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

Abstract

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]₂²⁻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.