Spectroscopic and Computational Studies of Ionic Clusters as Models of Solvation and Atmospheric Reactions
<p>Ionic clusters are useful as model systems for the study of fundamental processes in solution and in the atmosphere. Their structure and reactivity can be studied in detail using vibrational predissociation spectroscopy, in conjunction with high level ab initio calculations. This thesis pre...
| Summary: | <p>Ionic clusters are useful as model systems for the study of fundamental processes in solution and in the atmosphere. Their structure and reactivity can be studied in detail using vibrational predissociation spectroscopy, in conjunction with high level ab initio calculations. This thesis presents the applications of infrared spectroscopy and computation to a variety of gas-phase cluster systems.</p>
<p>A crucial component of the process of stratospheric ozone depletion is the action
of polar stratospheric clouds (PS Cs) to convert the reservoir species HCl and chlorine
nitrate (CIONO₂) to photochemically labile compounds. Quantum chemistry was used to
explore one possible mechanism by which this activation is effected: <br />
CI⁻ + CIONO₂ → CI₂ + NO₃⁻ (1)<br />
Correlated <i>ab initio</i> calculations predicted that the direct reaction of chloride ion with CIONO₂ is facile, which was confirmed in an experimental kinetics study. In the reaction a weakly bound intermediate Cl₂--NO₃⁻ is formed, with ~70% of the charge localized on the nitrate moiety. This enables the Cl₂--NO₃⁻ cluster to be well solvated even in bulk solution, allowing (1) to be facile on PSCs.</p>
<p>Quantum chemistry was also applied to the hydration of nitrosonium ion (NO⁺), an important process in the ionosphere. The calculations, in conjunction with an infrared spectroscopy experiment, revealed the structure of the gas-phase clusters NO⁺(H₂O)ₙ. The large degree of covalent interaction between NO⁺ and the lone pairs of the H₂O ligands is contrasted with the weak electrostatic bonding between iodide ion and H₂O.</p>
<p>Finally, the competition between ion solvation and solvent self-association is explored for the gas-phase clusters CI⁻(H₂O)ₙ, and CI⁻(NH₃)ₙ. For the case of water, vibrational predissociation spectroscopy reveals less hydrogen bonding among H₂O ligands than predicted by <i>ab initio</i> calculations. Nevertheless, for n≥5, cluster structure is dominated by water-water interactions, with CI⁻ only partially solvated by the water cluster. Preliminary infrared spectra and computations on CI⁻(NH₃)ₙ indicate that NH₃ preferentially binds to CI⁻ ion instead of forming inter-solvent networks.</p> |
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