Electron transfer to 5-bromouracil (5-BrU) from nucleobase (N) electron adducts (and their protonated forms) has been studied by product analysis and pulse radiolysis. When an electron is transferred to 5-BrU, the ensuing 5-BrU radical anion rapidly loses a bromide ion; the uracilyl radical thus formed reacts with added t-butanol, yielding uracil. From the uracil yields measured as the function of [N]/[5-BrU] after gamma-radiolysis of Ar-saturated solutions it is concluded that thymine and adenine electron adducts and their heteroatomprotonated forms transfer electrons quantitatively to 5-BrU. Like the electron adduct of adenine, those of cytosine and guanine are rapidly protonated by water. The (protonated) electron adduct of guanine does not transfer an electron to 5-BrU, and in the case of the (protonated) cytosine electron adduct only partial electron transfer is observed. The results can be modelled if the protonated electron adduct (protonated at N(3) or at the amino group) of cytosine, CH., which can transfer its electron to 5-BrU (k approximately 2 x 10(7) dm3 mol-1 s-1) is transformed in a slow tautomerization reaction (k approximately 2.5 x +/- 10(3) s-1) into another form C'H. (possibly protonated at C(6) or C(5)) which does not transfer an electron to 5-BrU. There is also electron transfer from the electron adduct of thymine to cytosine and guanine which serve as electron sinks. The rate constant of electron transfer from the thymine electron adduct to cytosine is about 250 times greater than that of the reverse reaction. The heteroatom-protonated electron-adduct of thymidine transfers an electron to 5-BrU more slowly (k = 2.3 x 10(7) dm3 mol-1 s-1) than the electron-adduct itself (k = 7.2 x 10(8) dm3 mol-1 s-1). Phosphate buffer-induced protonation of the electron-adduct of thymine at carbon (C(6)) prevents electron transfer to 5-BrU. Such phosphate catalysis is also observed as an intramolecular process (k approximately 2 x 10(4) s-1) with thymidine-5'-phosphate but not with the 3'-phosphate. Phosphate-induced protonation at carbon also reduces transfer efficiency for the electron adducts of dinucleoside phosphates such as dTpdT and dTpdA. The data raise the question whether in DNA the guanine moiety may act as the ultimate sink of the electron in competition with other processes such as protonation at C(6) of the thymine electron adduct.

The number of investigations of room temperature ionic liquids (RTILs) has been developing rapidly in recent years, e.g., due to their potential use as "green solvents" in many industrial applications. A large body of data has been accumulated on physico-chemical properties including ultrafast dynamics of these liquids. This review deals both with the generation and the subsequent relaxation dynamics of selected transient species following photolysis or pulse radiolysis of RTILs with special emphasis on the exceptional character of imidazolium based systems. Although considerable progress has been made, the understanding of the photochemistry of ionic liquids is just at its embryonic stage. Nevertheless, a brief comparison with high temperature ionic liquids (HTILs) such as alkali metal doped alkali halide mixtures already reveals important differences.

x-Monoalkoxyalkyl radicals produced from 1,4-dioxane ( 1 OO%), 1,3-dioxane (56%), tetrahydrofuran (92%) and dimethyl ether (100%) by H-abstraction by hydroxyl radicals generated in the radiolysis of water were found to react with 4-nitrobenzonitrile (NBN) by addition to give N-alkoxyaminoxyl-type radicals, which have absorption maxima at about 310 nm and decay very slowly (k = 0.4 -1.0 s-l). On the other hand, the reaction of the a-dialkoxyalkyl radical, 1,3-dioxan-2-yl 3 [from the reaction of hydroxyl radicals with 1,3dioxane (32%)] with NBN leads to the rapid formation of the radical anion NBN'-.The N-alkoxyaminoxyl-type radicals (A in the case of 1,4-dioxane and D in the case of dimethyl ether) react with ascorbate (k z 2 x lo4 dm3 mol-' s-'). They have a very low reactivity with oxygen (k < lo3 dm3 mol-' s-' in the case of tetrahydrofuran). On the other hand, they are rapidly reduced by NBN'-(k z lo9 dm3 mol-' s-' as observed with A and with B derived from 1,3-dioxane).The products [G values (in parenthesis) in units of lop7 mol J-'1 in the y-radiolysis of N,O-saturated solution of 1,4-dioxane in the presence of NBN are 1,4-dioxan-2-0ne (0.3), 2-hydroxy-l,4-dioxane (2.5), ethane-1,2-diol monoformate (2. l), ethane-1,2-diol diformate (0.7), formaldehyde (2. I), 4-nitrosobenzonitrile and other reduction products of 4-nitrobenzonitrile. These products are accounted for as resulting from the fragmentation of the aminoxyl radical A by (a) heterolysis of the C-0 bond (45%, leading to the one-electron oxidation of the 1,4-dioxan-2-y1 radical) and (b) homolysis of the N-0 bond (55%, leading to the formation of the 1,4-dioxan-2-0xyl radical which undergoes further fragmentation).The products from the reaction of methoxymethyl radicals with NBN under y-radiolysis conditions are formaldehyde (5.71, methanol (2.5) and methyl formate (1.3). It is concluded that also in this case the decay of the aminoxyl radical D occurs by two pathways: the heterolysis route (46%) and the homolysis route (54%).In the presence of oxygen the 1,4-dioxan-2-y1 radicals are converted into the corresponding peroxyl radicals. Their bimolecular decay (2k = 2.0 x lo8 dm3 mol-' s-l) yields the same products as in the case of NBN (albeit with a different product distribution and the formation of some peroxides): 1,4-dioxan-2-0ne (0.4), 2-hydroxy-l,4-dioxane (0.4), ethane-l,2-diol monoformate (0.6), ethane-l,2-diol diformate (2.8) and formaldehyde (0.6).These results indicate that fragmentation reactions involving the carbon-skeleton of organic radicals are important not only in the case of peroxyl radicals but they can also be induced by nitroaromatic sensitizers. In cells, reduction of the long-lived sensitizer adduct radicals by reducing agents such as ascorbate to give (toxic) hydroxylamine type products may compete with the homolytic or heterolytic fragmentation of the N-alkoxyaminoxyl radicals.

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