Fast Reactions

Fast reactions occurring in times shorter than the time needed to mix reactants pose a unique challenge for investigation. In a liquid-phase continuous-flow system, reactants A and B are swiftly pushed into the mixing chamber, where mixing occurs within 1 ms. The reaction mixture then flows through an observation tube, and one measures light absorption to determine species concentrations at various points of the tube. This method is most appropriate when relatively large volumes of reactants are available and when steady-state monitoring along the flow path is desired.

The stopped-flow method is a modification where reactants are mixed and rapidly flow into a receiving syringe, which halts the flow after reaching a stopping barrier. This method, essentially a static approach with rapid mixing, is beneficial for studying rapid enzyme-catalyzed reactions

For reactions with half-lives in the range of 10¹ to 10⁻³ seconds, continuous-flow and stopped-flow methods are applicable. In relaxation methods, the mixing problem is eliminated by perturbing a system in reaction equilibrium and observing its approach to a new equilibrium position. The temperature-jump (T-jump) method is a common relaxation method involving a sudden discharge of a high-voltage capacitor to raise the solution's temperature.

On the other hand, flash-photolysis and shock-tube methods generate reactive species for study. Flash photolysis exposes a system to a high-intensity, short-duration flash of light, triggering reactions that are followed by measuring light absorption. With a laser, pulse durations as short as 0.1 ps allow studying processes in the picosecond range, such as those involved in photosynthesis and vision.