Quadrupolar moment calculations and mesomorphic character of model dimeric liquid crystals

Model dimeric compounds CH3(CH2)nCOOROOC(CH2)pCOOROOC(CH2)nCH3, where (R=C6H4C(CH3)HCC6H4), of semiflexible liquid crystal polymers (LCP) have been studied by molecular modeling techniques. First, a conformational analysis of the molecules was made in order to find the lowest energy conformation structure. In a second stage, geometry optimization calculations of these conformers were performed at the quantum mechanics semiempirical PM3 level. Some molecular parameters (electric dipolar and quadrupolar moments) were obtained at the improved ab initio Hartree–Fock HF/6-31G(d)//PM3 level and are related with the mesomorphic character of these systems. It is found that the trace of the quadrupole moment tensor correlates with the spacer ((CH2)p-linkage) length, and this can be an indication of the trend that this magnitude exhibits in aromatic liquid crystal systems.