DEPENDENCE ON ELECTRONIC-STRUCTURE OF THE SITE OF PROTON-TRANSFER FROM ALKANE RADICAL CATIONS - NATURE OF HEPTYL RADICALS FORMED BY PROTON-TRANSFER FROM HEPTANE RADICAL CATIONS TO HEPTANE MOLECULES IN CCL3F MATRICES AT 77-K

A study is made of ESR spectra obtained after gamma-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 CCl3F 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 CCl3F, 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.