Radiation-induced transformations of methanol molecules in low-temperature solids: a matrix isolation study

Radiation-induced transformations of methanol in inert solids at 6 K reveal remarkable matrix effects, and mechanisms and astrochemical implications are discussed.

Hydroxymethyl radical self-recombination in high-temperature water

The self-recombination reaction of (•)CH2OH radicals in neutral aqueous solution has been studied at temperatures up to 300 °C at a pressure of 220 bar using pulse radiolysis and transient absorption. (•)CH2OH species decay by second-order kinetics independent of the applied dose, with a rate constant at 22 °C of 2k = 1.4 ± 0.1 × 10(9) M(-1) s(-1). The recombination follows Arrhenius behavior with the activation energy (E(a)) 12.7 ± 0.9 kJ/mol and pre-exponential factor of 1.9 ± 0.4 × 10(11) M(-1) s(-1). The overall recombination is significantly slower than the diffusion limit at elevated temperature, meaning that both disproportionation and dimerization channels have significant activation barriers. Ab initio calculations support the inference that the dimerization channel has no energy barrier, but has a large negative activation entropy barrier. The disproportionation channel (giving aqueous formaldehyde) almost certainly involves one or more specific water molecules to lower its activation energy relative to the gas phase.

Gamma radiolytic eradication of methoxychlor in aqueous media: the degradation pathways using HPLC and SPME-GC-MS

The gamma radiation-induced degradation of environmental pollutant methoxychlor in water was investigated. A 60Co gamma radiation source with a dose rate of 372 Gy h−1 was used for gamma irradiation of 1 mg L−1 and 10 mg L−1 methoxychlor in water with a varied absorbed dose of 1–5 kGy. A single step clean up and pre-concentration procedure based on solid phase micro-extraction was optimized. The extent of radiolytic degradation was monitored by reversed phase HPLC-UV and GC-ECD. The trace and ultra trace level degradation products were identified using GC-MS-SPME by comparing their mass spectra with the NIST 98 m mass spectral library. Most of the generated products for 4 kGy dose are substituted chlorophenols. The reaction pathways of these substituted chlorophenols and benzophenone formation are also proposed. However, generated chlorophenols disappeared along with methoxychlor for an absorbed dose of 5 kGy. The attained degradation of methoxychlor is ∼ 95% that reflects the potential use of ionization radiation for wastewater treatment.

Radiolytic degradation of methoxychlor in methanol and monitoring of radiolytic products by HPLC and GC-MS

Degradation of priority organic pollutant methoxychlor in methanol solution by gamma irradiation under varied experimental conditions has been optimized. The solution of methoxychlor was air saturated before irradiation. The extent of radiolytic degradation efficiency was monitored by reversed phase HPLC-UV; two major and two minor degradation products were detected. For 5 kGy gamma radiation dose at a rate of 200 kGy h−1≥95% methoxychlor was degraded. The degradation was also monitored by GC-ECD and the degradation products were identified using GC-MS after comparing their mass spectra with the NIST 98 m mass spectral library. It is proposed that major degradation occurs through dechlorination, dehydrochlorination, by the detachment of methoxyphenyl from methoxychlor and by interaction of other radicals generated by the methanol radiolysis. The probable reaction schemes for the formation of products have been proposed. Most of the generated products were methoxy substituted, probably due to the availability of the methoxy radical from methanol radiolysis. The identified radiolytic products of methoxychlor and the removal efficiency have been compared with those of UV photolysis. It is observed that although the source of degradation is somewhat different, the end products or radical generated species are of similar nature.

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

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.

The radiation chemistry of liquid methanol. I. The oxidizing radical

Methanolic solutions of I - , Br - , Cl - , SCN - and TMPD have been investigated by pulse radiolysis. Except for Cl - solutions oxidation of the solutes occurs and the transient products I 2 - , Br 2 - , (SCN) 2 - and TMPD + have been identified by their absorption spectra ( λ max = 380, 360, 470 and 565 nm respectively). For the Br - and SCN - cases the oxidation occurred only in acidified solution. These results are attributed to the occurrence of the reactions X - + CH 3 O → CH 3 O - + X, X + X - ⇌ X 2 - , where X - - I - , Br - , SCN - or TMPD. In pure methanol the methoxy radicals react to form the CH 2 OH radical. The dependences of the yields of I 2 - and TMPD + on the concentration of I - and TMPD respectively indicate that the yield of scavengable methoxy radicals G (CH 3 O) = 2.0 and that k (CH 3 O + X - )/ k (CH 3 O + CH 3 OH) = (1.4±0.1) x 10 4 . The presence of 0.1 mol HClO 4 /l or saturation of the solutions with N 2 O increases G (CH 3 O) by 0.5, an effect which is attributed to methoxy radicals which would otherwise react with electrons within the spurs. Solutions saturated with CO 2 do not show the increased yield presumably because of the occurrence of the back reaction: CO 2 - + CH 3 O → CH 3 O - + CO 2 . The yield of ethoxy radicals, G (C 2 H 5 O) derived from measurement of I 2 - formed in ethanolic KI solutions is estimated to be 1.5±0.1. Formate ion was found to have no effect on the yield of I 2 - from methanolic solutions of iodide and it is concluded that the reaction between methoxy radicals and formate ion is slow. The similar lack of effect of nitrite ion and iodide ion on the TMPD system is attributed to the reactions TMPD + NO 2 → TMPD + + NO 2 - , TMPD + I 2 - → TMPD + + 2I - .

Alkoxy radical intermediates in the thermal and photochemical oxidation of alcohols

Radical scavenging by nitrone and nitroso compounds, used in conjunction with e. s. r. spectroscopy, has demonstrated that alkoxy radicals are important intermediates during the thermal and photochemical oxidation of primary and secondary aliphatic alcohols: R 1 R 2 CHOH+X(ox) → R 1 R 2 CHȮH + + X(red) R 1 R 2 CHȮH + +R 1 R 2 CHOH → R 1 R 2 CHO. + R 1 R 2 CHOH + 2 R 1 R 2 CHO. + PhCH ═ NBu t → R 1 R 2 CHO─CHPh─.O─NBu t ─R 1 R 2 CHoH→R 1 R 2 C─OH + Bu t N ═ O → R 1 R 2 C─N─Bu t ─OH─O. Oxidants include the herbicidal paraquat ion, peroxydisulphate ion, the peroxydisulphatesilver (ɪ) couple, lead tetraacetate and uranyl nitrate. Independet generation of alkoxy radicals (photolysis of the corresponding alkyl nitrites R 1 R 2 CHONO) and their hydroxyalkyl isomers (H abstraction from the parent alcohol) confirms the e. s. r. assignments: mechanistic consequences of the results are discussed.

Absorption spectra of highly purified liver microsomal cytochrome P-450 in non-equilibrium conformational states at low temperatures

Absorption spectra of highly purified liver microsomal cytochrome P-450 in non-equilibrium states were obtained at 77 K by reduction with trapped electrons, formed by gamma-irradiation of the water-glycerol matrix. In contrast to the equilibrium form of ferrous cytochrome P-450 with the heme iron in the high-spin state the non-equilibrium ferrous state has a low-spin heme iron. The absorption spectrum of the non-equilibrium ferrous cytochrome P-450 is characterized by two bands at 564 (alpha-band) and 530 nm (beta-band). When the temperature is increased to about 278 K this non-equilibrium form of the reduced enzyme is relaxed to the corresponding equilibrium form with a single absorption band at 548 nm in the visible region characteristic for a high-spin heme iron.