Hydrogen transfer quantum diffusion

The effects of transfer of atoms by tunneling may play an essential role in a number of phenomena involving the transfer of atoms and atomic groups in the condensed phase. One may expect that these effects may exist not only in the proton transfer reactions considered above but also in such processes as the diffusion of hydrogen atoms and other light ions (e.g., Li+) in liquids, tunnel inversion and isomerization in some molecules, quantum diffusion of defects and light atoms in the electrode at cathodic incorporation of the ions, ion transfer across the liquid/solid interface, and low-temperature chemical reactions. 

Intra- and intermolecular hydrogen transfer processes are important in a wide variety of chemical processes, ranging from free radical reactions (which make up the foundation of radiation chemistry) and tautomeriza-tion in the ground and excited states (a fundamental photochemical process) to bulk and surface diffusion (critical for heterogeneous catalytic processes). The exchange reaction H2 + H has always been the preeminent model for testing basic concepts of chemical dynamics theory because it is amenable to carrying out exact three-dimensional fully quantum mechanical calculations. This reaction is now studied in low-temperature solids as well. 

A few relatively recent applications of PI-QTST are summarized in this subsection. For other applications and extensions of the theory, the reader is referred to the growing list of PI-QTST papers in such areas as electron transfer theory [102-105] and simulation [50,98-100,102,106], proton transfer theory [107] and simulation [46,77,107-111], hydrogen diffusion in [112] and on [113-116] metals, molecular diffusion [117] and adsorption [118,119] on metals, and in the theory of condensed-phase effects in quantum activated dynamics [43,63, 66, 96, 97,120-122],... 

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. 

---