General interpretative models have been devised for the collective treatment of the electronic spectra given by a group of structurally related molecules. The use of symmetry relations is illustrated by an analysis of the long wavelength absorption of di-and tri-arylmethyl ions, and the Hiickel orbitals of homonuclear ~r-electron models are employed to interpret the spectra of analogous heteronuclear conjugated molecules. The general scope of the spectroscopic applications of the Hiickel theory is discussed.
HITHERTO, the more rigorous applications of quantum chemistry to the interpretation of molecular electronic spectra have been confined largely to two main groups of compounds; firstly, diatomic molecules, 1 and a few particularly simple polyatomic systems, such as formaldehyde, z ethylene, 3 and acetylene, 4 and, secondly, homonuclear conjugated 7r electron systems, of which the aromatic hydrocarbons are the outstanding example, 5 though, in principle, polyenes, polyacetylenes, cumulenes, and related oddatom systems may be treated at corresponding levels of approximation. The spectra of the compounds in these groups constitute only a small fraction of the electronic absorption spectra recorded in the literature, from which, for example, some 45,000 spectra of organic molecules have been collected 6 for the period 1946-1955 alone.
The extension of the more rigorous quantum mechanical methods to heteronuclear conjugated ~v-electron systems, which comprise the majority of organic compounds for which electronic absorption spectra are available, involves, for the most part, the treatment of each molecule as a particular case, and any resultant unity lies more in the formalism of the treatment rather than in the structural chemistry of the compounds studied. A more economic and chemically more informative procedure consists in the treatment of a group of related absorption spectra derived from a particular class of organic compound in terms of a general model embodying the common structural features of the molecules investigated.