G. A. Salmon

Primary Products and Processes in the

From a study of the u. v., visible, near i. r. and e. s. r. spectra induced by γ-irradiation at 77°K in glassy MTHF and in glassy MTHF containing various additives and from a study of controlled temperature increases on these spectra, the following conclusions are drawn. (1) The primary products of the radiolysis are electrons (e-) and positive ions (MTHF+) which undergo a rapid ion-molecule reaction to give O CH3 radicals (R⋅). (2) e- can either be trapped in the glassy MTHF matrix or can be captured by either napththalene, ferric chloride, carbon tetrachloride, nitrous oxide or trans-stilbene if these substances are present. (3) The e-T are bleachable by light or heat and disappear independently of the radicals R⋅ without either augmentation of R⋅ or the production of any new radical species. (4) e-T and R⋅ disappear thermally and independently by second-order reactions, the rate constants being Ke- + e-(M-1 S-1) = 1012⋅4±1⋅1 exp ─ [0⋅85 ± 0⋅10 kcal/mole/R(T ─ 75)] and KR˙ + R˙(M-1 S-1) = 1013⋅3±1⋅4 exp ─ [1⋅20 ± 0⋅15 kcal/mole/R(T ─ 75)]. These rate expressions suggest that both reactions are diffusion controlled at low temperatures in the glassy phase. (5) The kinetics of the thermal bleaching of e-T indicate that the electrons migrate distances of about 150 Å from their parent positive ions before being trapped in the matrix. (6) The effect of FeCl3 in reducing the formation of e-T at 77°K and its lack of effect on the thermal bleaching of e-T suggests that the reaction e- + FeCl3 → FeCl2 + Cl- only occurs before the electron is thermalized.

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The methods of pulse radiolysis, low temperature optical and electron spin resonance spectroscopy and chemical analysis of stable products have been applied to an investigation of the γ-radiolysis of liquid and glassy methanol. Processes induced by generation of presumed intermediates by photoionization of solutes have been studied using the same techniques. Electrons are trapped in glassy methanol after γ-irradiation at 77 K with a yield G(e¯t) = 2.7 ± 0.3 and on warming are rendered mobile, ultimately forming an amount of hydrogen equal to 1/2G(e¯t). In contrast, illumination of the irradiated sample at 77 K with visible light decomposes the trapped electron, forming hydrogen and CH2OH or CH2O- in yield equal to G(e¯t). The solvated electron in liquid methanol also reacts with the solvent, at a rate faster than the corresponding reaction in water. The addition of sodium methoxide to liquid methanol results in a marked increase in both the yield and lifetime of the solvated electron observed after 0.2 μs irradiation pulse. The optical absorptions and decay parameters of CH2OH and CH2O- in liquid methanol were obtained; the ultraviolet absorption observed in irradiated methanol glass after removal of the trapped electrons by photo-bleaching is probably due to these two species. Solutions of methanol glass containing silver perchlorate exhibit complex optical and e. s. r. spectra, which provide evidence for several different species, but a complete analysis has not been possible. The radicals expected from the initial ionization event are considered. The results of this work together with published data are used to predict the reaction of the free radicals CH3O. , CH2OH, CH2O-, H. and e¯s or e¯t in the systems of interest and the relevance of these reactions to some aspects of the radiation chemistry of methanol is discussed.

-Radiolysis of 2-Methyltetrahydrofuran

The concentration dependences of the yield of excited triplet states, G(3S*), and the concentration dependences of the relative yield of radiation-induced fluorescence obtained on pulse radiolysis of solutions of naphthalene, anthracene, pyrene and 2,5-diphenyloxazole (PPO) are reported. The yields of excited singlet state solute molecules, G(1S*), formed on pulse radiolysis of naphthalene-cyclohexane and naphthalene-benzene solutions has been determined by comparing the intensity of the radiation-induced fluorescence with that obtained on photo-excitation. It is concluded that intersystem crossing (i.s.c.) from the excited singlet state is an important process in the formation of the high yield of triplet excited states of the solute. Under certain conditions this process accounts for up to 50 % of G(3S*) in cyclohexane and the entire G(3S*) in benzene solutions.

The Radiation Chemistry of Liquid and Glassy 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 I2-, Br2-, (SCN)2- and TMPD+ have been identified by their absorption spectra (λmax = 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- + CH3O → CH3O- + X, X + X- ⇌ X2-, where X- - I-, Br-, SCN- or TMPD. In pure methanol the methoxy radicals react to form the CH2OH radical. The dependences of the yields of I2- and TMPD+ on the concentration of I- and TMPD respectively indicate that the yield of scavengable methoxy radicals G(CH3O) = 2.0 and that k(CH3O + X-)/k(CH3O + CH3OH) = (1.4±0.1) x 104. The presence of 0.1 mol HClO4/l or saturation of the solutions with N2O increases G(CH3O) by 0.5, an effect which is attributed to methoxy radicals which would otherwise react with electrons within the spurs. Solutions saturated with CO2 do not show the increased yield presumably because of the occurrence of the back reaction: CO2- + CH3O → CH3O- + CO2. The yield of ethoxy radicals, G(C2H5O) derived from measurement of I2- 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 I2- 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 + NO2 → TMPD+ + NO2-, TMPD + I2- → TMPD+ + 2I-.

The Radiation-Induced Formation of Excited States of Aromatic Hydrocarbons in Benzene and Cyclohexane. II. Yields of Excited Singlet and Triplet State Solute Molecules

Optical and electron spin resonance spectroscopy have been applied to a study of the species trapped in glassy 1:1 mixtures of triethylamine and 3-methylpentane following γ-irradiation at 77 K. Two distinct bands are discernible in the optical spectrum, a band commencing at about 500 nm and with a peak at 1670 nm which decays rapidly in the temperature interval 90 to 96 K and which is readily photobleached by light of λ > 400 nm is attributed to the trapped electron, e¯t. The effect of diphenyl on this band indicates that G(e¯t) ≥ 2. 2. A short wavelength band rising from 500 nm to the limit of observation at 285 nm which persists to a higher temperature, ca. 103 K, and which is not photobleached by visible light is assigned to free radicals, probably C6H13∙ and (C2H5)2NCH∙CH3. The decay of both bands was studied over a range of temperatures. The e. s. r. spectrum of the glassy mixture comprises a narrow singlet (∆H = 3.5 G) which has the properties expected of the signal due to e¯t and a broad (170 G) poorly resolved signal may be attributed to the radicals responsible for the u. v. absorption band. The optical absorption spectrum of the (C2H5)2NCHCH3 radical was obtained by pulse radiolysis of triethylamine at room temperature and the kinetics of the decay of this species have been characterized.

The Radiation Chemistry of Liquid Methanol. I. The Oxidizing Radical

The contribution to naphthalene and pyrene triplet state formation of intersystem crossing from the excited singlet state has been determined for both cyclohexane and benzene solutions using the singlet state quenchers nitromethane and xenon. In agreement with the conclusions reported in part II, intersystem crossing plays an important role; under certain conditions up to 50 % of the total triplet yield in naphthalene-cyclohexane solutions has this origin. The extinction coefficient for naphthalene triplet absorption in cyclohexane at λmax = 412.5 nm is 20 000 ± 5000 1 mol-1 cm-1.

Radiolysis of triethylamine-3-methylpentane glasses

γ-irradiation of glassy solutions of naphthalene (Naph) or diphenyl (ɸ2) in n-propanol at 77 K produces both the anion of the solute and a neutral radical Naph.H or ɸ2H. At 79 K the reaction Naph- + EtCH2OH → Naph.H + EtCH2O- is slow but ɸ2- is stable reacting rapidly with the solvent only at temperatures > 90 K. These reactions can be accelerated by light which, as experiments with two-solute systems show, causes photodetachment of electrons from the anions and their capture by the second solute. The decay of Naph.H at 110 to 125 K measured at λmax = 332 nm is second-order and the rate constant is given by k = 10(13.0±1)exp ((11.3±0.8kJ mol-1) / (R(T - 72))) mol-1 l s-1.

The radiation-induced formation of excited states of aromatic hydrocarbons in benzene and cyclohexane. III. Effect of singlet state quenchers

The irradiation of glassy n-propanol at 77 K with 60Co γ-rays has been investigated by the use of low temperature optical and e.s.r. spectroscopy to identify intermediates and gas chromatography to measure yields of gaseous products. Trapped electrons (et-) are formed with a yield G(et-) = 1.7 ± 0.2. Controlled warming and illumination with light λ > λmax suggest that electrons are initially immobilized in cavities surrounded by a non-equilibrium configuration of solvent molecules. The decay of et- at 100 to 110 K is first-order, the Arrhenius parameters being lg A (s-1) = 12.5 ± 1.5 and Ea = 31.3 ± 2.9kJ/mol. For photobleaching with light λ 509nm , (ɸє)509 = 460 ± 501 mol-1 cm -1 independent of the fraction of et- bleached. Unlike thermal bleaching, photobleaching does not change the total radical concentration as observed by e.s.r. The thermal and photolytic destruction of et- are explained by the following reactions: et-→ EtCH2O- + H 2 et- + hv→EtCHOH + EtCH2O- + H2 →EtCHO-. + H2, and the effects of additives are discussed. The thermal decay of the u.v. absorption after complete photobleaching of et- showed evidence of a species stable at 120 K but which decayed slowly at room temperature. An assignment of the u.v. absorptions to the species EtCHOH and EtCHO- is discussed. Some results are presented on the photolysis of γ-irradiated glasses with light of λ 254 nm.