Molecular shape bond length

Problem 20.25 Give the electronic structure of the cyanamide anion, (NCN) Discuss its molecular shape, bond lengths, and location of charge. 

In the solid state, molecules line up in a pattern forming a crystal lattice similar to that of an ionic solid, but with less attraction between particles. The structure of the crystal lattice depends on the shape of the molecule and the type of intermolecular force. Most information about molecules, including properties, molecular shape, bond length, and bond angle, has been determined by studying molecular solids. 

Fig. 3.2). The bond angle in H20, for instance, is the angle (104.5°) between the two O—H bonds. Molecular shape, bond angles, and bond lengths can now be predicted by calculations based on the Schrodinger equation. These calculations are sometimes based partly on experimental information, when they are called semiempirical methods, and sometimes are purely theoretical predictions, when they are called ab initio methods. We shall see some of their output later in the chapter. 

Of the various geometric parameters associated with molecular shape, the one most nearly constant from molecule to molecule and most nearly independent of substituent effects is bond length. Bond lengths to carbon depend strongly on the hybridization of the carbon involved but are little influenced by other factors. Table 1.2 lists the interatomic distances for some of the most common bonds in organic molecules. The near constancy of bond lengths from molecule to molecule reflects the fact that the properties of individual bonds are, to a good approximation, independent of the remainder of the molecule. 

The next most important aspect of a molecular compound is its shape. The pictorial representations of molecules that most accurately show their shapes are images based on computation or software that represents atoms by spheres of various sizes. An example is the space-filling model of an ethanol molecule shown in Fig. C.2a. The atoms are represented by colored spheres (they are not the actual colors of the atoms) that fit into one another. Another representation of the same molecule, called a ball-and-stick model, is shown in Fig. C.2b. Each ball represents the location of an atom, and the sticks represent the bonds. Although this kind of model does not represent the actual molecular shape as well as a space-filling model does, it shows bond lengths and angles more clearly. It is also easier to draw and interpret. 

As we describe in Section 94, the bond length of a covalent bond is the nuclear separation distance where the molecule is most stable. The H—H bond length In molecular hydrogen is 74 pm (picometers). At this distance, attractive interactions are maximized relative to repulsive interactions (see Figure 9-2). Having developed ideas about Lewis structures and molecular shapes, we can now examine bond lengths In more detail. 

Conformational analysis consists in investigations concerned with the determination of molecular shapes, commonly described by bond angles and bond lengths. Among the various methods generally used68 for the estimation of these parameters, X-ray analysis provides... 

As shown in Fig. 4.69, the HfFLi- alkene complex exhibits expected parallels with the HfFLi- H2 complex (Fig. 4.59), both in terms of molecular shape and in terms of valence interactions. The characteristic features of such weak dative bonding include long Hf—C distances (2.82 A), normal C=C bond length (1.34 A), planar alkene bond angles, and small binding energy (15.1 kcalmol-1)-... 

General theories of molecular structure have been advocated by different groups in the past but their applicability has been restricted to the shape, Le. bond lengths and bond angles, of small molecules. Two types of approaches deserve special mention ... 

Chemical oxidation (by AgC104) affords the monocation [Cu(OETPP)]+ the molecular structure of which has been solved.102b An additional ruffling of the original saddle shape arises as a consequence of the one-electron removal, but the bond lengths substantially remain unvaried (Cu-N=1.97 A Ca-N=1.38 A ... 

With these five typical bond lengths, and the typical bond angles for tetrahedral, trigonal, and linear arrays, it becomes possible to construct molecular models to predict a molecule s size and shape. This may be achieved via a molecular model kit, or by computer graphics. 

I. A property present in a molecular entity (or a transition state) if the energy of that entity or state is enhanced due to unfavorable bond lengths, bond angles, or dihedral angles relative to some appropriate standard. It is the standard enthalpy of a structure relative to a strainless structure (real or hypothetical). 2. The change of volume or shape of a body, or portion of a body, due to the influence of one or more applied forces. 

Steric parameters include bond length, bond angle, bond torsion, conformation, topology or connection among atoms in the molecule, symmetry, moment of inertia, molecular diameter, molecular surface area, molecular volume, and molecular shape. 

The contents of this chapter are fundamental in the applications of molecular orbital theory to bond lengths, bond angles and molecular shapes, which are discussed in Chapters 3-6. This chapter introduces the principles of group theory and its application to problems of molecular symmetry. The application of molecular orbital theory to a molecule is simplified enormously by the knowledge of the symmetry of the molecule and the group theoretical rules that apply. 

Throughout the book, theoretical concepts and experimental evidence are integrated An introductory chapter summarizes the principles on which the Periodic Table is established and describes the periodicity of various atomic properties which are relevant to chemical bonding. Symmetry and group theory are introduced to serve as the basis of all molecular orbital treatments of molecules. This basis is then applied to a variety of covalent molecules with discussions of bond lengths and angles and hence molecular shapes. Extensive comparisons of valence bond theory and VSEPR theory with molecular orbital theory are included Metallic bonding is related to electrical conduction and semi-conduction. 

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