Molecular motion/dynamics, solid-state motionally averaged interactions

In recent years there has been considerable interest in the application of solid state NMR techniques to obtain structural and dynamic information on a wide variety of metal carbonyl complexes since solid state NMR can act as a bridge between crystallographic information and structures in solution. The NMR spectra of solids are, in principle, more complex than those in solution state because molecular motions in the latter case cause averaging of many interactions which are orientation dependent. This motional averaging brings several advantages in the acquisition of high-resolution spectra but there is also considerable loss of information in solution compared with solid state spectra. 

Solid-state NMR spectroscopy is well suited for studies of intramolecular dynamics because side chain motions can be analyzed Independent of overall molecular reorientation in crystalline samples. It is an alternative spectroscopic strategy to solution NMR because partial motional averaging of the anisotropic spin interactions occurs. Dipolar, chemical shift, and quadrupolar interactions can be used to describe the dynamics of aromatic rings of proteins and peptides. 

Above 200 K changes in line shape as shown in Fig. 26 reveal the onset of molecular dynamics (the simulations (solid lines) are carried out assuming a particular motional model, the moderate jump model, which is discussed below.) The line width has dropped to about 12 G, which is known to result from intramolecular interactions manifested as unresolved proton hyperfine splittings [132]. The dipolar interactions are now averaged out by the molecular dynamics. The remarkable intensity behavior (increasing intensity at higher temperature) is caused by the reversible conversion of DTBN moleeules from the ESR-inactive to the ESR-active state and is discussed elsewhere [133]. 

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