Solids molecular

Molecular solids may exhibit either crystalline or amorphous structures, depending on the complexity of the individual molecules comprising the bulk material. As with all solids, the more complex the subunits are, the harder it is for them to organize themselves in a repeatable fashion, resulting in an amorphous structure. Unlike purely ionic solids, molecular compounds may be soluble in either nonpolar or polar solvents, as long as the solvent polarity between solute and solvent is matched ( like dissolves like ). 

Both dipole-dipole and London Dispersion forces are subclasses of van der Waal interactions. When two polar molecules approach one another, a natural attraction known as dipole-dipole forces is created between oppositely charged ends. The relative intensity of dipole-dipole forces may be represented by Eq. 2  

In addition to the mutual attraction between polar molecules, there may also be an interaction between the solute molecules and liquid or gaseous solvent. In highly polar solvents such as water or alcohols, a dense shell of solvent molecules will surround the polar molecules. Although this solute/solvent interaction assists in the solubility of the molecules in the solvent, the dipole-dipole interactions between individual molecules are suppressed. 

In contrast to dipole-dipole forces, London Dispersion interactions are much weaker in nature since they involve nonpolar molecules that do not possess permanent dipole moments. The only modes for molecular attraction are through polarization of electrons, which leads to the creation of small dipole-dipole interactions and mutual attractive forces. Since electron polarization occurs much more readily for electrons farther from the nucleus, this effect is more pronounced for molecules that are larger with a greater number of electrons, especially positioned on atoms with a high atomic number, consisting of more diffuse orbitals. These induced dipole forces are responsible for the liquefaction of gases such as He and Ar at low temperatures and pressures. The relative strength of London Dispersion forces is described by Eq. 3  

If both polar and nonpolar molecules are present, a dipole-induced dipole interaction may occur. For this situation, the strength of association may be represented by Eq. 4, which is dependent on both the dipole moment of the polar molecule, and the polarizability of the nonpolar component. Once again, this relation does not include the interactions between the polar molecule and solvent molecules  

Molecular solids consist of atoms or molecules held together by dipole—dipole forces, dispersion forces, and/or hydrogen bonds. Because the.se intermolecular forces are weak, molecular solids are soft and have relatively low melting points (usually below 200 °C). Most substances that are gases or liquids at room temperature form molecular solids at low temperature. Examples include Ar, H2O, and CO2. 

Graphite is used as a lubricant and as the lead in pencils. The enormous differences in physical properties of graphite and diamond—both of which are pure carbon—arise from differences in their three-dimensional structure and bonding. 

In chapter 7, all works discussed on model molecular systems for conjugated polymers refer to condensed molecular solid ultra-thin films, generally prepared by condensation of molecules from the effusion of a Knudsen-type cell, in UHV, on to clean metallic substrates held at low temperatures. Clean is defined as atomically clean as determined by core-electron level XPS, such that there is intimate contact between the molecules at the substrate-film interface, without the influence of, for example, a metallic oxide, hydrocarbon 

In which substance, benzene or toluene, intermolecular forces stronger In which the molecules pack more efficiently  

Comparison of Atomic Separations Within Molecules (Covalent Bonds) and Between Molecules (Intermolecular Interactions) 

The forces that exist among the molecules in a molecular solid depend on the nature of the molecules. Many molecules such as CO2, h, P45 and Sg have zero dipole moments, and the intermolecular forces are London dispersion forces. Because these forces are usually small, we might expect all these substances to be gaseous at 25°C, as is the case for carbon dioxide. However, as the size of the molecules increases, the London forces become quite large, causing many of these substances to be solids at 25°C. 

Ionic solids are stable, high-melting-point substances held together by the strong electrostatic forces that exist between oppositely charged ions. The principles governing the structures of ionic solids were introduced in Section 13.5. In this section we will review and extend these principles. 

There are three types of holes in closest packed structures  

Trigonal holes are formed by three spheres in the same layer [Fig. 16.36(a)]. 

This class of solids features discrete molecules that are held together by rather weak intermolecular forces such as dipole-dipole, London Dispersion, and hydrogen 

So far we have considered solids in which atoms occupy the lattice positions. In most cases such crystals can be considered to consist of one giant molecule. However, there are many types of solids that contain discrete molecular units at each lattice position. A common example is ice, where the lattice positions are occupied by water molecules [see Fig. 16.12(c)]. Other examples are dry ice (solid carbon dioxide), some forms of sulfur that contain Ss 

When molecules do have dipole moments, their intermolecular forces are typically greater, especially when hydrogen bonding is possible. Water molecules are particularly well suited to interact with each other, because each mol- 

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These days security is at the top of everyone s iist of important concerns, especiaiiy for those peopie who are responsibie for the safety of our transportation systems. In particuiar, airports need speedy and sensitive detectors for expiosives. Piastic expiosives are especiaiiy tricky to detect because they do not respond to metai detectors, and they can be shaped into innocent-iooking objects to avoid X-ray detection. However, a team of scientists at Oak Ridge Nationai Laboratory ied by Thomas Thundat has just pubiished a description of an inexpensive device that is extremeiy sensitive to two N-containing compounds found in piastic expiosives. The key part of this detection device is a tiny (180-micrometer), V-shaped cantiiever made of siiicon. The cantiiever is shown in the accompanying photo next to a human hair for size comparison. 

This detector looks like a very promising addition to our arsenal of security devices. 

When explosive compounds bind to these V-shaped cantilevers, the microscopic structures, which are about the width of a hair, bend and produce a signal. 

So far we have considered solids in which atoms occupy the lattice positions. In some of these substances (network solids), the solid can be considered to be one giant molecule. In addition, there are many types of solids that contain discrete molecular units at each lattice position. A conunon example is ice, where the lattice positions are occupied by water molecules [see Fig. 10.12(c)], Other examples are dry ice (solid carbon dioxide), some forms of sulfur that contain Sg molecules [Fig. 10.32(a)], and certain forms of phosphorus that contain P4 molecules [Fig. 10.32(b)]. These substances are characterized by strong covalent bonding within the molecules but relatively weak forces between the molecules. For example, it takes only 6 kJ of energy to melt 1 mole of solid water (ice) because only intermolecular (H2O—H2O) interactions must be overcome. However, 470 kJ of energy is required to break 1 mole of covalent O—H bonds. The differences between the covalent bonds within the molecules and the forces between the molecules are apparent from the comparison of the interatomic and intermolecular distances in solids shown in Table 10.6. 

The forces that exist among the molecules in a molecular solid depend on the nature of the molecules. Many molecules such as CO2,12, P4, and Sg have no dipole moment, and the intermolecular forces are London dispersion forces. Because these forces are 

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When molecules do have dipole moments, their intermolecular forces are significantly greater, especially when hydrogen bonding is possible. Water molecules are particularly well suited to interact with each other because each molecule has two polar O—H bonds and two lone pairs on the oxygen atom. This can lead to the association of four hydrogen atoms with each oxygen two by covalent bonds and two by dipole forces  

Note the two relatively short covalent oxygen-hydrogen bonds and the two longer oxygen-hydrogen dipole interactions that can be seen in the ice structure in Fig. 10.12(c). 

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One molecular solid to which a great deal of attention has been given is ice. A review by Fletcher [74] cites calculated surface tension values of 100-120 ergs/cm (see Ref. 75) as compared to an experimental measurement of 109 ergs/cm [76]. There is much evidence that a liquidlike layer develops at the ice-vapor interface, beginning around -35°C and thickening with increasing temperature [45, 74, 77, 78]. 

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.)... 

The Debye model is more appropriate for the acoustic branches of tire elastic modes of a hanuonic solid. For molecular solids one has in addition optical branches in the elastic wave dispersion, and the Einstein model is more appropriate to describe the contribution to U and Cj from the optical branch. The above discussion for phonons is suitable for non-metallic solids. In metals, one has, in addition, the contribution from the electronic motion to Uand Cy. This is discussed later, in section (A2.2.5.6T... 

Chronister E L and Crowell R A 1991 Time-resolved coherent Raman spectroscopy of low-temperature molecular solids in a high-pressure diamond anvil cell Chem. Phys. Lett. 182 27... 

Shirley E L 1998 Many-body effects on bandwidths in ionic, noble gas, and molecular solids Phys. Rev. B 58 9579-83... 

Hesselink W H and Wiersma D A 1983 Theory and experimental aspects of photon echoes in molecular solids... 

It can be readily confirmed thaf by decreases as the number of bonds N increases and/or llieir length (r ) decreases. This relationship between the bond strength and the number of neighbours provides a useful way to rationalise the structure of solids. Thus the high coordination of metals suggests that it is more effective for them to form more bonds, even though each individual bond is weakened as a consequence. Materials such as silicon achieve the balance for an infermediate number of neighbours and molecular solids have the smallest atomic coordination numbers. 

Theoretical Aspects and Computer Modelling of the Molecular Solid State A. Gavezotti, Ed., John Wiley Sons, New York (1997). 

It should be loted that with low-energy surfaces the sudden fall in the heat of adsorption is absent. This is illustrated in Fig. 2.15, where the contrast between the behaviour of nitrogen on the carbons (high-energy surfaces) and on the molecular solids (low-energy surfaces) is very clear. 

A typical temperature dependence of is shown in fig. 53. Clough et al. [1981] have found a universal correlation between the temperature at which has a minimum, r in, and A, when the measurements are performed at the same Zeeman frequency. This correlation, demonstrated in fig. 54, holds for all molecular solids studied so far, with A covering a range of four orders... 

Pertsin, A.J. and A.I. Kitaigorodskii, 1987, The Atom-Atom Potential Method. Application to Organic Molecular Solids (Springer, Berlin). 

Since the structure and properties of fullerene solids are strongly dependent on the structure and properties of the constituent fullerene molecules, we first review the structure of the molecules, which is followed by a review of the structure of the molecular solids formed from Ceo, C70 and higher mass fullerenes, and finally the structure of Cgo crystals. 

Pure NI3 has not been isolated, but the structure of its well-known extremely shock-sensitive adduct with NH3 has been elucidated — a feat of considerable technical virtuosity.Unlike the volatile, soluble, molecular solid NCI3, the involatile, insoluble compound [Nl3.NH3] has a polymeric structure in which tetrahedral NI4 units are comer-linked into infinite chains of -N-I-N-I- (215 and 230 pm) which in turn are linked into sheets by I-I interactions (336 pm) in the c-direction in addition, one I of each NI4 unit is also loosely attached to an NH3 (253 pm) that projects into the space between the sheets of tetra-hedra. The stmcture resembles that of the linked Si04 units in chain metasilicates (p. 349). A further interesting feature is the presence of linear or almost linear N-I-N groupings which suggest the presence of 3-centre, 4-electron bonds (pp. 63, 64) characteristic of polyhalides and xenon halides (pp. 835-8, 897). 

When an ionic solid such as NaCl dissolves in water the solution formed contains Na+ and Cl- ions. Since ions are charged particles, the solution conducts an electric current (Figure 2.12) and we say that NaCl is a strong electrolyte. In contrast, a water solution of sugar, which is a molecular solid, does not conduct electricity. Sugar and other molecular solutes are nonelectrolytes. 

Although the purpose here is not to give a full understanding of photoeleciron spectroscopy, it can be useful to discuss some of the specific features in a photoelectron spectrum which can be helpful for the understanding of the different examples discussed in this chapter. The main emphasis in the background to PES will be focused on the molecular solids aspect since this chapter deals with condensed conjugated systems. The interested reader can find a more in-depth discussion on the technique, relative to organic polymeric systems, in Refs. [4, 9, 10]. 

The remaining four elements form molecular solids. The atoms of white phosphorus, sulfur, and chlorine are strongly bonded into small molecules (formulas, P4, S8, and Cl2, respectively) but only weak attractions exist between the molecules. The properties are all appropriate to this description. Of course there is no simple trend in the properties since the molecular units are so different. 

The principles of equilibrium have wide applicability and great utility. For example, they aid us in understanding and controlling the solubility of solids and gases in liquids. We shall consider, first, the solubility of a molecular solid in a liq-... 

Expression (4) is applicable to the solubility equilibria of some substances in water, but not of all. Contrast, for example, water solutions of sugar, salt, and hydrochloric acid. Sugar forms a molecular solid and, as it dissolves in water, the sugar molecules remain intact. These molecules... 

Hydrochloric acid, HC1, is similar. This substance is a gas at normal conditions. At very low temperatures it condenses to a molecular solid. When HC1 dissolves in water, positively charged hydrogen ions and negatively charged chloride ions are found in the solution. As with sodium chloride, a conducting solution containing ions is formed ... 

The examples just mentioned include two elements, fluorine and lithium. Fluorine forms a weakly bound molecular solid. Lithium forms a metallic solid. Let us see how we can account for this extreme difference, applying the principles of bonding treated in Chapter 16. 

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, 102, 301, 306 Molecular substances and van der Waals forces, 306... 

The Lewis structure of the product, a white molecular solid, is shown in (32). In this reaction, the lone pair on the nitrogen atom of ammonia, H3N , can be regarded as completing boron s octet in BF3 by forming a coordinate covalent bond. 

Molecular solids are assemblies of discrete molecules held in place by intermolecular forces. 

In this part of the chapter, we begin with molecular solids and distinguish them from network solids. Then we examine metallic solids, which, if consisting of a single element, are built from identical atoms stacked together in orderly arrays. The structures of ionic solids are based on the same kinds of arrays but are complicated by the need to take into account the presence of ions of opposite charges and different sizes. 

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