The C2H7+ potential energy surface: a Fourier transform ion cyclotron resonance investigation of the reaction of methyl cation with methane

Fourier transform ion cyclotron resonance experiments have been carried out to probe the nature of the potential energy surface for the reaction of methyl cation (CH3+) with methane (CH4) to give the ethyl cation (C2H5+). Product distributions for reactions of CH3+ and CD3+ with CD4 and CH4, respectively, are found to give a near statistical distribution of ethyl cation products, in good agreement with previous work. When the methyl cation is initially coordinated to HF in the form of a methylfluoronium ion, however, the product distributions are decidedly nonstatistical and are indicative of a reaction which is very nearly thermoneutral. Thermochemical data for relevant species suggest that the reaction is very slightly endothermic. All of the experiments support the intermediacy of a C2H7+ complex in the reactions. Ab initio calculations, in conjunction with all of the experimental data, reveal that there are likely three different stable forms of C2H7+ involved in the potential energy surface for the reaction. One of the forms, a nonclassical C2H5+ coordinated to H2 in a proton bound dimerlike structure, has not previously been considered to play a role in this reaction. The existence of this structure is supported by infrared multiphoton dissociation experiments on C2H7+ previously carried out by Lee and co-workers (J. Am. Chem. Soc. 111 (1989) 5597) and the high pressure mass spectrometric experiments of Hiraoka and Kebarle (J. Am. Chem. Soc. 98 (1976) 6119).