Choice of Parameters for the Hückel π-Electronic Structure of Furan: Tentative Extension to Pyrrole and Thiophene

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Abstract
A comparison of the experimental bond lengths of furan with empirical bond length/bond energy relations for carbon-oxygen bonds leads to the conclusion that the ratio of the carbon-oxygen resonance integral to the carbon-carbon resonance integral of furan is between about 0.1 to 0.3. Use of this value along with a Coulombic integral parameter based upon the ultraviolet absorption spectrum of furan leads to a Hückel molecular orbital picture of the π-electronic structure of furan which is compatible with the known chemical reactivity of the molecule. The same approach is used to suggest resonance and Coulombic parameters for the Hückel molecular orbital treatment of pyrrole and thiophene. The treatment yields atom localization energies and para-localization energies consistent with the relative chemistries of furan, pyrrole, and thiophene.