Infrared Spectroscopy of Hydrated Bicarbonate Anion Clusters: HCO_3~-(H_2O)_(1-10)

摘要

Infrared multiple photon dissociation spectra are reported for HCO_3 ~-(H_2O)_(1-10) clusters in the spectral range of 600-1800 cm~(-1). In addition, electronic structure calculations at the MP2/6-311 +G(d,p) level have been performed on the n = 1 -8 clusters to identify the structure of the low-lying isomers and to assign the observed spectral features. General trends in the stepwise solvation motifs of the bicarbonate anion can be deduced from the overall agreement between the calculated and experimental spectra. The most important of these is the strong preference of the water molecules to bind to the negatively charged CO_2 moiety of the HCO_3~- anion. However, a maximum of four water molecules interact directly with this site. The binding motif in the most stable isomer of the n = 4 cluster, a four-membered ring with each water forming a single H-bond with the CO_2 moiety, is retained in all of the lowest-energy isomers of the larger clusters. Starting at n = 6, additional solvent molecules are found to form a second hydration layer, resulting in a water-water network bound to the CO_2 moiety of the bicarbonate anion. Binding of a water to the hydroxyl group of HCO_3 is particularly disfavored and apparently does not occur in any of the clusters investigated here. Similarities and differences with the infrared spectrum of aqueous bicarbonate are discussed in light of these trends.