Substituent Effects on Keto–Enol Equilibria Using NMR Spectroscopy

Manbeck, Kimberly A.; Boaz, Nicholas C.; Bair, Nathaniel C.; Sanders, Allix; Marsh, Anderson L.

doi:10.1021/ed1010932

Cited by 38 publications

(22 citation statements)

References 11 publications

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“…This indicates a higher content of the diketo tautomer in the tautomeric equilibrium in this compound that will affects its fluorescence. This is consistent with that the alkyl substituents at the central carbon atom in β-diketones increase the diketo content [52].…”

Section: The Electronic Spectra Of the Studied Compoundssupporting

confidence: 88%

“…It is known that in β-diketones alkyl groups attached to the αposition (central carbon atom) produces a decrease of the enol percent meanwhile stabilizing the enol form [51]. On the other hand, the electron withdrawing nature of the Cl group in βdiketones leads to the favor of the enol form due to the stabilization of the conjugate base enolate anion by the presence of the Cl, while in contrary the electron donating nature of the methyl group leads to favor the keto form [52]. Table 1 shows the trans-enol is less stable even than the diketo form.…”

Section: Structure and Energiesmentioning

confidence: 99%

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The investigation of the photophysical properties of α-chlorocurcumin and α-methylcurcumin

Saeed

2015

Eur. J. Chem.

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The electronic properties of α-chlorocurcumin and α-methylcurcumin was theoretically investigated at the B3LYP/6-311++G(d,p) level of theory. The thermodynamics quantities were estimated by calculating the frequencies of the molecules. Three main isomers were predicted after full geometry optimization of various suggested isomers within the tautomeric mixture of each molecule; the cis-enol, trans-enol and the trans-diketo isomers. Their stability was in the sequence: cis-enol > trans-diketo > trans-enol. The stabilization energy for the cisenol with respect to trans-diketo and trans-enol in chlorocurcumin is 8.44 and 12.59 kcal/mol, respectively, while in methylcurcumin, it is 4.80 and 10.79 kcal/mol, respectively. The fluorescence spectra were recorded for the investigated compounds in several protic and aprotic solvent with different dielectric constants and H-bonding abilities. The emission maxima are within the range 487 to 571 nm in ethylene glycol, while they are within the range 475 to 557 nm in n-hexane. The fluorescence quantum yields of both compounds are low and lower than those of curcumin. The quantum yield of chlorocurcumin ranges from ΦFl = 0.008 in MeOH to ΦFl = 0.058 in toluene, while for methylcurcumin it ranges from ΦFl = 0.007 in DMF to ΦFl = 0.0524 in ethylene glycol. The fluorescence of both compounds quenched by water and their fluorescence life times are estimated from the slopes of the linear curves that obtained from Stern-Volmer relationship to be 1.44 and 1.40 psec for chlorocurcumin and methylcurcumin, respectively.

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Section: The Electronic Spectra Of the Studied Compoundssupporting

confidence: 88%

Section: Structure and Energiesmentioning

confidence: 99%

The investigation of the photophysical properties of α-chlorocurcumin and α-methylcurcumin

Saeed

2015

Eur. J. Chem.

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“…Variations in the electron‐donating groups (EDGs) and electron‐withdrawing groups (EWGs) of molecules are effective on different physical and chemical properties. Computational investigations of substituent effects on the structure and properties of the molecules have been reported 39–68 …”

Section: Introductionmentioning

confidence: 99%

Computational investigation of the substituent effect in the [2 + 4] Diels–Alder cycloaddition reactions of HSi≡Si(para‐C₆H₄X) with benzene

Ahraminejad

Ghiasi

Mohtat

et al. 2021

J Chinese Chemical Soc

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In this paper, the DFT method was applied at the B3LYP/6-311+G (d,p) level of theory to investigate the [2 + 4] Diels−Alder cycloaddition reactions of HSi≡Si(para-C 6 H 4 X); X = NH 2 , OH, Me, H, F, CCH, OCF 3 with benzene. It was attempted to show how the substituent affects the barrier height and thermodynamic parameters of these reactions. To illustrate the interaction between two fragments at transition states Activation Strain Model (ASM) and Energy Decomposition Analysis (EDA) were utilized. The Wiberg bond indices were used to check the progress of the reactions. The rate constant of the reaction was computed at 300 K, in the gas phase. Also, the synchronicity values of this reaction were determined. K E Y W O R D S rhenabenzyne, phosphine ligand, natural bond analysis (NBO), Tolman cone angles, Tolman's electronic parameter (TEP), para-delocalization index (PDI)

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“…15 Related derivatives cover a chemical shift range of 13–17 ppm 21. The distinct NMR parameters of the enol versus keto forms of such compounds find frequent use in undergraduate laboratory experiments set up to study the tautomer equilibrium, and its solvent dependence, by NMR spectroscopy 22. An interesting related system is 2‐hydroxyacetophenone, where a CC bond is in conjugation with an H‐bonding carbonyl group but also part of an aromatic 6‐membered ring; the chemical shift of the OH proton is 12 ppm 23.…”

Section: Introductionmentioning

confidence: 99%

“…[21] The distinct NMR parameters of the enol versus keto forms of such compounds find frequentu se in undergraduate laboratory experiments set up to study the tautomer equilibrium,a nd its solventd ependence, by NMR spectroscopy. [22] An interesting relateds ystemi s2 -hydroxyacetophenone, where aC =Cbond is in conjugation with an H-bonding carbonyl group but also part of an aromatic 6-membered ring;t he chemical shift of the OH protoni s1 2ppm. [23] Av ery large protons hift exceeding 20 ppm has been found for the internal H-bond of the hydrogen succinate anion (HSA), ap artially deprotonated dicarboxylic acid, and relateds ystems.…”

Section: Introductionmentioning

confidence: 99%

Analysis of Proton NMR in Hydrogen Bonds in Terms of Lone‐Pair and Bond Orbital Contributions

Sutter

Aucar

Autschbach

2015

Chemistry A European J

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NMR spectroscopic parameters of the proton involved in hydrogen bonding are studied theoretically. The set of molecules includes systems with internal resonance-assisted hydrogen bonds, internal hydrogen bonds but no resonance stabilization, the acetic acid dimer (AAD), a DNA base pair, and the hydrogen succinate anion (HSA). Ethanol and guanine represent reference molecules without hydrogen bonding. The calculations are based on zero-point vibrationally averaged molecular structures in order to include anharmonicity effects in the NMR parameters. An analysis of the calculated NMR shielding and J-coupling is performed in terms of "chemist's orbitals", that is, localized molecular orbitals (LMOs) representing lone-pairs, atomic cores, and bonds. The LMO analysis associates some of the strong de-shielding of the protons in resonance-assisted hydrogen bonds with delocalization involving the π-backbone. Resonance is also shown to be an important factor causing de-shielding of the OH protons for AAD and HSA, but not for the DNA base pair. Nitromalonamide (NMA) and HSA have particularly strong hydrogen bonds exhibiting signs of covalency in the associated J-couplings. The analysis results show how NMR spectroscopic parameters that are characteristic for hydrogen bonded protons are influenced by the geometry and degree of covalency of the hydrogen bond as well as intra- and intermolecular resonance.

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Substituent Effects on Keto–Enol Equilibria Using NMR Spectroscopy

Cited by 38 publications

References 11 publications

The investigation of the photophysical properties of α-chlorocurcumin and α-methylcurcumin

The investigation of the photophysical properties of α-chlorocurcumin and α-methylcurcumin

Computational investigation of the substituent effect in the [2 + 4] Diels–Alder cycloaddition reactions of HSi≡Si(para‐C₆H₄X) with benzene

Analysis of Proton NMR in Hydrogen Bonds in Terms of Lone‐Pair and Bond Orbital Contributions

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Substituent Effects on Keto–Enol Equilibria Using NMR Spectroscopy

Cited by 38 publications

References 11 publications

The investigation of the photophysical properties of α-chlorocurcumin and α-methylcurcumin

The investigation of the photophysical properties of α-chlorocurcumin and α-methylcurcumin

Computational investigation of the substituent effect in the [2 + 4] Diels–Alder cycloaddition reactions of HSi≡Si(para‐C6H4X) with benzene

Analysis of Proton NMR in Hydrogen Bonds in Terms of Lone‐Pair and Bond Orbital Contributions

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Computational investigation of the substituent effect in the [2 + 4] Diels–Alder cycloaddition reactions of HSi≡Si(para‐C₆H₄X) with benzene