DOES ANTIAROMATICITY IMPLY NET DESTABILIZATION

An analysis is presented of the results of earlier ab initio computati onal studies of cyclobutadiene, cyclooctatetraene, and 1,4-dihydropyra zine. The first and third of these are normally categorized as antiaro matic. All three molecules are polyenes, even when the last two are fo rced into planar conformations. There is no driving force for extensiv e pi delocalization, even when it would appear to have been facilitate d. Calculated isodesmic energies show a net destabilization only in th e case of cyclobutadiene, which we attribute to strain and repulsion b etween the pi electrons of the C=C double bonds. The other two molecul es have negative isodesmic energies, indicative of net stabilizing eff ects. We conclude that the concept of antiaromaticity is useful for id entifying molecules that resist the apparent opportunity for extensive pi delocalization, but that it does not intrinsically imply net desta bilization. (C) 1994 John Wiley and Sons, Inc.