The mechanisms of elementary physicochemical processes: An introductory report

Abstract

Most of the work of theoretical physical chemists and chemical physicists relates at least indirectly to the mechanisms of physicochemical processes. Therefore it is important to examine the meaning and the scope of that notion in the context of recent developments in computational studies. After a brief mention of the meaning of the expression "elementary" physicochemical processes (EPCP), the authors adopt as a definition of mechanism a cause-effect description of an EPCP based on metastable and transient states corresponding to minima and saddle points of the potential energy surface; these states transform into one another according to appropriate selection rules. The so-called reaction-path Hamiltonian can be seen as the starting point for a quantum interpretation of the mechanism concept. On this basis the reaction coordinate, the mechanism profile, and the transition state can be fitted into the same framework. Selection rules are illustrated on the symmetry rules, with a few recent examples of applications which also show their limitations. "Propensity" rules allowing surmises on the nature of a transition state from a static picture of the initial state are also considered and their connection with "reactivity indices" emphasized. Processes involving excitation of electronic states as well as environmental effects are briefly examined. Finally, a specific example taken from surface studies is described in some detail to provide the grounds for further reflection. What Is a Mechanism?The meaning and scope of the concept of mechanism of a physicochemical process such as-a chemical reaction requires a careful discussion because it can be introduced at various levels and because there is much confusion in the literature especially as regards its relation with the detailed description in spacetime of the given process. This report is intended as a contribution to that general discussion. The realm of physicochemical processes, however, is so vast that a preliminary restriction of the field to be covered is mandatory. We shall only consider processes involving stable or metastable states separated by energy barriers which are passed by modifications of the relative positions of atoms in molecules, groups of molecules, or admolecule-substrate complexes. We further assume that these processes can be described in terms of energy hypersurfaces defined in the framework of the Born-Oppenheimer [l] or equivalent approximations, so that for a given electronic state of the system the energy may be represented as a function of the nuclear positions. Within this approximation the considerations presented here will be mostly limited to continuous and smooth surfaces ; this presupposes the definition of suitable domains [2], which exclude points where the energy is not diff erentiable-typically at the intersection of energy hypersurfaces [3,4]-as well as physically meaningless points like