Building on theoretical studies of Mulliken on the nature of electrophilic aromatic substitution, three levels of theory in computational chemistry are used to expand upon and quantify his central hypothesis that hyperconjugation (HC) can play an important role in determining structures, spectra, and reactivities in chemical systems. In this work benzenium, and four isomeric forms of toluenium, as well as certain butenolides, are studied using self-consistent field, configuration interaction and density functional theory. Results for energetics, geometric parameters, atomic populations, dipole moments, and polarizabilities are reported. The linking of the CH<sub>2</sub> and CH<sub>3</sub> groups in toluenium is consistent with the outgrowth of a strongly stabilizing quinoidal structure as a consequence of the HC, as was predicted by Mulliken. The calculations here show that the charge coming from toluene protonation migrates predominantly to carbon atoms meta to the methyl group, as exhibited by reduced negative net charges compared to the parent molecules. The optimized geometrical parameters of the three structural isomers of toluenium indicate that the position of the proton does not strongly impact the strength of the HC. In the six butenolide structures examined here, there is an energetic preference for the ester group outside the ring to be co-planar with the ester group that is part of the butenolide ring, and also, therefore, with the α,b π bond of the ring. The results generally support the predictions that hyperconjugation leads to key bond length shortenings, decreased polarizabilities and electron density shifts that can impact reactivities in substituted aromatic molecules.

Computational studies of hyperconjugation in chemical structures, spectra, and reactivities of molecules. benzenium, toulenium and butenolides

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