Structure of propanoic acid dimers as studied by means of MIR and FIR spectroscopy

Infrared spectra of propanoic acid isolated in the neon and argon matrixes were obtained and studied with the aid of the DFT calculations. The geometric structures of the cyclic and open dimers of PA were elucidated. The calculations predict that among various types of the open dimers only one type with additional hydrogen bond between oxygen of hydroxyl group of a monomer and hydrogen of hydroxyl group of other monomer (OH dimer) is energetically favorable. Structure of this dimer differs from that of formic and acetic acids, where open dimers with the hydrogen bond between oxygen of hydroxyl group and hydrogen of terminal methylene group (CH dimer) are formed. This finding explains fact why formic and acetic acids in crystalline state form polymer chains while higher member of homological row of the monocarboxylic acids (starting from propanoic) acid crystallizes as cyclic dimers. Due to the low barrier between the open and cyclic dimers of propanoic acid the open dimer is experimentally observed only in the neon matrix formed by rapid freezing of the gaseous matrix mixture from 300 K down to 2.8 K. Similar structures were not observed in the conventional 15 K argon matrix.