Quadrupolar moment calculations and mesomorphic character of model dimericliquid crystals

Model dimeric compounds CH3-(CH2)(n)COO-R-OOC-(CH2)(p)-COO-R-OOC(CH2)(n)-CH 3, where (R = -C6H4-C(CH3)double bond HC-C6H4-), of semiflexible liquid cry stal polymers (LCP) have been studied by molecular modeling techniques. Fir st, a conformational analysis of the molecules was made in order to find th e lowest energy conformation structure. In a second stage, geometry optimiz ation calculations of these conformers were performed at the quantum mechan ics semiempirical PM3 level. Some molecular parameters (electric dipolar an d 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 t hese systems. It is found that the trace of the quadrupole moment tensor co rrelates with the spacer (-(CH2)(p)-linkage) length, and this can be an ind ication of the trend that this magnitude exhibits in aromatic liquid crysta l systems. (C) 2001 Elsevier Science B.V. All rights reserved.