Halogen bonds in crystal TTF derivatives: an ab initio quantum mechanical study.

The stabilisation energies of five ionic and neutral organic crystal structures containing various halogen bonds (I···I, Br···Br, I···Br, I···S and Br···S) were calculated using the DFT-D3 method (B97D/def2-QZVP). Besides them, the ionic I3(-)···I2 and neutral I2···I2, complexes (in the crystal geometries) were also studied. The nature of the bonds was deduced from the electrostatic potential evaluated for all subsystems. In almost all the cases, the σ-hole was positive; it was negative only for the ionic I3(-) system (although more positive than the respective belt value). The strongest halogen bonds were those that involved iodine as a halogen-bond donor and acceptor. Among ionic X···I3(-) and neutral X···I2 and X···Y dimers, the neutral X···I2 complexes were, surprisingly enough, the most stable; the highest stabilisation energy of 13.8 kcal mol(-1) was found for the I2···1,3-dithiole-2-thione-4-carboxylic acid complex. The stabilisation energies of the ionic I3(-)···I2 and neutral I2···1,3-dithiole-2-thione-4-carboxylic acid (20.2 and 20.42 kcal mol(-1), respectively) complexes are very high, which is explained by the favourable geometrical arrangement, allowing the formation of a strong halogen bond. An I···I halogen bond also exists in the neutral I2···I2 complex, having only moderate stabilisation energy (3.9 kcal mol(-1)). This stabilisation energy was, however, shown to be close to that in the optimal gas-phase L-shaped I2···I2 complex. In all the cases, the dispersion energy is important and comparable to electrostatic energy. Only in strong halogen bonds (e.g. I3(-)···I2), the electrostatic energy becomes dominant.