Back to structural formulae

In Chapter 8, we reconciled the realistic visualization of molecules in terms of a global electron density distribution around an equilibrium arrangement of nuclei, as required by experiment, with the classical description of molecular structure in terms of individual atoms bonded to their neighbours. For a comprehensive analysis of the various ways of representing molecular structures, readers are invited to consult ref. 125. 

In theoretical terms, the total electron density in a molecule is easily expressed in terms of the occupied molecular orbitals. Additional information is gained from the m.o. approach especially regarding the electronic energy for ground and excited states and the detailed features (e.g. phase) of individual m.o.s. Molecular orbitals are mathematical functions that can be constructed as linear combinations of orbitals of the contributing atoms, in a process where the atoms lose their individuality, except for the respective nuclei and, perhaps, the core electrons. The valence electrons are described by functions which, in general, extend to several atoms or even to the whole molecule. 

I is in general no direct relation between such functions and ionization energies or electron excitation this is because they are not eigenfunctions of a hamiltonian, hence they cannot be associated with an energy. For that reason, we kept the usual designation localized molecular orbitals but with [ the last word in inverted commas orbitals . However, for the interpretation of some other molecular properties, the minimized residual interactions i between quasi-localized molecular orbitals are not very importaint and, so, the direct use of a localized bond description is quite justified. That is the [ Case for properties such as bond energies and electric dipole moments, as well as the features of the total electron density distribution with which those properties are directly associated. 

In Chapter 9, we found that some geometrical arrangements of nuclei do not allow an equivalent set of localized molecular orbitals to be defined. In such cases, there are non-localizable canonical m.o.s these structures are presented as resonant hybrids in the classical valence-bond description. 

We have previously (Chapter 8) discussed the valence-shell electron pair repulsion method as a predictive model of molecular geometry which. 

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