Pulsed studies observed growth

In basic solutions ascorbate is apparently oxidized preferentially by the electron transfer process, which goes to completion in less than 2 fts after termination of the electron pulse (see Structure I). In nitrous-oxide-saturated acid solutions (pH 3.0-4.5), A and two other species which were shown to be OH-radical adducts were observed (37), thus confirming earlier observations (18,19,23, 25). The ascorbate radical anion was identified by its doublet of triplets spectrum that maintains its line position from pH 13 to 1. One OH-radical adduct (IV) shows a doublet, the lines of which start to shift below pH 3.0 it has a pK near 2.0, a decay period of about 100 fxs, and probably does not lead to formation of A". The other OH-radical adduct (II) is formed by addition of the OH radical to the C2 position its ESR parameters are = 24.4 0.0002 G and g == 2.0031 0.0002. Time growth studies suggest that this radical adduct converts to the ascorbate anion radical (III) with r 15 fxs, and accounts for 50% of the A signal intensity 40 fxS after termination of the electron pulse. The formation of the three radicals can be summarized as shown in Scheme 1. 

Amorphous Ti02/MV systems behave quite differently upon photo-excitation. MV2+ is not adsorbed on amorphous Ti02 and so instantaneous formation of MV+ is not observed. However, in the presence of electron donors such as polyvinyl alcohol (PVA), or halide ions, a slow growth of MV+ formation is observed over a period of several us (Fig. 10). In the case of PVA, a permanent reduction of MV2+ is observed as reported by GrStzel et al. (16), but in the case of Cl no permanent reduction is observed. Pulse photolysis studies show large yields of initially formed MV+, but rapid back reaction of MV+ and Cl2 yield the starting materials. In 0.1 M HClO the reduction of MV2+ is not efficient, as electron transfer from ClO - to the Ti02 hole is not efficient. 

Cluster properties, mostly those that control electron transfer processes such as the redox potential in solution, are markedly dependent on their nuclearity. Therefore, clusters of the same metal may behave as electron donor or as electron acceptor, depending on their size. Pulse radiolysis associated with time-resolved optical absorption spectroscopy is used to generate isolated metal atoms and to observe transitorily the subsequent clusters of progressive nuclearity yielded by coalescence. Applied to silver clusters, the kinetic study of the competition of coalescence with reactions in the presence of added reactants of variable redox potential allows us to describe the autocatalytic processes of growth or corrosion of the clusters by electron transfer. The results provide the size dependence of the redox potential of some metal clusters. The influence of the environment (surfactant, ligand, or support) and the role of electron relay of metal clusters in electron transfer catalysis are discussed. 

A pulse radiolysis study of the formation, decay, and absorption spectra - of transients produced by reactions of DNA and of some of its constituents with the OH- free radical is presented in this paper. Solutions were saturated with N20 so that the only significant reactive species produced by radiation was the OH- free radical. Rates of formation were determined by direct observation of the growth of transient absorption. Compounds and conditions were selected to permit investigation of (a) the sites of attack by OH- on pyrimidine bases, nucleosides, and DNA under various conditions (b) charge effects on reaction rates (c) comparative rates of addition to pyrimidine bases, and abstraction from... 

The reactivity of short-lived bimetallic clusters has also been studied by the kinetics method. Under conditions when a transient alloyed cluster of Ag-Au was formed,reactivity with the electron donor MV+ was probed and compared with that of monometallic Ag clusters previously observed. Just after the pulse a mixed solution of Ag and Au cyanides is partially reduced into atoms Ag and Au , while is partially reduced to the redox probe MV+. It is observed that in the first 20 ms the kinetics, at 400 nm, of cluster growth are the same as in the absence of the probe. Thus the coalescence of atoms to form an alloyed small cluster is, at first, not affected. The mechanism should be the same as in Eqs. (20)-(23). After this period, however, the decay of MV" at 700 nm starts in correlation with the increase of the cluster absorbance which results from electron transfer (Fig. 12). When the bimetallic cluster formed reaches the critical size where its potential becomes slightly higher than °(MV +/MV+ ), it acts as a nucleus that initiates a catalyzed growth fed alternately by electron transfer from the donor and the adsorption of excess Ag or Au ions. For i +J > ny. 

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