Abstract

European Journal of Mass Spectrometry
Volume 10 Issue 6, Pages 909–920 (2004)
doi: 10.1255/ejms.700

Li⁺-coordinated polyglycol radicals: ion–molecule reactions with alkenes and ethers

Michael J. Polce, Jody M. Modarellia and Chrys Wesdemiotis
Department of Chemistry, The University of Akron, Akron, Ohio 44325, USA. E-mail: wesdemiotis@uakron.edu

The Li⁺ complexes of the radicals HOCH₂CH₂O· ("oxy" radical), HOCH₂CH₂OCH₂· ("methyl" radical) and HOCH₂CH₂OCH₂CH₂· ("ethyl" radical) are produced in the gas phase by fast atom bombardment of poly(ethylene glycol) 200 and the ion–fmolecule reactions of these novel distonic ions, [R·+Li]⁺, with 1-butene, dimethyl ether and methyl vinyl ether are examined in the hexapole collision cell of a sector/quadrupole tandem mass spectrometer. The products arising from these reactions depend on the reagent used and the type of radical site present in [R·+Li]⁺. With 1-butene, predominantly radical-site addition–feliminations take place, initiated by addition of the unpaired electron of [R·+Li]⁺ to the olefinic double bond; fragmentations, induced by the unpaired electron of the adduct, follow to create stable closed-shell products. Dimethyl ether attaches to the charge site of [R·+Li]⁺ (i.e. at the metal ion) and the adduct reacts further by elimination of (mainly) small radicals from either the original [R·+Li]⁺ segment or the added reagent. Methyl vinyl ether, which blends the structural features of alkenes and ethers, gives rise to a combination of such radical- and charge-site addition–fragmentation processes. The dissociations promoted by the unpaired electron involve b-scissions and ·H rearrangements, which are the typical reactions of neutral radicals. All lithiated radicals undergo addition to double bonds. A significant fraction of the oxy radical also reacts via hydrogen atom abstraction from the added reagent. The charge site (metal ion) does not directly participate in the radical-site reactions observed, but facilitates specific channels by transiently coordinating to dissociating ligands, so that inter-ligand hydrogen atom rearrangements between these ligands can occur. This type of metal ion involvement in radical reactivity could be particularly important in biological milieus where metal ions abound.

Keywords: radicals, distonic ions, ion–molecule reactions, addition–fragmentation pathways, polyglycol, tandem mass spectrometry

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