Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

INTRODUCTION 115equilibrium position after a single small jump is followed, and the stationary methodsin which the interaction is measured between the chemical system and anoscillating constraint.A technique which is finding increasing application in solution reactions is theuse of spectral line-broadening. Work has been done especially with NMR spectra,but the ideas used here have been also applied to ESR and Raman signals. The reactionswhich may be studied in this way are somewhat restricted in scope since theirhalf-lives must be of the same order of magnitude as the relaxation timcs of thephysical process involved. But where they can be used, these methods are oftenpowerful; for example, NMR line-broadening can be used for reactions, such as theexchange of solvent between the coordination shell of a metal and the bulk medium,which involve no net chemical change. These reactions are very difficult to studyby other methods. In Section 4 the emphasis is placed on the use of NMR becausemost of the work in this area has been done using this technique; it also illustratesthe principles rather well.In what is perhaps the competition method par excellence, the process againstwhich the chemical reaction is pitted is the fluorescence of one of the reactants.Absorption of low-wavelength light by a solution of a species A may produce excitedmolecules A* which can return to their ground states by the emission of fluorescence.If the illumination is continuous and of constant intensity, a steady statesituation will arise in which the rate of formation of A* molecules is balanced bythe rate of their deactivation. Under these conditions the intensity of fluorescence isproportional to the steady-state concentration of A*. The excited state of A (thefirst excited singlet) is attained within about lo-’’ sec of the interaction of A withthe photon, and so the lower time limit of reactions which can be studied by thismethod lies near what might be loosely described as the boundary of “chemical”reactions. The upper time limit is the average lifetime of the fluorescing species,which is about sec. In the absence of a chemical reaction A* loses its excessenergy by various processes, the most important of which is frequently deactivationby solvent molecules. It is possible to relate the rate coefficients of the fluorescencequenching reactions with the fluorescence intensities and the concentration of thequenching species; the way in which this is done is considered in Section 5. Becauseof the requirement that one species fluoresce and the restrictions on the reactionhalf-life, this method is not one of the most generally useful for following fast reactionsin solution. It has, however, yielded very important results among the fastestof the chemical reactions.The type of reaction which can be studied by electrochemical methods is limitedin several respects. The systems must be good electrical conductors (which hastended to limit them to aqueous solutions to which large amounts of inert electrolytehave been added), one of the species involved must be electrolytically reducibleat conveniently applied potentials and the rate coefficients must fall within certainboundaries. The electrolytic reduction of a species in a reaction system where theseReferences pp. 176-179