Is delocalization a driving force in chemistry ? Benzene, allyl radical, cyclobutadiene and their isoelectronic species.
Résumé
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 delocaliaztion is, then, seldom expected to be a significant driving force in chemistry. By this principle, the pi-components of delocalized species, like C6H6 or C3H5, are predicted to be distortive electronic systems that are trapped, within rigidly symmetric sigma frames, and are thereby delocalized despite their opposite inherent tendency. The predictions are examined by means of ab initio investigations at the level of STO-3G, 6-31G and 6-311G with extensive correlation (CI) calculations (up tp 6*10+6 determinants). sigma-pi energy partitions show that the pi-components of C6H6 and C3H5 are indeed distortive much like the pi-electrons of C4H4, and all the pi-components resemble, in turn, their isoelectronic Hn (n=3,4,6) species in the common reluctance to adopt geometries that lead to electronic delocalization. Electronic delocalization in C3H5 and C6H6 turns out to be a byproduct of the sigma-imposed geometric symmetry and not a driving force by itself. The pi-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 pi-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.