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 trichlorofluoromethane matrixes at 77 K

Abstract

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.

Hydrocarbon σ‐radical cations

“…A number of computational81,82 and experimental ESR data are available for n ‐butane55,78,83–85 and isobutane radical cations 55,78,82. The structural changes in the radical cations relatively to the respective neutrals are substantial as the relaxation energies, i.e., the differences between the vertical I p (11.0–11.2 eV) and adiabatic I a (10.3–10.6 eV) ionization potentials are large,86 but still lower than that of methane63 ( I a –12.6 eV, I p –13.6 eV87).…”

Preferential solvent-to-solute proton transfer upon ionization by .gamma.-irradiation of solid heptane/octane-d18 systems: proton transfer as an alternative process to hole transfer in irradiated solid binary alkane systems

“…The lateral parts of the spectrum, on the other hand, are solely due to 1 -heptyl radicals. 9 The two characteristic features in this region are a "double-humped" curve and, more centrally situated, a rather narrow ESR absorption. These features are predicted by theoretical considerations on the hyperfme interaction in alkyl radicals.…”

Multiphoton processes in cyclohexane and trans-decalin and the formation of high-mobility cations

“…RH + 2hv or 3hv -RH,+* + e" (6) RH'+* + RH -R' + RH2+ (7) RH,+* -H2 + olefin*+ (8) studies establish that reaction 7 occurs, although excitation may not be required, in which case reaction 7 becomes identical with reaction 5. The results of the present study are completely consistent with reaction 7, and this is supported by the preponderance of relevant radiolysis studies which are referred to in the Introduction.…”