Structure and Diamagnetic Susceptibility of Cyclo-octatetraene

ACCORDING to Pascal's empirical formulæ¹, if cyclo-octatetraene has a structure corresponding with a cyclic system of conjugated double and single bonds, which may be puckered, the molecular susceptibility to be expected is - 50.2 × 10^-6 C.G.S. On the other hand, if it contains eight `aromatic' carbon atoms with eight pooled electrons, the molecular susceptibility to be expected would be - 73.4 × 10<sup>-6</sup> C.G.S. This characteristic increase in diamagnetic susceptibility when a pool of `aromatic' electrons is present² makes a useful index of aromatic character. In the case of cyclo-octatetraene, recently published data for the heat of formation³ lead to a calculated resonance energy of 25.4 kcal./mole. Although this resonance energy is smaller than in the case of benzene (approximately 39 kcal./mole)⁵, it is sufficiently high to leave some doubt as to whether cyclo-octatetraene may have its structure stabilized by the formation of an aromatic pool of electrons. Optical evidence⁴ is not quite decisive on this point. Measurements have now been made on the magnetic susceptibility of cyclo-octatetraene, to test for an aromatic pool of electrons by the enhanced diamagnetism.