| Popis: |
The lattice energy of tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (C6H4S4.C12H4N4) with space group P21/c and segregated stacks was minimized for four interstack parameters; the unit-cell parameters a, c and β, and the angle, ϕ, between the projection of the longitudinal molecular axis on the ac plane and a. The lattice energy of a model with space group C2/m and mixed stacks was minimized for the unit-cell parameters a, b and β and the intrastack molecular distance, c. Van der Waals, repulsive and electrostatic interactions were calculated from two sets of atom-atom potentials and atomic charges. The minimum of the P21/c lattice was achieved at a structure with parameters deviating 0.2-2.5% from their observed values. The minimum of the C2/m lattice was achieved at a structure with parameters reasonably close to the observed values of related TCNQ compounds. The latter structure provided to be 9.2-16.8 kJ mol-1 more stable than the P21/c modification, indicating the possibility of the preparation of mixed-stack structures. |
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