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Can O=NOOH undergo homolysis?

W H Koppenol1, R Kissner

  • 1Laboratorium für Anorganische Chemie, Eidgenössische Technische Hochschule-Zürich, Switzerland. koppenol@inorg.chem.ethz.ch

Chemical Research in Toxicology
|March 25, 1998
PubMed
Summary

Homolysis of peroxynitrous acid (O=NOOH) is unlikely to form hydroxyl radicals and nitrogen dioxide. New thermodynamic data indicate its homolysis rate is slower than isomerization, contradicting previous calculations.

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Area of Science:

  • Chemical Kinetics
  • Thermodynamics
  • Free Radical Chemistry

Background:

  • Previous thermodynamic calculations suggested peroxynitrous acid (O=NOOH) homolysis yields hydroxyl radical (HO•) and nitrogen dioxide (NO2•).
  • This was partly based on the assumption of a near-zero enthalpy of ionization for O=NOOH.
  • The ionization of O=NOOH is sensitive to its environment and its isomerization rate to nitrate.

Purpose of the Study:

  • To reinvestigate the temperature dependence of the ionization constant of O=NOOH.
  • To determine the thermodynamic parameters governing the homolysis of O=NOOH.
  • To assess the likelihood of O=NOOH homolysis versus isomerization.

Main Methods:

  • Experimental determination of the temperature dependence of the ionization constant of O=NOOH.
  • Thermodynamic calculations of the Gibbs energy and rate of homolysis.
  • Comparison of homolysis rates with isomerization rates.

Main Results:

  • The determined enthalpy of ionization (ΔH°) for O=NOOH is 4 ± 2 kcal mol⁻¹.
  • This yields a standard Gibbs energy of homolysis of 16 kcal mol⁻¹.
  • Calculated homolysis rates (1 x 10⁻⁴ to 0.6 s⁻¹) are slower than the isomerization rate (1.2 s⁻¹ at 25°C).

Conclusions:

  • Homolysis of O=NOOH is unlikely to be a significant reaction pathway.
  • The rate of homolysis is slower than the rate of isomerization to nitrate.
  • Previous suggestions of significant O=NOOH homolysis are inconsistent with new thermodynamic data.

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