Electrostatic energy of aromatic ion radical crystals

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Abstract
The electrostatic energy of aromatic charge-transfer (CT) and free radical (FR) crystals is obtained by using the many-electron site representation and the modified Hubbard model previously developed to correlate the unusual magnetic, electric and optical properties of CT and FR crystals. Both π-electron overlap between adjacent ion radicals in a stack and configuration interaction with CT states are included, thus improving on the Madelung approximation of disjoint charge distributions. π-electron overlap leads to a Coulomb exchange energy E ex which stabilizes both ionic CT and FR crystals, while configuration interaction scales the ground state charge densities, and thus the Madelung energy, in CT crystals. E ex is obtained for 1 : 1 CT crystals and a numerical estimate of 10–15 kcal/mole is found for both TMPD-TCNQ and for TMPD-chloranil.