Intermolecular Interactions And Crystal Stabilities Of Tetrathiafulvalene-Tetracyanoquinodimethane

Abstract

The crystal structure of the organic conductor derived from tetrathiofulvalene (TTF) and tetracyanoquinodimethane (TCNQ) is composed of segregated stacks of donor (TTF) and acceptor (TCNQ) molecules and ions. Charge transfer from donor to acceptor stacks gives a stable crystal where ions of the same charge pack with parallel molecular planes at short intermolecular distances. An ab initio quantum mechanical calculation using a 6-31G** basis set and the DMOL method has been carried out to examine the intermolecular bonding and Coulombic interactions in this crystal. Molecular relationships at the calculated energy minima are close to the observed crystal structure. Formation of extended intermolecular orbitals within each type of stack correlates with the intermolecular bonding which appears to be present in this material as shown by the short interplanar distances. A complete evaluation of the crystal stability could not be carried out owing to the complexity of the problem but the results show that as a stack of TCNQ anions is extended the energy of the highest occupied intermolecular orbital is reduced. Conversely, as a stack of TTF cations is extended, the energy of the highest occupied orbital is raised. The calculation is in agreement with the observed properties of the TTF-TCNQ crystal and suggests that interactions between molecular ions can be treated only by quantum mechanical methods.