Modelling the structure and IR spectra of micro-hydrated ions

Abstract : Ion micro-hydration has been modeled by a combination of classical and quantum methods. The structures, dynamics and IR spectra of micro-hydrated cluster ions ranging from 1 to 216 water molecules have been considered with special emphasis on the comparison to recent experimental data. Quantum modeling has been used to provide reference values against which the polarizable AMOEBA force field could be calibrated. Extension of the parameter set was carried out in some cases, in particular when improvement of the electrostatic and polarization energy terms was deemed necessary. Classical molecular dynamics was then used with two different aims: (1) extensive exploration of potential energy surfaces for large cases with numerous low energy minima, for which statistical sampling is required, and (2) computation of IR spectra through Fourier transform of the Dipole moment AutoCorrelation Function (DACF), at various temperatures. The hydration of Zn2+ was studied in detail. While the coordination number (CN) of zinc cation is firmly established to be six in bulk solution, recent experimental and computation studies in the gas phase have unveiled a complex pattern for small clusters with up to 12 water molecules with a CN more near 5 or maybe even 4. Our calculations, performed from 6 to 216 water molecules, allow a precise description of CN evolution with cluster size. Sodiated tryptamine with one or two water molecules attached was investigated in order to interpret gas phase IR spectra of these species. Particular care was taken of electrostatics - especially atomic charges - and of starting structures in molecular dynamics simulations. It was then possible to investigate temperature-dependent dynamics in detail and to obtain good agreement with IRPD spectra. Finally, the hydration of the sulfate anion with up to 100 water molecules was analyzed focusing on the existence of dangling O-H bonds on the cluster surface as a function of cluster size. In agreement with IRPD experiments, we find that small clusters are compact with no dangling bond. The latter appear when ca. 25 water molecules are present with a characteristic vibrational band. Overall, these results underline the requirements for accurate modeling of ion hydration. These involve refined treatment of electrostatic and polarization interactions, extensive sampling of the potential energy surfaces, careful treatment of cluster boundaries including a confining repulsive wall, and long-time simulations to attain converged radial distribution functions. With such developments at hand, it is now possible to obtain a realistic picture of structures and dynamics of large micro-hydrated cluster ions, as can be judged from the comparison to experiment, especially vibrational spectroscopy.
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Submitted on : Friday, February 27, 2015 - 6:30:03 AM
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Chandramohan Jana. Modelling the structure and IR spectra of micro-hydrated ions. Theoretical and/or physical chemistry. Ecole Polytechnique, 2014. English. ⟨tel-01120935⟩

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