The Madelung energy, intermolecular charge transfer, and disorder of HMTTF-TCNQ (hexamethylene-tetrathiafulvalenium-tetracyanoquinodimethane)

The Madelung energy of HMTTF-TCNQ (hexamethylene-tetrathiafulvalenium-tetracyanoquinodimethane) has been calculated to be 2.62 eV per donor–acceptor pair with use of a set of CNDO/2 fractional atomic charges. It is suggested that this relatively small value of the electrostatic binding energy is characteristic of incompletely charge transfered 1:1 segregated stack compounds. An examination of the experimental bondlengths of HMTTF-TCNQ also indicates that the material is incompletely charge transferred, in agreement with the results of recent diffuse x-ray scattering measurements. Regions of observed one-dimensional disordered are shown to be close in electrostatic energy to the normal orthorhombic phase. The complete atomic coordinates for the HMTTF-TCNQ structure are given.