Free radicals and diradicals in the reaction between nitrous acid and bisulfite in acid aqueous media

Abstract

4) Walters, V. A.; Haddad, C. M.; Thiel, Y.; Colson, S. D.; Wiberg, K. B.; Johnson, P. M.; Foresman, B. J. J. Am. Chem. Soc. 1991, 113, 4782. (5) Bent, G. D.; R w i , A. R.Rates of the prompt formation of (HSO,),NO'(F) in the reaction of nitrous acid with bisulfite were measured by stopped-flow electron spin resonance spectrometry at pH = 3.0-5.8, 298 K. Initial rates have a positive dependence on (HONO] and [HSOy] but decrease markedly with pH. The induction periods observed in air-saturated solutions are s u p p d by addition of 2 mM hydroxylaminedisulfonate, (S03-)2NOH (FH), revealing a free-radical precursor to F. The spin trap 5.5-dimethyl-1-pyrroline N-oxide (DMPO) also inhibits the formation of F but withour producing newparamagneric species, ruling out monoradical intermediates. We show that these observations are consistent with the previously proposed participation of a nitrene, HON:, in this system. Present evidence suggests that the title reaction is a potential source of free radicals in natural environments.

Role of Nitrogen Dioxide in the Production of Sulfate during Chinese Haze-Aerosol Episodes

“…It is expected to be favored at the high relative humidity and high S / V aerosol densities that are prevalent during HEs. We show that the photolysis of HONO and NO 2 – can be a significant source of OH radicals even under hazy conditions and point out that the thermal reaction between NO 2 – and HSO 3 – also proceeds via free radicals, both of which can initiate oxidative chains in the presence of O 2 . These findings suggest that future field campaigns should focus on OH radical measurements during winter HEs, an issue that has not been properly addressed in the literature.…”

“…Reductions of HONO and NO 2 – to NO [ E °(HONO + H + + e = NO + H 2 O) = 0.75 V, and E °(NO 2 – + 2 H + + e = NO + H 2 O) = 1.08 V] or to N 2 O [ E °(2HONO + 4H + + 4e = N 2 O + 3H 2 O) = 1.06 V, and E °(2NO 2 – + 6H + + 4e = N 2 O + 3H 2 O) = 1.04 V] could also drive the thermal (dark) oxidation of HSO 3 – to HSO 4 – [ E °(HSO 3 – + H 2 O = HSO 4 – + 2H + + 2e) = 0.15 V] at pH 4, depending on the concentrations of the reactants and products. The oxidation of HSO 3 – by HONO and NO 2 – , however, is complex and proceeds slowly via a free radical mechanism through S- and N-containing intermediates. − In the presence of air, O 2 could participate in this process (see below) …”

The heterogeneous formation of N2O in the presence of acidic solutions: Experiments and modeling

“…It is important to point out that NO is always present in the system due to HONO decomposition. These results are surprising because the role of oxygen in trapping the intermediates of the N(III)/S(IV) reaction such as HON was confirmed in a recent study [21]. This trapping reaction is rapid with a rate constant on the order of the diffusion-controlled limit.…”

“…The redox reaction of HONO and SO 2 results in S(VI) and HON, which undergoes fast secondary reactions such as dimerization /dehydration to result in N 2 O [16]. The Raschig mechanism (Scheme I) describes the reduction of HONO by bi-sulfite ion leading to an initial adduct nitroso-sulfonate ( ) which further reacts with an additional bi-Ϫ ONSO 3 sulfite ion to form hydroxylaminedisulfonate (HADS) or undergoes hydrolysis to form nitroxyl (HON) in a competitive reaction [20,21]. Nitroxyl is a key intermediate of N(+I) leading to nitrous oxide and its dehydrative dimerization to N 2 O occurs in solution, but the corresponding rate constant was not accurately estimated [22,23].…”

“…Nitroxyl is a key intermediate of N(+I) leading to nitrous oxide and its dehydrative dimerization to N 2 O occurs in solution, but the corresponding rate constant was not accurately estimated [22,23]. Nitroxyl having a triplet ground state ( 3 AЈ) is expected to react rapidly with O 2 [21] forming the singlet diradical peroxynitrite which either rearranges into nitrate at high pH or dissociates to form at lower pH [24] according to the follow-OH ϩ NO 2 ing reactions:…”