Of molecular solids

Finally, it has been possible to obtain LEED patterns from films of molecular solids deposited on a metal-backing. Examples include ice and naphthalene [80] and various phthalocyanines [81]. (The metal backing helps to prevent surface charging.)... 

We have seen that the pure elements may solidify in the form of molecular solids, network solids, or metals. Compounds also may condense to molecular solids, network solids, or metallic solids. In addition, there is a new effect that does not occur with the pure elements. In a pure element the ionization energies of all atoms are identical and electrons are shared equally. In compounds, where the most stable electron distribution need not involve equal sharing, electric dipoles may result. Since two bonded atoms may have different ionization energies, the electrons may spend more time near one of the positive nuclei than near the other. This charge separation may give rise to strong intermolecular forces of a type not found in the pure elements. 

Molecular solids have their lattices composed of molecules held in place by London forces, dipole-dipole forces, and hydrogen bonding. Solid methane and water are example of molecular solids. 

O. Herzberg, M. Epple. Formation of Polyesters by Thermally Induced Polymerization Reactions of Molecular Solids. Eur. J. Inorg. Chem. 2001, 1395-1406. 

FE data have been collected from the fracture of a wide variety of single and multi-component solids, ranging from single crystals of molecular solids to fiber-reinforced composites, and also from the peeling of adhesives 0-16 ). In this paper, we will restrict our attention to FE arising from the failure of polymer composites (fibrous and particulate), and the individual components thereof (fibers and matrix resins). 

The paramount advantage of molecular solids over their more classical inorganic counterparts is that their constituents, the building blocks, are molecules or clusters that can be designed and synthesized in other words they can be intentionally modified. Therefore, we can talk about molecular and crystal engineering and the goal is to be able to produce materials with predetermined physical properties. We are not yet at this desired level but the scientific and technical bases are certainly at hand. 

The traditional approach for structure solution follows a close analogy to the analysis of single-crystal XRD data, in that the intensities 1(H) of individual reflections are extracted directly from the powder XRD pattern and are then used in the types of structure solution calculation (e.g. direct methods, Patterson methods or the recently developed charge-flipping methodology [32-34]) that are used for single-crystal XRD data. As discussed above, however, peak overlap in the powder XRD pattern can limit the reliability of the extracted intensities, and uncertainties in the intensities can lead to difficulties in subsequent attempts to solve the structure. As noted above, such problems may be particularly severe in cases of large unit cells and low symmetry, as encountered for most molecular solids. In spite of these intrinsic difficulties, however, there have been several reported successes in the application of traditional techniques for structure solution of molecular solids from powder XRD data. 

Scotland, UK). Maybe the youngest branch of crystallography, combining photochemistry and X-ray diffraction, viz. photocrystallography, is introduced by Professor Pance Naumov (Osaka, Japan) with the chapter Chemical X-Ray Photodiffraction Principles, Examples, and Perspectives . This concise five chapter volume is closed by an in-depth treatise of the recent advances in the field of X-ray powder diffraction by Professor Keruieth Harris (Cardiff, Wales, UK) titled Powder Diffraction Crystallography of Molecular Solids . 

Electron-Induced Reactions—HREELS Measurements. Novel LEE-induced chemistry has also been observed in HREEL measurements of molecular solids and molecules physisorbed on the surface of RGS. For example, Lepage et ah, building on the initial observations of Jay-Gerin et al. [141], have employed HREELS to measure in situ, neutral dissociation products arising from the impact of low-energy electrons on thin multilayer films of methanol [37] and acetone [38]. The technique is similar to that developed earlier by Martel et al. [258] for chemisorbed systems, in that the same electron beam is used for both the production and the detection of the neutral fragments. However, in the work of Lepage... 

Crystals of molecular solids—solids composed of individual molecules— have structures that reflect the fact that their molecules are not spheres... 

To predict the sign of A S, look to see if the process involves a phase change, a change in the number of gaseous molecules, or the dissolution (or precipitation) of a solid. Entropy generally increases for phase transitions that convert a solid to a liquid or a liquid to a gas, reactions that increase the number of gaseous molecules, and dissolution of molecular solids or salts with +1 cations and —1 anions. 

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