Infrared spectroscopy of CO₃^•−(H₂O)_1,2 and CO₄^•−(H₂O)_1,2

Hydrated molecular anions are present in the atmosphere. Revealing the structure of the microsolvation is key to understanding their chemical properties. The infrared spectra of CO₃^•−(H₂O)_1,2 and CO₄^•−(H₂O)_1,2 were measured via infrared multiple photon dissociation spectroscopy in both warm and cold environments. Redshifted from the free O–H stretch frequency, broad, structured spectra were observed in the O–H stretching region for all cluster ions, which provide information on the interaction of the hydrogen atoms with the central ion. In the C–O stretching region, the spectra exhibit clear maxima, but dissociation of CO₃^•−(H₂O)_1,2 was surprisingly inefficient. While CO₃^•−(H₂O)_1,2 and CO₄^•−(H₂O) dissociate via loss of water, CO₂ loss is the dominant dissociation channel for CO₄^•−(H₂O)₂. The experimental spectra are compared to calculated spectra within the harmonic approximation and from analysis of molecular dynamics simulations. The simulations support the hypothesis that many isomers contribute to the observed spectrum at finite temperatures. The highly fluxional nature of the clusters is the main reason for the spectral broadening, while water–water hydrogen bonding seems to play a minor role in the doubly hydrated species.