Tunneling mechanisms, hydrogen bonds proton transfer

The double hydrogen bond in principle facilitates two mechanisms of double proton transfer stepwise and concerted. Studying the transfer rates within the semi-classical tunneling approximation of the variational transition state theory, Kim found that the two protons are transferred synchronously across the transition state with D2h symmetry [194], The actual proton-tunneling distance is considerably reduced due to the contraction of the hydrogen bonds, i.e., heavy atom motion promotes the proton transfer. On the other hand, the ab initio path-integral Car-Parinello calculations predict that the motion of the two protons in the vicinity of the potential minima is... 

It should be kept in mind that conventional AIMD simulation techniques, both BOMD and CPMD, are not able to describe all types of dynamics encountered in chemistry. One thing they lack is an ability to handle dynamics that can only be explained with frill quantum mechanics. Proton tunneling and ion dispersion, for example, are purely quantum effects that can play a fundamentally important role in biological systems, in polymer electrolyte fuel cells, and in many other water-containing systems. In fact, the commonly accepted mobility mechanism is the so-called stmctural diffusion or Grotthuss mechanism, in which solvation stmctures diffuse through the hydrogen bond network via sequential proton transfer reactions. 

Tunnelling spectroscopy is unique to observing quantum nonlinear dynamics in crystals. Evidence for proton transfer along hydrogen bonds is another outstanding contribution of INS. It is another manifestation of the decoupling of proton dynamics from the crystal lattice. The quantum nature of proton transfer dynamics even at room temperature is quite unforeseen and contrasts with mechanisms based on semiclassical diffusion jumps. 

A similar effect of the symmetry of the double-well potential is predicted by calculations on the systems with an intramolecular hydrogen bonding. Thus, in the case of 3-oxy-2-methyl-2-propenal XVII, a proton tunnelling with the frequency of 7 x 10 s is expected only for the conformation XVII(s) of the transition state, while a 90° rotation of the 2-methyl group totally prevents the operation of this transfer mechanism [75] ... 

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