The SO−·4-induced Chain Reaction of 1,3-dimethyluracil with Peroxodisulphate

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Abstract
The sulphate radical .SO4- reacts with 1,3-dimethyluracil (1,3-DMU) (k = 5 .times. 109 dm3 mol-1 s-1) thereby forming with .gtoreq. 90 percent yield the 1,3-DMU C95)-OH adduct radical 4 as evidenced by its absorption spectrum and its reactivity toward tetranitromethane. Pulse-conductometric experiments have shown that a 1,3-DMU-.SO4- adduct 3 as well as the 1,3-DMU radical cation 1, if formed, must be very short-lived (t1/2.ltoreq. 1 .mu.s). The 1,3-DMU c(5)-OH adduct 4 reacts slowly with peroxodisulphate CK = 2.1 .times. 105 dm3 mol-1 s-1). It is suggested that the observed new species is the 1,3-DMU-5-OH-6-SO.ovrhdot.4 radical 7. At low dose rates a chain reaction is observed. The product of this chain reaction is the cis-5,6-dihydro-5,6-dihydroxy-1,3-dimethyluracil 2. At a dose rate of 2.8 .times. 10-3 Gy s-1 a G value of .apprx. 200 was observed ([1,3-DMU] = 5 .times. 10-3 mol dm-3; [S2O82-] = 10-2 mol dm-3; [t-butanol] = 10-2 mol dm-3). The peculiarities of this chain reaction (strong effect of [1,3-DMU], smaller effect of [S2O82-]) is explained by 7 being an important chain carrier. It is proposed that 7 reacts with 1,3-DMU by electron transfer, albeit more slowly (k .apprxeq. 1.2 .times. 104 dm3 mol-1 s-1) than does .SO4-. The resulting sulphate 6 is considered to hydrolyse into 2 and sulphuric acid which is formed in amounts equivalent to those of 2. Computer simulations provide support for the proposed mechanism. The results of some SCF calculations on the electron distribution in the radical cations derived from uracil and 1-methyluracil are also presented.

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