Abstract
This work is concerned with the application of recent theoretical methods in elucidating the spectra of heterocyclic systems. It is now generally accepted that a satisfactory picture of the first few excited states can only be obtained by admitting limited configuration interaction. But this picture is also dependent upon the precise choice of orbitals - which, in a heterocyclic molecule, may range from Huckel molecular orbitals of the parent hydrocarbon to self-consistent orbitals of the substituted system and scarcely any information is available concerning the relative merits of different choices. In the present paper (Part I) general methods are summarized, seven different choices of orbitals are suggested and discussed, and careful numerical comparisons are made in applications to pyridine and pyrazine. Useful conclusions emerge, in the light of which it is possible to consider more complicated systems. Such developments are taken up in a companion paper by Peacock (Part II).
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