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

1972 ◽  
Vol 327 (1570) ◽  
pp. 305-316 ◽  
Keyword(s):  
Absorption Spectra ◽  
Pulse Radiolysis ◽  
Radiation Chemistry ◽  
Back Reaction ◽  
Oh Radical ◽  
Nitrite Ion ◽  
Methoxy Radicals ◽  
Iodide Ion ◽  
Pure Methanol

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 - .

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

1972 ◽  
Vol 328 (1572) ◽  
pp. 23-36 ◽  
Keyword(s):  
Aqueous Solution ◽  
Rate Constants ◽  
Pulse Radiolysis ◽  
Nanosecond Pulse ◽  
Forward Rate ◽  
Mechanism Of Formation ◽  
Methoxy Radicals ◽  
Nanosecond Pulses ◽  
Radiation Induced ◽  
Pure Methanol

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 organic amides I. A pulse radiolysis study of solvated electrons and alkali-metal-electron species in cyclic amides

1989 ◽  
Vol 421 (1860) ◽  
pp. 169-178 ◽  
Keyword(s):  
Alkali Metal ◽  
Absorption Spectra ◽  
Pulse Radiolysis ◽  
Near Infrared ◽  
Radiation Chemistry ◽  
Alkali Metal Salts ◽  
Solvated Electrons ◽  
Electron Scavenger ◽  
Cyclic Amide ◽  
Metal Electron

Pulse radiolysis of the cyclic amides N -methylpyrrolidinone (NMP), N -ethylpyrrolidinone (NEP), 1,3-dimethyl-2-imidazolidinone (DMI) and 1,3-dimethyl-2-oxo-hexahydropyrimidine (DMH) and the non-cyclic amide tetramethylurea (TMU) yielded absorption spectra in the near infrared that are attributed to solvated electrons. Addition of a variety of alkali-metal salts caused no detectable change in the absorption spectrum of e - s and no new absorptions attributable to alkali-metal anions were detected. The effect of dose on the decay of e - s in NMP was studied in detail. The yields of e - s in these amides were estimated by using trans -stilbene as an electron scavenger. Absorption spectra, which are not removed by saturation with N 2 O and CO 2 , are observed in the wavelength range 300-500 nm.

Phenylalanine: Its ˙OH and SO4˙⁻-Induced Oxidation and Decarboxylation. A Pulse Radiolysis and Product Analysis Study

1993 ◽  
Vol 48 (6) ◽  
pp. 761-770 ◽  
Author(s):  
Degui Wang ◽  
Heinz-Peter Schuchmann ◽  
Clemens von Sonntag
Keyword(s):  
Radical Cation ◽  
Pulse Radiolysis ◽  
Side Reactions ◽  
Intramolecular Electron Transfer ◽  
Oh Radical ◽  
Product Analysis ◽  
Water Elimination ◽  
Deprotonation Reaction ◽  
Type Radical ◽  
Radical Yield

Phenylamine has been oxidized by radiolytically generated hydroxyl and sulfate radicals, the ensuing intermediates and their reactions have been studied by pulse radiolysis and product analysis in the absence and presence of oxidants such as Fe(CN)63- and O2. Upon OH radical attack, hydroxycyclohexadienyl-type radicals are mainly formed while Η-abstraction reactions can be neglected. In the presence of Fe(CN)63- these radicals are for the most part oxidized to the corresponding tyrosines (80%), except for the ipso-OH-adduct radicals (≈ 20%). It is concluded that ˙OH-addition is almost random, but with a slight avoidance of the metaposition relative to the ortho-, para- and ipso-positions. Oxygen adds reversibly to the OH-adduct radicals (kf = 1.8 × 108 dm3 mol-1 s-1, kr = 5.4 × 104 s-1). In this case, tyrosine formation occurs by HO2˙-elimination. However, due to side reactions, tyrosine formation only reaches 52% of the OH radical yield. The tyrosine yield drops to 10% in the absence of an oxidant.Upon SO4˙⁻-attack, decarboxylation becomes a major process (33% of SO4˙⁻) alongside the production of tyrosines (43%). Here, with Fe(CN)63- as the oxidant the formation of p-Tyr (18.5%) and m-Tyr (16.5%) is preferred over o-Tyr formation (8.5%). It is believed that in analogy to other systems a radical cation is formed immediately upon SO4˙⁻-attack which either reacts with water under the formation of hydroxycyclohexadienyl-type (“OH-adduct”) radicals, or decarboxylates after intramolecular electron transfer. The radical cation can also arise indirectly through H+-catalysed water elimination from the ˙OH-adduct radicals. At pH 2 and a dose rate of 0.0046 Gy s-1 CO2 formation matches the OH radical yield when ˙OH is the attacking radical. Below pH 2, G(CO2) decreases with falling pH. This indicates the occurrence of another, unimolecular, pathway under these conditions competing effectively with decarboxylation. This appears to be a relatively slow deprotonation reaction of the carboxylprotonated phenylalanine radical cation which gives rise to the benzyl-type radical.

Reactions of OH Radicals with Acetylacetone in Aqueous Solution. A Pulse Radiolysis and Electron Spin Resonance Study

1982 ◽  
Vol 37 (3) ◽  
pp. 368-375 ◽  
Author(s):  
R. K. Broszkiewicz ◽  
T. Söylemez ◽  
D. Schulte-Frohlinde
Keyword(s):  
Double Bond ◽  
Pulse Radiolysis ◽  
Visible Region ◽  
Alkaline Condition ◽  
Main Reaction ◽  
Electron Spin Resonance Study ◽  
Oh Radicals ◽  
Oh Radical ◽  
Keto Form ◽  
Optical Absorbance

Abstract Pulse radiolysis experiments monitoring optical absorbance as well as conductivity and in-situ ESR radiolysis studies show that the OH radical reacts with the enol (k=8.6 x 109 M-1 s-1) and the enolate (k = 7.4 X 109 M-1 s-1) forms of acetylacetone by addition to the C = C double bond in aqueous N2O saturated solution. The OH reaction with enol leads to equal amounts of two radicals, CH3COCHOHCOHCH3 (2) and CH3COCHC(OH)2CH3 (4), as determined by scavenger reactions. At pH less than 1 the radical CH3COCHCOCH3 (1) is observed by ESR spectroscopy showing that radical 2 and/or 4 eliminate water by proton catalyzed reactions. Under alkaline condition the OH adducts to the enolate eliminate OH -with rate constants larger than 105 s-1 leading to radical 1. G(OH-) is determined to be 5.6 showing that addition is the main reaction of OH with enolate. To a much smaller degree the OH radical is proposed to abstract an H atom from that CH3 group which is attached to the C -C double bond in enol and enolate, producing substituted allyl radicals which absorb in the visible region. The reaction of OH with the keto form has not been observed indicating that the rate constant of this reaction is significantly smaller than those with enol and enolate.

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