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In Situ Observation of Non-Classical 2-Norbornyl Cation in Confined Zeolites under Ambient Temperature.

Carbonium ions are an important class of reaction intermediates, but their dynamic evolution is difficult to be monitored by in situ techniques under experimental conditions because of their extremely short life. Probably the most famous case is 2-norbornyl cation (2NB + ): its existing form (classical or non-classical) had been debated for decades, until the concrete proof of non-classical geometry was obtained by X-ray crystallographic characterization under ultra-low temperature (40 K) and super acidic environment. However, we are in lack of understanding to 2NB + under ambient conditions. Herein, by taking advantage of the confinement effect and delocalized acidic environment of zeolites, we successfully stabilized 2NB + and unequivocally confirmed its "non-classical" structure inside the ZSM-5 zeolite by ab initio molecular dynamics simulations and 13 C solid-state nuclear magnetic resonance experiments. It is the first time to in situ observe the non-classical 2NB + without the super acidic environment under ambient temperature, which provides a new strategy to expand the carbocation chemistry.