Electron spin resonance evidence for the ring-closed and ring-opened forms of a substituted cyclopropane radical cation

Abstract

Gas-phase experiments1,2 and solid-state ESR studies1 23 have provided evidence that the radical cation derived from ethylene oxide (oxirane) is the ring-opened isomer CH2OCH2+', a con-

Radical cations of disubstituted cyclopropanes: stereoelectronic effects on hyperfine coupling

“…The a H values for trans ‐ 5 •+ show excellent agreement with those derived experimentally by Williams and co‐workers (Table ) . The calculations indicate positive a values for protons interacting via π,σ‐delocalization (hyperconjugation), i.e.…”

“…As for the electronic structure of cis ‐ 5 •+ , the a values calculated for the methyl protons a Me,avg = +20.2 G (vs. 20.5 G, deduced from EPR) and the tertiary protons, a tert = –11.0 G (vs. 10.8 G, deduced from EPR), respectively, are in good agreement with the EPR experimentally derived splittings . On the other hand, a substantial discrepancy is observed for the geminal protons: the calculations for these geometrically nonequivalent protons indicate significantly different spin populations for the protons cis and trans, respectively, relative to the two methyl groups, viz ., a trans = +30.4 G, a cis = +10.6 G; yet the simulation of the experimental EPR spectrum was achieved with identical hfcs ( a gem = 20.5 G; a value close to the average of calculated a 3cis and a 3trans ). We note that the different a values calculated for H 3cis and H 3trans cannot be rooted in a dissymmetrical distribution of the unpaired spin density between the tertiary carbons: C 1 and C 2 have identical spin densities (ρ 1,2 = 0.485).…”

“…the geminal and methyl protons: a gem = +19.2 G (vs. 21.8 G, deduced from ESR); and a Me,avg = +20.2 G (vs. 21.8 G, deduced from ESR) . For the tertiary protons, coupled via π,σ‐polarization, somewhat weaker negative a values are indicated, viz ., a tert = −11.5 G, again in excellent agreement with the ESR experiment (11.9 G) . Essentially, the entire spin density resides on C 1 and C 2 (ρ 1,2 = 0.478), distributed symmetrically between them.…”

1,n-Radical ions: an abinitio study of racemization and isomerization of the cyclopropane radical cation

“…The reported esr parameters for the 3-chloropropyl radical are very similar (within fractions of a gauss) to those parameters determined from the esr spectrum of the radical in the annealed matrix (24). In the case of the tetramethylcyclopropane radical cation (21) it was noted that the radical CC13CHZC(CH3),. has the same esr parameters as the species observed in the annealed matrix.…”

“…Recently, Williams and co-workers have reported esr evidence for the 90,O conformer of the cyclopropane radical cation (20) and the tetramethylcyclopropane radical cation (21). The structural assignments were based on analysis of the esr spectra that were irreversibly produced upon annealing a Freon matrix containing the radical cation in the 90,90 conformation.…”