No abstract

Pulse radiolysis of air-free aqueous methyl viologen (MV2+) solutions was carried out at various pH. The attack of eiq on MV2+, with k(eiq+MV2+) = 7.5 x 1Olo dm3 mo1-l s-l, leads to the formation of the long-lived radical cation (MV' +), which possesses two absorption maxima at 392.5 nm (E,,,,, = 4200 m2 mol-') and 600 nm ( E ~, , = 1450 m2 mol-l). The H-atoms react with MV2+ at pH 1 forming two species, which have superimposed absorption bands. By means of a computer simulation they are resolved in the absorptions belonging to: (1) a protonated form of the radical cation (MV'+H+), which is produced with k(H + MV2+) = (3.5 f 0.2) x lo8 dm3 mol-l s-l, has 2 absorption maxima at 390 nm (E,,, = 1700 m2 mol-I) and 595 nm ( E ~, : , = 760 m2 mol-l) and decays by second-order kinetics with k = 3.5 x lo9 dm3 mol-' s-l; (2) an H-adduct (MVe2+H) on the ring carbon, which is formed with 2467 80-2

The behaviour of aqueous MV2+ towards oxidizing radicals (OH and 0-) has been investigated in the pH range from 6 to 14 by means of pulse radiolysis. A semi-linear optimization method was applied for resolving the complex reaction mechanism. In the pH range from 6 to 8 the rate constant for attack by OH is k = (2.5 f0.2) x lo8 dm3 mol-' s-l. The resulting transient absorbs at Amax = 470 nm ( E ~, ~ = 1600 f 70 m2 mol-l) and decays with 2k = (1.3 f0.2) x lo8 dm3 mol-l s-l.In strongly alkaline solutions (pH 2 13.8) the 0-radical anion reacts preferentially by hydrogen abstraction from the methyl group, k = (1.4f0.2) x lo9 dm3 mol-l s-l, forming a radical which then decays by reaction with OH-(k = 2.8 x 106 dm3 mol-l s-l) to produce a modified radical cation; this has absorption maxima at 392 and 605 nm (&3g2 = 4300 m2 mol-l, E,,, = 1500 m2 mol-l) and is relatively long lived.The remaining part (< 10%) of 0attacks MV2+ at the ring carbon atom, k = (l.Ok0.4) x 10" dm3 mol-1 s-l, resulting in an 0adduct, which has Amax = 470 nm

No abstract

Using hydrogen at high pressures of up to 150 bar (0.12 mol dmP3 H,) as an OH scavenger in aqueous MV2+ solutions (pH 1) it is possible to differentiate between two kinds of transient formed simultaneously by H-atom attack on methyl viologen. One of them is assigned to an H adduct on the N atom, MV'+H+ ( k = 3.1 x los dm3 mol-l s-l), with absorption bands identical to those of the radical cation, MV'+, but with E,,,., = 3200 m2 mo1-I and E ~, , = 1100 m2 mol-I. The MV'+H+ species deprotonates with k = 2 x lo4 s-l, forming the long-lived radical cation, MV'+. The second type of transient produced, with k = 2.9 x lo8 dm3 mol-l s-l, is attributed to an H-adduct on the ring carbon, MVa2+H, with E,,, = 570 m2 mol-l and E,,, = 920 m2 mol-l, decaying by second-order kinetics with 2k = (6.0 1) x lo8 dm3 mol-l s-l. The formation of MV'+ by electron transfer from the propan-2-01 radical has been reinvestigated (pH 0-7); its absorption spectrum does not change in this pH range.Recently we have reported1 that hydrogen atoms react with methyl viologen (MV2+) in acidic aqueous solution (pH 1) forming two types of transient, one attributed to an adduct on the nitrogen atom (A,,, at 390 and 595 nm) and the other to an adduct

No abstract

No abstract

The semireduced transients of proflavin were studied in airfree acid aqueous solutions by a combined pulse radiolysis-computer optimization procedure. The main product, semiquinone (C-radical; 75%), formed with k=6.4 × 10