Terephthalate and Fricke dosimetry have been carried out to determine the sonolytic energy yields of the OH free radical and of its recombination product H2O2 in aqueous solutions under various operating conditions (nature of operating gas, power, frequency, temperature). For example, in the sonolysis of Ar-saturated terephthalate solutions at room temperature, a frequency of 321 kHz, and a power of 170 W kg-1, the total yield [G(.OH) + 2 G(H2O2)], equals 16 x 10(-10) mol J-1. This represents the total of .OH that reach the liquid phase from gas phase of the cavitating bubble. The higher the solute concentration, the lower the H2O2 production as more of the OH free radicals are scavenged, in competition with their recombination. Fricke dosimetry, in the absence and presence of Cu2+ ions, shows that the yield of H atom reaching the liquid phase is much lower, with G(H.) of the order of 3 x 10(-10) mol J-1. These sonolytic yields are smaller in solutions that are at the point of gas saturation, and increase to an optimum as the initial sonication-induced degassing and effervescence subsides. The probing of the sonic field has shown that the rate of sonolytic free-radical formation may vary across the sonicated volume depending on frequency and power input.
Abstract Hydroxyl radicals from the radiolysis of N2O/O2 (4:1)-saturated aqueous solutions of acetone (10−3 mol dm −3) react with the solute by H-abstraction thereby forming acetonyl radicals. As shown by pulse radiolysis, the acetonyl radicals rapidly add oxygen (k = 3 x 109 dm3mol−1s−1) thereby forming acetonylperoxyl radicals which decay by second-order kinetics (2 k = 8 x 108 dm3mol−1s−1). Under steady-state radiolysis conditions (0.24 Gy s−1) the products (G values in parentheses) are methylglyoxal (2.5), hydroxyacetone (0.5), form aldehyde (1 .6), acids (1 .7), carbon dioxide (0 .5), organic (hydro)peroxides (0.4), hydrogen peroxide (2 .2), and G (oxygen uptake) has been determined at 5.2. In the decay of the acetonylperoxyl radicals the major process (slightly less than one half) leads in a concerted reaction to two molecules of methyl glyoxal and one molecule of hydrogen peroxide. The Russell mechanism which yields one molecule of methylglyoxal, hydroxyacetone and oxygen contributes about one fifth as does the breakdown into two molecules of form aldehyde, one molecule of oxygen and two acetyl radicals. Carbon dioxide, formaldehyde and acetic acid are among the products which result from the acetylperoxyl radicals formed upon oxygen addition to the acetyl radicals. A bout one tenth of the organic peroxyl radicals is converted into hydroperoxides by O2⨪ produced as a byproduct in the water radiolysis.
Pulse radiolysis of N2O-saturated solutions of poly(U) in the presence of tetranitromethane showed that 81 per cent of the radicals formed are reducing in nature. Using data from other sources it has been estimated that 70 per cent of the OH radicals add to the base at C(5) and 23 per cent at C(6) while only 7 per cent abstract an H-atom from the sugar moiety. To a large extent the C(5) OH adduct radicals attack the sugar moiety of poly(U) thereby inducing strand breakage and base release. G (base release) = 2.9 can be subdivided into three components: (a) immediate (20 per cent), (b) fast (50 per cent) and (c) slow (30 per cent). The immediate base release must occur either during the free-radical stage or as a result of the rapid (t1/2 less than 4 min at 0 degree C) decomposition of a diamagnetic product. The fast and the slow processes are only readily observable at elevated temperatures, e.g. at 50 degrees C the half lives are 83 min and 26 h, respectively (Ea (fast) = 68 kJ mol-1, Ea (slow) = 89 kJ mol-1, A (fast) = 1.5 X 10(7) s-1, A (slow) = 1.9 X 10(9) s-1. It is concluded that there are three different types of sugar lesions giving rise to base release, structures for which are tentatively proposed.
Abstract Hydroxyl radicals from the radiolysis of N2O/O2 (4:1 v/v)-saturated aqueous solutions have been reacted with acetate ions (10-2M). As measured by pulse radiolysis, the resulting ·CH2CO2- radicals react with oxygen yielding the corresponding peroxyl radicals, ·O2CH2CO2- (k = 1.7 x 109 M-1s-1).These peroxyl radicals decay bimolecularly (2k = 1.5 x 108 M-1s-1) giving rise to the products (G values in brackets) glyoxylic acid (2.7), glycolic acid (0.7), formaldehyde (1.4), carbon dioxide (1.4), organic hydroperoxide (0.7) and hydrogen peroxide (2.5). Oxygen is consumed with a G value of 5.3. Aided by data from pulse radiolysis it is concluded that the intermediate tetroxide formed upon the bimolecular decay breaks down by various routes to yield:(i) hydrogen peroxide and two molecules of glyoxylic acid (ca. 27%);(ii) oxygen, glycolic acid and glyoxylic acid (ca. 25%);(iii) hydrogen peroxide and two molecules of formaldehyde, carbon dioxide and OH- (25%).These reactions do not involve free radicals as intermediates;(iv) There is some O⨪2 (G ≈ 0.5) formed in the decay of the peroxyl radicals, which is attributed to the decay of intermediate oxyl radicals (tetroxide → O2 + 2 ·OCH2CO2-) by 1,2-H shift, oxygen addition and HO2· elimination, a reaction sequence which gives rise to glyoxylic acid (10%); (v)The reaction of O2⨪ with the organic peroxyl radical yields the hydroperoxide (13%).Reaction (iii) is a novel peroxyl radical reaction.
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