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 interconversion steps is extended and applied to discuss the fundamentals of hydrogen bonding and proton transfer. A chemical reaction, being a reshuffling of charges, is always coupled to an electromagnetic field, and corresponds to a change in quantum populations of a global Hamiltonian. The evolution goes via subsets of electronic quantum states defining bottleneck regions which, in turn, characterize the mechanisms. The elementary interconversion step is identified with quantum-dynamical processes where linear superpositions of relevant electronic quantum states couple the precursor (activated reactant) via bottleneck states to those defining successor (activated products) complexes. The coupling between different electronic states is made via the interaction with the electromagnetic field. Pictorially speaking, all interconverting species share the stationary nuclear geometry around which the bottleneck spectrum is built. This approach led to a non-BO mechanism for chemical interconversions. For steps mediated by ground-state-less molecular Hamiltonians (modelled, for instance, by saddle points at a Born-Oppenheimer (BO) level of computation) the reactants (products) must be moulded into the geometry of the bottleneck for the interconversion to take place as a Franck-Condon-like process. At the lowest level, the theory predicts the physical existence of collision (diffusion) pairs different from the hydrogen-bonded complexes. Discussions of experimental data show that the present theory gives a rationale to most of the phenomenological approaches developed to describe the properties of water (liquid and solid) and the prototropic mechanism in water. (C) 1998 Elsevier Science B.V. All rights reserved.