Covalent molecular crystals

Covalent molecular crystals. The crystal structure of a covalent molecular... 

RELATING IDEAS Explain why ionic crystals melt at much higher temperatures than typical covalent molecular crystals. 

In most covalent compounds, the strong covalent bonds link the atoms together into molecules, but the molecules themselves are held together by much weaker forces, hence the low melting points of molecular crystals and their inability to conduct electricity. These weak intermolecular forces are called van der WaaFs forces in general, they increase with increase in size of the molecule. Only... 

The red compound 80484, obtained by fusing equimolar amounts of the elements, is a covalent molecular species which can be crystallized from benzene. Similar procedures yield Se2Sg, 8087 and TeS7, all of which are structurally related to Sg (p. 654 see also p. 763). 

Chapter 6. Covalent, Inter metallic, and Molecular Crystals 553... 

However, It has been found that in many cases, simple models of the properties of atomic aggregates (monocrystals, poly crystals, and glasses) can account quantitatively for hardnesses. These models need not contain disposable parameters, but they must be tailored to take into account particular types of chemical bonding. That is, metals differ from covalent crystals which differ from ionic crystals which differ from molecular crystals, including polymers. Elaborate numerical computations are not necessary. 

The electrodynamic forces proposed for stabilizing jellium provide the principal type of bonding in molecular crystals such as solid methane, rare gas crystals, solid anthracene, and the like. These forces also form the inter-chain bonding of long-chain molecules in polymeric materials (the intra-molecular bonding within the chains is usually covalent). 

In molecular crystals, there are two levels of bonding intra—within the molecules, and inter—between the molecules. The former is usually covalent or ionic, while the latter results from photons being exchanged between molecules (or atoms) rather than electrons, as in the case of covalent bonds. The hardnesses of these crystals is determined by the latter. The first quantum mechanical theory of these forces was developed by London so they are known as London forces (they are also called Van der Waals, dispersion, or dipole-dipole forces). 

The term molecular crystal refers to crystals consisting of neutral atomic particles. Thus they include the rare gases He, Ne, Ar, Kr, Xe, and Rn. However, most of them consist of molecules with up to about 100 atoms bound internally by covalent bonds. The dipole interactions that bond them is discussed briefly in Chapter 3, and at length in books such as Parsegian (2006). This book also discusses the Lifshitz-Casimir effect which causes macroscopic solids to attract one another weakly as a result of fluctuating atomic dipoles. Since dipole-dipole forces are almost always positive (unlike monopole forces) they add up to create measurable attractions between macroscopic bodies. However, they decrease rapidly as any two molecules are separated. A detailed history of intermolecular forces is given by Rowlinson (2002). 

The principal intention of the present book is to connect mechanical hardness numbers with the physics of chemical bonds in simple, but definite (quantitative) ways. This has not been done very effectively in the past because the atomic processes involved had not been fully identified. In some cases, where the atomic structures are complex, this is still true, but the author believes that the simpler prototype cases are now understood. However, the mechanisms change from one type of chemical bonding to another. Therefore, metals, covalent crystals, ionic crystals, and molecular crystals must be considered separately. There is no universal chemical mechanism that determines mechanical hardness. 

In order to treat hardness quantitatively, it is essential to identify the entities (energies) that resist dislocation motion as well as the virtual forces (work) that drive the motion. These are the ying and yang of hardness. They are very different in pure metals as compared with pure covalent solids, and still different in salts and molecular crystals. 

Other Compounds. The molecular crystal I2 has been studied by Pasternak, Simopoulos, and Hazony (26). By measuring the temperature dependence of the recoilless fraction they obtained an effective Moss-bauer temperature, Om = 60°K., which is considerably less than the range found for the alkali iodides, Om = 100° to 120 °K. Because the covalent intramolecular bonding in I2 is much stronger than the intermolecular bonding, it is reasonable to assume for data interpretation that the recoil energy is taken up by the entire I2 molecule. 

As already mentioned in Section 1.1, chemists regard molecular crystals as supermolecules. This is fully justihed since molecules are built by connecting atoms through covalent bonds and crystals are built by connecting molecules with inter-molecular interactions. Crystal engineering can be dehned as the understanding of... 

Two later sections (1.6.5 and 1.6.6) look at the crystalline structures of covalently bonded species. First, extended covalent arrays are investigated, such as the structure of diamond—one of the forms of elemental carbon—where each atom forms strong covalent bonds to the surrounding atoms, forming an infinite three-dimensional network of localized bonds throughout the crystal. Second, we look at molecular crystals, which are formed from small, individual, covalently-bonded molecules. These molecules are held together in the crystal by weak forces known collectively as van der Waals forces. These forces arise due to interactions between dipole moments in the molecules. Molecules that possess a permanent dipole can interact with one another (dipole-dipole interaction) and with ions (charge-dipole interaction). Molecules that do not possess a dipole also interact with each other because transient dipoles arise due to the movement of electrons, and these in turn induce dipoles in adjacent molecules. The net result is a weak attractive force known as the London dispersion force, which falls off very quickly with distance. 

Examples of molecular crystals are found throughout organic, organometallic, and inorganic chemistry. Low melting and boiling temperatures characterize the crystals. We will look at just two examples, carbon dioxide and water (ice), both familiar, small, covalently bonded molecules. 

The ionic radius of chlorine has the value 1.81 A (Chap. 13). The following distances between chlorine atoms of different molecules have been observed in the molecular crystal 1,2,3,4,5,(Whexachlorocyclo-hexane.-57 3,60, 3.77, 3.82 A these are close to twice the ionic radius. Similar agreement is shown by many other organic crystals and inorganic covalent crystals. Cadmium chloride, for example, consists of... 

A supramolecular synthon represents a reproducible, frequently occurring kind of non-covalent interaction found in molecular crystal structures. It has predictive power and may be used in crystal design. Supramolecular synthons are distinct from tectons, the molecules or the building blocks of the crystal. 

The advantages of SQMF however can be understood only in comparison with the purely empirical schemes for normal mode calculations. Despite of their wide diversity VFF is always present as a compulsory element, as far as compounds with well pronounced covalent bonding are considered (isolated molecules, molecular crystals, polymers, etc.). In the context of the SQMF technique VFF is also an important model from a conceptual point of view. For this reason in the Introduction we will consider the main ideas which underlie the VFF model. The SQMF method itself will be considered in the following sections. 

Ans. (a)Ba—denser electron sea (b) Si—covalently bonded network versus molecular crystal (c)Xe—higher atomic number means stronger London forces (d) MgF2—cations and anions both smaller. 

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