Ab initio calculation of molecular structure and dipole moment for selected divinyl and diepoxy molecules

The ab initio quantum-chemical calculations were used to obtain the components of the dipole moment of four molecules composed of a rigid central segment and two symmetric flexible endings. The recently synthesized molecules are of interest due to their liquid-crystalline properties. Two different central segments and two kinds of endings (vinyl and epoxy) were studied. As a first step of calculations the geometry of each individual segment was optimized in order to obtain the structure of the lowest energy. The optimization of one of the central segments revealed two conformations with similar energy. Due to several possibilities of joining the endings with the central segment a series of possible conformers may be created for each of the molecules. The lowest-energy conformer was finally optimized for each structure. The relation of the total molecular dipole moment to the value and relative orientation of the dipole moments of individual segments is discussed. For the molecules in which nematic mesophase was detected it was found that replacing vinyl endings with epoxy endings results in doubling of the dipole moment. The direction of the dipole moment also strongly depends on the type of ending. The polarizability contribution to the total dipole moment is negligible for the fields lower than 10(6) V/m. (C) 2002 Elsevier Science B.V. All rights reserved.