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Decay and protonation of the 5‐chlorouracil radical anion in aqueous solutions

1975; Volume: 79; Issue: 7 Linguagem: Inglês

10.1002/bbpc.19750790707

0005-9021

Burkhard O. Wagner, Dietrich Schulte‐Frohlinde,

Advanced oxidation water treatment

Abstract The pulse radiolysis of 5‐chlorouracil (ClUr) in airfree aqueous solution has been investigated. The rate constants for the reactions of the formation and the decay of the 5‐chlorouracil radical anion presented in the following reaction scheme have been measured at 21 °C: magnified image k 1 =(2.2 ± 0.4) · 10 10 M −1 s −1 , k 2 =(9 ± 2) · 10 4 s −1 , k 3 =(3.3 ± 0.6) · 10 10 M −1 s −1 , k ‐3 = (3.3 ± 0.6) · 10 5 s −1 , 2 k 4 = (6.6 ± 1.2) · 10 9 M −1 s −1 ; k 1 agrees with the values so far measured [1, 2]; the other rate constants have not been determined before. – The dissociation of ClUr. (reaction (2)) proceeded with an activation energy E a = (10.5 ± 1) kcal/mol (44 kJ/mol) and an entropy of activation ΔS ≠ (25°C) = ‐(1 ± 5) eu. – The value of the dissociation constant was determined by two independent methods which lead to the same value ( pK 3 = 5.0 ± 0.2 at 21°C). – Computer simulations using the obtained k values gave good agreement for the decay and the subsequent rise of the conductivity after pulse. – G‐values under 60 Co‐γ‐irradiations of 5‐chlorouracil in aqueous de‐aerated solutions containing tert.‐butanol were: G (‐ClUr) = 3.2 ± 0.3/100 eV, G (Ur) = 2.7 ± 0.2/100 eV and G (Cl − ) = 2.7 ± 0.1/100 eV, at p H values of 6.5 (buffered) and 4.7 (unbuffered). – Under pulse conditions the yield of chloride was G (Cl − ) = 2.7 ± 0.2/100 eV in neutral and alcaline solutions, but was lower in acidic solutions ( p H < 5) and at higher dose rates. This result was interpreted in terms of reaction (4).