Reactions of the dications C7H62+, C7H72+, and C7H82+ with methane: Predominance of doubly charged intermediates

The reactions of methane with the dications C7H62+, C7H72+, and C7H82+ generated by electron ionization of toluene are studied using mass-spectrometry tools. It is shown that the reactivity is dominated by the formation of doubly charged intermediates, which can either eliminate molecular hydrogen to yield doubly charged products or undergo charge-separation reactions leading to the formation of a methyl cation and the corresponding C7Hn+1+ monocation. Typical processes observed for dications, like electron transfer or proton transfer, are largely suppressed. The theoretically derived mechanism of the reaction between C7H62+ and CH4 indicates that the formation of the doubly charged intermediate is kinetically preferred at low internal energies of the reactants. In agreement, the experimental results show a pronounced hydrogen scrambling and dominant formation of the doubly charged products at low collision energies, whereas direct hydride transfer prevails at larger collision energies. (C) 2008 Elsevier B.V. All rights reserved.