The Magnetic Properties of Several Pseudo-Jahn-Teller Nonbenzenoid Aromatic Hydrocarbons

The effect of bond length alternation on the magnetic properties of several Pseudo-Jahn-Teller nonbenzenoid aromatic hydrocarbons with 4ν carbon atoms will be investigated. The magnetic moment and the susceptibility of a cyclic polyene will first be formulated in terms of the bond alternation parameter. Appropriate cross-links for forming nonbenzenoid aromatic hydrocarbons, such as pentalene and heptalene, will then be treated as perturbations. It will be shown that the anomalously large paramagnetic susceptibilities of these molecules predicted by Pullman et al. (“Les Theories Electroniques de la Chimie Organique,” Masson, Paris (1952), p. 527) arise mainly from the neglect of the bond alternation inherent in them. It will then be concluded that the magnetic susceptibility is a very sensitive indicator of the bond alternation, and that the diamag-netically-induced ring currents in these molecules are very much impeded.

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