Crystal Coulomb energies. VI. Madelung energies of simple TCNQ salts. What is the electron affinity of TCNQ (7,7,8,8-tetracyanoquinodimethan) ?

Abstract

The Madelung energy EC (classical interionic crystal Coulomb energy in the fractional point-charge approximation) has been calculated for Na+TCNQ−, Rb+TCNQ−(I), Rb+TCNQ−(II), and for NH4+TCNQ−. The values obtained are EC=−5.29, −4.71, −4.64, and −4.82 eV/molecule, respectively. When these values are coupled with available thermochemical data in a Born–Haber cycle, then the best recent experimental estimate of the electron affinity of TCNQ, 2.8 eV (obtained by Cs beam collisional ionization) is not verified, because apparently in these organic ionic crystals, as many as 2.5 eV of binding must be due to other interactions which our Madelung calculations ignore at present. It is likely that charge-induced dipole interactions may supply the several electron volts that are missing from the calculated crystal binding energy.