Dynamic phenomena solid state

First introduced to polymer chemistry by Schaefer and collaborators, CP-MAS spectroscopy has already yielded interesting results in both stractural and dynamic studies. The comparison of spectra in solution and in bulk permits identification of frozen conformations, distinction between spectra of crystalline and amorphous phases and measurement of the rate of several eonformational transitions. For example, the C spectrum of the poly(phenylene oxide), 74, in solution consists of five signals while the CP-MAS spectrum displays six. In the solid state the resonance of the aromatic CH appears split into two components. The phenomenon is attributed to the forbidden rotation of the benzene ring around the O. .. O axis, which makes the two carbon atoms indicated with an asterisk no longer equivalent. 

Chaos was also investigated in solid-state lasers, and the important role of a pump nonuniformity leading to a chaotic lasing was pointed out [42]. A modulation of pump of a solid-state NdPsOn laser leads to period doubling route to chaos [43]. The same phenomenon was observed in the case of laser diodes with modulated currents [44,45]. Also a chaotic dynamics of outputs in Nd YAG lasers was also discovered [46 18]. In semiconductor lasers a period doubling route to chaos was found experimentally and theoretically in 1993 [49]. 

It is possible to perturb a spin equilibrium by photoexciting one of the isomers. Among the possible radiative and nonradiative fates of the excited state is intersystem crossing to the manifold of the other spin state. Internal conversion within this manifold ultimately results in the nonequilibrium population of the ground state. If these processes are rapid compared with the relaxation time of the spin equilibrium, then the dynamics of the ground state spin equilibrium can be observed. This experiment was first performed for spin equilibria with a coordination-spin equilibrium of a nickel(II) complex (85). More recently a similar phenomenon has been observed in the solid state at low temperatures (41). The nonequilibrium distribution can be trapped for long periods at... 

An interesting phenomenon whereby achiral compounds occupy chiral cavities has been reported. Steroidal host compounds give rise to the attachment of definite chiral conformations of achiral compounds within cavities, making it possible to observe solid-state circular dichroism spectra. Gdaniec and Polonski reported this type of property for the inclusion compounds of DCA and CA with various aromatic ketones [40a] and benzil [40c], Furthermore, it is possible for the selected conformers to maintain their chiral state temporarily in solution. That is, soon after the inclusion compounds are dissolved, the chirality may be retained for some time. /V-Nitrosopiperidines were found to display this type of dynamic chiral recognition in DCA and CA inclusion compounds [40b], In this case, one can observe the decay of the circular dichroism signal after dissolution of these inclusion compounds in methanol. 

As most organometallic compounds, lithium enolates are highly polar entities susceptible to combine in various types of (eventually solvated) aggregates that undergo dynamic equilibria in solution. This phenomenon explains why enolate solutions are difficult to describe by the classical spectroscopic, physicochemical or theoretical methods, a difficulty enhanced by the sensitivity of these equilibria to many physicochemical factors such as the concentration, the temperature or the presence of complexing additives (lithium halides, amides, amines, HMPA,. ..). The problems due to dynamics are avoided in the solid state where many clusters of lithium enolates, alone or co-crystallized with exogenous partners, have been identified by X-ray crystallography. 

In the case of the particles accommodating amine ligands, a new phenomenon has been evidenced, namely, a dynamic exchange at the NMR timescale between free and coordinated amines. It has been correlated to the TEM and HREM results, which show that, at the early stage of the reaction, the particles display a spherical aspect and a small size (ca. 2-3 nm), and that after a few hours, the particles coalesce into elongated wormlike particles, still constituted of pure, unoxidized hep ruthenium. This NMR observation is particularly interesting since it evidences for these particles a fluxionality similar to that of molecular clusters, which is well documented. The ruthenium nanoparticles contain coordinated mobile surface hydrides, as recently demonstrated by a combination of NMR techniques in solution, gas phase, and in the solid state. ... 

A well-known and important phenomenon in the area of nuclear-spin resonance (NMR) in gases, liquids, or solid samples is dynamic nuclear-spin polarisation (DNP) (see e.g. [M6]). This term refers to deviations of the nuclear magnetisation from its thermal-equilibrium value, thus a deviation from the Boltzmann distribution of the populations of the nuclear Zeeman terms, which is produced by optical pumping (Kastler [31]), by the Overhauser effect [32], or by the effet solide or solid-state effect [33]. In all these cases, the primary effect is a disturbance of the Boltzmann distribution in the electronic-spin system. In the Overhauser effect and the effet solide, this disturbance is caused for example by saturation of an ESR transition. Owing to the hyperfine coupling, a nuclear polarisation then results from coupled nuclear-electronic spin relaxation processes, whereby the polarisation of the electronic spins is transferred to the nuclear spins. 

It is important to mention at this point that, in the solid state, some of the above spectral parameters are not only determined by molecular motions, but also by the dynamics of the spin system. As a consequence, in bulk polymers at temperatures below the glass-transition temperature, the competition between molecular dynamics and spin dynamics may introduce some difficulties in the data interpretation. On the other hand, the spin dynamics phenomenon itself can be a most useful tool for studying the solid-state organization of heterogeneous materials. 

As described above, high-resolution C NMR spectra are obtainable also for solid samples, combining techniques such as DD, MAS and CP. It is, then, possible to investigate the molecular chain conformation characteristics of the solid state via the chemical shift of individual carbons. Furthermore, if one measures simultaneously the relaxation of C magnetization, one also expects to obtain information on molecular chain dynamics. This section describes the relaxation phenomenon. 

The possibility of dynamic behavior for single groups or entire molecules in the solid state when weak noncovalent interatomic forces are present is quite general and this phenomenon has a pronounced influence on the macroscopic physical properties of the supramolecular systems. 

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