Infrared Photodissociation Spectroscopy of Protonated Formic Acid−Water Binary Clusters, H+·(HCOOH)n·H2O (n = 1−5). Spectroscopic Study of Ion Core Switch Model and Magic Number

Infrared spectra of protonated formic acid−water binary clusters, H+·(HCOOH)n·H2O (n = 1−5), are investigated by infrared photodissociation spectroscopy and ab initio molecular orbital calculations. The asymmetric OH stretching vibration of water is observed in the infrared photodissociation spectra of the clusters with n = 1−3; it disappears in the spectra of the n = 4 and 5 clusters. On detailed comparison of the observed infrared spectra with calculated ones, the most stable geometric structures are obtained for the n = 1−5 clusters. These results suggest that the clusters switch the ion cores from HCOOH2+ for n = 1−3 to H3O+ for n = 4 and 5. The n = 5 cluster has a cyclic-type structure; the H3O+ ion core is fully surrounded and stabilized by five formic acid molecules. This characteristic nature produces a magic number of the n = 5 cluster.