IR Spectroscopy of Gas-Phase Protonated Water–Acetonitrile Clusters: Comparison of the Clusters in the Condensed Phase and Gas Phase

Abstract We performed infrared photodissociation spectroscopy and theoretical computations on gas-phase protonated water–acetonitrile clusters, H+[(H2O)m(CH3CN)n] (m = 1, n = 1–3; m = 2, n = 1–6; and m = 3, n = 5), to determine their cluster structures and compared their infrared spectra with previously reported spectra of protonated water clusters in acetonitrile solutions. For the (m = 2, n = 4) cluster, which serves as a model for the protonated water dimer in an acetonitrile solution, the band center of the hydrogen-bonded OH stretch absorption was observed at a lower frequency than that in the solution spectra. This indicates that the formation of the second and subsequent solvation shells significantly weakens the hydrogen bonding between the protonated water dimer (ion core) and the first solvation shell in the solution phase. On the other hand, the spectrum of the (m = 3, n = 5) cluster, a model of the protonated water trimer with a completed first solvation shell, showed a much better agreement with that of the solution phase, suggesting a tendency where an increase in the protonated cluster size reduces the influence of the second solvation shell.