The role of precursor states in the stereo-dynamics of elementary processes.

The present perspective review focuses on the role of the precursor state, controlling the dynamical evolution of elementary processes, whose structure and stability are often difficult to characterize on quantitative grounds. In particular, such a state depends on the critical balance of weak intermolecular forces operative at long and intermediate separation distances. In this paper, a complementary problem has been properly addressed, concerning the suitable formulation of the intermolecular forces involved, defined in terms of a limited number of parameters and applicable in the whole space of the relative configurations of interacting partners. Important help to the solution of such a problem has been provided by the phenomenological method which adopts semi-empirical and empirical formulas to represent the basic features of the leading interaction components. Such formulas are defined in terms of a few parameters directly or indirectly related to the fundamental physical properties of the interacting partners. In this way, the basic features of the precursor state controlling its stability and its dynamical evolution have been defined in an internally consistent way for several elementary processes, having apparently different natures. Particular attention has been paid to the chemi-ionization reactions: they are treated as prototype oxidation processes for which all electronic rearrangements affecting stability and evolution of the precursor state, coincident with the reaction transition state, have been characterized in great detail. The obtained information appears to be in the perspective of general interest for many other elementary processes, difficult to investigate in the same detail since many other effects mask their basic features.