Valence bond theory Bonding multiple bonds

Before considering how SMIRKS can be used to carry out transformations with multiple reactants, first consider simpler unimolecular transformations. These are discussed separately because of the important use of unimolecular transformations to enforce the consistent use of SMILES throughout the database. This improves the integrity of the data in a chemical sense, rather than a relational database sense as discussed previously. The root of the issue is this There are multiple ways to represent the same molecular structure due to the limitations of valence bond theory. In valence bond theory, upon which SMILES is based, atoms have formal charges, most often zero. The bonds between atoms are shared pairs of electrons and may consist of multiple shared pairs giving rise to double, triple, or possibly even higher-order bonds between atoms. This simple theory, while quite powerful and applicable to a majority of chemical structures, leads to certain ambiguities. 

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

Applying Valence Bond Theory (Multiple Bonding)... 

The bonding and geometry in a molecule can be described in terms of valence bond theory. In this theory, a bond is formed by the overlap of orbitals from two atoms. Hybrid orbitals, a set of equivalent orbitals formed by combining atomic orbitals, are often needed to describe this bond. Multiple bonds occur via the overlap of atomic orbitals to give cr bonds and tr bonds. Cis-trans isomers result from the molecular rigidity imposed by a tt bond. 

Jaffe, H. H. Studies in Molecular Orbital Theory of Valence. III. Multiple Bonds Involving d-Orbitals. J. physic. Chem. 58, 185—190 (1954). 

The crystal structure of 864(118207)2 90, 91) has shown 864 to be square planar with an 8e-8e bond distance of 2.283(4) A, significantly less than that of 2.34(2) A found in the 8eg molecule (92), indicating some degree of multiple bonding. 8uch a result is consistent with a valence bond description of the molecule involving four structures of type VII. Alternatively the structure can be understood in terms of molecule orbital theory. The circle in structure VIII denotes a closed-shell (aromatic ) six-w-electron system. Of the four tt molecular orbitals,... 

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. 

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