Isoconjugate Spectra and Variconjugate Sequences

Abstract

Two molecules are isoconjugate or, more strictly, iso-π-electronic if they have the same number and geometrical arrangement of π-electrons bound about equally tightly in a planar conjugated system. Their π-electron spectra then show close resemblances. Example: aniline-styrene. A variconjugate sequence is a series of such molecules in which the binding of some of the π-electrons and the appearance of the spectra slowly changes. Example: fluorobenzene (iso-π-electronic with benzene), phenol, aniline (iso-π-electronic with styrene). Here the heteroatoms are formally saturated and no transitions of nonbonding electrons (n-bands) are observed. Second example: phenyl cyanide (almost iso-π-electronic with benzene), benzaldehyde, phenyl azomethane (iso-π-electronic with styrene). Here the heteroatoms are formally conjugated and n-bands appear at long wavelengths in addition to the π-spectra. The location of the π-bands partly determines that of the n-bands. The π-spectrum of every organic molecule with a closed-shell ground state lies on a variconjugate sequence between the π-spectra of two conjugated hydrocarbons. Several such sequences involving 90-odd compounds are displayed, showing the different binding of π- and n-electrons by different heteroatoms, and the interaction of n-bands of neighboring heteroatoms. An examination is made of the general problem of assigning observed electronic transitions in organic-molecule spectra to particular theoretically predicted transitions. It is shown how variconjugate sequences could be useful in making such assignments.