A new view of the quantum dynamics for proton transfer along hydrogen bonds: Vibrational spectroscopy with neutrons

Inelastic neutron scattering is an ideal tool to observe proton transfer dynamics along hydrogen bonds in the quantum regime. Tunnelling' transitions associated with the quantum transfer of a single proton have been observed for potassium hydrogen carbonate, KHCO 3 , and similar hydrogen bonded complexes in the solid state. More recently, the tunnelling transition was observed for a polypeptide: polyglycine II, The tunnelling bands are rather sharp. Therefore, the proton transfer is largely decoupled from the heavy atom dynamics. Force-field calculations including INS intensities confirm that the proton dynamics are almost totally decoupled from those of the surrounding heavy atoms. This is in contrast to the vibration assisted tunnelling model that supposes a large modulation of the double minimum potential by the low frequency modes of the hydrogen bond. The double minimum potential functions goveming the proton transfer are discussed.