Chapter 18: Reaction Dynamics

The chapter presents multiple theoretical frameworks for calculating rate constants in elementary bimolecular reactions, with transition-state theory serving as the primary model for understanding reaction rates in solution. Transition-state theory proposes that reactants first aggregate into an activated complex or transition state, a high-energy intermediate whose structural and energetic properties can be characterized through statistical analysis of its populated energy levels. From this theoretical foundation emerges the Eyring equation, which relates the reaction rate constant to fundamental thermodynamic quantities including the enthalpy and entropy of activation, providing valuable predictive power for reactions occurring in condensed phases and enabling researchers to interpret how factors like ionic strength influence reaction rates through the kinetic salt effect. The chapter also demonstrates how transition-state theory illuminates reaction mechanisms through analysis of kinetic isotope effects, which reveal the extent to which atomic mass influences reaction rates and thereby identify which bonds are broken or formed during the rate-determining step. At a more detailed mechanistic level, the chapter explores potential energy surfaces as mathematical representations of the energy landscape governing reactant trajectories and molecular encounters. These surfaces, which plot energy against reaction coordinates and internal molecular geometries, reveal the topographical features that control whether and how efficiently reactive collisions proceed. Experimental investigation of reaction dynamics through molecular beam methods provides empirical evidence for the predictions of theoretical models by directly measuring how reactant collision outcomes depend on the distribution of available energy, particularly the allocation between vibrational excitation and translational kinetic energy, and how these factors interact differently with attractive versus repulsive potential energy surface topographies.