Quantum Mechanical Formulation of Rate Theory

Abstract

We shall now proceed to an outline of a formal quantum mechanical description of elementary chemical processes. Two approaches would be expected to provide the conceptual basis. Thus, chemical processes between separate species may be viewed as a scattering process, the time evolution of which is formulated in terms of the states of the infinitely separated reactants (33,113,141). On the other hand, relaxation processes in isolated molecular species or intramolecular processes in species trapped in solid media are usually viewed as the decay of a metastable excited ‘zero-order’ electronic state by the influence of an intramolecular or medium-induced perturbation (2–4). However, the essentials of both approaches consist in the choice of a suitable set of ‘zero-order’ functions of the system followed by a description of the time evolution of this set under the influence of some perturbation which arises from interactions which were not included in the zero order Hamiltonians. Considering at first the former approach we notice that the sequence of events in chemical reactions between mobile species can be divided in three stages (33,94). Firstly, in the ‘infinitely remote’ past the reactants are located sufficiently far from each other that all interreactant interactions can be ignored.