Vibration Spectra of Carboxylic Acids by Neutron Spectroscopy

Neutron energy-loss spectra in the range 400–1400 cm−1 are reported for formic, acetic, propionic, chloroacetic, trichloroacetic, trifluoroacetic, pentafluoropropionic, oxalic, malonic, succinic, maleic, and tartaric acids. The measurements were taken in the solid state and are used to throw light on the hydrogen-bonding process. Much of the data follow the expected pattern for carboxylic acids, but there are indications from the γ(OH) wavenumbers that the chloro and fluoro acids form weaker hydrogen bonds than the corresponding hydrogeneous acids, a fact that is unexpected in view of the chemical evidence that these are strong acids.