A chemical approach towards the spectroscopy of carboxylic acid dimer isomerism

The vibrational dynamics and hydrogen bond topology of excited isomers of carboxylic acid dimers is elucidated by an FTIR study of mixed acetic acid-methyl acetate clusters in supersonic jet expansions. The partial esterification prevents a second OH-O hydrogen bond in the dimer and replaces it by a weak CH-O contact. Vibrational transitions due to mixed acid-ester dimers are observed in the O-H, C=O, and C-O stretching range. Similarities between the mixed dimer spectrum and weak bands in the spectrum of pure acetic acid clusters suggest a common hydrogen bond pattern for both species. It is the hydrogen bond pattern observed between two adjacent monomers in solid acetic acid. The conclusions are supported by quantum-chemical calculations.