Formation of Dimer Cations of Aromatic Hydrocarbons

Abstract

STABLE molecules and inert gas atoms, M, form “unstable” dimers, M₂, which are held together only by van der Waals forces. When the dimer is ionized, an anti-bonding electron is removed and the species M₂⁺ should be stable with respect to (M⁺ + M). Such species have long been known for M = inert gas (dissociation energy, D, ∼ 1 eV) and M = N₂, CO, etc. (D∼0.5 eV). Dimer cations of derivatives of naphthalene^1–3, anthracene^1,2 and benzene³ have recently been detected by electron spin resonance (e.s.r.)^1,2 and optical absorption spectroscopy³. The e.s.r. evidence^1,2 suggests that the dimer cations have symmetrical sandwich structures similar to those postulated for excimers, M₂^* (ref. 4). These species may be formed by reaction (1) whenever the monomer ions are produced chemically, by high energy radiation, or by electrolysis. Our aim here is to assess the possible importance of these species in solution at room temperature, by estimating the equilibrium constant, K, of (1), for a number of compounds.