The Dimethylene Ketene Radical Cation

Abstract

Molecular orbital calculations (at the Becke3LYP/6-31G(d) level of theory) indicate that the distonic ion •CH2C(CH2)CO+ is planar and has C2v symmetry. Most of the positive charge resides at the carbonyl carbon while the odd spin is delocalized over the allyl group. A similar spin distribution was calculated for the allyl radical. Hence, the reactivity of •CH2C(CH2)CO+ toward spin traps may be expected to resemble that of the neutral allyl radical. This issue was examined experimentally by using Fourier transform ion cyclotron resonance mass spectrometry. Similar to other carbon-centered free radicals, the gaseous •CH2C(CH2)CO+ abstracts H• from benzeneselenol, CH3Se• from dimethyl diselenide, and I• from allyl iodide. However, some of these reactions occur at efficiencies that suggest catalysis by the charged group. Further, the reaction of •CH2C(CH2)CO+ with dimethyl disulfide was found to proceed in an unprecedented manner. In addition to the expected formation of CH3SCH2C(CH2)CO+ and CH3S• (via CH3S• abstraction), a new distonic ion, CH3SCH•C(CH2)CO+, is generated. This ion is likely produced upon H• abstraction by CH3S• from CH3SCH2C(CH2)CO+ within the collision complex (net CH2S abstraction). The latter reaction has not been reported for distonic radical cations with localized radical sites. Hence, the reaction must be driven by the regeneration of the delocalized allylic radical. This proposal is supported by the observation of CH2S abstraction for the product ion CH3SCH•C(CH2)CO+ that also contains a delocalized radical site.