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The molecular orbital (LCAO) method has been used to calculate the electron delocalization energies, bond orders and freevalence indexes of some cyclic small-ring hydrocarbons and free radicals including a number of cyclobutadiene derivatives. It is concluded that the (4n + 2) π-electron rule of aromatic stability can only be justified by the simple molecular orbital treatment for monocyclic conjugated polyolefins.
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Roberts et al. (1952) studied this question.
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