A DFT-based analysis of the grossly varying reactivity pattern in room-temperature activation and dehydrogenation of CH4 by main-group atomic M+ (M=Ga, Ge, As, and Se).
Insight into the mechanism(s) of the recently reported thermal reactions of methane with the main-group atomic cations Ga+, Ge+, As+ and Se+ (A. Shayesteh et al. J. Phys. Chem. A 2009, 113, 5602) has been obtained from DFT-based calculations including relativistic effects. Excellent agreement with the experimental findings has been achieved, and a coherent description of the reactivity is provided. For example, the chemical inertness of the Ga+/CH4 couple is due to an endergonic formation of an encounter complex as well as a high endothermicity for the dehydrogenation of methane (DeltaG298=84.7 kcal mol(-1)). For Ge+/CH4, formation of an encounter complex is slightly exergonic (DeltaG298=-8.9 kcal mol(-1)) and thus is observed with low efficiency; the dehydrogenation path, however, is not accessible (DeltaG298=13.6 kcal mol(-1)). For the Se+/CH4 couple, the significantly enhanced rate of adduct formation is suggested to result from a spin-orbit-mediated quartet-doublet spin flip that generates the insertion product [(H)Se(CH3)]+, 2Se-3, in its doublet state rather than forming a simple encounter complex. For the dehydrogenation of 2Se-3 to generate 2[Se(CH2)]+, various mechanistic variants have been explored, all of which, however, involve transition structures that are located energetically above the entrance channel, thus preventing the reaction from occurring. In contrast, facile and efficient dehydrogenation of CH4 by ground-state As+ (3P0) takes place, and this reaction constitutes a textbook example of the operation of a two-state reactivity scenario. Here, a triplet-singlet conversion occurs along the oxidative insertion of As+ into the C-H bond of CH4 to generate [(H)As(CH3)]+ in its singlet state. From this intermediate, which constitutes the global minimum on the As+/CH4 potential energy surface, dehydrogenation is brought about by a sequence of alpha-hydrogen migration and reductive elimination of molecular hydrogen from the hydride carbene complex 1As-5. The potential energy surfaces for the As+/CH4 couple are remarkably similar irrespective of including or ignoring relativistic effects in the calculations.
Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany.
Xinhao
X
Zhang
Helmut
H
Schwarz