QUANTUM-MECHANICS AND THE THEORY OF HYDROGEN-BOND AND PROTON-TRANSFER- BEYOND A BORN-OPPENHEIMER DESCRIPTION OF CHEMICAL INTERCONVERSIONS

A recent quantum-mechanical theory of elementary chemical interconvers ion steps is extended and applied to discuss the fundamentals of hydro gen bonding and proton transfer. A chemical reaction, being a reshuffl ing of charges, is always coupled to an electromagnetic field, and cor responds to a change in quantum populations of a global Hamiltonian. T he evolution goes via subsets of electronic quantum states defining bo ttleneck regions which, in turn, characterize the mechanisms. The elem entary interconversion step is identified with quantum-dynamical proce sses where linear superpositions of relevant electronic quantum states couple the precursor (activated reactant) via bottleneck states to th ose defining successor (activated products) complexes. The coupling be tween different electronic states is made via the interaction with the electromagnetic field. Pictorially speaking, all interconverting spec ies share the stationary nuclear geometry around which the bottleneck spectrum is built. This approach led to a non-BO mechanism for chemica l interconversions. For steps mediated by ground-state-less molecular Hamiltonians (modelled, for instance, by saddle points at a Born-Oppen heimer (BO) level of computation) the reactants (products) must be mou lded into the geometry of the bottleneck for the interconversion to ta ke place as a Franck-Condon-like process. At the lowest level, the the ory predicts the physical existence of collision (diffusion) pairs dif ferent from the hydrogen-bonded complexes. Discussions of experimental data show that the present theory gives a rationale to most of the ph enomenological approaches developed to describe the properties of wate r (liquid and solid) and the prototropic mechanism in water. (C) 1998 Elsevier Science B.V. All rights reserved.