richter and T. Springer, eds, 1987, Quantum Aspects of Molecular Motions in Solids. 

Kashiwabra, H., Shimada, S., Hori, Y. and Sakaguchi, M. ESR Application to Polymer Physics — Molecular Motion in Solid Matrix in which Free Radicals are Trapped. Vol. 82, pp. 141 -207. 

With concern to the high internal mobility of the molecules in the high temperature solid state phase, some parallelism to n-alkanes can be stated. In the pseudohexagonal (rotator) phase the latter are also characterized by fast molecular motions. For discrimination and according to Pfitzer 14) and Dale 13) in the following the term pseudorotator phase is used for the mobil crystalline state of cyclic molecules. 

The polymer at the gel point is in a critical state [3], and the name critical gel [4] is appropriate for distinguishing polymers at the gel point from the various materials which commonly are called gels. The critical gel has no intrinsic size scale except for the size of its oligomeric building block, and molecular motions are correlated over large distances. The combination of liquid and solid... 

Solid-state NMR methods have been much used to study the characteristics of the network chains themselves, particularly with regard to orientations [265], molecular motions [266], and their effects on the diffusion of small molecules [267], Aspects related to the structures of the networks include the degree of cross-linking [268,269], the distributions of cross-links [270] and stresses [271], and topologies [272,273]. Another example is the use of NMR to clarify some issues in the areas of aging and phase separation [274],... 

Atoms and molecules in solid state are subject to restricted motions, so that all the interactions which are orientation dependent are not averaged by molecular motion and the spectra are complex it is therefore fundamental to apply approaches able to increase the resolution... 

When the course taken by a given solid-state reaction is determined by geometrical details of the crystal lattice, the reaction type falls under the general category of topochemistry. In a topochemical reaction, the reaction takes place in the solid state with a minimum amount of molecular motion. For example, bimolecular reactions are expected to take place between nearest neighbors, which then suggests that the product of the reaction would be a function of the geometric relation in the crystal structure of the reactant molecules. 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

Fig. 6 (a) SQ and (b) DQ rotor-synchronized 2H MAS NMR spectra of sodium tetrathionate dihydrate-d4 (solid lines). The dashed line in (a) represents the exact numerical simulation of the SQ spectrum for random molecular motion with the rate constant k given in the figure, (c) The corresponding experimental and simulated static 2H quadrupolar-echo spectra, (d) Simulated SQ (solid line) and DQ (dashed line) linewidths as functions of k. (Reproduced with permission from [88])... 

Throughout the cross-polarization pulse sequence, a number of competing relaxation processes are occurring simultaneously. The recognition and understanding of these relaxation processes are critical in order to apply CP pulse sequences for quantitative solid state NMR data acquisition or ascertaining molecular motions occurring in the solid state. 

FIG. 23 A schematic illustration of the molecular motions and associated T2 relaxation curve behavior for the three major domains in foods—liquid, viscous liquid, and solid (crystalline and glassy). Typical H T2 NMR relaxation time values observed in these domains, and values specific for water in liquid and crystalline domains, are listed. 

---