Solution photochemistry of the uranyl ion

Various physical techniques have been applied to the study of the primary processes involved in the photoreaction of uranyl ions with a variety of organic molecules. Electron spin resonance spectroscopy has been used to identify the primary radical products in many solid matrices at temperatures of down to 77 K and in the liquid state at 200 - 300 K in a slow flow system. In general, primary alcohols, R CH20H, yield R CHOH in the solid state whilst secondary and tertiary alcohols at 77 K undergo C - C cleavage to give alkyl radicals. Upon softening the matrices by warming, the radicals derived from tertiary alcohols attack parent molecules. Carboxylic acids undergo both abstraction of an α-hydrogen atom and C - C fission, the former process predominating with simple mono carboxylic acids and their esters and some dicarboxylic acids. The H-abstraction mechanism is also important with aldehydes, ketones and amides, the latter preferring to lose a hydrogen atom from N-alkyl substituents where applicable. Phosphorus-centred radicals exhibiting considerable g- and a- tensor anisotropy have been produced in the solid state. The radicals derived from primary alcohols and carboxylic acids in the flow apparatus are essentially the same as those observed at 77 K. n-Butyl lactate is shown to yield two conformational isomers of the radical CH3C(OH)C02C4H9 and mixtures of radicals were obtained from the ethers 1, 2-dimethoxyethane and 2-methoxyethanol. The mechanism of photochemical reaction of U(VI) with organic molecules is discussed. The final section of this work concerns the identification of the (excited state) absorption spectrum of the photoreactive state of the uranyl ion by (spectrographic) microsecond and (kinetic spectroscopic) nanosecond flash photolysis. By monitoring the decay of this absorption at ~590 nm, absolute values for second -order rate constants for the photoreaction with alcohol molecules have been determined and these are compared with the results of luminescence intensity and lifetime quenching by these molecules (the latter by the single-photon counting technique). The appreciable deuterium kinetic isotope effects on the rate constants are discussed in terms of the nature of the primary photochemical step