Intermolecular energy and structure of tetrathiafulvalene (TTF) stacks from atom–atom potentials

Abstract

The lattice energy of isolated, regular tetrathiafulvalene stacks was minimized for a longitudinal slip of the molecules relative to each other at constant intermolecular separation and transverse slip. The van der Waals and repulsive interactions were calculated from atom-atom potentials. A simple expression is presented for the electrostatic interaction in neutral and charged stacks. This electrostatic contribution was calculated from CNDO/2 atomic point charges. The latter contribution proved to be negligible for stacks built up from neutral molecules. For these stacks the minimum of the lattice energy is achieved at a slip of 0.1-0.2 Å below the observed values. Eclipsed stacks, with zero slip, appeared to be only 3.35 kJ mol-1 less stable than slipped ones. In stacks built up from positively charged molecules the van der Waals and repulsive contributions are dominated completely by the electrostatic interaction. These stacks tend to a structure with infinite slip.