EPR Detection of HNO2•-in the Radiolysis of Aqueous Nitrite and Quantum Chemical Calculation of Its Stability and Hyperfine Parameters

Abstract

Experimental and theoretical evidence is presented to support a prior suggestion [Lymar et al. J. Phys. Chem. A 2002, 106, 7245] that radiolytically generated hydrogen atoms attack at the nitrogen, rather than the oxygen, of nitrite ions in aqueous solution. Time-resolved electron spin resonance detection was used to unambiguously identify the HNO2 ?- radicals formed. At pH 9 the radicals live about 10 ?s, and have quite broad (0.6 G) lines. The observed hyperfine splitting at nitrogen was aN 19.6 G, with each of the three nitrogen lines further split by the small hydrogen coupling, aH 4.5 G. The g factor for the radical is 2.0053. Although this is the first observation of this radical in fluid solution, the ESR parameters are consistent with previous observations in the solid phase. The identity of the radical was also confirmed by quantum chemical calculation of the ESR parameters, including the g factor. It was necessary to take into account vibrational modulation of the computed hyperfine parameters when comparing theory to experiment because of the large-amplitude motion of the hydrogen atom in the pyramidal radical. The yield of HNO2 ?- radicals was estimated at 70% of the available H atoms by a kineticmore » method. Computed thermodynamic parameters confirm that, in the gas phase, both HNO2 ?- and HONO?- are stable relative to the asymptotes H + NO2- and OH- + NO, with HNO2 ?- 40 kJ mol-1 above HONO?- but protected from rearrangement by a large barrier. In solution, calculations indicate that while HNO2 ?- is still bound with respect to dissociation into H + NO2-, it is now only 3.2 kJ mol-1 above HONO?- which, in turn, lies about 73 kJ mol-1 above OH- + NO, and dissociation is driven by the strong solvation of the hydroxide ion.« less