H/D isotope exchange between methane and magic acid (HSO3F–SbF5): an in situNMR study

Abstract

The kinetics of hydron exchange between methane and a series of DSO₃F–SbF₅ superacids were measured by in situ²H decoupled ¹H NMR spectroscopy. The rates of exchange showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation ΔG^# (30 °C) decreasing from 97 to 84 kJ mol⁻¹ over the range of concentration 19 to 49 mol % SbF₅. The constant free enthalpy of activation ΔH^# (ca. 65 kJ mol⁻¹) and the decreasing entropy of activation ΔS^# seem to indicate that an increase in acidity of the superacid system does not substantially change the nature of the transition state but rather acts on its solvation.