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

Summary

The sulphate radical SO⁻ ·₄ reacts with 1,3-dimethyluracil (1,3-DMU) (k = 5 × 10⁹ dm³ mol⁻¹ s⁻¹) thereby forming with ⩾ 90 per cent yield the 1,3-DMU C(5)-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-SO⁻ ·₄ adduct 3 as well as the 1,3-DMU radical cation 1, if formed, must be very short-lived (t_1/2 ≤ 1 μs). The 1,3-DMU C(5)-OH adduct 4 reacts slowly with peroxodisulphate (k = 2·1 × 10⁵ dm³ mol⁻¹ s⁻¹). It is suggested that the observed new species is the 1,3-DMU-5-OH-6-SO·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 × 10⁻³ Gy s⁻¹ a G value of ∼ 200 was observed ([13-DMU] = 5 × 10⁻³ mol dm⁻³; [S₂O²⁻₈] = 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 [S₂O²⁻₈]) 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 ≈ 1·2 × 10⁴ dm³ mol⁻¹ s⁻¹) than does SO⁻ ·₄. 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.

• 1,3-dimethyluracil
• peroxodisulphate
• sulphate radical
• chain reaction
• model for direct effect