Dependence on conformation of the site of proton transfer from alkane radical cations. Nature of the octyl radicals formed by proton transfer from octane radical cations to octane molecules in CCl3F matrices at 77 K

Abstract

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.

Intrinsic Acceptor Site Selectivity in the Proton Transfer from Alkane Radical Cations to Alkane Molecules. Evidence in γ-Irradiated CCl3F/Undecane

“…As a result of this aggregation, a gradual transformation (with respect to the cationic species) takes place of alkane radical cations into protonated alkanes by proton transfer to neutral alkane molecules. The occurrence of such proton transfer in irradiated CCl 3 F/alkane systems at cryogenic temperatures has clearly been established by ESR spectroscopy. , …”

“…Evidence has been derived first and foremost from ESR experiments on the nature of the alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules in various irradiated systems ( viz. , hexane and octane in synthetic zeolites, heptane and octane in CCl 3 F, and octane in pentane- d 12 ), which clearly show that the radical site in such radicals is related very strictly to the structure of the semioccupied molecular orbital of the parent cations. It is indeed observed that a high unpaired-electron density in a particular carbon−hydrogen bond leads to proton transfer from that site, giving rise to a particular alkyl radical.…”

“…The occurrence of a reaction between saturated hydrocarbon radical cations and molecules is clearly indicated by the observed dependence on solute concentration of the signal intensity of fluorescence-detected magnetic resonance (FDMR) spectra of different hydrocarbon radical cations in liquid systems and in particular by γ-irradiation of alkanes in CCl 3 F matrices at 77 K, , using ESR spectroscopy. Such experiments also clearly indicate that for the higher alkanes the reaction occurs essentially by proton transfer to and not by hydrogen abstraction from the neutral alkane molecules.…”

Proton Transfer from Alkane Radical Cations to Alkanes

“…at or around the onset of alkane aggregation and alkyl radical formation [18,25]. Working around the onset of alkyl radical formation is essential because otherwise all radicals are transformed into the thermodynamically most stable ones by intermolecular radical-site transfer.…”

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

“…Effects of conformation, which affect the electronic structure of alkane radical cations, on the site of proton transfer from such cations, have also been observed. 10 In -alkane radical cations in the extended conformation, in which there is high unpairedelectron density in the carbon-carbon -bonds as well as in the 0022-3654/93/2097-1477S04.00/0 two in-plane chain-end carbon-hydrogen bonds, 1-alkyl radicals are very selectively formed by proton transfer.…”