Is Delocalization a Driving Force in Chemistry? Benzene, Allyl Radical, Cyclobutadiene, and Their Isoelectronic Species

Sason S. Shaik*, Philippe C. Hiberty, Jean michel Lefour, Gilles Ohanessian

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

274 Scopus citations

Abstract

The VB correlation diagram model (Figure 1) is used to answer the title question. It is shown that only atoms that form weak two-electron bonds with low triplet excitation energies may generate delocalized species that are stable toward a localizing distortion. Electronic delocalization is, then, seldom expected to be a significant driving force in chemistry. By this principle, the 7r-components of delocalized species, like C6H6and C3H5, are predicted to be distortive electronic systems that are trapped, within “rigidly” symmetric tr-frames, and are thereby delocalized despite their opposite inherent tendency. The predictions are examined by means of ab initio investigations at the levels of STO-3G, 6-31G, and 6-311G with extensive correlation (Cl) calculations (up to 6 X 106determinants). <r-ir energy partitions show that the 7r-components of C6H6and C3H5are indeed distortive much like the 7r-electrons of C4H4, and all the ?r-components resemble, in turn, their isoelectronic H„ (n = 3, 4, 6) species in the common reluctance to adopt geometries that lead to electronic delocalization. Electronic delocalization in C3H5and C6H6turns out to be a byproduct of the a-imposed geometric symmetry and not a driving force by itself The 7r-distortive propensities are shown to coexist harmoniously with the thermochemical stability of benzene and the rotational barrier of allyl radical. Further application of the model shows that 7r-delocalization, per se, is seldom expected to be a driving force in organic molecules containing C, N, and O. In this manner the delocalization problem is unified and shown not to be merely a matter of electron count and mode of delocalization.

Original languageEnglish
Pages (from-to)363-374
Number of pages12
JournalJournal of the American Chemical Society
Volume109
Issue number2
DOIs
StatePublished - 1 Jan 1987
Externally publishedYes

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