Hydrogen bonding and quantum dynamics in the solid state

Abstract

Hydrogen bonding is of great importance to many fields in physics, chemistry and biology. However, a comprehensive view of this interaction is still far from being achieved. Recent developments of experimental techniques such as neutron diffraction, inelastic neutron scattering (INS), quasi-elastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) T1 measurements provide new information on the structure and dynamics of hydrogen bonds. The INS technique is unique to observing proton dynamics in the quantum regime. In the most favourable cases, potential functions for the proton motions are determined accurately and can be compared with those derived from other techniques or calculated with quantum chemistry methods. Two classical examples are presented to illustrate recent developments. First, tautomerism in centrosymmetric dimers is a prototypical example for proton transfer. The potential functions determined with vibrational spectroscopy or derived from QENS or NMR T1 measurements are quite different. This is tentatively related to the characteristic time scale for each technique. Second, the strong symmetric hydrogen bond in the potassium hydrogen maleate is a model for the intermediate state in the proton transfer chemical reaction. The potential function for the proton determined with INS is quite different from the single minimum potential normally anticipated. The new concept of hydrogen bonding-antibonding vibrational state is emphasized. In all cases, advanced techniques yield a rather complex view of hydrogen bonds dominated by quantum effects that cannot be yet rationalized with the available theoretical tools of quantum chemistry.