The electronic spectra of substituted aromatic hydrocarbons

Abstract

IN THIS paper I wish to give a critical comparison of the quantum mechanical models which may be used for interpreting the spectra of substituted aromatic hydrocarbons. The first point to be decided when entering into a calculation of electronic energy levels, is what kind of orbitals are to be used to build up the electronic states. I f one is going to work within the valence bond framework, then one uses just the atomic orbitals, and constructs wave functions from these according to the Heit ler-London description of the electron pair bond. This approach will give satisfactory wave functions and energies if the basic set of states is large enough. However, the crude theory (which we may refer to as resonance theory), in which one includes only those wave functions which are associated with chemical structures having most of the electrons paired, and an electron distribution which is favoured by the electronegativity of the atoms in the molecule, does not give sensible results.1 For example, if benzene is substituted by one donor group (e.g. OMe, NH2) and one acceptor (e.g. NO2, COOH) then one finds that the spectra of the ortho and meta substituted compounds are very similar to one another but different from that of the para compound. Moreover, the para compound absorbs at shorter wavelengths than the other two. Resonance theory however, would say that it is the meta compound which should be the odd one out, since it is the only one which cannot be given a quinonoid structure of the type.