| Sumario: | The orbital symmetry forbidden thermal electrocyclic equilibria between a series of cyclophanedienes and dimethyldihydropyrenes (CPDs→←DDPs) were studied using density functional theory (DFT). These reactions are important not only because of their fundamental interest but also in how they restrict the potential utility of the DDP photoswitches by limiting the thermal lifetime of the CPDs. The transition states (TS5) for these reactions could not be modeled using restricted OFT (RB3LYP) but were located using unrestricted OFT (UB3LYP). Each TS possesses significant biradical character as indicated by their spin contaminated wave functions, (S²) ≠ 0. Specific substitution by nitrile or trifluoromethyl group(s) is predicted to strongly affect the magnitude of the activation barriers for these reactions. In particular, replacing the internal methyl groups of the CPDs/ODPs with nitrile groups is predicted to have the maximum effect and to raise the activation barriers and lifetimes of the CPDs considerably.
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