Calorimetric Titrations in anhydrous acetonitrile at 298.15K have been performed to give the complex stability constants and thermodynamic properties for the complexation reactions of sodium thiocyanate and potassium thiocyanate with dibenzo-18-crown-6(Ⅰ), dibenzo-20-crown-6 (Ⅱ) and dibenzo-22-crown-6(Ⅲ ). The complex stability constanta, reaction enthalpies and (entropies were calculated directly by using calorimeter connected to an CA-033 Ancrocomputer. Data analyses assuming 1:1 stoichiometry were sussessfully applied to all of the crown ether-cation combinations employed. The thermodynamic parameters obtained and examination of CPK molecular models reveal that the less-symmetrical arrangement of donor oxygen induced by increasing methylene in dibenzo-18-crown-6 molecule and lead to an unfavorable conformation for complexation compared with parent crown ether. The complex stability constants are lower than those of dibenzo-18-crown-6(Ⅰ) for the ligands (Ⅱ ) and (Ⅲ) with Na+ and K+ but the relative cation selectivity for K+/Na+ are increased respectively. The effects of molecular structure of benzo crown ether and cation diameter upon complex stability are discussed from a viewpoint of thermodynamics.
