Abstract
A series of crystalline 1:1 hydrogen-bonded complexes between 2,4-dichlorobenzoic acid (DCBA) and various quinolines and pyridines was prepared. Fourier-transform infrared spectra (FTIR) of these complexes are presented and analyzed in the nu(OH) and nu(C=O) regions. The correlation between the gravity center of the protonic vibrations nu(cg) (cm(-1)), the quantity Deltanu/Deltanu(0) and the pK(a) values of the amines are presented. The integrated intensities in solution A (Lmol(-1) cm(-2)) and in the crystalline state A (kg mol(-1), cm(-2)) in the nu(C=O) region were correlated with the pK(a) values of the investigated amines. Striking anomalies are observed at the critical region (50% proton transfer). These results can be interpreted by assuming that quasi-symmetrical O..H..N bridge is formed with a single, flat minimum potential in the solid state and a double minimum potential in solution.