Electron spin resonance spectra of ether radical cations generated by radiolysis

Abstract

Exposure of dilute solutions of ethers in trichlorofluoromethane (freon) to ⁶⁰Co γ-rays at 77 K gave the corresponding ether cations or derivatives thereof, as judged by their e.s.r. spectra. Dimethyl and diethyl ethers gave the primary cations, the unpaired electron being largely confined to oxygen but with considerable delocalisation onto β-protons. For n-propyl and larger alkyl substituents, hydrogen shifts occur to give alkyl radicals. Several cyclic monoethers gave primary cations again exhibiting very large β-proton hyperfine coupling constants. For example, the oxetane cation had four equivalent β-protons with coupling constants of 64 G. However, ethylene oxide (oxirane) was exceptional, with four equivalent β-protons with couplings of only 16 G. It is suggested that either the orbital on oxygen is switched from the normal π(b₁) orbital to the in-plane σ(a₁) orbital or ring opening has occurred. For tetrahydropyran the product had two protons with 34.5 G splitting, two with 15 G splitting, two with 11 G splitting and two with ca. 2.5 G splitting. We suggest that the β-proton coupling is low because of the conformation of the cation.

Acetals exhibited remarkably large hyperfine coupling to the two methylene protons (ca. 145 G as a result of σ-π delocalisation within this unit. For s-trioxane cations only two of the three oxygen atoms participate strongly in the SOMO with two strongly coupled protons. However, a second radical species was detected in this system which showed an unusually large g-value variation (g_x= 2.0286, g_y= 2.0067, g_z= 2.0023) as well as very large proton coupling. It is suggested that this species is essentially an alkoxide radical formed by ring opening.

For 1,2-diethers weak O[graphic omitted]O σ* interaction is thought to occur. Thus, 1,4-dioxane, for example, exhibits only small proton splittings, which is expected in a σ* system.

The cation formed from the vinyl ether EtOCHCH₂, which is of interest since such cations are thought to be of importance as intermediates in the free-radical oxidations of vinyl ethers, has e.s.r. parameters which suggest that the limiting structure Et[graphic omitted]CH–ĊH₂ dominates, the spin-density on the methylene group being ca. 89%. Cations formed from (RO)₂CH–CHCH₂ rearranged to give the stable (R[graphic omitted])₂C–ĊHMe radicals. Also for MeOCCH, the cation has a structue close to the limiting Me[graphic omitted]CĊH form.

We have also studied the effect of ionizing radiation on solutions of ethers in sulphuric acid. Electron addition gave alkyl radicals but in several cases there was evidence for the formation of cations R₂O⁺˙(R₂OH⁺+ HSO₄˙→ R₂O⁺˙+ H₂SO₄) on annealing above 77 K. If this is correct, the extent of electron delocalisation on to β-protons for these ions in sulphuric acid is reduced relative to the cations in freon.

All attempts to prepare alcohol cation radicals, RȮH⁺, by these procedures failed. For freon solutions, this is thought to be because of extensive association due to hydrogen bonding in even the most dilute solutions. This leads to intermolecular hydrogen transfer giving the radicals R₂ĊOH which were detected by e.s.r. spectroscopy. An alternative explanation is that the radicals RȮH⁺ can transfer protons to the solvent.