Kinetic applications of electron paramagnetic resonance spectroscopy

none triplet with the nonbonding electrons of N. Acknowledgment. This work was ... Kinetic Applications of ElectronParamagnetic Resonance. Spectroscop...
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quenching by tert-butylamine of phosphorescence of b e n z ~ p h e n o n e7.0 , ~ ~X lo7 M-l sec-'. That the value is lower than that for 2-butylamine may be due to a less f a v o r a b l e steric factor for interaction of the benzophenone triplet with the nonbonding electrons of N.

Acknowledgment. This work was supported by Fellowships under Title IV of the National Defense Education Act and from the Petroleum Research Fund, administered by the American Chemical Society, and by the National Science Foundation, GP 9247.

Kinetic Applications of Electron Paramagnetic Resonance Spectroscopy. 11. Self-Reactions of N-Alkyl Nitroxides and N-Phenyl Nitroxide'92 D. F. B ~ w m a n J. , ~ L. Broken~hire,~ T. Gillan,* and K. U. Ingold* Contribution from the Division of Chemistry, N a t i o n a l Research Council of Canada, O t t a w a , C a n a d a . Received January 22, 1971

Abstract: The self-reactions of methyl, isopropyl, fert-butyl, a n d phenyl nitroxides in solution have been examined by epr spectroscopy. T h e reaction is a disproportionation t o yield a hydroxylamine and a nitroso compound: ZRNHO. .-,R N H O H RNO. Rate constants for this reaction have been measured in several solvents. I n benzene at 25 O the rate constants are in the range 4 x 107 t o 3 X lo6M-' sec-I. For R = phenyl and R = tert-butyl the reaction was shown t o b e reversible and the equilibrium constants were measured over a range of temperature. T h e measured enthalpy change for the reaction with R = tert-butyl is in good agreement with that calculated (for R = CHJ from the heats of formation of the various species involved. Evidence is presented for hydrogen bonding of methyl nitroxide in methanol. The rate constant for the slow conversion of phenyl nitroxide t o azoxybenzene has also been measured over a range of temperature in benzene.



has been known for some time from qualitative observations made during the recording o f the electron paramagnetic resonance spectra o f nitroxide radicals that N-alkyl and N-aryl nitroxides are much less stable than the majority of N,N-dialkyl or N,N-diary1 nitroxides. The epr spectra of p h e n y l nitroxide5-'0 and several alkyl nitroxides (e.g., MeNHO. iPrNH0.,13 t e r t - B u N H 0 . , 1 3 1 ' 4 PhCH2NH0.15) have been reported but there have been no quantitative studies of the stabilities or of the decay o f any of these radicals. In Part I 2 we described an epr study of the kinetics and mechansim of the self-reaction of N,Ndiethyl n i t r o x i d e in solution. The present paper describes a similar study of the self-reactions of methyl, isopropyl, tert-butyl, and phenyl nitroxides.

Experimental Section The general experimental procedure has been described previously. * Nitroxide radicals were produced from the appropriate hydroxylamine in the cavity of a Varian E3 epr spectrometer. The hydroxylamine was oxidized to nitroxide by rert-butoxy radicals formed by the photolysis of di-tert-butyl peroxide (0.050.5 A4 generally)

, 1 1 j 1 2

(1) Issued as NRCC No. 12239. (2) Part I : I