Capacitive Energy and the Ionization of Aromatic Hydrocarbons

Abstract

Large polynuclear aromatic molecules ionize like fragments of graphite, and their ionization potentials approach the sum of the graphite work function v_gr and the electrostatic work of charging a conductor the size and shape of the molecule. The capacitive work for a single charge is u=e²/2C, where C is the molecular capacitance. In the limit of large size, the work to remove n electrons is W_n=nv_gr+n²u. This limiting law implies a relation among successive ionization potentials and the electron affinity, which is well satisfied by anthracene and phenanthrene, and moderately well by naphthalene and even benzene. For larger molecules, C, and therefore u, can be estimated classically by assuming the conducting region is bounded by the carbon skeleton, and computed by analog measurements on metallic models; results are tabulated for a series of aromatics. The relation between the limiting law and more detailed quantal formulations is discussed, and the magnitude and trend of some expected deviations are explored with the help of a simple free electron picture.