Molecular solids, changes state

The mechanisms of static friction and stick-slip motion, as discussed in the last section, are supposed to be a good description of dry friction. Another case, perhaps more general in engineering practices, to be addressed in this section is lubricated sliding where liquid lubricant, consisting of a few molecule layers, is confined between two solid walls. Both experimental and theoretical studies indicate, as we have discussed in Chapter 5, that there are substantial changes in rheology of the confined lubricant, and the liquid may transit practically to a solid-like state when film thickness becomes molecularly thin [32,33]. 

The transducing mechanism of semiconductor luminescence involves the modification of the semiconductors surface electrical properties through molecular adsorption. Changes in solid-state electro-optical properties result from adsorption of the molecule of interest onto the semiconductor surface. 

Typical results are shown in Fig. 6 for U-methane in graphite pores of H =7.5 at T=114 K. At p/ps=l the system is solid-like at this temperature, but a discrete change in density occurs around p ps ca.0.5. The self diffiisivity along axial direction also shows drastic change at this point. Further examination of various characteristics of molecular state such as snapshots, in-plane pair correlations and static structure factors confirmed that this change in density is the result of a phase transition from solid-like state to liquid-like one, or melting. Since the critical condensation condition for this pore is far lower than this transition point to stay around p ps= ca.0.2, the liquid-like state is not on metastable branch but thermodynamically stable. Thus a solid-liquid coexistence point is found for this temperature. 

Solid-state SS-NMR and XRPD take advantage of different phenomena to evaluate polymorphism. XRPD is sensitive to differences in unit cell dimensions, whereas SS-NMR is sensitive to conformational changes and differences in magnetic environments.94 One can think of two hypothetical situations. If two polymorphic forms differ exclusively in their unit cell dimensions but the conformation of the molecule is preserved, then the XRPD patterns are likely to be substantially different, whereas SS-NMR may not register any changes. On the other hand, if the unit cell dimensions are preserved while the molecular conformation changes substantially, SS-NMR is likely to pick up the differences and the XRPD patterns might be the same. In this sense, both techniques provide information that is complementary to the other.94... 

Intuitively, one would expect dramatic changes in the atomic and electronic properties of a heterogeneous molecular solid under a violent assault. One plausible mechanism is electronic excitation electrons are light, fast and quantal, thus responding first to exterior perturbations. Following electronic excitation, however, several processes could be at play to induce chemical reactions. These include dissociation on the excited state surface, radiative processes, and nonradiative energy deactivation. 

The valinomycin study just described illustrates the value of close comparison of NMR and X-ray diffraction data. Correlation of NMR assignments with X-ray data for known structures may allow the subsequent elucidation of conformational features for related materials on the basis of NMR data alone. It is notoriously difficult to relate solid-state structures determined by X-ray diffraction to the solution conformation observed by conventional NMR spectroscopy, although data from the former technique are often the only type available for work on receptor recognition. However, solid-state NMR may provide a link between these two techniques by indicating whether molecular conformation changes do occur when the physical state is altered. In contrast to the valinomycin complexes discussed above, it has been found that significant differences in the solution and solid-state spectra of morphine... 

Are changes in state physical or chemical changes for molecular solids Why ... 

In plastics, as in many solids, increasing temperature at constant elongation results in a strain level drop. To phrase this differently, the amount of force required to elongate these materials decreases. Fig. 15. This reduction in elasticity does not, however, follow a uniform curve. On the other hand, nor are the sudden changes in state in evidence that are observed when low molecular substances change from one state of aggregatiiMi to another. 

Molecular solids Molecular solids tend to have relatively low melting and boiling temperatures (for example, nitrogen, methane, carbon dioxide, ammonia are in the gaseous state at room temperature), as the bonds between molecules are weak and the bonds within molecules do not need to be broken for the change of state. 

The relation between the EA response and the macroscopic nonlinear properties of the material has been drawn by a classical approach. In Section 19.3.3, the mechanism behind the field-induced variation will be explored by a quantum mechanical approach. In molecular solids and in conjugated polymers, where the states are localized to a high degree, the change in a with the electric field is in general ascribed to a Stark shift of the molecular energy levels. 

---