Cryst. Res. Technol. 35 2000 11-12 1321–1330 The dimer molecular structure of 4,n-alkyloxybenzoic acids displaying nematic and smectic C liquid crystal phases is investigated. AB INITIO molecular orbital calculations at HF/STO-3G level of theory are applied in order to calculate the molecule of in its closed dimer, open dimer and monomer states. The molecular geometry parameters (stereoparameters), the thermochemical quantities and molecular polarizability are calculated. The microscopical calculations give a base for development of a mean- field theory where the constituent molecule can not be approximated as a rigid rod. Keywords: liquid crystals, dimers, molecular structure (Received June 5, 2000; Accepted October 4, 2000) I. Introduction The general structural property of the constituent liquid crystal molecules is the “hard” part consisting of two or more benzene rings and flexible alkyl chains, thus expressing a rod-like molecular form. There are, however, structural units forming dimers by hydrogen bonds and also displaying liquid crystal state. One peculiarity of these liquid crystals (like 4-n- alkyloxybenzoic acids) is the variation of the molecular structure induced by variation of the temperature or by surface action or by some external affects destroying the hydrogen bonds. The dimer ring formed at linking of the two monomers in closed dimers is also flexible. Thus in the structural units of the 4-n-alkyloxybenzoic acids there are two flexible parts – centers – the dimer ring and the alkyl chains. The influence of the flexibility of the alkyl chains on the liquid crystal properties has been investigated by many authors (ARNOLD; MARCELIJA; GRAY; GRULER; LIPPMANN, WERBER; PINES et al), however the role of the dimer ring flexibility is still not clarified. The existence of two centers of flexibility along the molecular length obviously produces more complex properties of the liquid crystal state. As a result, the nematic phase of liquid crystal organized by hydrogen bonded in dimers molecules expresses peculiar macroscopic properties – some anomalies in the temperature trends of typical liquid crystal constants (CARR; GRULER, MEIER; PETROV et al, 1992; PETROV, DURAND; PETROV, SIMOVA). Consequently, in order to understand and describe the nematic state of liquid crystals built by dimer molecules the internal degree of freedom of the constituent molecule must be taken into account. The variation of the concentration of the three basic molecular forms (monomers, closed dimers and open dimers) by temperature variation in the nematic range of 4-n-alkyloxybenzoic acids was demonstrated in (PETROV et al, 1994). Recently (PETROV et al, 1992) it was also demonstrated, that monomers could also realize not only linkage between them by hydrogen bonds, but also such hydrogen coupling with the hydrophilic surfaces like ITO or PVA (polyvinylalkohol). In such way the bulk P. BOBADOVA-PARVANOVA, V. PARVANOV*, M. PETROV**, L. TSONEV** Department of Chemistry, Sofia University, Sofia, Bulgaria *Institute of Catalysis, Bulgarian Academy of Sciences, Sofia, Bulgaria **Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria Molecular Structure of the Nematic Liquid Crystals made by Hydrogen Bonded in Dimers Molecules