Polyatomic molecules multiple bonds

The contents of Part 1 is based on such premises. Using mostly 2x2 Hiickel secular equations, Chapter 2 introduces a model of bonding in homonuclear and heteronuclear diatomics, multiple and delocalized bonds in hydrocarbons, and the stereochemistry of chemical bonds in polyatomic molecules in a word, a model of the strong first-order interactions originating in the chemical bond. Hybridization effects and their importance in determining shape and charge distribution in first-row hydrides (CH4, HF, H20 and NH3) are examined in some detail in Section 2.7. 

Resonance structures Two or more Lewis structures for a single molecule or polyatomic ion that differ in the positions of lone pairs and multiple bonds but not in the positions of the atoms in the structure. It is as if the molecule or ion were able to shift from one of these structures to another by shifting pairs of electrons from one position to another. 

In diatomic molecules it was observed that the vibration frequency increased with the multiplicity of the bond and similar behaviour occurs in polyatomic molecules. The frequency of the C—G bond lies in the range 800 to 1,200 cm"1 and increases to 1,600 cm"1 for the C=C bond and to 2,100 cm"1 for C=C. Similar behaviour is observed in bonds between other atoms and the data are summarized... 

Chapter 4 Valence bond theory multiple bonding in polyatomic molecules 105... 

In the previous section, we emphasized that hybridization of some or all of the valence atomic orbitals of the central atom in an XY species provided a scheme for describing the X—Y f7-bonding. In, for example, the formation of sp, sp and sp d hybrid orbitals, some p or d atomic orbitals remain unhybridized and, if appropriate, may participate in the formation of TT-bonds. In this section we use the examples of C2H4, HCN and BF3 to illustrate how multiple bonds in polyatomic molecules are treated within VB theory. Before considering the bonding in any molecule, the ground state electronic configurations of the atoms involved should be noted. 

MO theory is extensively used for those polyatomic molecules in which multiple bonding occurs. We have already considered how N02 is formulated in VB theory as a resonance hybrid (3-V). In MO theory it can be treated in the following way. We first assume that a set of a bonds is formed using four electrons and that several other electron pairs are non-bonding. Thus we write a framework of nuclei and a electrons (3-VII). There are still four electrons to be assigned and each of the three atoms has an empty pn orbital (one whose nodal plane coincides with the molecular plane). If we start with... 

Johnson KH (1966) Multiple-scattering model for polyatomic molecules. J Chem Phys 45 3085-3095 Johnson KH (1973) Scattered-wave theory of the chemical bond. Adv Quantum Chem 7 143-185 Joly Y (2001) X-ray absorption near-edge stmcture calculations beyond the muffin-tin approximation. Phys Rev B63 125120-1-10... 

Many polyatomic molecules contain multiple bonds. Ethylene, C2H4, the structural unit of the plastic polyethylene, has a double bond between two carbon atoms. There is a triple bond between carbon atoms in acetylene, C2H2, the fuel used in a welder s torch. The carbon atom in carbon dioxide, CO2, is double bonded to two oxygen atoms. The Lewis diagrams for these compounds are... 

LCAO-MO theory is specially appropriate for describing polyatomic molecules in which multiple bonds occur. Strict application of the theory often demands very complex calculations in which all the valence orbitals in a field formed by the nuclei and the rest of the electrons should be considered. However there are some simplifications that make its application easier. 

Electron-dot formulas can be used to diagram the sharing of valence electrons in molecules and polyatomic ions. The presence of multiple bonds can be identified, and possible resonance structures can be drawn. From the electron-dot formulas, we can predict the three-dimensional shapes and polarities of molecules. Then we examine how the different attractive forces between the particles of ions and molecules influence their physical properties, such as melting and boiling point. Finally, we discuss the physical states of solids, liquids, and gases and describe the energy involved in changes of state. 

Up to now, we have looked at covalent bonding in molecules or polyatomic ions that have only single bonds. However, in some molecular compounds, atoms share two or three pairs of electrons to complete their octets. A double bond occurs when two pairs of electrons are shared in a triple bond, three pairs of electrons are shared. Atoms of carbon, oxygen, nitrogen, and sulfur are most likely to form multiple bonds. Atoms of hydrogen and the halogens do not form double or triple bonds. 

When a molecule or polyatomic ion contains multiple bonds, it may be possible to draw more than one electron-dot formula. We can see how this happens when we draw the electron-dot formula for ozone, O3, a component in the stratosphere that protects us from the ultraviolet rays of the Sun. 

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