Solid-state electronic structure of bis(tetramethyltetraselenafulvalene)-hexafluorophosphate

The band structure and solid-state electronic properties of bis(tetramethyltetraselenafulvalene)-hexafluorophosphate (TMTSF)2PF6 are investigated by a crystal orbital (CO) approach of the complete neglect of differential overlap (CNDO) type. (TMTSF)2PF6 crystallizes in the triclinic space group P1̄ with TMTSF molecules that are almost perpendicular to the crystallographic a axis. We analyze the atomic net charges qi, so-called Wiberg bond indices Wij, two-center energies Eij of bonded atomic pairs in the organic donor unit, as well as the dispersion curves εi(k) and the electronic density of states (DOS) distribution. We predict a charge transfer (CT) of 1.237e to the hexafluorophosphate unit. In the organic π system a pronounced bond order alternation occurs. The CC bonds within each pentagon exhibit almost perfect double-bond character while the CC bridge between both pentagons is of partial double-bond character only. The SeC bonds are essentially of the single-bond type. Electrostatic Madelung contributions should be important for the stabilization of the crystal. The band structure is characteristic for a molecular crystal with weak covalent intermolecular interactions. The CO data suggest the possibility of a dimensionality crossover of (TMTSF)2PF6 as a function of temperature.