Aggregation of molecules in liquid pyridine and its solutions: Raman spectra and quantum-chemical calculations

In solutions of pyridine-propan-2-ol the ring vibration band of the hydrogen-bonded aggregates (H-aggregates) is shifted towards greater frequencies and changes its position with the change of pyridine concentration. The application of the mixture diluting method in a large amount of a neutral solvent (triple mixture) allows one to “resolve” the aggregates band structure. The observed doublet structure of the band is connected with the presence of two types of H-aggregates in liquid mixture. Quantum-chemical calculations confirm the high-frequency shift of 992 cm− 1 band of pyridine in the formation of aggregates with propan-2-ol. Similar calculations for isolated aggregates of pyridine-methyl alcohol show that several types of aggregates are possible including closed three-molecular H-aggregates of pyridine molecule with two molecules of methyl alcohol. Calculations show also that the pyridine molecules can form among themselves a weak dimer aggregate with two intermolecular hydrogen bonds.