Molecular and Ionic Crystals

The relatively weak intermolecular forces can be easily disrupted by thermal vibrations. The molecular crystals differ from ionic and covalent crystals in lower melting points, lower densities, and relatively lower strength. 

According to empirical evidence, the Lennard-Jones potential (11.26) describes the interaction between molecules well. The Lennard-Jones potential does not depend on a bonding direction, therefore the molecular solids tend to have the largest possible number of neighbors. SoUd molecular crystals often have the face-centered cubic lattice or close hexagonal crystal lattice. 

Examples of molecular crystals are ice, solid noble gases, oxygen, nitrogen, halogens (p2, CI2, Br2,12), sulfur as an Sg ring, phosphorus as a P4 tetrahedron, and the hydrocarbons and organic compounds. 

The molecular crystals are important for science and widely used in industry. 

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Because of the vastness of the subject matter, we shall focus our attention on hydrogen bonding interactions between ions and on the possibilities and limitations of their use in the design and construction of molecular materials of desired architectures and/or destined to predetermined functions. Obviously, the crystal engineer (or supramolecular chemist) needs to know the nature of the forces s/he is planning to master, since molecular and ionic crystals, even if constructed with similar building blocks, differ substantially in chemical and physical properties (solubility, melting points, conductivity, mechanical robustness, etc.). 

Thus, the interaction between structural entities of molecular and ionic crystals does not lead to appreciable delocalization of electrons strongly bonded to these entities. Therefore, the electronic structure of molecular and ionic crystals is practically not influenced by the size of such crystals. Size effects arise only in M/SC crystals with covalent (or at least partly covalent) bonds and depend on a relationship between the crystal size a and the length of delocalization (or, otherwise, electronic correlation) for valent electrons in a lattice. 

Molecular and ionic crystals are the easiest systems to apply to these schemes. The task becomes far more difficult in the case of covalent solids. However, this technique has been recently updated for calculations of covalent solids both at the semiempirical37 and at the ab initio levels.38... 

The lattice energy of a crystal is the energy required to separate the crystal into its component atoms, molecules, or ions at 0 K. In this section we examine the calculation and measurement of lattice energies for molecular and ionic crystals. 

Calculate lattice energies of molecular and ionic crystals (Section 21.4, Problems 35-38). 

The nature of plasticity is rupture and rearrangements of interatomic bonds which in crystalline objects involve peculiar mobile linear defects, referred to as dislocations. Temperature dependence of plasticity may significantly differ from that of Newtonian fluids. Under certain conditions (including the thermal ones) various molecular and ionic crystals, such as NaCl, AgCl, naphthalene, etc., reveal a behavior close to the plastic one. The values of x typically fall into the range between 10s and 109 N m 2. At the same time, plastic behavior is typical for various disperse structures, namely powders and pastes, including dry snow and sand. In this case the mechanism of plastic flow is a combination of acts involving the establishment and rupture of contacts between dispersed particles. Plastic object, in contrast to a liquid, maintains the acquired shape after removal of the stress. It is worth... 

Chapter 15 is devoted to the bonding nature in molecular and ionic crystals. We recall the dipole-dipole, dipole-induced and dispersion intermolecular forces. The van der Waals and hydrogen bonds are considered. We discuss intermolecular structure and strength of ice and the solid noble gases. The description of organic molecular crystals is presented. In conclusion we consider ionic crystals and calculate their interatomic bonding. 

We should add that molecular and ionic crystals are generally brittle because they fracture easily along crystal planes. MetalUc crystals, by contrast, are malleable that is, they can be shaped by hammering (Figure 11.28). 

Hartree-Fock Organic and main-group inorganic compounds, molecular and ionic crystals One hundred Ground state and transition state structures, thermochemical properties (e.g., binding energies), vibrational properties Most commonly used ab initio quantum chemical method, very well tested for organic molecules, conceptual difficulties with metals... 

We have selected a few cases of molecular and ionic crystals as tests for our new program. They include crystals of Ar, KCl... 

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