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The Catalytic Role of a Single Water on Keto-Enol Tautomerization Explored by Fourier Transform Microwave Spectroscopy and Ab Initio Calculations

  • Author / Creator
    Gao, Jiao
  • Keto-enol tautomerization plays important roles in biochemistry, atmospheric chemistry, and crystallography. It has attracted much attention for a long time. Solvation (hydration) is found to play essential roles in controlling this tautomerization process. Here, the effects of a single water molecule on the keto-enol tautomerization process were explored using chirped-pulse and cavity-based Fourier transform microwave spectroscopy with the aid of high-level ab initio calculations. Systematic studies on selected neutral molecular complexes involving keto-enol tautomerization provide new insights into understanding how hydration is responsible for making changes in distribution among tautomers.
    I first explored how water affects the keto-enol tautomerization of acetone using microwave spectroscopy. I assigned the quantum numbers to observed rotational
    transitions of the most stable keto form of the acetone-water complex and experimentally determined the internal rotation barriers of the two methyl groups in acetone, which are inequivalent due to the hydrogen-bonded water unit.
    Next, I investigated the keto-enol tautomeric and conformational changes of the cyclohexanone monomer and its monohydrate by Fourier-transform microwave
    spectroscopy and ab initio calculations. Ten isotopologues, including all six single 13C substitutions observed in natural abundance and four different isotopic species of water (H2O, D2O, DOH, and HOD) were measured for the most stable structure of the chair conformer of the keto tautomer-water complex. The experimental structure of cyclohexanone-water complex was determined directly using this isotopic information.
    Finally, I explored the keto-enol tautomerization of two β-diketones, i.e., acetylacetone and benzoylacetone. I only observed the enol form of acetylacetone-water complex, and
    the internal rotation barrier of the methyl group involved in hydrogen bonding in the complex was determined experimentally. For the benzoylacetone monomer, the
    experimental and theoretical results suggest that the experimental structure is an average of the two most stable enol tautomers, i.e., the proton is near the middle position between the two carbonyl groups. The study also shows that the proton transfer between the two carbonyl groups in the benzoylacetone monomer is coupled with the internal rotation of the methyl group. Apart from that, two isomers out of six of the benzoylacetone-water complexes also were detected using the chirped-pulse Fourier-transform microwave spectrometer. The study shows that both isomers of the enol tautomer exist in its water complexes, which further confirms that two enol tautomers should coexist in its monomer.
    Overall, these studies are important contributions to understanding how a single water molecule catalyzes the keto-enol tautomerization and changes relative energies. This work
    establishes a solid foundation for future work on larger ketone-(water)N clusters.

  • Subjects / Keywords
  • Graduation date
    Fall 2019
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/r3-xrj6-w547
  • License
    Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.