Hannah

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

Figure 1:Basic reaction of keto- and enol- acetylacetone

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)

Figure 2: Tautomers of nucleic acid bases

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

Figure 3: 3D structure of keto-acetylacetone

Proton NMR

Figure 4: Proton NMR for keto-acetylacetone

Proton NMR shifts:File:NMR-Chemical-Shift-PPM-Range-and-Values-Table-.png

Enol Acetylacetone

Figure 5: 3D structure of enol-acetylacetone

Proton NMR

Figure 6: Proton NMR of enol-acetylacetone

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