to a radical anion results in protonation, i.e. the sequence of reduction followed by protonation is equivalent to hydrogenation. For instance, the anthracene radical anion forms mainly (but not exclusively) 9,10-dihydroanthracene. Radical anions and their protonation are central to the Birch reduction. Coordination to metal ions Radical anions of polycyclic aromatic compounds function as ligands in organometallic chemistry. Radical cations Cationic radical species are much less common than the anions. Denoted , they appear prominently in mass spectrometry. When a gas-phase molecule is subjected to electron ionization one electron is abstracted by an electron in the electron beam to create a radical cation M+.. This species represents the molecular ion or parent ion. A typical mass spectrum shows multiple signals because the molecular ion fragments into a complex mixture of ions and uncharged radical species. For example, the methanol radical cation fragments into a methenium cation CH3+ and a hydroxyl radical. In naphthalene the unfragmented radical cation is by far the most prominent peak in the mass spectrum. Secondary species are generated from proton gain (M+1) and proton loss (M-1). Some compounds containing the dioxygenyl cation can be prepared in bulk. Organic conductors Radical cations figure prominently in the chemistry and properties of conducting polymers. Such polymers are formed by the oxidation of heterocycles to give radical cations, which condense with the parent heterocycle. For example, polypyrrole is prepared by oxidation of pyrrole using ferric chloride in methanol: n C4H4NH + 2 FeCl3 → (C4H2NH)n + 2 FeCl2 + 2 HCl Once formed, these polymers become conductive upon oxidation. Polarons and bipolarons are radical cations encountered in doped conducting polymers. References Reactive intermediates Mass spectrometry