The structure of the C4H6CH3+ cation 13C and 1H NMR spectroscopic investigation of equilibrium isotope effects in deuterated methylbicyclobutonium ions

Deuterium equilibrium isotope effects (EIE) in fast rearranging C4H6CH3+ cations are measured by 13C and 1H NMR spectroscopy. At -1530 C the cation is static and shows no EIE. Comparison of the EIE in model cations with cyclopropylmethyl and cyclobutyl structures taken together with the EIE evidence for hypercoordination in the parent C4H7+ cation shows that the minimum energy structure of the C4H6CH3+ cation is a threefold degenerate pentacoordinated methylbicyclobutonium ion. Rapid interconversion through a planar 1-methylcyclobutyl cation transition state equilibrates the geminal methylene protons. Therefore only averaged isotope effects are observable in monodeuterated C4H6CH3+ cations whereas distinct effects for configurationally stable exo and endo monodeuterated C4H7+ cations have been reported previously. 16 refs.; 5 figs