A kinetic isotope effect of the intramolecular hydrogen migration within 2-oxy-3,6-di-tert-butylphenoxy radical.

Prokof’ev, A. I.; Бубнов, Н. Н.; Солодовников, С. П.; Кабачник, М. И.

doi:10.1016/s0040-4039(01)96183-0

Cited by 27 publications

(7 citation statements)

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“…Of course, we can not exclude the possibility that this species is initially formed, since it would immediately rearrange to the observed thermodynamically more stable radical. [9] As far as the second point is concerned, the neglecting of the entropy of reaction in the specific case of catechol 1 A was an extrapolation based on previous experiments showing that the entropy of a hydrogen exchange reaction for several pairs of monophenols is negligible.…”

Section: Resultsmentioning

confidence: 99%

A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the OH Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants

Lucarini

Pedulli

Guerra³

2004

Chemistry A European J

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New experimental results on the determination of the bond dissociation enthalpy (BDE) value of 3,5-di-tert-butylcatechol, a model compound for flavonoid antioxidants, by the EPR radical equilibration technique are reported. By measurement of the equilibrium constant for the reaction between 3,5-di-tert-butylcatechol and the 2,6-di-tert-butyl-4-methylphenoxyl radical, in UV irradiated isooctane solutions at different temperatures, it has been shown that the thermodynamic parameters for this reaction are DeltaH degrees = -2.8+/-0.1 kcal mol(-1) and DeltaS degrees = +1.3+/-0.2 cal mol(-1) K(-1). This demonstrates that the entropic variations in the hydrogen exchange reaction between phenols and the corresponding phenoxyl radicals are also negligible when one of the reacting species is a polyphenol and that the EPR radical equilibration technique also allows the determination of the Obond;H BDEs in intramolecularly hydrogen-bonded polyphenols. The BDE of 3,5-di-tert-butylcatechol (78.2 kcal mol(-1)) was determined to be identical to that of alpha-tocopherol. Through use of the group additivity rule, this piece of data was also used to calculate the strength of the intramolecular hydrogen bond between the hydroxyl proton and the oxygen radical centre in the corresponding semiquinone radical (5.6 kcal mol(-1)), which is responsible both for the excellent antioxidant properties of catechols and for the BDE of catechol (81.8 kcal mol(-1)). These values are in poor agreement with those predicted by DFT calculations reported in the literature (9.5 kcal mol(-1) and 77.6 kcal mol(-1), respectively). Extensive theoretical calculations indicate that the BDE of catechol is reproduced well (81.6 kcal mol(-1)) by use of diffuse functions on oxygen and the CCSD method.

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Section: Resultsmentioning

confidence: 99%

A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the OH Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants

Lucarini

Pedulli

Guerra³

2004

Chemistry A European J

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“…10 The value a( 1 H) ) 3.17 G for the silver complex is close to the values of a( 1 H) ) 3.0-3.3 G for SQ •-radicals with alkali metals, 10 which shows that the O-Ag chemical bond in the silver complex is essentially ionic. For comparison, we can mention that, for 3,6-di-tert-butyl-2-Roxyphenoxide radicals (SQ • R) with covalent chemical bonds O-R, a( 1 H) > 3.90 G. 12,13,17 As can be seen from Figure 1, with the temperature lowering, the central component (with total value of nuclear spin projection ∑m I ( 1 H) ) 0) of the triplet due to the two protons experiences faster broadening than the components with ∑m I ( 1 H) ) (1. This can be visualized by noticing the change in the amplitude ratio of the proton triplet components in the EPR spectra (marked with dots in Figure 1) from 1:2:1 at high temperatures to about 1:1:1 at 190 K in toluene ( Figure 1).…”

Section: Resultsmentioning

confidence: 99%

Structure and Dynamics of Bis(triphenylphosphine)-3,6-di-tert-butyl-o-semiquinone Radical Anion Complexes of Ag⁺ and Cu⁺

et al. 2003

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Complexes of Ag + and Cu + with 3,6-di-tert-butyl-o-semiquinone radical anion (SQ •-) and two triphenylphosphine (PPh 3 ) ligands have been investigated by electron paramagnetic resonance (EPR) in organic solvents. High-temperature EPR spectra of the Ag + and Cu + complexes contain lines of hyperfine splittings due to two equivalent 1 H nuclei in the 4,5-positions of the aromatic ring of the SQ •-ligand, two equivalent 31 P nuclei from the two PPh 3 ligands, and 107,109 Ag and 63,65 Cu nuclear isotopes, respectively. On the basis of the temperature dependence of the 31 P hyperfine interaction constant a( 31 P), the structure of the Ag + complex at low temperature was determined to be planar, while at higher temperatures the increase in the values of a( 31 P) shows that phosphorus atoms of the two PPh 3 ligands move out of the π-plane of the SQ •-ligand. EPR spectra of the Ag + complex with variable temperature exhibit alternating line width effect in hyperfine splitting due to the two aromatic protons in the SQ •-ligand. At low temperatures these two protons become nonequivalent. This effect reveals a dynamic charge redistribution within Ag + and two oxygen atoms of the SQ •-ligand. In the Cu + complex, we have found a temperature-independent symmetric charge distribution within two oxygen atoms of the radical anion and Cu + , but different positions of the two PPh 3 ligands at low temperatures. EPR spectra of the Cu + complex at variable temperature reveal the dynamic exchange in the positions of PPh 3 ligands. The rates of this process are on the order of solvent reorientation frequencies, while intramolecular electron exchange rates in the Ag + complex are about 2 orders of magnitude lower. The rate of the electron exchange reaction dramatically decreases in viscous mineral oil, which shows that the intramolecular charge redistribution reaction is controlled by nonpolar solvent dynamics.

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“…When 3,6‐di‐ tert ‐butyl‐2‐hydroxyphenoxyl and its deuterated analog are dissolved in heptane the Arrhenius diagram of Fig. 4 is obtained by Bubnov et al 80, 81. The kinetic isotope effect is about 10 at room temperature.…”

Section: Experimental Examplesmentioning

confidence: 95%

“… Arrhenius curves for the tautomerism of 3,6‐di‐ tert ‐butyl‐2‐hydroxyphenoxyl in heptane. The data were taken from Bubnov et al 80, 81. The solid lines were calculated using the parameters listed in Table 2 …”

Section: Experimental Examplesmentioning

confidence: 99%

Heavy atom motions and tunneling in hydrogen transfer reactions: the importance of the pre‐tunneling state

Limbach

Schowen

2010

J of Physical Organic Chem

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Arrhenius curves of selected hydrogen transfer reactions in organic molecules and enzymes are reviewed with the focus on systems exhibiting temperature‐independent kinetic isotope effects. The latter can be rationalized in terms of a ‘pre‐tunneling state’ which is formed from the reactants by heavy atom motions and which represents a suitable molecular configuration for tunneling to occur. Within the Bell–Limbach tunneling model, formation of the pre‐tunneling state dominates the Arrhenius curves of the H and the D transfer even at higher temperatures if a large energy Em is required to reach the pre‐tunneling state. Tunneling from higher vibrational levels and the over‐barrier reaction via the transition state which lead to temperature‐dependent kinetic isotope effects dominate the Arrhenius curves only if Em is small compared to the energy of the transition state. Using published data on several hydrogen transfer systems, the type of motions leading to the pre‐tunneling state is explored. Among the phenomena which lead to large energies of the pre‐tunneling state are (i) cleavage of hydrogen bonds or coordination bonds of the donor or acceptor atoms to molecules or molecular groups in order to allow the formation of the pre‐tunneling state, (ii) the occurrence of an energetic intermediate on the reaction pathway within which tunneling takes place, and (iii) major reorganization of a molecular skeleton, requiring the excitation of specific vibrations in order to reach the pre‐tunneling state. This model suggests a solution to the puzzle of Kwart's findings of temperature‐independent kinetic isotope effects for hydrogen transfer in small organic molecules. Copyright © 2010 John Wiley & Sons, Ltd.

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A kinetic isotope effect of the intramolecular hydrogen migration within 2-oxy-3,6-di-tert-butylphenoxy radical.

Cited by 27 publications

References 3 publications

A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the OH Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants

A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the OH Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants

Structure and Dynamics of Bis(triphenylphosphine)-3,6-di-tert-butyl-o-semiquinone Radical Anion Complexes of Ag⁺ and Cu⁺

Heavy atom motions and tunneling in hydrogen transfer reactions: the importance of the pre‐tunneling state

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A kinetic isotope effect of the intramolecular hydrogen migration within 2-oxy-3,6-di-tert-butylphenoxy radical.

Cited by 27 publications

References 3 publications

A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the OH Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants

A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the OH Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants

Structure and Dynamics of Bis(triphenylphosphine)-3,6-di-tert-butyl-o-semiquinone Radical Anion Complexes of Ag+ and Cu+

Heavy atom motions and tunneling in hydrogen transfer reactions: the importance of the pre‐tunneling state

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Structure and Dynamics of Bis(triphenylphosphine)-3,6-di-tert-butyl-o-semiquinone Radical Anion Complexes of Ag⁺ and Cu⁺