The two studies reported in this issue are from three experimental groups who each used different methods of producing, cooling, and mass-selecting the large protonated water clusters. Nevertheless, the results of their work are in excellent agreement. The vibrational transitions probed by both studies are the OH stretch fundamentals of the cluster, which show exquisite sensitivity to the hydrogen-bonding environment of water’s OH groups and the chemical structure of the core ion. The two OH bonds in an isolated water molecule produce two OH stretch fundamentals that appear at 3657 and 3756 cm− 1, which are in-phase and out-of-phase vibrations, respectively, of the two OH bonds. When an OH group is placed in a hydrogen bond to another water molecule (OH… O), the frequency of the hydrogen-bonded OH stretch vibration is lowered and the intensity of its IR fundamental is increased. In bulk liquid water or ice, almost all water molecules are involved in four hydrogen bonds with neighbors: Each molecule donates its OH group to two hydrogen bonds and uses its lone electron pairs on the oxygen atom to accept two hydrogen bonds from its neighbors. However, in clusters of the size reported here, many of the water molecules in the cluster are on the surface of the cluster, and are involved in either two or three hydrogen bonds. When only a single OH group is hydrogen bonded, the two OH bonds on a given molecule become decoupled, forming localized oscillations due to the hydrogenbonded OH and free OH groups. Ironically, in the studies of Miyazaki et al.(2) and Shin et al.(1), it is the free OH stretch fundamentals that contain the most distinctive signatures regarding the structure of the clusters. The interpretation of these signatures is aided greatly by theoretical calculations of co-authors Christie and Jordan (1). In particular, the frequency of the free OH stretch fundamental shifts characteristically, depending on whether the water molecule is two-or three-coordinated. Over much of the cluster size range studied (n= 10 to 20 and 23 to 27), there are wellresolved transitions due to both types, indicating that these clusters possess a complicated hydrogen-bonded network containing both two-coordinate (single acceptor–single donor, AD) and three-coordinate (AAD and ADD) water molecules. However, at n= 21, the absorption due to two-coordinated water molecules disappears, pointing to a highly symmetric structure for the n= 21 cluster. Other features of the spectrum, supported by calculation, suggest that this structure consists of a dodecahedral cage with a single interior water. Like so many important problems in science, the present chapter on protonated water clusters does not close the book on the subject. In fact, these first spectral data on large protonated water clusters raise nearly as many questions as they answer.

The calculations predict that the proton in the lowest energy structures for the n= 21 cluster exists as a hydronium ion (H3O+), supporting the Eigen model. However, the predicted intense and highly shifted OH stretch transitions due to the H3O+ ion are not observed in the spectrum. The position of the H3O+ ion, interior to the cage or taking up a surface site, is also not established. Furthermore, the n= 22 cluster also shows no measurable AD free OH stretch band, despite the presence of the extra water molecule that would seem to spoil the dodecahedral cage. Finally, it is possible that the spectra reported here contain contributions from more than one structural isomer of a given cluster size. We can anticipate that these groups and others, aided by increasingly sophisticated and accurate calculations, will characterize these …