Electron spin resonance study of liquids during photolysis. XVI. Kinetics of radicals showing emission from tartaric acid

Several free radicals are formed during continuous photolysis of aqueous solutions of tartaric acid, one of which is HOOCCH(OH)CH(OH), radical A. This radical has eight hyperfine lines which are in emission in concentrated solutions of tartaric acid where the radical lifetime is short and which appear in absorption in dilute solutions where the lifetime is longer. The photolytic light source was interrupted with a rotating sector in order to study kinetic behavior. When the light is turned on, radical A always gives a transient signal in the direction of emission before steady state is achieved. There is also a transient when the light is turned off. This behavior has been studied in several concentrations of tartaric acid, a phenomenological theory developed for the shape of the curves, the chemical lifetime of the radical evaluated, and the thermal relaxation time measured. The electron spin states are polarized at the time the radical is formed, and the degree of polarization is − 0.142 (difference of populations of spin states divided by the sum) as compared to 0.00076 for thermal equilibrium. The lifetime of radical A is limited by its transformation to HOOCCHCHO, radical C, by an acid catalyzed rearrangement. Kinetic data on radical C show a large inductive effect due to consecutive, slow chemical reactions. When oxalic acid is added to the system there is electron transfer from various radicals in solution to the oxalic acid giving a reduced form of oxalic acid, radical D. Although this radical always appears in absorption at steady state, the sector results show initial polarization in the emissive direction before steady state is reached. The polarization of A transfers to the oxalic acid in forming radical D.