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Introduction
Based on Experiment 44: NMR determination of keto-enol equilibrium constants p522
Shoemaker, D. P., Garland, C. W., & Nibler, J. W. (1989). Experiments in physical chemistry (5. ed). McGraw Hill.
Tautomers of Acetylacetone (C5H8O2)
- Keto = ketone
- Enol = alcohol
- -may also be referred to as an alkenol
- Usually the keto tautomer is favored
Why Tautomerization Matters
- Different solvents may change the equilibrium of the keto- and enol- forms
- -Keto/enol equilibrium can be determined using proton NMR, as the spectra are different enough to distinguish between the tautomers
- Tautomers in DNA replication have been hypothesized to contribute to mutagenic mispairings
- "whereas in RNA, minor tautomeric forms have been proposed to enhance the structural and functional diversity of RNA enzymes and aptamers" (https://rnajournal.cshlp.org/content/21/1/1.full.html)
WebMO
- All calculations were done using HF/6-31G(d) for geometry optimization
- The products of the geometry optimizations were used to calculate the NMR spectra using the same basis sets
Keto Acetylacetone
Proton NMR
Proton NMR shifts:File:NMR-Chemical-Shift-PPM-Range-and-Values-Table-.png
Enol Acetylacetone
Proton NMR
Proton NMR shifts:File:NMR-Chemical-Shift-PPM-Range-and-Values-Table-.png
N.B. Hydrogen bonding deshields the proton, further increasing the chemical shift