Assigning Predissociation Infrared Spectra of Microsolvated Hydronium Cations H3O+·(H2)n (n=0, 1, 2, 3) by Ab Initio Molecular Dynamics

摘要

Messenger predissociation spectroscopy is an important experimental method to obtain vibrational spectra of molecular ions or complexes such as protonated water clusters H+·(H2O)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 H2-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 H3O +·(H2)n (n=0, 1, 2, 3). We find that microsolvation with H2 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. How do messengers modify vibrational spectra of small gas-phase molecules in messenger predissociation spectroscopy? By applying ab initio molecular dynamics to H2- microsolvated hydronium ion H3O+·(H 2)n (n=0, 1, 2, 3), these interactions (see picture) are investigated as a function of an increasing number of messengers n.