Solvated CH5+ in liquid superacid

The transition states for methane activation in liquid superacid have been studied by experimentally determined secondary kinetic deuterium isotope ef fects (SKIEs) and computational chemistry. For the first time, the SKIEs on hydrogen/deuterium exchange of methane have been measured by using the met hane isotopologues in homogeneous liquid superacid ((HF)-H-2/ SbF5). To ach ieve high accuracy of the SKIEs, the rate constants for pairs of methane is otopologues were simultaneously measured in the same superacid solution by using NMR spectroscopy Density functional theory (DFT and high-level ab ini tio methods have been employed to model possible intermediates and transiti on states, assuming that the superacids involved in the exchange reactions are H2F- ions solvated by HF Only the unsolvated superacid H2F- is found to be strong enough to protonate methane. yielding the methonium ion solvated by HF as a potential energy minimum. In contrast, the (HF)(x)-solvated H2F - superacids (x = 1-4) do not appear to be strong enough to yield stable so lvated methonium ions. However, such ions show up as parts of the transitio n slates of the exchange in which the methonium ions are solvated by (HF)(x ). The calculated DFT activation barrier is in good agreement with that exp erimentally observed.