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.