Dinitrogen as a Sensor for Metastable Carboxylic Acid Dimers and a Weak Hydrogen Bond Benchmarking Tool

Molecular nitrogen as a weak hydrogen bond acceptor is added to formic and acetic acid and their monodeuterated isotopologues. FTIR spectroscopy of supersonic expansions in the O-H stretching region reveals the formation of the weakly bound N₂-carboxylic acid complexes. Their respective spectral downshifts from the monomer fundamental vibration are used to benchmark electronic structure calculations and vibrational perturbation theory. The small size of the investigated systems allows for a wide range of electronic structure levels to be explored. The O-H stretching vibration of an open dimer of acetic acid can be discriminated from the cyclic dimer vibrations by its higher susceptibility to coexpanded nitrogen.