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Studies in radiation chemistry

University of British Columbia

The experimental work presented in this dissertation consists of two separate parts. Firstly, a study of the reaction of hydrated electrons with molecular nitrogen is reported. Secondly, the results of an investigation of the effects of Ƴ-radiation on the polar aprotic solvent, propylene carbonate; (a) in the glassy solid state at 77 °K, and (b) as a liquid at room temperature, are presented. Hydrated electrons were generated by Ƴ-radiolysis of aqueous solutions containing H₂ and 0H⁻ and also containing N₂ at concentrations up to 0.1 M (200 atm pressure). Significant yields of ammonia were obtained, but by completely eliminating the gas space above the solution it was shown that the majority of the NH₃ arose through "direct action" of the radiation on dissolved N₂. Although the hydrated electron is one of the most powerful and reactive reducing agents, it is unable to cause reduction fixation of molecular nitrogen. An upper limit of k₁<18 M⁻¹s⁻¹was estimated for the rate constant of the reduction reaction. Conversion of the hydrated electrons to H atoms in acid solution did not affect the ammonia yield, implying that H atoms are also unable to reduce nitrogen. In the glassy solid state at 77 °K, Ƴ-irradiation of propylene carbonate produced species identified as trapped electrons. They were characterised by a narrow [formula omitted], Gaussian shaped ESR line at g = 2.0028 and an optical absorption band with ʎmax∼ 370 nm . The electrons were unstable at 77 °K and decayed via a non-homogeneous process believed to be reaction with positive ions. Also formed by the Ƴ-radiation were four unidentified trapped radicals, all characterised by doublet ESR signals centered at g = 2.0023 and with hyperfine splittings of 42, 58, 83, and 124 G . Ultraviolet photolysis of the irradiated glasses at 77 °K produced new radicals identified as CO₃ , HCO and CH₃. The CO₃ radical gave a single ESR line at g∼2.015 and a broad visible optical absorption band with ʎmax∼600 nm. HCO was identified by its asymmetric doublet ESR signal with hyperfine splitting of about 130 G and a multi-line vibronic absorption spectrum in the 500 - 750 nm region. The methyl radicals were unstable in the matrix and were identified by their characteristic 1:3:3:1 quartet ESR spectrum with hyperfine splitting of about 21 G . Ƴ-radiolysis of liquid propylene carbonate at 25 °C produced H₂ , CO and CO₂ as the major gaseous "molecular" products with yields: GH₂= 0.75 ± 0.05, GCO = 1.2 ±0.1, and GCO₂ = 3.2 ±0.3. Methane was also produced via a secondary process involving methyl radicals with a yield: G(CH₄) = 0.20±0.02 . Scavenger experiments with N₂0, I₂, methanol and acid indicated that an anionic reducing species was formed by the radiation with a yield of GX - = 2.0±0.2 . This species was probably a solvated electron although the possibility of it being a reactive molecular anion could not be excluded on the basis of the steady state radiolysis data. A transient optical absorption at 630 nm was observed on pulse radiolysis of propylene carbonate with 3 nsec pulses of 0.5 MeV electrons. However, either solvated electrons or the C0₃⁻ radical ion could have been responsible for the absorbance.

Text

2011-04-21

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http://hdl.handle.net/2429/33900

Chemistry

University of British Columbia

Graduate