Some Basic Properties of H and eaq

Much of basic free-radical chemistry of DNA and its constituents have been elucidated with the help of radiation techniques. This requires one to address briefly the properties of the H atom and the hydrated electron, eaq , which are important intermediates in the radiolysis of water (Chap. 2.2). 

The OH radical, which is also generated under these conditions, maybe converted into H by reacting it with excess H2 (Christensen and Sehested 1983). This may require a special pressure cell (Christensen and Sehested 1980). The scavenging of OH with fBuOH (k = 6 x 108 dm3 mol-1 s 1) is often the more convenient approach. Under adequate conditions, this leaves H largely untouched, since its rate of reaction with fBuOH is low (k = 1.7 x 10s dm3 mol-1 s 1 Buxton et al. 1988 recently revised at 1.15 x 106 dm3 mol-1 s 1 Wojnarovits et al. 2004). 

H is the conjugate acid of eaq [pKa(H ) = 9.1 reactions (1), k = 2.2 x 107 dm3 mol-1 s-1 and (2), k = 23 x 1010 dm3 moU1 s 1 (Buxton et al. 1988), for the thermodynamic properties of this system, see Hickel and Sehested (1985)]. Thus, in pure water, the lifetime of eaq is quite long (Hart et al. 1966), even long enough to monitor its presence spectrophotometrically under steady-state 60Co-y-radi-olysis conditions (Gordon and Hart 1964). 

Reaction (1) is best described as a proton transfer from the weak acid H to the strong base OH (Han and Bartels 1992). For the rapid conversion of eaq into H in neutral solution (i.e., at low 11+ concentration), phosphate buffer maybe used [reaction (3) k = 1.1 x 107 dm3 mol-1 s 1 (Grabner et al. 1973)]. The rate constant depends somewhat on the phosphate concentration, and at 1 mol dm-3 phosphate (pH -5.7) the reported value is 1.85 x 107 dm3 mol-1 s 1 (Ye and Schuler 1986). 

The hydrated electron is characterized by its strong absorption at 720 nm (e = 1.9 x 104 dm3 mol-1 cm-1 (Hug 1981) the majority of the oscillator strength is derived from optical transitions from the equilibrated s state to the p-like excited state (cf. Kimura et al. 1994 Assel et al. 2000). The 720-nm absorption is used for the determination of its reaction rate constants by pulse radiolysis (for the dynamics of solvation see, e.g Silva et al. 1998 for its energetics see, e.g Zhan et al. 2003). IP only absorbs in the UV (Hug 1981), and rate constants have largely been determined by EPR (Neta et al. 1971 Neta and Schuler 1972 Mezyk and Bartels 1995) and competition techniques (for a compilation, see Buxton et al. 1988). In many aspects, H and eaq behave very similarly, which made their distinction and the identification of eaq difficult (for early reviews, see Hart 1964 Eiben 1970 Hart and Anbar 1970), and final proof of the existence of the 

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