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Kinetic and thermodynamic properties of the aminoxyl (NH2O center dot) radical
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
2006 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 110, no 1, 192-197 p.Article in journal (Refereed) Published
Abstract [en]

The product of one-electron oxidation of (or H-atom abstraction from) hydroxylamine is the H2NOcenter dot radical. H2NOcenter dot is a weak acid and deprotonates to form HNO-center dot; the pK(a)(H2NOcenter dot) value is 12.6 +/- 0.3. Irrespective of the protonation state, the second-order recombination of the aminoxyl radical yields N-2 as the sole nitrogen-containing product. The following rate constants were determined: k(r)(2H(2)NO(center dot)) = 1.4 x 10(8) M-1 s(-1), k(r)(H2NOcenter dot + HNO-center dot) = 2.5 x 10(9) M-1 s(-1), and k(r)(2HNO(-center dot)) = 4.5 x 10(8) M-1 s(-1). The HNO-center dot radical reacts with 02 in an electron-transfer reaction to yield nitroxyl (HNO) and superoxide (O-2(-center dot)), with a rate constant of k(e)(HNO-center dot + O-2 -> HNO + O-2(-center dot)) = 2.2 x 10(8) M-1 s(-1). Both O-2 and O-2(-center dot) seem to react with deprotonated hydroxylamine (H2NO-) to set up an autoxidative chain reaction. However, closer analysis indicates that these reactions might not occur directly but are probably mediated by transition-metal ions, even in the presence of chelators, such as ethylenediamine tetraacetic acid (EDTA) or diethylenetriamine pentaacetic acid (DTPA). The following standard aqueous reduction potentials were derived: E degrees(H2NO center dot,2H(+)/H3NOH+) = 1.25 +/- 0.01 V; E degrees(H2NOcenter dot,H+/H2NOH) = 0.90 +/- 0.01 V; and E degrees(H2NOcenter dot/H2NO-) = 0.09 +/- 0.01 V. In addition, we estimate the following: E degrees(H2NOH+center dot/H2NOH) = 1.3 +/- 0.1 V, E degrees(HNO, H+/H2NOcenter dot) = 0.52 +/- 0.05 V, and E degrees(HNO/HNO-center dot) = -0.22 +/- 0.05 V. From the data, we also estimate the gaseous O-H and N-H bond dissociation enthalpy (BDE) values in H2NOH, with BDE(H2NO-H) = 75-77 kcal/mol and BDE(H-NHOH) = 81-82 kcal/mol. These values are in good agreement with quantum chemical computations.

Place, publisher, year, edition, pages
2006. Vol. 110, no 1, 192-197 p.
Keyword [en]
electron-spin resonance, acid perchlorate solutions, pulse-radiolysis, aqueous-solution, alkaline solutions, hydroxylamine, oxidation, nitroxyl, reduction, mechanism
URN: urn:nbn:se:kth:diva-15368DOI: 10.1021/jp054747tISI: 000234520300025ScopusID: 2-s2.0-31144460773OAI: diva2:333409
QC 20100525Available from: 2010-08-05 Created: 2010-08-05Bibliographically approved

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