Vibrational signatures of carboxylic acid microhydration for the HyDRA project

HyDRA aims at the systematic experimental characterisation of water vibrations in different microsolvation situations for the purpose of quantum chemistry benchmarking, where quantum chemistry involves suitable combinations of electronic structure and nuclear dynamics treatments. The current HyDRA database largely lacks acid microsolvation data. This is cured by the present contribution, which characterises neutral microhydrates of formic and acetic acid by supersonic jet FTIR and Raman spectroscopy. The centroid of the bound water OH stretching fundamental in the monohydrate of formic acid is located at 3535(4) cm^-1. Literature monohydrate spectra of benzoic acid are confirmed. A surprisingly abundant metastable monohydrate of the acetic acid dimer is identified. The empirical relationship between harmonic B3LYP carboxylic acid complexation shifts of water and the experimental OH stretching shifts is remarkably robust.