Nanosecond pulse radiolysis of methanolic and aqueous solutions of readily oxidizable solutes

Abstract
The formation of I ̅ 2 and (CNS) ̅2 has been observed in aqueous solution of KI and KCNS respectively following irradiation with nanosecond pulses of 3 MeV electrons. In both cases it is necessary to invoke the intermediate and consecutive formation of two species which do not absorb light at the monitoring wavelength. The following mechanism is invoked for the formation of X ̅ 2 (where X ̅ = I ̅ or CNS ̅ ): OH + X ̅ → HOX ̅ , (5a) HOX ̅ → OH ̅ + X , (5b) X ̅ + X ̅ ⇌ X ̅ 2 (6) For the iodide solutions the rate constants were evaluated as k5a = k6 = (1.21 + 0.08) x 1010 1 mol-1 s-1 and k5b = (1.2 ± 1.0) x 108 s-1. In the case of the thiocyanate solutions k5a = k6 = (1.08 ± 0.10) x 1010 1 mol-1 s-1 and HOCNS ̅ is estimated to have a lifetime of about 5 ns. The radiation induced oxidation of N, N , N', N'-tetramethyl-p-phenylenediamine (TMPD) to Wurster’s Blue cation (TMPD+) has been observed by nanosecond pulse radiolysis of solutions of TMPD in methanol. It is concluded that the oxidation of TMPD is by methoxy radicals and the rate constants kCH2O. +TMPD and kCH2O.+CH3OH are evaluated to be (6·10 ± 0·05) x 109 1 mol-1 s-1 and 2·63 + 0·10 x 105 1 mol-1 s-1 respectively. Thus the half-life of methoxy radicals in pure methanol is 106 ns. The formation of I ̅2 was observed in methanolic solutions of KI. The oxidizing species is thought to be the m ethoxy radical and the mechanism of formation of I ̅2 is by the reactions CH3O + I ̅ → CH3O ̅ + I ̅ , I + I ̅ ⇌ I ̅ 2. The rate constant of reaction (1) and the forward rate of the equilibrium (2) are estimated to be (3·7 ± 0.3) x 109 1 mol-1 s-1 an d (2·6 ± 0·4) x 1010 1 mol-1 s-1 respectively. Observations on the transient u.v. absorption band of pulse irradiated methanol suggest that the spectra of CH3O and CH2OH are very similar for λ = 250 to 320 nm.

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