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The carbocations 1,5-and 1,6-dimethylcyclododecyl are postulated to have a p-hydrido-bridged structure. This assignment is based on the observation of a single very high-field hydrogen in the 'H NMR, anomalously low coupling constants involving this hydrogen, and very small isotope perturbation shifts using Saunders' criterion for equilibrating and resonance structural distinction. Both positional and conformational isomerization is possible in these bridged ions, but no evidence for cis-trans isomerization was found. The p-hydrido bridging is best treated as an "open" three-center, two-electron resonance system involving a single potential minimum for the bridged hydrogen. The bridging hydrogen is not electrophilic, and the bridged ions cannot be prepared by protonating the central bond of decalins nor can the p-hydrogen be removed as a proton. Numerous theoretical calculations have been performed and correlated with the experimental data.We have recently reported the direct observation of a series of secondary medium-ring cycloalkyl cations for which a p-hydrido-bridged structure 1 (R, = R2 = H ) was m \[I I R2 1, m = n = 3; m = 3, n = 4; m = 3, n = 5; m =4, n = 5Although these secondary cation investigations are very relevant in terms of previously observed transannular reactivity in medium rings: they are not ideal species for looking in detail at the evidence for this p-hydrido bridging. The corresponding ditertiary analogues, e&, R1 = Rz = CH3, have two major advantages: (1) they are thermally more stable, and (2) the proposed p-hydrido bridge can be fixed geometrically simply by positioning the two alkyl groups in a known relationship.In this work, we report a detailed investigation of the dimethylcyclodecyl ~y s t e m .~

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From the observation of very low temperature line broadening in the NMR spectrum of the methylcyclobutyl cation 1, one is forced into a major reinterpretation regarding the structure of this ion. Contrary to the latest literature interpretation, the methylcyclobutyl cation is not in a populated equilibrium with the a-methylcyclopropylcarbinyl cation. In order to rationalize the very unusual I3C+ chemical shift in 1, we consider the possibility of sp3 hybridization for this cation center. This possibility is examined by considering four additional criteria which might allow one to distinguish sp2-and sp3-hybridized carbonium ions: (1) comparisons of the NMR spectra of 1 with spectra of other substituted cyclobutyl cations which are shown to be normal sp2-hybridized cations; (2) comparisons of IH-IH coupling in 1 with coupling of this type in known sp2-hybridized cations; (3) comparisons of I3C-l3C coupling constants in 1 and in analogous five-and six-membered rings; and (4) comparison of +C-CH3 'H chemical shifts in 1 and in sp2-hybridized model compounds. The possibility of an sp3-hybridized carbonium ion structure for 1 is considered in light of previous solvolysis results and previous molecular orbital calculations.

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