Of the chemical reactions that can occur in aqueous solution, acid-base reactions are among the most pervasive and important. Although it is easy to specify the balanced chemical equation for such a reaction [HX(aq) oH+(aq) + X(aq), where H is a proton and Xis the conjugate base of the acid HX], it is far more difficult to characterize the structures of these solvated ions, particularly the solvated proton.
On page 1137 of this issue, Shin et al. (1) and Miyazaki et al. on page 1134 (2) use the powerful tool of infrared (IR) spectroscopy to probe protonated water clusters H+(H2O)nwith n = 6 to 27. By isolating and mass-selecting the large protonated water clusters in the gas phase, the researchers are able to record the IR spectra of each cluster size in this range, and thereby follow the development of the IR spectrum as a function of the number of water molecules in the cluster.