Spur decay kinetics of the hydrated electron following picosecond pulse radiolysis of water have been measured using a time-correlated transient absorption technique with an asynchronous mode-locked laser. The 11 ns time window afforded by this signal-averaging technique is ideal to match up with more conventional transient absorption measurements taken to microsecond time scales. The precise data recorded in this study require a revision downward of the “time zero” solvated electron yield to approximately 4.0 per 100 eV of energy absorbed, to match the best available scavenger product measurements.

The reaction of OH radicals with a number of substituted halobenzenes (C, H, -, X, Y, where X = F, CI or Br and Y = CH3, CH,CI, CHCI,, CF, or OCH,) produces a transient absorption band whose A,,, is in the range 31 0-340 nm. This band is assigned to the OH adduct and decayed with second-order kinetics, the bimolecular rate constant being in the region of 0.6 x lo9 -5.0 x lo9 dm3 mol-' s-I. The OH radical reacts mainly by addition to the benzene ring. The rate constants for the reaction of OH radicals, as determined from formation kinetics studies, are between 1.7 and 9.3 x I O9 dm3 mol-' s-l, and are found to depend on the nature of the substituents. A linear correlation is observed with the Hammett parameter.

Radiation chemical reactions of • OH, O •-, and SO 4 •-with benzaldehyde, acetophenone, and benzophenone have been studied using both pulse and steady-state radiolysis techniques. The observed rates for the • OH addition (k ) (2.6-8.8) × 10 9 M -1 s -1 ) are higher than those found for the SO 4 •-reaction (k ) (0.7-4.0) × 10 9 M -1 s -1 ). The rate for the reaction of O •-with benzaldehyde is higher than that found for • OH, while a reverse trend is observed in the case of the two ketones. Optical absorption spectra of the intermediate transients formed in the reactions of • OH and SO 4 •-with all three compounds are similar with a peak around 370-380 nm. The absorption spectra from the O •-reaction have shown a major peak at 310 nm and are somewhat different from those obtained in the reaction of • OH. The yields of the phenolic products formed in the reaction of • OH with benzaldehyde and acetophenone in the presence of 0.1 mM ferricyanide corresponded to only 30% and 50% • OH yields, respectively. Benzoic acid is a major product formed with benzaldehyde in the reaction of • OH as well as SO 4 •-with G values of 2.1 and 1.3 per 100 eV, respectively. The formation of the exocyclic OH adduct is a major pathway in the reactions of • OH (by addition) and of SO 4 •-from hydrolysis of the initially formed radical cation (k ) 2.4 × 10 4 s -1 ) with benzaldehyde. The exocyclic OH adduct undergoes disproportionation to give benzoic acid. The formation of the exocyclic OH adduct of acetophenone is possibly hindered owing to the bulky -COCH 3 group.

The reactions of hydrated electrons e-aq with hypoxanthine and inosine were followed using pulse radiolysis methods. In a neutral solution the electron adduct of inosine is immediately protonated at the heteroatoms of the purine ring by water (k >> 2.5 x 10(6)s-1) to give In(N,O-H).. These N,O-protonated intermediates have a single absorption maximum at 300 nm. In basic solution the protonation of the electron adduct of inosine by water leads to other intermediate products with an absorption maximum at 350 nm. These intermediates are believed to be the C-protonated electron adducts of inosine (In(N,O-H).). In (N,O-H). and In(C-H). differ strongly in their ability to reduce p-nitroacetophenone (PNAP). In(N,O-H). are strong reductants and reduce PNAP quantitatively to PNAP.-. Based on the pH dependence of PNAP.- yields, two types of tautomers of In(C-H). could be distinguished. One of the tautomers can reduce PNAP, albeit with slower rate than In(N,O-H)., the other tautomer has no reducing properties. The latter is the one with the higher pKa and therefore thermodynamically more stable. The absorption spectrum of the intermediates produced in the reaction of e-aq with hypoxanthine at neutral pH is very similar to that of In(N,O-H). with a maximum at 300 nm. However, no build-up at 350 nm was observed in basic solution as in the case of the electron adduct of inosine. The reaction of H atoms with inosine produces in basic solution intermediate radicals with the same absorption spectrum as the C-protonated electron adducts of inosine. It is suggested that both the reactions of e-aq and H. with inosine in basic solution produce the same radical, namely the H-adduct of inosine (In(C-H)) with the highest pKa. alpha-Hydroxyalkyl radicals were found to react very slowly with purine bases and nucleosides in neutral to basic solutions. In acidic solution their reactivity increases and a number of rate constants were determined by pulse radiolysis measurements at pH 0.4. The intermediates from the reaction of 2-hydroxy-2-propyl radicals with inosine could be observed pulse spectrometrically in neutral and in basic solutions. In basic solution this reaction leads to intermediates with the same absorption maximum at 350 nm as that of the H-adduct of inosine. Furthermore, the yield of acetone was found to increase strongly in basic pH.(ABSTRACT TRUNCATED AT 400 WORDS)

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