Unusual Reactivity of the Radical Cations of Some Simple Trivalent Organophosphorus Compounds toward Dimethyl Disulfide and Dimethyl Diselenide

Abstract

Dimethyl disulfide and dimethyl diselenide are known to readily undergo charge exchange with gaseous conventional radical cations containing oxygen, nitrogen, and sulfur functionalities. In sharp contrast, the radical cations of trimethylphosphine and trimethyl phosphite rapidly abstract CH3S• and CH3Se• groups from dimethyl disulfide and dimethyl diselenide, respectively, in a dual-cell Fourier-transform ion cyclotron resonance mass spectrometer. These sorts of abstraction reactions have been reported earlier only for distonic radical cations (ions with spatially separated charge and radical sites). Isomerization of the organophosphorus radical cations to their distonic forms prior to or during the reaction was ruled out by demonstrating that the connectivity in (CH3)3P•+ does not change during the reaction: the CH3S• abstraction product has the structure (CH3)3P+−SCH3. Instead, the abstraction reactions are likely initiated by thermoneutral charge exchange. The neutral phosphorus compound then replaces a CH3S• or CH3Se• group in ionized dimethyl disulfide and ionized dimethyl diselenide, respectively. In support of this mechanism, three different neutral phosphorus compounds were shown to replace CH3S• in the radical cation of dimethyl disulfide. Phosphorus radical cations with high recombination energies were found to react with dimethyl disulfide by exclusive charge exchange. Hence, the abstraction reactions require a radical cation with a recombination energy close to the ionization energy of dimethyl disulfide (8.1 eV) and dimethyl diselenide (7.9 eV). Further, the reactions seem to be limited to phosphorus-containing ions since radical cations with nitrogen and sulfur functionalities do not undergo these reactions even when their recombination energies are close to 8.1 eV.