Selective cleavage of the C–O bonds in alcohols and asymmetric ethers by dissociative electron attachment

Abstract

Dissociative electron attachment spectra of 20 saturated compounds containing ether and hydroxyl groups are presented. Two groups of fragmentation processes are identified: (i) one or two bands mediated by shape resonances in the 1–5 eV range and (ii) three bands (often overlapping) in the 5–12 eV range, assigned to Feshbach resonances with a hole in either the oxygen ‘nonbonding’ orbitals n_O or _O, or in one of the available σ orbitals. The main result of this paper is the discovery of unexpected selectivity in the cleavage of C–O bonds in asymmetric ethers, R₁–O–R₂, within the range of the σ-Feshbach resonances, where the loss of a neutral alkyl group, R₁, peaks at a given energy (9.1, 8.4, 8.0 and 8.8 eV for R₁ = ethyl, propyl, butyl and t-butyl, respectively), independently of R₂ in all compounds studied. This empirical rule indicates an unexpected conclusion, that the excitation (a hole and two excited electrons) of Feshbach resonances responsible for the fragmentation is spatially localized on one alkyl group or the other. This interpretation is supported by correlation with ionization energies of the alkanes, R₁H. The methyl group behaves differently from the larger alkyl groups—it is not split off at all (except in methanol), thus acting as a ‘protective group’. A number of other observations were made: the previously observed lack of cleavage of C–O bonds (in contrast to O–H bonds) in the n_O and _O Feshbach resonance bands was confirmed in all compounds containing one oxygen atom, but it is now shown that the C–O bond can be broken under certain circumstances in compounds with two oxygen atoms, either in cyclic hydrogen-bonded structures or when the neutral fragment contains oxygen.