A quantum-chemical study of hydride transfer from alkanes to carbenium cations in the gas phase: A comparison with liquid- and solid-acid catalytic systems

An ab initio investigation of the gas-phase hydride transfer reaction R-H + R(+) --> [R-H-R](+) --> R(+) + R-H has been performed. Four examples of this reaction, with R = methyl, ethyl, sec-propyl and tert-butyl fragments were considered. The potential-energy surfaces (PES) for all these cases found at the MP2 level have potential wells which correspond to the symmetric [R-H-R](+) complexes. Such PES are different from those for hydride transfer in liquid super acids and especially in heterogeneous zeolite catalysts. Indeed, solvation effects change the relative stability of carbenium ions R(+) and carbonium ions [R-H-R](+) in liquid super acids. Therefore, the latter species correspond to small barriers on PES for hydride transfer, instead of potential wells in the gas phase. In zeolite catalysts, the interaction of carbenium ions R(+) with zeolite surfaces is much stronger and results in formation of covalent surface esters instead of ion pairs, whereas carbonium ions [R-H-K](+) represent high-energy activated complexes.