Collect. Czech. Chem. Commun. 1986, 51, 2675-2684

Mass spectrometric and electron spin resonance study of allylic peroxyl radicals in the gas-phase reaction between allylic radicals and dioxygen

Karel Mach, Jana Nováková, Vladimír Hanuš and Zdeněk Dolejšek

The J. Heyrovský Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 121 38 Prague 2


Mass spectrometric measurement of the pyrolyzed (790-1 030 K) mixture of 1,5-hexadiene or 2,5-dimethyl-1,5-hexadiene with dioxygen afforded no evidence for the presence of allylic peroxyl radicals or aldehydes in the gas phase though the allylic radicals were readily detected at low ionization potentials. On the other side, the electron spin resonance measurement revealed that the allylic radicals were converted quantitatively to allylic peroxyl radicals during condensation of pyrolytic mixtures at 77 K, and acrolein and methacrolein were found mass spectrometrically after thawing of the condensed pyrolytic material as stable products of the allylperoxyl and 2-methylallylperoxyl radical, respectively. These results are compatible with the temperature dependent dissociation equilibrium of the allylic peroxyl radicals. The allylic peroxyl radicals were trapped at 77 K using co-condensation with adamantane from the gas phase in yields as high as 30-50% of the radicals produced in pyrolysis. The electron spin resonance spectra showed partial motional freedom of the peroxyl radicals when they were trapped in the low-contaminated adamantane matrix at 77 K, but in matrices highly contaminated with the biallyl hydrocarbon and products of pyrolysis the ESR spectra indicated rigidly trapped peroxyl radicals. Free rotation of the allylic peroxyl radicals in both types of matrices occurred at 110 K.