Reactions of dichloride anion radicals with pyrimidine and its derivatives.
Rate constants for the reactions of dichloride anion radicals with pyrimidine and its derivatives were determined by a conventional flash photolysis. Kinetic treatment was carried out by assuming that second-order self decay of the anion radicals and the reaction with a substrate proceed simultaneously. Values of the rate constants were found within 1.1 × 107 to 4.1 × 109 M-1 s-1; the lowest reactivity was observed for pyrimidine and the highest for 2, 4, 6triaminopyrimidine. Comparison of the rate constants with those for hydroxyl radicals suggests that the dichloride anion radicals undergo a different reaction with pyrimidine bases from that of the hydroxyl radicals. A transient absorption due to an intermediate produced in the reaction with the anion radicals was observed for 5-aminouracil. This is only a case where relatively long-lived absorption was observed. The absorption was used to determine the rate constants for the reactions with the anion radicals.
Transient intermediate produced from 5-aminouracil in reaction with dichloride anion radical
A transient absorption, which is relatively long-lived, was found in a flash photolysis of aqueous potassium chloride containing 5-aminouracil. The transient absorption with a peak at about 335 nm was assigned to an intermediate produced from 5-aminouracil through a one-electron oxidation by dichloride anion radical. For the intermediate, a possible structure is proposed to explain its long life-time. On the basis of the transient absorption, a determination of the rate constant for the reaction of 5-aminouracil with dichloride anion radical was carried out by use of a competition kinetics. The value of (2.9±0.2)×108 dm3 mol−1 s−1 was obtained as the rate constant on the basis of rate constants of several reference compounds. By comparing the values obtained by the competition method with those estimated by a direct analysis of the decay rate of dichloride anion radical, the validity of the competition method was examined.
Pulse radiolysis of aqueous thiocyanate and iodide solutions
The transient absorption produced by the pulse radiolysis of CNS– is shown to be due to (CNS)2 formed by the reactions : [graphic omitted] The rate constants of reactions (1), (2) and (3) at 22°C determined from kinetic and equilibrium measurements are 2.8 × 1010, 7.0 × 109 M–1 sec–1 and 3.4 × 104 sec–1, respectively. For iodide ion, which forms I2 in the same way, the equilibrium constant for I2 formation is 1.13 × 105 M–1. The corresponding kinetic constants cannot be determined as accurately as for CNS– but are estimated to be 3.4 × 1010, 7.6 × 109 M–1 sec–1 and 6 × 104 sec–1.