Theoretical study on mesogenic core structures of nematic liquid crystalline compounds

The structures and intermolecular interaction energies of 10 dimers, included in the mesogenic core structures of typical liquid crystalline (LC) compounds, are obtained at the MP2/6-31G(d) level of theory. It is proved that the dispersion energy significantly contributes to the total interaction energy of these dimers. Even when bulky substituents are introduced into the core part, the interaction energy is still large. It is also revealed that when a long intermolecular distance is provided by a high steric repulsion originating from the linkage of two phenyl groups, the dispersion energy is significantly small. However, in this range of intermolecular distances, the electrostatic energy caused by a strong quadrupole-quadrupole attractive interaction plays a dominant role, and as a result, a rather stable dimer is formed. In all 10 dimers, the dispersion, electrostatic, and exchange-repulsion energies strongly depend on the geometrical orientation of the molecules. The calculated interaction energies of these dimers are also compared with the corresponding experimentally measured viscosities. The results suggest an explicit linear relationship between the interaction energies and viscosities.