Collision-induced dissociation and neutralization-reionization mass spectra of dimethylhalonium ions

Abstract

The collision-induced dissociation (CID) and neutralization-reionization (NR) mass spectra of dimethylhalonium ions ((CH3)2X+, X = F, Cl, Br and I) with 8 keV translational energy have been recorded. The (CH3)2F+ ion decomposes following collision with O2 predominantly by loss of methane. The loss of a methyl radical competes with methane elimination in the reactions of the (CH3)2Cl+ and (CH3)2Br+ ions, whereas the (CH3)2I+ ion fragments mainly by elimination of a methyl radical. Formation of an X+ ion either by a reductive elimination forming ethane as the neutral product and/or by the consecutive loss of two methyl radicals occurs to some extent for the (CH3)2Br+ and (CH3)2I+ ions. Neutralization of the dimethylhalonium ions by collision with Xe does not result in dimethylhalogen radicals which persist on the microsecond time scale of the experiment and/or survive reionization by collision with O2. The NR process for the dimethylfluoronium ions results mainly in the formation of CH2F+ ions and to a lesser extent of CH3F.+ ions, whereas the CH2X+ ions are less abundant than the CH3X.+ radical cations in the NR experiments with the other halonium ions. The formation of relatively more abundant CH3X.+ ions than of CH2X+ ions in the NR experiments as compared with the CID experiments suggests that the predominant reaction accompanying neutralization of the halonium ions with Xe is a simple cleavage to form a methyl radical and a CH3X molecule. The NR process yields in all instances X+ ions, whose relative abundance increases on going from (CH3)2F+ to (CH3)2I+.