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Photodehydrocyclizations, analogous to the formation of phenanthrene from stilbene, appear to be possible only if the sum of the free valence numbers of the proper atoms in the first excited state exceeds a critical value. Localization energies and superdelocalizabilities also set bounds to the occurrence of this type of reaction. Calculation of the relative magnitudes by the simple Hückel MO method appears to be satisfactory only in alternating compounds.
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Laarhoven et al. (1968) studied this question.
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