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Radiation chemistry of dimethylsulfoxide Cooper, Terry Kenneth

Abstract

A comprehensive study of the radiation chemistry of dimethyl- sulfoxide (DMSO), and DMSO-H₂0 mixtures, by steady-state γ-radiolysis, pulse radiolysis and low temperature matrix isolation techniques has revealed several significant features about the transient ionic products of the radiolysis. Very short-lived (τ[sub ½] < 25 nsec), weakly bound solvated electrons are formed in quite high yields in these liquids at room temperature. Solvated electrons in the binary DMSO-H₂O mixtures exhibit single optical absorption bands with maxima which vary smoothly with composition from pure water (λ[sub max] = 720 nm) to pure DMSO (λ[sub max] > 1500 nm). The electron stability varies with composition, showing a minimum lifetime coinciding with DMSO-H₂O mixtures which exhibit maximum inter-molecular structure. In glassy mixtures at 77°K, electrons generated radiolytically or photochemically are not trapped aid stabilized. The positive ions formed from irradiated DMSO are longer lived (τ[sub ½] ~ 2 µsec) than the solvated electons and have a strong absorption band centered at λ[sub max] 550 nm. In DMSO-H₂O mixtures this same absorption band is formed. The cationic yield is directly proportional to the fraction of the total dose absorbed initially by DMSO which suggests there is no charge transfer, proton exchange or positive ion scavenging in these mixtures. The yield of both e[sup - / sub s] and cations was ~ 30% higher in the fully deuterated DMSO than in the hydrogenated material indicating some alteration in the electron escape probability due to isotopic substitution. Most of the above observations are interpreted in terms of the physical properties of DMSO and its aqueous mixtures. The radiation yield of free ions is measured to be 1.30 ± 0.15 which is in reasonably good accord with a liquid having a dielectric constant of 48. The yield of solvated electrons was ascertained by observing the formation of anthracene anions in solutions of anthracene and corresponded very closely to the yield of positive ions determined from the formation of in KBr₂⁻ solutions. The near infrared absorption band of solvated electrons in DMSO indicates a weakly stabilized species which is consistent with the very poor solvating power for negative ions afforded by DMSO due to the aprotic nature of the solvent. On the other hand the lack of charge transfer or proton exchange with H₂O shown by the DMSO cation is consistent with the dipolar character of DMSO and its strong solvation of positive ions. A variety of free radicals have been observed by electron spin resonance in γ-irradiated polycrystalline DMSO at 77°K amongst which the •CH₃ and a sulfur containing radical are distinguished. Under γ-radiolysis liquid DMSO produces H₂, CH₄, C₂S₆ and (CH₃)₂S with G values of 0.20 ± 0.01, 3.3 ± 0.1, 0.49 ± 0.03 and 1.2 ± 0.2 respectively.

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