Potential mesogens based on pyridine derivatives: The geometric structure, conformational properties and characteristics of intermolecular hydrogen bonds

Abstract Conformational properties of the main part (excluding OC 3 H 7 radicals) of the p-n -propyloxybenzoic (A 1 ) and p-n -propyloxycinnamic (A 2 ) acids molecules (relating to mesomorphic compounds) as well as p-n -propyloxybenzoic acid pyridine ester (B 1 ) and p-n -propyloxyphenylazopyridine (B 2 ) molecules (relating to non-mesomorphic compounds) were studied by DFT(B3LYP)/cc-pVTZ method. It was shown that the main parts of A 1 and A 2 acids are rigid. The barrier to internal rotation of pyridine fragment in the B 1 and B 2 molecules depends on the nature of the bridging group. It was determined that all studied A 1 ⋯B 1 , A 2 ⋯B 1 and A 2 ⋯B 2 complexes are characterized by a strong hydrogen bond. The binding energy of complexes (≈14 kcal/mol, with BSSE corrections, DFT(B97D)/6-311++G**) exceeds the energy per hydrogen bond in the corresponding acid dimers (≈10 kcal/mol). The structural non-rigidity of A⋯B complexes is mainly caused by possibility of OC 3 H 7 radicals internal rotation and A and B molecules rotation about the (H)O⋯N line. The characteristics of intermolecular hydrogen bonds were determined by NBO-analysis. The obtained results indicate that examined complexes correspond to the basic requirements to mesogen molecular forms. The thermodynamic functions of the gas-phase complexation reactions (idealized model of the complexes formation in the condensed state) were calculated. Preliminary studies of mesogen–non-mesogen A 1 ⋯B 2 system by differential scanning calorimetry and polarizing optical microscopy, showed that it has mesomorphic properties.