Larry D. Snow

ESR evidence for the formation of the ring-opened cation CH2OCH2± from ethylene oxide

Abstract ESR results show that the radical cation formed from ethylene oxide in the solid state is the ring-opened 2-oxa-trimethylene cation with a symmetrical (C2ν) planar structure similar to that of the isoelectronic allyl radical. In contrast, the trimethylene oxide radical cation retains the ring structure of the parent molecule and its ESR parameters are characteristic of an oxygen-centred species.

An ESR study of the acetaldehyde radical cation in freon matrices: Evidence for halogen superhyperfine interaction

Abstract The substructure associated with the doublet ESR spectrum of the acetaldehyde radical cation in Freon matrices below 100 K is shown to arise from a matrix interaction and not, as previously proposed, from coupling to the hydrogens of the methyl group. Since the reversible loss of this substructure at higher temperature is accompanied by almost no change in the large isotropic 1 H coupling(136 G) to the aldehydic hydrogen, the matrix perturbation appears to have a negligible effect on the spin distribution in the radical cation and is according interpreted as a superhyperfine interaction

On the structure of the radical cation formed from ethylene oxide in the CFCl3 matrix

Abstract ESR results are presented and considered in the light of a recent novel suggestion that the radical cation produced by radiolytic oxidation of ethylene oxide in the CFCl 3 matrix possesses a ring-closed o ( n ) structure corresponding to the highly excited 2 2 A 1 state. Both the unusually large temperature dependence observed for the parallel component of the 13 C hyperfine tensor and the essentially isotropic g -factor (2.0023) close to free spin are inconsistent with this proposed oxygen-centred radical. On the other hand, these results and the 1 H hyperfine coupling constant of 16.2 G are naturally accommodated by the ring-opened oxallyl structure which has been calculated to be at least 20 kcal mol −1 more stable than any of the ring-closed states in the gas phase.

Reply to comment “possible structures for trimethylene radical cations in freon matrices”☆

Abstract The possible existence of an interaction between the ring-opened cyclopropane radical cations and the CFCl 2 CF 2 Cl solvent has been probed by ESR studies. No definitive evidence for this interaction has been obtained.

Spin delocalization in radical cations of oxygen-containing organic compounds as revealed by long-range hyperfine interactions and solvent effects

E.s.r. evidence is presented for long-range 1H hyperfine interactions and thermally reversible solvent effects involving mainly the radical cations of aldehydes and ketones in Freon matrices. Long-range couplings to δ protons are found to be surprisingly large in the cations of acyclic, cyclic and polycyclic carbonyl compounds. The conformational requirements for these large couplings are discussed in terms of facile spin transmission via a trans arrangement of C—C and C—H σ bonds, the assumption of this trans effect providing a rigorous and consistent analysis of the experimental results for both non-rigid and rigid molecular geometries. The matrix perturbation observed at low temperatures is believed to arise from a weak complex formed between the radical cation and the solvent, the hyperfine interaction occurring mainly with a single chlorine nucleus. The reversible loss of the e.s.r. substructure associated with this complex at elevated temperatures is attributed to motional averaging of the chlorine hyperfine anisotropy rather than to dissociation of the complex.

Long-range proton hyperfine couplings in radical cations of carbonyl compounds

Large non-aldehydic 1H hyperfine couplings in the radical cations of four carbonyl compounds are assigned to long-range hyperfine interactions relayed through a trans(W-plan) arrangement of C–C and C–H σ bonds; for example, the two strongly coupled hydrogens in the cyclohexanone radical cation are identified by deuterium substitution as the ‘δ’ equatorial pair at C-3 and C-5 rather than the ‘γ’ equatorial pair at C-2 and C-6 assigned previously.