| Abstract: | A computational study has been undertaken to elucidate the mechanism of the bicyclopropylidene radical cation (BCP?+) rearrangement into the tetramethyleneethane radical cation (TME?+). A stepwise mechanism is found for the first ring opening, with an activation energy of 7.3 kcal mol?1, while the second ring opening proceeds with no activation energy. Each ring opening is combined with a striking pyramidalization of one carbon atom in the central bond. In a natural bond orbital (NBO) analysis, the dominating reaction coordinate during the ring opening is found to be the olefinic carbon atom rehybridization, which also favors the continued bond breaking. Widely different ESR parameters are computed for the two sets of four protons in BCP?+, in excellent agreement with the observed spectrum, which are interpreted in the NBO analysis in terms of two hyperconjugative effects. Two minimum energy structures are located for TME?+, separated by a cusp on the internal rotation path of this cation, both of which show ESR parameters in good agreement with the observed spectrum for TME?+. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 |
