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Common intermediates in the OH-radical-induced oxidation of cyanide and formamide

Abstract:

OH radicals generated in the pulse radiolysis of N2O-saturated water react with formamide (by H-abstraction) and with cyanide (by addition and rearrangement) to give the same radicals, the main radical being the ̇CONH2 radical 1 (85-95%), the other most likely being the HC(O)NḢ radical 2 (5-15%). Quantum-chemical calculations support the preferred formation of 1 from formamide as well as from cyanide. Radical 1 reduces tetranitromethane (TNM) to the nitroform anion (2.8 × 108 dm3 mol-1 s-1). In the presence of oxygen, it rapidly adds oxygen (2.7 × 109 dm3 mol-1 s-1) to give the corresponding peroxyl radical ̇OOCONH2 (3) which absorbs more strongly in the wavelength region 250-400 nm than radical 1 [ε(320 nm) ≈ 180 dm3 mol-1 cm-1]. Peroxyl radical 3 deprotonates in basic solution (pKa ≈ 9.6), and its anion rapidly eliminates O2̇- (106 s-1) to give cyanic acid. Product studies under γ-radiolysis conditions show that in the absence of O2 less than half of radical 1 decays by disproportionation to produce cyanate and formamide. In basic solution and in the presence of O2, the G values of cyanate and H2O2 confirm that all of the peroxyl radical 3 decays by O2̇--elimination.