Bonding in Simple Molecules

The many different conformers resulting from rotation around the carbon-carbon bonds in simple molecules like ethane and w-butane may be shown by Newman projections (Figure 2.7). The most stable is the anti or trans projection where the steric hindrance is minimized. There are a number of eclipsed and gauche arrangements of which only one of... 

These studies form part of a programme on hydrogen bonding in simple molecules being carried out in the Cavendish Laboratories under the supervision of Dr. Cochran. The work on ammonium bifluoride has been done by T. R. R. McDonald, that on theophylline and... 

Dr. Schneider s paper has given an introduction to the subject of the nuclear resonance study of hydrogen bonding, which will not be repeated here. Our initial aim was to study molecular complexes in aqueous solution, with particular emphasis on complexes of biological importance. It soon became apparent that the phenomena were so complex that a fundamental study of hydroxyl-type hydrogen bonds in simple molecules was first required. 

Bonding in Simple Molecules That Contain Transition Metals... 

Unlike sulphur oxygen seldom forms more than two covalent bonds in simple molecules or ions. There are numerous ionic crystals in which 0 forms three bonds (rutile, phenacite, etc.). In salt hydrates where H2O is bonded to a metal atom, (a), or in hydroxides where OH bridges two metal atoms, (b), the bonds to the metal atoms presumably have appreciable ionic character, and this is probably also true in AlOCl (p. 408) and [Ti(0R)4] 4 (p. 942). Three equivalent (covalent )... 

This chapter shows the connection between the one-electron orbital configuration of the elements and their positions in the Periodic Table and their characteristic or group oxidation numbers, their variable valances and their abilities to form ionic and covalent bonds in simple molecules. 

Figure 4. Electron pair bonding in simple molecules. Each H atom shares two electrons, each of the other atoms has four pairs.

Valence shell electron-pair repulsion (VSEPR) is an approach that provides a method for predicting the shape of molecules based on the electron-pair electrostatic repulsion described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957 and in the succeeding decades. Despite this method s simple approach, based on Lewis electron-dot strnctnres, the VSEPR method in most cases predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules. The most common method of determining the actual structures is X-ray diffraction, although electron diffraction, nentron diffraction, and many spectroscopic methods are also used. In Chapter 5, we will provide molecular orbital approaches to describe bonding in simple molecules. 

The group orbital approach described in this chapter, despite its modest use of group theory, conveniently provides a qualitatively useful description of bonding in simple molecules. Computational chemistry methods are necessary for more complex molecules and to obtain wave equations for the molecular orbitals. These advanced methods also apply molecular symmetry and group theory concepts. 

Photoelectron spectroscopy has confirmed the essential features of the MO description of bonding in simple molecules however, the proper MOs are not always easy to visualize. A localized bond representation, involving hybrid AOs which overlap little with each other, accounts for most of the chemically important properties of most molecules and the localized bonds are easy to visualize. The localized bond picture can be related to the proper (approximate) description through perturbation theory. The localized bond model is generally not applicable to electron-deficient molecules, conjugated systems, or transition states. The application of perturbation theory to the description of these three cases where the localized bond model breaks down is the subject of the following chapters. 

The molecular structure input requires atom types to be assigned, which are not the same from one force field to the next. The input also includes a list of bonds in the molecule. There is not a module to automatically assign atom types. Most of the modules use a Cartesian coordinate molecular structure, except for a few that work with torsional space. The same keyword file is read by all the executables. A little bit of input is obtained by the program either interactively or from an ASCII file piped to standard input, which makes for a somewhat cryptic input file. This system of common input files and the user choosing which executables to run give TINKER the ability to run very sophisticated simulations while keeping the input required for simple calculations fairly minimal within the limitations mentioned here. 

Before considering the special case of rotation about bonds in polymers it is useful to consider such rotations in simple molecules. Although reference is often made to the free rotation about a single bond, in fact rotational energies of the order of 2kcal/mole are required to overcome certain energy barriers in such simple hydrocarbons as ethane. During rotation of one part of a molecule about... 

Ethylene oxide, the simplest epoxide, is an intermediate in the manufacture of both ethylene glycol, used for automobile antifreeze, and polyester polymers. More than 4 million tons of ethylene oxide is produced each year in the United States by air oxidation of ethylene over a silver oxide catalyst at 300 °C. This process is not useful for other epoxides, however, and is of little value in the laboratory. Note that the name ethylene oxide is not a systematic one because the -ene ending implies the presence of a double bond in the molecule. The name is frequently used, however, because ethylene oxide is derived pom ethylene by addition of an oxygen atom. Other simple epoxides are named similarly. The systematic name for ethylene oxide is 1,2-epoxyethane. 

This chapter and the next describe chemical bonding. First, we explore the interactions among electrons and nuclei that account for bond formation. Then we show how atoms are connected together in simple molecules such as water (H2 O). We show how these connections lead to a number of characteristic molecular geometries, hi Chapter fO, we discuss more elaborate aspects of bonding that account for the properties of materials as diverse as deoxyribonucleic acid (DNA) and transistors. 

Many of the Lewis structures in Chapter 9 and elsewhere in this book represent molecules that contain double bonds and triple bonds. From simple molecules such as ethylene and acetylene to complex biochemical compounds such as chlorophyll and plastoquinone, multiple bonds are abundant in chemistry. Double bonds and triple bonds can be described by extending the orbital overlap model of bonding. We begin with ethylene, a simple hydrocarbon with the formula C2 H4. 

It is easy to describe cis and trans forms in simple molecules. When four different groups are attached to a carbon-carbon double bond the cis form is that which contains the longest carbon chains on the same side of the double bond. 

B. D. Darwent. Bond Dissociation Energies in Simple Molecules. National Standard Reference Data Series, National Bureau of Standards 31 U.S. Department of Commerce Washington, D.C., 1970. 

Few examples have been reported (5, 8, 9, 10, 12, 24) of cage recombination of simple alkoxy or acyloxy radicals to form O—O bonds in isolable molecules. This paper explores further the implications of the observed (17, 22, 23) scrambling of label seen in acetyl peroxide carbonyl-18O recovered after partial decomposition. 

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