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Vibrational Spectroscopy of Gaseous Hydrogen-Bonded Clusters: On the Role of Isomer-Specificity and Anharmonicity


Heine,  Nadja
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Heine, N. (2014). Vibrational Spectroscopy of Gaseous Hydrogen-Bonded Clusters: On the Role of Isomer-Specificity and Anharmonicity. PhD Thesis, Freie Universität, Berlin.

Cite as: http://hdl.handle.net/11858/00-001M-0000-0024-67E0-4
Gas phase clusters typically serve as model systems for studying properties of more complex systems under well-defined conditions in the absence of perturbing interactions with an environment. However, some clusters themselves play crucial roles in relevant processes. Charged clusters, for example, represent key precursors in the formation of aerosols in the atmosphere. In order to ultimately improve our understanding of atmospheric processes, in general, and climate simulations, in particular, novel experimental techniques yielding molecular-level insight into their structure, energetics, reactivity and dynamics are required. The studies presented in this thesis aim at shedding new light on the solvation behavior of hydrogen-bonded cluster ions. Gas phase vibrational spectra are measured by means of mass-selective infrared photodissociation (IRPD) spectroscopy and structures are assigned, based on a comparison between experimental and simulated spectra of different isomers derived from electronic structure calculations. Often multiple isomers are present in the experiment. In order to isolate their individual contributions to the IRPD spectrum, IR/IR double-resonance (IR2MS2) spectroscopy is performed. To this end a custom ion trap triple mass spectrometer was conceived, designed and constructed, which allows measuring isomer-specific vibrational spectra over nearly the entire IR spectral range as a function of cluster size, composition and internal temperature. The capabilities of the new instrument are demonstrated by measuring isomer-specific IR2MS2 spectra of the Eigen-type and Zundel-type conformers of the protonated water hexamer from 260 to 3900 cm-1. Comparison to \textit{ab initio} molecular dynamics simulations (AIMD) not only provides insight into the mechanism responsible for the characteristically broad IR absorptions of hydrated protons, but also allows for the first experimental identification of hydrogen-bond stretching vibrations in protonated water clusters (in the terahertz region). This study is then extended to larger protonated water clusters H+(H2O)n, addressing the question of how the number of isomers evolves with the size of the hydration shell. The structure, stability and solvation behavior of atmospherically-relevant nitrate-containing anions is studied. These experiments follow how the hydrogen-bonded solvent network evolves, one solvent molecule at a time. Hydrogen dinitrate contains a surprisingly stable equally-shared proton motif (O2NO-H+ -ONO2), which is eventually disrupted upon solvation. The spectra of larger nitrate/nitric acid/water complexes already converge to those of the condensed phase and are furthermore discussed in the context of "IRMPD transparent" bands. Finally, anharmonic effects in the IRPD spectra of the singly-hydrated complexes are investigated, aided by state-of-the-art AIMD simulations as well as vibrational configuration interaction calculations.