Theoretical and Experimental Studies of the Spin Trapping of Inorganic Radicals by 5,5-Dimethyl-1-pyrroline N-Oxide (DMPO). 3. Sulfur Dioxide, Sulfite, and Sulfate Radical Anions

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• 作者：Pedro L. Zamora ; Frederick A. Villamena
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：July 5, 2012
• 年：2012
• 卷：116
• 期：26
• 页码：7210-7218
• 全文大小：554K
• 年卷期：v.116,no.26(July 5, 2012)
• ISSN：1520-5215

文摘

Radical forms of sulfur dioxide (SO₂), sulfite (SO₃^{2鈥?/sup>), sulfate (SO₄2鈥?/sup>), and their conjugate acids are known to be generated in vivo through various chemical and biochemical pathways. Oxides of sulfur are environmentally pervasive compounds and are associated with a number of health problems. There is growing evidence that their toxicity may be mediated by their radical forms. Electron paramagnetic resonance (EPR) spin trapping using the commonly used spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), has been employed in the detection of SO₃鈥⑩€?/sup> and SO₄鈥⑩€?/sup>. The thermochemistries of SO₂鈥⑩€?/sup>, SO₃鈥⑩€?/sup>, SO₄鈥⑩€?/sup>, and their respective conjugate acids addition to DMPO were predicted using density functional theory (DFT) at the PCM/B3LYP/6-31+G**//B3LYP/6-31G* level. No spin adduct was observed for SO₂鈥⑩€?/sup> by EPR, but an S-centered adduct was observed for SO₃鈥⑩€?/sup>and an O-centered adduct for SO₄鈥⑩€?/sup>. Determination of adducts as S- or O-centered was made via comparison based on qualitative trends of experimental hfcc鈥檚 with theoretical values. The thermodynamics of the nonradical addition of SO₃2鈥?/sup> and HSO₃鈥?/sup> to DMPO followed by conversion to the corresponding radical adduct via the Forrester鈥揌epburn mechanism was also calculated. Adduct acidities and decomposition pathways were investigated as well, including an EPR experiment using H₂17O to determine the site of hydrolysis of O-centered adducts. The mode of radical addition to DMPO is predicted to be governed by several factors, including spin population density, and geometries stabilized by hydrogen bonds. The thermodynamic data supports evidence for the radical addition pathway over the nucleophilic addition mechanism.}