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

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.