The 4 K radiolysis of linear alkanes as studied by electron spin resonance spectroscopy: selective formation of terminal alkyl radicals in the primary process

Ion-molecule reactions of primary radical cations in the liquid-phase radiolysis of n-alkanes: An EPR study

“…In accordance with published data [4,6,7] and the results of this work, the following reaction scheme can be proposed for the formation of alkyl radicals. Primary ionization yields excited (unrelaxed) radical cations (reaction (5)).…”

“…For example, in the case of n-hexane, the yield of n-hexyl radicals in liquid-phase radiolysis was G( ) ≤ 1.0 per 100 eV; however, the total yield including fragment radicals was as high as G( ) = 1.7 per 100 eV, which is consistent with our data. The total yields of radicals in the liquid phase, which were determined previously with the use of various scavengers [12,13], as well as the yields measured in this work by spin trapping with BNB, G(R) ≈ 5.0 per 100 eV are close to the yields of trapped radicals in irradiated crystalline alkanes [3,4], and they depend only slightly on the alkane chain length.…”

“…It is well known [4] that only fully extended chains, in which gauche-conformers cannot be formed, occur in the crystallites of linear alkanes. Selective deprotonation of gauche-conformers from the ë 2 position with the maximum spin density results in the formation of only 2-alkyl radicals.…”

“…111 phase until very recently. Then, it was found that were formed in a noticeable yield in odd linear alkanes, starting with ë 11 ç 24 , γ -irradiated at 77 K. Iwasaki et al [4] studied a series of the crystallites of linear alkanes from ë 6 to ë 25 irradiated at 4 K. They found that the yields of terminal radicals, as well as the total yield of radicals, in linear alkanes varied in a markedly alternating manner at odd and even numbers of C atoms [4]. It was found that in even homologues were converted into on heating to 77 K, whereas they were stable in the odd ones.…”

“…The selective formation of 1-alkyl radicals from radical cations with the extended zigzag structure and the stabilization of 1-alkyl radicals in low-temperature crystalline alkanes with yields higher than those for the formation of these radicals on random ruptures of C-H bonds by H atoms indicate that the ion-molecule reactions of primary radical cations play an important role in the radiolysis of n -alkanes. The high yields of n -alkyl radicals at 4 K [4] are due to the fact that only the transtrans extended-zigzag conformers occur in the crystallites of pure linear alkanes, whereas the gauche -conformers are absent. However, the gauche -conformers can occur in considerable amounts in the liquid phase, and the ratios between the yields of different alkyl radicals in liquid and solid phases can differ noticeably.…”