While characterizing the compound resulting from deprotonation of cyclooctatriene by H NMR spectroscopy, the group observed that the resonance corresponding to two protons bonded to the same methylene bridge carbon exhibited an astonishing degree of separation in chemical shift.
From this observation, Pettit, et al. concluded that the classical structure of the cyclooctatrienyl cation must be incorrect. Instead, the group proposed the structure of the bicyclo[5.1.0]octadienyl compound, theorizing that the cyclopropane bond located on the interior of the eight-membered ring must be subject to considerable delocalization, thus explaining the dramatic difference in observed chemical shift. Upon further consideration, Pettit was inclined to represent the compound as the "homotropylium ion," which shows the "internal cyclopropane" bond totally replaced by electron delocalization. This structure shows how delocalization is cyclic and involves 6 π electrons, consistent with Huckel's rule for aromaticity. The magnetic field of the NMR could thus induce a ring current in the ion, responsible for the significant differences in resonance between the exo and endo protons of this methylene bridge. Pettit, et al. thus emphasized the remarkable similarity between this compound and the aromatic tropylium ion, describing a new "homo-counterpart" to an aromatic species already known, precisely as predicted by Winstein.
Subsequent NMR studies undertaken by Winstein and others sought to evaluate the properties of metal carbonyl complexes with the homotropylium ion. Comparison between a molybdenum-complex and an iron-complex proved particularly fruitful. Molybdenum tricarbonyl was expected to coordinate to the homotropylium cation by accepting 6 π electrons, thereby preserving the homoaromatic features of the complex. By contrast, iron tricarbonyl was expected to coordinate to the cation by accepting only 4 π electrons from the homotropylium ion, creating a complex in which the electrons of the cation are localized. Studies of these complexes by H NMR spectroscopy showed a large difference in chemical shift values for methylene protons of the Mo-complex, consistent with a homoaromatic structure, but detected virtually no comparable difference in resonance for the same protons in the Fe-complex.