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

  • Microwave spectroscopy
  • ab initio calculation
  • Keto-Enol tautomerization
  • microsolvation

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-xrj6-w547

• License

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