Crystal packing of TCNQ anion π-radicals governed by intermolecular covalent π-π Bonding: DFT calculations and statistical analysis of crystal structures

On the basis of a thorough Cambridge Structural Database survey, we present a statistical analysis of the packing of TCNQ anion π-radicals in TCNQ charge transfer salts, which reveals three packing motifs between neighboring TCNQs: one with a zero longitudinal offset and an approximate 1 Å transversal offset, another with an approximate 2 Å longitudinal offset and zero transversal offset, and the third with a relatively long σ-bond in the length of r = 1.6-1.7 Å connecting two TCNQ fragments. Along with the statistical analysis of the crystal structures, we also present density functional theory calculations of the total energy, covalent π-π bonding interaction energy, and Coulombic repulsion energy for the [TCNQ] ₂^2-π-dimers with various packing geometries. We find that the interactions between TCNQ anion π-radicals include contributions from intermolecular covalent π-π bonding interaction and local dipole repulsions, in addition to Coulombic repulsion, van der Waals and the attractive electrostatic forces between counter-cations and TCNQ anions pointed out recently by other groups for TCNE anion radicals. We describe an approximate formula for intermolecular interaction energy, E_int = E _coul + E_bond + E_vdW, for systems in vacuum, while in the solid state E_coul is compensated by the attractive electrostatic forces between counter-cations and TCNQ anions. We conclude that the crystal packing of TCNQ molecules in their charge transfer salts is predominantly determined by the intermolecular covalent π-π bonding term, E_bond.