The technique of pulse radiolysis has been utilized to study the absorption spectra of the transient free radical adducts which result from reactions of ·OH and$e_{{\rm aq}}{}^{-}$ with the pyrimidine, uracil. The ·OH attaches to the 5,6 bond of uracil to form an adduct (·UOH) which has peaks at 390 nm and ∼240 nm. Changes in the spectrum of ·UOH with pH have led us to the conclusion that at least one isomer of ·UOH has${\rm pK}_{{\rm a}}\text{'}$s of ∼7 and 9.5. The ·UOH decays by a second order kinetics with rate constant,$2k=(1.1\pm 0.2)\times 10^{9}\ M^{-1}\ s^{-1}$ which is independent of ionic strength. There is a residual spectrum which remains after the decay of the ·UOH absorption. This residual spectrum is different at pH 5 and 8.5, and shows a similar pH dependence to that of ·UOH itself. The$e_{{\rm aq}}{}^{-}$ adds to uracil to form an adduct which has protonated ($\cdot {\rm UH}_{1}$) and unprotonated ($\cdot {\rm U}^{-}$) forms which are in equilibrium with a pK a of 7. The spectra of both$\cdot {\rm UH}_{1}$ and$\cdot {\rm U}^{-}$ both have peaks near 250 nm with shoulders near 300 nm. The spectrum$\cdot {\rm U}^{-}$ is more intense than that of$\cdot {\rm UH}_{1}$. The rate constant for protonation of$\cdot {\rm UH}_{1}$ is$(5.7\pm 0.4)\times 10^{10}\ M^{-1}\ s^{-1}$. Both$\cdot {\rm UH}_{1}$ and$\cdot {\rm U}^{-}$ decay by second order kinetics with rate constants,$2k=(2.5\pm 0.5)\times 10^{9}\ M^{-1}s^{-1}$ and$2k=(1.6\pm 0.3)\times 10^{9}\ M^{-1}s^{-1}$ respectively. Evidence is presented which indicates that a reaction between$\cdot {\rm UH}_{1}$ (or$\cdot {\rm U}^{-}$) and an absorbing product may occur. In Ar bubbled solution with no scavengers present, the observed spectrum agrees closely with the calculated sum of the spectra of the individual adducts. This spectrum also decays by second order kinetics with the same halflife at all wavelengths implying that the adducts decay by reaction with themselves and with each other.

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