H/D isotope exchange between methane and magic acid (HSO3F-SbF5):: an in situ NMR study

The kinetics of hydron exchange between methane and a series of DSO3F-SbF5 superacids were measured by in situ H-2 decoupled H-1 NMR spectroscopy. The rates of exchange showed a strong dependence on antimony pentafluoride concentration, with the free energy of activation DeltaG(#) (30degreesC) decreasing from 97 to 84 kJ mol(-1) over the range of concentration 19 to 49 mol % SbF5. The constant free enthalpy of activation DeltaH(#) (ca. 65 kJ mol(-1)) and the decreasing entropy of activation DeltaS(#) 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.