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 ̅ , (5 a ) HOX ̅ → OH ̅ + X , (5 b ) X ̅ + X ̅ ⇌ X ̅ 2 (6) For the iodide solutions the rate constants were evaluated as k 5 a = k 6 = (1.21 + 0.08) x 10 10 1 mol -1 s -1 and k 5 b = (1.2 ± 1.0) x 10 8 s -1 . In the case of the thiocyanate solutions k 5 a = k 6 = (1.08 ± 0.10) x 10 10 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 k CH 2 O. +TMPD and k CH 2 O.+CH 3 OH are evaluated to be (6·10 ± 0·05) x 10 9 1 mol -1 s -1 and 2·63 + 0·10 x 10 5 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 CH 3 O + I ̅ → CH 3 O ̅ + 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 10 9 1 mol -1 s -1 an d (2·6 ± 0·4) x 10 10 1 mol -1 s -1 respectively. Observations on the transient u.v. absorption band of pulse irradiated methanol suggest that the spectra of CH 3 O and CH 2 OH are very similar for λ = 250 to 320 nm.

Photochemistry of the Nonspecific Hydroxyl Radical Generator,N-Hydroxypyridine-2(1H)-thione

“…Due to its absorption properties, • OH cannot be observed directly. Thiocyanate ions (SCN - ) were used to probe the presence of • OH: the reaction with • OH, according to eqs 7 and 8, gives (SCN) 2 •- , which is easily detectable by its absorption around 480 nm …”

“…The transient spectrum recorded following flash photolysis of a neutral solution of N-HPT in the presence of KSCN displays a broad band with a maximum around 480 nm, which is characteristic of (SCN) 2 •- …”

Rate constant and activation energy determination for reaction of e−(aq) and •OH with 2-butanone and propanal

“…Concern has been raised previously (17) about a potential problem with this procedure in that the formation of (SCN)? '-involves the intermediate formation of SCN' and SCN0H'- (18)(19)(20). If either of these species can oxidise MEK, then less (SCN)2'-will be formed, leading to an artificially high rate constant for 'OH reaction with MEK.…”

Photochemistry ofN-Hydroxy-2(1H)-pyridone, a More Selective Source of Hydroxyl Radicals ThanN-Hydroxypyridine-2(1H)-thione

“…The spectrum recorded after pulsed excitation (λ exc = 308 nm) of an acidic aqueous solution of N-HP in the presence of KSCN (0.2 M) was found to display a strong absorption band with a maximum at 480 nm . This band is indicative of the trapping of • OH and was not formed when • OH scavengers ( t- BuOH, DMSO, ascorbic acid, or phenol) were added to the solution.…”

“…In aqueous media, several acceptors can be used to probe the formation of • OH. Thiocyanate ions are known to react with • OH, according to eqs 9 and 10, to give (SCN) 2 •- , which is easily detectable by its absorption around 480 nm …”