Transfer integrals, anion-cation interaction and molecular vibrations in the organic salts (RTCF) 2X: Structural and theoretical investigation

Abstract

The transfer integrals of a large series of organic salts, including symmetrical and unsymmetrical molecules and anions of different geometries are presented. We outline the variation of these integrals with the physical constraints and show that even small geometrical changes may induce large variations of the electronic interactions. The general features of the delocalized model are presented on the grounds of modifications of the Fermi surface. These results are compared with the localized approach based on interacting, isolated dimers. An attempt to correlate these models is made, showing that the TMTTF salts may behave differently from their selenium analogs. A careful examination of crystallographic structures shows that the molecular vibrations (especially those of the C=C and CS or CSe bonds) which interact with the molecular orbitals originating in the electronic band might be linked on adjacent chains by the anion atoms. Model calculations, based on a SCF semi-empirical method, of these interactions are presented, and the possible implications of this mechanism on the competition between electronic states are discussed.