Received: February 4, 2004

In Final Form: May 13, 2004

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 a_N = 19.6 G, with each of the three nitrogen lines further split by the small hydrogen coupling, a_H = 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 kinetic 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.

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