The cohesive energy of tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (TTF TCNQ) as a function of charge transfer

Abstract

The Madelung, polarization, charge–dipole, and dispersion contributions to the theoretical lattice energy of tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (TTF TCNQ) were computed for the uniform lattice as a function of charge transfer ρ and for the ρ = 1/2 Wigner lattice. The atom-in-molecule charges, local hybrid dipole moments, and polarizabilities used in this study were obtained from a finite-electric-field perturbation MINDO/3 calculation. Some improvements were obtained over the Madelung- only case, but the charge–dipole term tends to cancel the polarization energy term (or the Madelung energy term). The dispersion energies calculated with atom-in-molecule polarizabilities and molecular and ionic ionization energies are smaller than the dispersion energies obtained by using parametrized van der Waals coefficients for neutral hydrocarbons. The lattice energy terms at intermediate charge transfer were obtained by linear interpolation of the charges, local moments, and polarizabilities between their ρ = 0.0 values and their ρ = 1.0 values. The McConnell, Hoffman, and Metzger criterion for ionicity used along with all of the cohesive energy terms included here fails to yield the desired cohesive energy minimum at fractional charge transfer. Using the Soos estimate of the cost of ionization does provide a minimum at intermediate charge transfer, but not significantly far from the minimum obtained from using the Madelung energy alone.