Photokinetics of Bimolecular Reactions: Predictive Modelling of the Basic Bimolecular Photoreactions XXYY'(Φ,k) and XX'YY'(Φ,k).

The photokinetic behaviour of bimolecular photoreactions is usually considered to obey one of the three (zeroth, first, or second) orders of thermal kinetics. The underlying hypothesis here (de facto equating the kinetics of thermal and photoreactions) is not consistent with the major differences in the two physical systems (e.g., light and heat absorption by molecules are differently quantified). Also, while the rate laws of thermal reactions are, in principle, all analytically solvable, the non-linear ones of photoreactions are all (but one) unsolvable by known mathematical methods. This mathematical issue represents an important reason behind the underdevelopment of photokinetics along the general principles established in kinetics. To move beyond the current status quo, there is a need for a new strategy. A successful approach, previously confirmed for unimolecular photoreactions, is employed here, for the first time, to bimolecular photoreactions involving one or two reactants (X and X'), one or two products (Y' and Y) and two (photo- and thermal) processes. It consists of proposing a general integrated rate law (or model explicit equation), testing and validating it by both excellent fitting of the full kinetic traces of the reaction species obtained by Runge-Kutta (RK) numerical integration and a linear relationship between theoretical, RK- and fitting-calculated values of the initial reaction rates. The features of the reactions in the testing set (more than 200 cases) spanned a wide range of possible reaction conditions and properties. The kinetic behaviour of bimolecular photoreactions was proven to be well described by a model equation involving the typical Φ-order kinetics, α Log1+βe-γt, term. Furthermore, a detailed kinetic elucidation method was developed for the determination of the full set of reaction parameters for the two bimolecular photoreactions studied, XXYY'(Φ,k) and XX'YY'(Φ,k). The model equation, the trends observed in reactivity with the various reaction parameters, and the elucidation method represent the first complete characterisation of the bimolecular photoreactions' photokinetics ever published in the photochemistry literature. The present work contributes to the standardisation of photokinetics, its concepts and its investigative tools.