title: Assigning predissociation infrared spectra of microsolvated hydronium cations H3O(+)⋅(H2)n (n=0, 1, 2, 3) by ab initio molecular dynamics.
abstract: Messenger predissociation spectroscopy is an important experimental method to obtain vibrational spectra of molecular ions or complexes such as protonated water clusters H(+)⋅(H(2)O)(n) in the gas phase. However, the molecular properties and thus the linear infrared spectra may be modified upon microsolvation with typical messengers such as H(2) molecules or noble gas atoms. Employing ab initio molecular dynamics for the H(2)-microsolvated hydronium ion, we investigate these effects explicitly as a function of an increasing number of messengers up to filling the first microsolvation shell, that is, for H(3)O(+)⋅(H(2))(n) (n=0, 1, 2, 3). We find that microsolvation with H(2) lowers the inversion barrier of the hydronium core, which governs the inversion tunnel splitting due to umbrella motion, and thus accelerates the inversion dynamics. By comparison to experiment a comprehensive band assignment for the O-H stretching region is given, and thereby the observed blueshift of stretching bands with increasing n is explained. Furthermore, detailed analyses reveal intricate intra- and intermolecular anharmonic mode couplings induced by the messengers, which yield a rich vibrational dynamics in these, at first glance, simple systems. Finally, the virtues but also the shortcomings of the ab initio molecular dynamics approach to vibrational spectroscopy are discussed.
authors.affiliation: Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany.
authors.forename: Marcel
authors.initials: M
authors.lastname: Baer
authors.forename: Dominik
authors.initials: D
authors.lastname: Marx
authors.forename: Gerald
authors.initials: G
authors.lastname: Mathias
mesh_terms: D002412:Cations / Q000737:chemistry*; D005740:Gases / Q000737:chemistry; D006860:Hydrogen Bonding; D056004:Molecular Dynamics Simulation; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D014732:Vibration; D014867:Water / Q000737:chemistry*
chemical_list: D002412:Cations; D005740:Gases; D014867:Water