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|Title:||Modeling Ionophore Selectivity in the Gas Phase: IR Spectroscopy of Solvated Metal Ion-Crown Ether Complexes|
|Author(s):||Rodriguez, Jason David|
|Doctoral Committee Chair(s):||Lisy, James M.|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Crown ethers are a family of compounds that are closely related to a variety of biological and naturally-occurring ionophores. Thus, crown ethers have become the prototypical ionophore-model systems in chemistry and biology. Insight gained by studying the interaction of crown ethers with metal ions may lead to the design and development of new ionophores that can be used for specialized purposes. The key to unraveling the selectivity trends observed in crown ether systems is to characterize and classify the various competing non-covalent interactions. Gas-phase cluster spectroscopy offers a suitable avenue to study the role that these non-covalent interactions play in metal ion crown ether systems.
The major portion of this dissertation deals with studying M+ (12-crown-4 ether)(H2O)1--4 and M +(18-crown-6 ether)(H2O)1--4 systems, where M=Li, Na, K, Rb, and Cs, using a combination of infrared predissociation spectroscopy and density functional theory methods. Since the ultimate goal is to model condensed-phase and physiological systems, where hydration plays a central role, special emphasis is given to studying the role of microhydration in these systems. The results indicate that as the degree of hydration is increased there is a shift away from the crown ether···M + and crown ether···H2O interactions, which dominate in systems with one water, in favor of extensive H2O···H 2O hydrogen bonding interactions. These hydrogen bonding interactions lead to broad featureless spectra that make comparison with calculation and elucidation of structure difficult. One solution to this problem is to generate the argon-tagged analogs of the hydrated species, M+(12-crown-4 ether)(H2O)1--4Ar and M+(18-crown-6ether)(H 2O)1--4Ar. Since argon-tagged species are colder than their un-tagged counterparts, their spectra have sharp, well-defined peaks which allow for comparison with calculations. While differences in the infrared spectra for the tagged and un-tagged species exist (temperature effects) for both 12-crown-4 and 18-crown-6 systems, the most interesting aspects of the argon-tagged studies is the ability to generate, trap, and detect high energy conformers in our 18-crown-6 ether experiments. Upon irradiation with an infrared photon, these high-energy conformers undergo a rearrangement to the global-minimum energy conformers and liberate the excess energy via loss of all solvating ligands.
A custom electro-sonic spray ionization source has also been constructed to extend metal ion crown ether studies to systems with divalent ions. The initial study on Mn2+(18-crown-6 ether)(CH3OH) 1--3 reveals strong, charge-enhanced hydrogen bonding is present when the metal ion is a dication. Hydrogen bonds observed in the IR spectra shift by more than 500 cm-1 to lower frequency. This causes overlap between the hydrogen-bonded OH stretch of CH3OH and the CH stretching vibrations of 18-crown-6 ether. Using density functional theory calculations and deuterated studies, the experimental IR spectra are deconvoluted.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009.
|Date Available in IDEALS:||2014-12-17|