The spin-coupled description of aromatic, antiaromatic and nonaromatic systems

The correlated motion of π electrons in a range of aromatic, antiaromatic and nonaromatic systems is investigated using modern spin-coupled theory. In the particular case of benzene, the resulting wavefunction, which is superior to the simple MO description, resembles closely the classical VB picture, except for small (but crucial) deformations of the orbitals. It is argued, by reference to a multiconfiguration approach which subsumes both of the simple MO and VB models, that we should feel comfortable with switching between the two modes of description, according to which leads most directly to correct predictions for the particular problem at hand. The bonding in benzene is contrasted with that in cyclobutadiene and cyclooctatetraene, at various geometries. In the spin-coupled model, the distinguishing feature of antiaromatic species is the occurrence of triplet coupling of electron pairs. Of course, the singlet ground states of both of these molecules distort from the idealized high-symmetry geometries to nonaromatic situations with olefinic bonds.