Molecular solids crystalline

Gaseous carbon dioxide, CO2, when cooled sufficiently forms a molecular crystalline solid, which is illustrated in Figure 1.50. Notice that the unit cell contains clearly discernible CO2 molecules, which are covalently bonded, and these are held together in the crystal by weak van der Waals forces. 

Explain the differences among atomic, ionic, and molecular crystalline solids. 

Ice (H20) is a molecular crystalline solid. Six water molecules bond to each other to form a hexagonal pattern. This pattern is reflected in the hexagonal geometry exhibited in snowflakes. In Activity 4.1, you will construct models of basic crystalline solids and grow molecular-solid crystals. Then you will consider the basic structures of crystalline solids and look upon these structures as three-dimensional works of art. 

B. Prepare a molecular crystalline solid (choose one preparation, either potassium aluminum sulfate or sucrose) ... 

This activity includes models of atomic, ionic, and molecular crystalline solids. List the names of the solids represented by each of these models. Make a generalization about the shape of the solid crystal and the type of solid. 

When mechanical stress is exerted on molecular crystalline solids of coordination compounds, not only crystalline lattice but also the constituent molecules are subjected to distortion. The type and extent of distortion of each molecule is inevitably different due to the anisotropic nature of mechanical stressing. This, in turn, causes disproportionation of then-ligand fields to decrease their symmetry. 

Evaluating the consequences of the intermolecular dipole-dipole interactions in molecular crystalline solids. 

When ionic or molecular crystalline solids are mixed with a support of high specific surface area and then heated at a suitable temperature below their melting point for several hours, many of them disperse spontaneously onto the surface of the carrier [ 173]. This can be detected by the disappearance of their X-ray diffraction peaks. The formation of a (sub-)monolayer of the guest compound on the surface of the support has been proposed, and it has been claimed that this process is driven by a gain in entropy [173-177]. As described below, this phenomenon can be utilized for the incorporation of oxides into the pores of zeo-htes. 

Schemes for classifying surfactants are based upon physical properties or upon functionality. Charge is tire most prevalent physical property used in classifying surfactants. Surfactants are charged or uncharged, ionic or nonionic. Charged surfactants are furtlier classified as to whetlier tire amphipatliic portion is anionic, cationic or zwitterionic. Anotlier physical classification scheme is based upon overall size and molecular weight. Copolymeric nonionic surfactants may reach sizes corresponding to 10 000-20 000 Daltons. Physical state is anotlier important physical property, as surfactants may be obtained as crystalline solids, amoriDhous pastes or liquids under standard conditions. The number of tailgroups in a surfactant has recently become an important parameter. Many surfactants have eitlier one or two hydrocarbon tailgroups, and recent advances in surfactant science include even more complex assemblies [7, 8 and 9].

In chemicals like salol the molecules are elongated (non-spherical) and a lot of energy is needed to rotate the randomly arranged liquid molecules into the specific orientations that they take up in the crystalline solid. Then q is large, is small, and the interface is very sluggish. There is plenty of time for latent heat to flow away from the interface, and its temperature is hardly affected. The solidification of salol is therefore interface controlled the process is governed almost entirely by the kinetics of molecular diffusion at the interface. 

Calculations for Ceo in the LDA approximation [62, 60] yield a narrow band (- 0.4 0.6 eV bandwidth) solid, with a HOMO-LUMO-derived direct band gap of - 1.5 eV at the X point of the fee Brillouin zone. The narrow energy bands and the molecular nature of the electronic structure of fullerenes are indicative of a highly correlated electron system. Since the HOMO and LUMO levels both have the same odd parity, electric dipole transitions between these levels are symmetry forbidden in the free Ceo moleeule. In the crystalline solid, transitions between the direct bandgap states at the T and X points in the cubic Brillouin zone arc also forbidden, but are allowed at the lower symmetry points in the Brillouin zone. The allowed electric dipole... 

The structure of AICI3 is similarly revealing. The crystalline solid has a layer lattice with 6-coordinate Al but at the mp 192.4° the stmcture changes to a 4-coordinate molecular dimer Al2Clg as a result there is a dramatic increase in volume (by 85%) and an even more dramatic drop in electrical conductivity almost to zero. The mp therefore represents a substantial change in the nature of the bonding. The covalently bonded... 

Substituted Ammonium Ions. Like NH4C1 the substance NH3-(CH3JCI, where a CH3 group has been substituted for one hydrogen, forms a crystalline solid and so do the substances NH2(CH3)2C1 and NH(CH3)3C1. When one of these substances is dissolved in water, it is completely dissociated into Cl- ions and molecular positive ions corresponding to (NH4)+. Suppose now that such a solution contains an NH3 molecule, and consider the following proton transfer... 

The starting point for the synthesis of xenon compounds is the preparation of xenon difluoride, XeF2, and xenon tetrafluoride, XeF4, by heating a mixture of the elements to 400°C at 6 atm. At higher pressures, fluorination proceeds as far as xenon hexafluoride, XeFfi. All three fluorides are crystalline solids (Fig. 15.27). In the gas phase, all are molecular compounds. Solid xenon hexafluoride, however, is ionic, with a complex structure consisting of XeF< + cations bridged by F anions. 

In crystalline solids, the Raman effect deals with phonons instead of molecular vibration, and it depends upon the crystal symmetry whether a phonon is Raman active or not. For each class of crystal symmetry it is possible to calculate which phonons are Raman active for a given direction of the incident and scattered light with respect to the crystallographic axes of the specimen. A table has been derived (Loudon, 1964, 1965) which presents the form of the scattering tensor for each of the 32 crystal classes, which is particularly useful in the interpretation of the Raman spectra of crystalline samples. 

The diamine and diacid monomers used to make type AABB nylons are typically rather difficult to handle in their pure form. Diamines are liquids or semisolids at room temperature, while the diacids are crystalline solids. These monomers become much more manageable when they are combined to form nylon salts, as shown in Fig. 23.7 a). Nylon salts are solids that can be easily handled and ensure a stoichiometric balance between the diacid and diamine, which is necessary to produce high molecular weight polymers. In the case of nylon 66, the precursor salt is made by boiling adipic acid and hexamethylene diamine in methanol, from which the nylon salt precipitates. 

The majority of cluster technetium compounds are subject to thermal decomposition topochemically (i.e. their decomposition reaction occurs in the solid phase), [H(H20)2]2 [Tc8Br4( -Br)8]Br2 being an exception. This compound melts before decomposition (at 610-620 °C), which is good evidence in favour of the molecular crystalline structure of its dehydrated form [Tc8Br4(/i-Br)8]Br2. ... 

It should be noted that molecular complexes of the cyclodextrins may be isolated as crystalline solids for example, a crystalline complex is obtained with iodine (which resembles the well known blue complex between iodine and starch) as well as with a large number of other inorganic and organic guests. 

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