Crystal Coulomb Energies. II. The Electrostatic Binding Energy of Four Organic Donor-Acceptor Crystals by Ewald's Method and Charge Transfer in Organic Lattices

Abstract

In an effort to confirm the classification of four organic donor‐acceptor crystals into the almost fully ionic (``holoionic'') class, and the nearly neutral (``nonionic'') class, their electrostatic binding energies E_C, based on the point‐charge model, have been calculated rigorously. The E_C were found to be model independent, and were compared with 2ε₀, the cost of ionizing the lattices. $E_C{\text{+ 2ε}}_0\text{>0}$ was expected for the two crystals which are experimentally known to be nonionic: for (1:1)‐(hexamethylbenzene: para‐chloranil) the result was E_C=−4.4 eV against ${\text{2ε}}_0\text{=6.6\hspace{0.17em}}\mathrm{eV}$ , and for (1:1)‐(naphthalene:tetracyanoethylene) E_C=−4.3 eV vs ${\text{2 ε}}_0\text{=6.5\hspace{0.17em}}\mathrm{eV}$ . For the two crystals which are holoionic by experiment $E_C{\text{+ 2ε}}_0\text{<0}$ was expected, but for (1:1)‐(N, N, N′, N′, ‐tetramethyl‐para‐phenylenediamine⁺ 7, 7, 8, 8‐tetracyanoquinodimethan⁻) the result was E_C=−3.9 eV vs ${\text{2ε}}_0\text{=4.6\hspace{0.17em}}\mathrm{eV}$ , and for (1:1)‐(N, N, N′, N′‐tetramethyl‐para‐phenylenediamine⁺ ‐para‐chloranil⁻) E_C=−4.4 and ${\text{2ε}}_0\text{=4.9\hspace{0.17em}}\mathrm{eV}$ . Clearly in these crystals the total binding energy must include a nonnegligible exchange Coulomb energy.