Comment on ‘The correspondence between quantum and classical mechanics: An experimental demonstration of the smooth transition between the two regimes’ [Chem. Phys. Lett. 402 (2005) 519]

In a recent experimental study of proton transfer in benzoic acid, employing field cycling NMR relaxometry (Chem. Phys. Lett. 402 (2005) 519 ), the authors came to the conclusion that the transition between the quantum and classical regimes is smooth and that the spectral densities of proton transfer are characterised by a single correlation time. The analysis presented, however, of the same field cycling NMR relaxometry data in terms of Schrödinger’s equation shows that the proton transfer dynamics is characterised by two correlation times τov (Arrhenius) and τtu (Schrödinger), which characterise two fundamentally different motional processes, namely jumps over the barrier and tunnelling through the barrier. The temperature dependence of the (1/τtu) is the solution of Schrödinger’s equation, which also yields the temperature Ttun , where begins the tunnel pathway for proton transfer. Below Ttun the energy of the particle is lower than the value of the potential barrier that the particle has to pass over. Above Ttun the length of the de Broglie wave generated by thermal motion is too short to pass through the potential barrier.