Heptyl radicals formed by y-irradiation of n-C7Hl6, both in neat form and with various concentrations of n-CsDls added before irradiation, are investigated by ESR spectroscopy. It is observed that the presence of deuterated octane in protiated heptane results in a substantial increase in the relative contribution of protiated 1 -heptyl radicals to the radical spectrum. Their formation is attributed to the proton-transfer reaction (n-C7H16*+ + n-CsDl8 -1-C7H15* + n-CsDlgH+) which, in accordance with an earlier and experimentally verified postulate stating that a strict relation exists between the radical site in alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules and the structure of the semioccupied molecular orbital of the parent cation, is expected to yield l-heptyl radicals selectively. The increase in the relative importance of l-heptyl vs other isomeric heptyl radicals as a result of the presence of octane in heptane is confirmed by analysis of the tetradecanes formed by irradiation of heptane/ l-chlorohexane( 1 mol %) and heptane/lchlorohexane( 1 mol%)/octane(9 mol 7%) systems. As to the acceptor site, a study is made by gas chromatography of the yield and isomeric composition of chlorooctanes formed by y-irradiation of n-C.1Hl6, containing 1 mol 5 % l-C6H1&1 and various concentrations of n-CsHls. Formation of chlorooctanes in such systems is largely due to neutralization of protonated octane molecules by chloride ions (CsH19+ + C1--CsH17Cl+ H2). The isomeric composition yields direct information on the proton-acceptor site in the octane molecule. It is observed that mainly 2-chlorooctane is formed, with much smaller yields of 1-and 3-chlorooctane; the formation of 4-chlorooctane is still less pronounced. The selective formation of 2-chlorooctane, which can be rationalized on the basis of structural considerations and of the fact that the proton affinity for primary C-H protonation is smaller than for secondary C-H protonation, confirms that proton transfer to octane molecules takes place from the chain-end position in extended heptane radical cations and indicates that octane solute molecules are packed quite regularly and with conservation of the layer structure in the heptane crystal. Various alternative mechanisms for chlorooctane formation are discussed and are are discarded with respect to the selective formation of 2-chlorooctane on various grounds. The results obtained further support the postulate stating that a strict relation exists between the proton-donor site in the proton transfer from alkane radical cations to alkane molecules and the structure of the semi-occupied molecular orbital of the parent cation.

A study has been made by EPR spectrometry of octane radical cations and octyl radicals formed by y-irradiation of octane at various concentrations in CCI,F. The EPR spectrum at very low concentration (60.5 mol%) is essentially due to octane radical cations, which are largely in the gauche-at-C-2 conformation with large unpaired-electron density on one in-plane chain-end hydrogen and one in-plane hydrogen attached to C-2. Superimposed on the radical cation spectrum, a low intensity spectrum due to octyl radicals appears above 0.5 mot%. The signal intensity and relative contribution to the paramagnetic absorption of this spectrum increase very strongly with increasing concentration of octane. It is observed that at low octane concentration both primary and secondary octyl radicals are present in irradiated CCI,F-octane systems, and this is true from the very first appearance of octyl radicals in such systems. A t higher concentration, secondary octyl radicals become very much the dominant neutral radical species. The results indicate that: ( i ) primary and secondary octyl radicals are formed by proton transfer from octane radical cations (in the gauche-at-C-2 conformation) to octane molecules; (ii) with increasing concentration of octane in CC13F, the size of octane aggregates increases gradually from two-molecule to higher-order clusters, resulting in intermolecular radical site transfer with transformation of primary into secondary octyl radicals. The results confirm that the nature of the alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules is related to the structure of the semi-occupied molecular orbital of the parent cation.

A study is made of ESR spectra obtained after y-irradiation of heptane at various concentrations in CCl3F. At low concentration, the ESR spectrum mainly consists of a triplet due to heptane radical cations in the extended conformation. Superimposed on this triplet is a low-intensity spectrum due to heptyl radicals; the signal intensity and relative contribution to the paramagnetic absorption of this heptyl radical spectrum increases drastically with increasing concentration of heptane. The nature of the heptyl radicals formed is derived from comparisons with powder spectra of authentic isomeric heptyl radicals, obtained by irradiation of various bromoheptanes in perdeuterated cis-decalin, and appears to depend on heptane concentration. It is observed that at low heptane concentration only primary (chain-end) heptyl radicals are present in the system; in contrast, at higher heptane concentrations secondary heptyl radicals become the dominant neutral radical species. It is concluded that primary heptyl radicals are formed by proton transfer from heptane radical cations to heptane molecules in small heptane aggregates present in the CC13F matrix. The results clearly indicate that in the case of heptane the reaction RH'+ + RH -R' + RH2+ is essentially due to proton transfer from the radical cation to the neutral alkane and not to hydrogen abstraction by this cation. At higher heptane concentration in CC13F, the size of the heptane aggregates gradually increases, resulting in intermolecular radical site transfer with formation of secondary heptyl radicals. The results obtained unambiguously show that the nature of the heptyl radicals formed by proton transfer from heptane radical cations to heptane molecules is related very strictly to the structure of the semioccupied molecular orbital of the parent cation.

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A study is made of the effect of added octane-& on the ESR spectrum of heptane observed after y-irradiation at 77 K. It is found that the presence of deuterated octane in heptane upon y-irradiation results in a substantial increase in the relative contribution of 1-heptyl radicals to the paramagnetic absorption. This increase is due to the occurrence of proton transfer from heptane radical cations to deuterated octane, which is considerably more efficient than to the heptane matrix. An explanation based on solid-state structure and proton affinity data is presented. The results indicate that proton transfer may be an important alternative to hole transfer in irradiated solid binary alkane systems.