Much experimental and theoretical work has been directed elucidating the nature of the cationic intermediate(s) involved in cyclopropylcarbinyl, cyclobutyl, and allylcarbinyl interconversions under so-called “stable-ion" as well as solvolytic condition. Whereas all experimental evidence on C_4H_7^+ indicates that the species is a nonclassical cation, controversy continues regarding the equilibrium geometry of this cation, with some favoring the bicyclobutonium structure , and others the “bisected” cyclopropylcarbinyl arrangement (1b).
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Staral et al. (1978) studied this question.