Abstract
Disordered molecular solids present a rather broad class of substances of different origin—amorphous polymers, materials for photonics and optoelectronics, amorphous pharmaceutics, simple molecular glass formers, and others. Frozen biological media in many respects also may be referred to this class. Theoretical description of dynamics and structure of disordered solids still does not exist, and only some phenomenological models can be developed to explain results of particular experiments. Among different experimental approaches, electron paramagnetic resonance (EPR) applied to spin probes and labels also can deliver useful information. EPR allows probing small-angle orientational molecular motions (molecular librations), which intrinsically are inherent to all molecular solids. EPR is employed in its conventional continuous wave (CW) and pulsed—electron spin echo (ESE)—versions. CW EPR spectra are sensitive to dynamical librations of molecules while ESE probes stochastic molecular librations. In this review, different manifestations of small-angle motions in EPR of spin probes and labels are discussed. It is shown that CW-EPR-detected dynamical librations provide information on dynamical transition in these media, similar to that explored with neutron scattering, and ESE-detected stochastic librations allow elucidating some features of nanoscale molecular packing. The possible EPR applications are analyzed for gel-phase lipid bilayers, for biological membranes interacting with proteins, peptides and cryoprotectants, for supercooled ionic liquids (ILs) and supercooled deep eutectic solvents (DESs), for globular proteins and intrinsically disordered proteins (IDPs), and for some other molecular solids.
Highlights
Organic and biological solids with a disordered molecular structure are interesting from points of view of their practical importance and their unusual fundamental properties
From a fundamental point of view, disordered molecular solids possess many intriguing properties of a nature that still remains unclear. These media show anomalous thermal conductivity and a specific heat at cryogenic temperatures [5,6,7]. This anomaly may be described within a model of tunneling localized excitations or Two-Level Systems model (TLSs) [8,9,10], in which it is postulated that some atoms or groups of atoms have two equilibrium positions between which they can tunnel [7]
Numerous neutron scattering data show that in molecular disordered media dynamical transition appears as a sharp increase of the motional amplitude at a certain temperature, Td. [27,28,29,83]; a similar effect was observed in a Mössbauer absorption experiment [27,84]
Summary
Organic and biological solids with a disordered molecular structure are interesting from points of view of their practical importance and their unusual fundamental properties. From a fundamental point of view, disordered molecular solids possess many intriguing properties of a nature that still remains unclear These media show anomalous thermal conductivity and a specific heat at cryogenic temperatures [5,6,7]. Because of the weakness of intermolecular interactions in molecular solids, the cage boundaries may fluctuate (Figure 1b), especially at temperatures close to glass transition temperature (Tg). These fluctuations provide a source of stochasticity of motion, so librations may become stochastic. Because of the weakness of intermolecular bonds, the well potential may fluctuate stochastically, and stochasticity may appear because of random transitions between anharmonic sublevels
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