The pairing of electronic states in alternant hydrocarbons

The pairing of Hückel molecular orbitals in an alternant hydrocarbon holds in a more general sense for wave functions which make complete allowance for the correlation of π electrons within the atomic orbital scheme. The new pairing property holds exactly in the scope of Pariser, Parr, and Pople's approximations, and leads to an exact correspondence between every detail of the excited states, the electronic spectra and electron resonance spectra of positive and negative hydrocarbon ions. Neutral molecules have a half-filled electron shell in which electrons and holes are on an equal footing. This causes the electron distribution to be uniform in every electronic state, and leads to two kinds of excited state—‘even’ and ‘odd’—as Pariser first suggested. Transitions between states of the same parity are forbidden. In neutral radicals the spin density vanishes in all the bonds, and in both radicals and molecules the bond orders vanish between atoms of the same set (starred or unstarred).