We study a potential surface for the tetramethylene diradical, CH2CH2CH2CH2, a likely intermediate in the nonconcerted fragmentation of cyclobutane to two ethylenes. Outside a clearly defined region of a distorted cyclobutane we find two apparent minima, in geometry close to trans and gauche conformations of a model «-butane. These minima appear to be unstable with respect to fragmentation to two ethylenes. In fact, a motion in any degree of freedom away from the distorted cyclobutane region leads to a level crossing and a tetramethylene species whose orbitals resemble those of two interacting ethylenes. If by the term "intermediate" is meant a true minimum in the many dimensional potential surface then our calculations imply there is no such species intervening between cyclobutane and two ethylenes. We suggest that a large energetically flat region of a potential surface will in the context of modern collision theory have the same operational consequences as a true intermediate, and that such a region, termed a "twixtyl," may be a common feature of nonconcerted reactions.
The 2.3-dimethylenetetramethylene biradical has a variety of interesting collapse products. We report extended Huckel and Pariser-Parr-Pople calculations of the electronic structure of this molecule as a function of twist around its central bond. Though our calculations point to a twisted triplet ground state they are not definitive. In any conformation the ally1 moieties couple only weakly and the lowest singlet state is then a nearly equal mixture of two configurations. An orbital symmetry analysis of the interconversions of the biradical and its collapse products predicts the stereochemical consequences of concertedness in these transformations.
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