Bond Dissociation Enthalpy Values Research Articles

A computational study of CS bond dissociation enthalpies in petroleum chemistry

High-level theoretical methods (BMK, B3LYP, B98, B3P86, B3PW91, PBE1PBE, PBE1KIS, MPWPW91, MPW1KCIS, TPSS1KCIS, G3, G3//BMK, and CBS-Q) were utilized to study the carbon–sulfur bond dissociation enthalpies (BDEs) of hydrocarbons in petroleum chemistry. The performance of these methods was evaluated on the basis of a training set including the available experimental BDEs, and it was found that the BMK (Boese-Martin for Kinetics) method had the best agreement with experimental values. By using the BMK method to calculate CS BDEs of saturated hydrocarbon, the main factors, which determine the changing trend of BDE values, were discussed. Results revealed that the repulsive energies played an important role in determining a change in the trend of BDEs as well as the radical effect. Good agreements were obtained between further calculated BDEs and the experimental ones for CS and CO bonds. Moreover, the same calculation method was applied to predict CS BDEs for which the experimental values were still unavailable. A range of predicted bond dissociation enthalpy values were provided according to the calculations. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 22:97–105, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20662

A DFT study on the radical scavenging potential of selected natural 3′,4′-dihydroxy aurones

A series of 3′,4′-dihydroxy natural aurones differing in ring A substitution pattern have been studied with regards to their radical scavenging potential in the gas and in the liquid phase (benzene, water) using density functional theory (DFT) and the Becke's 3 Lee Yang Parr functional (B3LYP). Based on O–H bond dissociation enthalpy (BDE) and adiabatic ionization potential (IP) values, all tested aurones are expected to be more efficient hydrogen atom and electron donors than luteolin. The substitution pattern in ring A may have an impact on hydrogen atom donating activity of the compounds as phenolic and non-phenolic hydrogen atoms may be donated. Substitution affected polarity and in certain cases planarity of compounds, which may influence efficiency in real systems. Hydration at C-2 (auronols) affected polarity, electron donation and stability of the derived phenoxy radical at C-4′ and loss of planarity, rendering membrane penetration difficult. Change in C ring configuration (isoaurones) resulted in lower IP and O–H BDE value at C-6. The current findings suggest that aurones deserve more attention for food or pharmaceutical applications as antioxidants.

Quantitative structure–antioxidant activity relationship of trans-resveratrol oligomers, trans-4,4′-dihydroxystilbene dimer, trans-resveratrol-3-O-glucuronide, glucosides: Trans-piceid, cis-piceid, trans-astringin and trans-resveratrol-4′-O-β-D-glucopyranoside

By means of the accurate computations based on the density functional theory, the relationships between the molecular structure and free radical scavenging activity have been studied for dimer of trans-4,4′-dihydroxystilbene, trans-resveratrol-3-O-glucuronide, glucosides: geometric stereoisomers of piceid, trans-astringin, trans-resveratrol-4′-O-β-D-glucopyranoside and trans-resveratrol dimers: pallidol, geometric stereoisomers of ɛ-viniferin, stereoisomers of trans-δ-viniferin and trans-resveratrol trimer-gnetin H. Our results have shown that all oligomers, glucosides and trans-resveratrol-3-O-glucuronide exhibit stronger antioxidant activity than trans-resveratrol and that dimer of trans-4,4′-dihydroxystilbene is a stronger antioxidant than its monomer as well as that cis stereoisomers of piceid and ɛ-viniferin are weaker antioxidant than their trans stereoisomers. The homolytic bond dissociation enthalpy values calculated reveal the predominant H-transfer capacity of the OH groups in the trans-stilbene moiety. The hydrogen atom transfer mechanism of free radicals scavenging by the compounds studied is proved more preferable than the single-electron transfer mechanism in the mediums investigated. All the above-mentioned compounds have been proved to have significantly higher ability to electron donation in water medium than in the gas phase. The experimental observations are satisfactorily explained by the results obtained.

Electrochemical and Density Functional Theory Study on the Reactivity of Fisetin and Its Radicals: Implications on in Vitro Antioxidant Activity

Antioxidative properties of naturally occurring flavon-3-ol, fisetin, were examined by both cyclic voltammetry and quantum-chemical based calculations. The three voltametrically detectable consecutive steps, reflected the fisetin molecular structure, catecholic structural unit in the ring B, C3-OH, and C7-OH groups in the rings C and A. Oxidation potential values, used as quantitative parameter in determining its oxidation capability, indicated good antioxidative properties found with this molecule. Oxidation of the C3'C4' dixydroxy moiety at the B ring occurred first at the lowest positive potentials. The first oxidation step induced fast intramolecular transformations in which the C3 hydroxy group disappeared and the product of this transformation participated in the second oxidation step. The highest potential of oxidation was attributed to the oxidation of C7 hydroxy group. The structural and electronic features of fisetin were investigated at the B3LYP/6-311++G** level of theory. Particularly, the interest was focused on the C3' and C4'-OH sites in the B ring and on C3-OH site in the C ring. The calculated bond dissociation enthalpy values for all OH sites of fisetin clearly indicated the importance of the B ring and C3' and C4'-OH group. The importance of keto-enol tautomerism has also been considered. The analysis also included the Mulliken spin density distribution for the radicals formed after H removal on each OH site. The results showed the higher values of the BDE on the C7-OH and C3-OH sites.

Reliability of bond dissociation enthalpy calculated by the PM6 method and experimental TEAC values in antiradical QSAR of flavonoids

The applicability of the newly developed RM1 and PM6 methods implemented in the semiempirical quantum chemistry mopac2009™ software package in modeling free radical scavenging activity of flavonoids was examined. Bond dissociation enthalpy (BDE) of OH groups could be calculated much faster than with DFT method but with similar quality. Despite the known shortcomings of the Trolox equivalent antioxidant capacity (TEAC) assay, we show that taking into account the hydrogen atom transfer (HAT) mechanism of free radical scavenging of flavonoids encoded by minimal BDE values (BDE min) and the number of OH groups ( nOH), as well as experimental data, reasonable QSAR models could be developed. For TEAC values of 38 flavonoids measured by the ABTS free radical, a model based on BDE min and nOH was developed, having very good statistical parameters ( r = 0.983, r cv = 0.976). The applicability of this model to three different data sets of flavonoids and reliability of TEAC values measured in distinct laboratories were discussed. Finally, a reasonably good model of experimental vitamin C equivalent antioxidant capacity (VCEAC) of 36 flavonoids was obtained ( r = 0.954, r cv = 0.947), involving BDE min and nOH as descriptors. Additionally, all presented models have comparable fit and cross-validated statistical parameters, as well as significant regression coefficients.

Combustion Pathways of the Alkylated Heteroaromatics: Bond Dissociation Enthalpies and Alkyl Group Fragmentations

The bond dissociation enthalpies (BDEs) of the alkyl groups of the alkyl-substituted heterocycles have been studied and compiled using DFT methodology, with the intent of modeling the larger heterocyclic functionalities found in coal. DFT results were calibrated against CBS-QB3 calculations, and qualitative trends were reproduced between these methods. Loss of hydrogen at the benzylic position provided the most favorable route to radical formation, for both the azabenzenes and five-membered heterocycles. The ethyl derivatives had lower BDE values than the methyl derivatives due to increased stabilization of the corresponding radicals. Calculated spin densities correlated well with bond dissociation enthalpies for these compounds, while geometric effects were minimal with respect to the heterocycles themselves. Temperature effects on the bond dissociation enthalpies were minor, ranging by about 5 kcal/mol from 298 to 2000 K; the free energies of reaction dropped significantly over the same range due to entropic effects. Monocyclic heteroaromatic rings were seen to replicate the chemistry of multicyclic heteroaromatic systems.

Hydrogen‐Atom Transfer Reactions from ortho‐Alkoxy‐Substituted Phenols: An Experimental Approach

The role of intramolecular hydrogen bonding (HB) on the bond-dissociation enthalpy (BDE) of the phenolic O-H and on the kinetics of H-atom transfer to peroxyl radicals (k(inh)) of several 2-alkoxyphenols was experimentally quantified by the EPR equilibration technique and by inhibited autoxidation studies. These compounds can be regarded as useful models for studying the H-atom abstraction from 2-OR phenols, such as many lignans, reduced coenzyme Q and curcumin. The effects of the various substituents on the BDE(O-H) of 2-methoxy, 2-methoxy-4-methyl, 2,4-dimethoxyphenols versus phenol were measured in benzene solution as -1.8; -3.7; -5.4 kcal mol(-1), respectively. In the case of polymethoxyphenols, significant deviations from the BDE(O-H) values predicted by the additive effects of the substituents were found. The logarithms of the k(inh) constants in cumene were inversely related to the BDE(O-H) values, obeying a linear Evans-Polanyi plot with the same slope of other substituted phenols and a y-axis intercept slightly smaller than that of 2,6-dimethyl phenols. In the cases of phenols having the 2-OR substituent included in a five-membered condensed ring (i.e, compounds 9-11), both conformational isomers in which the OH group points toward or away from the oxygen in position 2 were detected by FTIR spectroscopy and the intramolecular HB strength was thus estimated. The contribution to the BDE(O-H) of the ortho-OR substituent in 9, corrected for intramolecular HB formation, was calculated as -5.6 kcal mol(-1). The similar behaviour of cyclic and non-cyclic ortho-alkoxy derivatives clearly showed that the preferred conformation of the OMe group in ortho-methoxyphenoxyl radicals is that in which the methyl group points away from the phenoxyl oxygen, in contrast to the geometries predicted by DFT calculations.

Using Molecular Strain and Aromaticity To Create Ultraweak C-H Bonds and Stabilized Carbon-Centered Radicals.

An approach based on relief of molecular strain in the parent hydrocarbon, extended conjugation in the radical, and the driving force toward aromaticity is used to design molecules with ultraweak C-H bonds. The molecular strain is generated by two fused rings containing (5,5)-, (5,6)-, or (6,6)-membered ring structures. Homodesmotic reactions are used to calculate the molecular strain enthalpy (MSE) of the parent hydrocarbons and the corresponding radicals, and to analyze how it changes through these reactions. B3LYP calculations are used to obtain the bond dissociation enthalpies (BDEs) for breaking one or more C-H bonds as well as the C-O bond formed after oxygen addition to the radical. Loss of a second H-atom can lead to very low R-H BDE values, especially when the ultimate product is aromatic. Molecular structures based on these ideas may be of interest as novel antioxidants based on carbon-centered radicals.

Bond Dissociation Enthalpies of Large Aromatic Carbon-Centered Radicals

Carbon-hydrogen bond dissociation enthalpy (BDE) values are computed for the class of benzylic radicals. An extended and representative set of large methylated polyaromatics has been submitted to an accurate computational study using various levels of theory. The hybrid B3P86 as well as two contemporary functionals (BMK and M05-2X) are applied. For a selection of species, the suitability of the DFT methods is validated through comparison with high-level G3(MP2)-RAD and SCS-ROMP2 results. The influence of the polyaromatic environment on the BDE results is thoroughly discussed. The results are compared with other hydrocarbon radical types in order to obtain a generalized radical stability scale. In order to complete this investigation, also carbon-carbon BDE values have been calculated, giving information about the influence of the local environment on removing the methyl group from the polyaromatic.

Bond dissociation enthalpies calculated by the PM3 method confirm activity cliffs in radical scavenging of flavonoids

Radical scavenging potency of flavonoids is associated with activity cliffs, i.e., small chemical modifications on flavonoid core can have a significant effect on activity. The presence or absence of the 3',4'-diOH and/or 3-OH group may serve as an activity switch for radical scavenging. The physicochemical background of such an indicator variable, defined previously (Amić et al. (2003) Croat Chem Acta 76:55-61), is confirmed by computation of bond dissociation enthalpies and selecting the minimal of all values relating to flavonoid OH groups. Bond dissociation enthalpies for hydrogen abstraction from OH groups for 29 flavonoids were calculated by the PM3 method. Minimal bond dissociation enthalpy values were obtained for OH groups attached to C-3, C-3' and C-4' positions, and they correspond to the previously introduced indicator variable. Taking into account some driving forces of the radical scavenging mechanism, it is possible to relate structural characteristics of flavonoids to their radical scavenging potency as well as to develop reliable structure-activity models.

Understanding the Toxicity of Phenols: Using Quantitative Structure−Activity Relationship and Enthalpy Changes To Discriminate between Possible Mechanisms

Experimental studies of the "extended toxicity" of substituted phenols are mainly of two types: the toxicity due to phenoxyl radical formation and the toxicity caused by metabolites, for example, the formation of quinones. Quantitative structure-activity relationship (QSAR) studies of phenol toxicity have dealt with the formation of phenoxyl radicals using bond dissociation enthalpy (BDE) of parent phenols, have obtained good correlations with experimental data, and have concluded that phenoxyl radicals are the toxic agent. However, the actual toxic mechanism has remained poorly defined. In this study, we follow the metabolic pathways of monosubstituted phenols to their quinone end products and calculate enthalpy changes for all relevant reactions. These enthalpy changes are first used as descriptors for a QSAR analysis. Many of these new descriptors, including some relevant to quinone formation, are highly correlated with the BDE values of the parent phenols. Therefore, a QSAR analysis by itself is inconclusive as to the mechanism of toxicity. To better define the problem, we have returned to a detailed analysis of net enthalpy changes. We show that the formation of phenoxyl radical is the rate-determining step: This step is slow for electron-withdrawing group substituted phenols (EWG-phenols), whereas it is fast for electron-donating group substituted phenols (EDG-phenols). The study of net enthalpy changes of reactions reveals that once the phenoxyl radical is present, the corresponding quinone is rapidly formed, so that quinone formation may be ultimately responsible for toxicity of EDG-phenols. We then demonstrate how the suggested mechanism (quinone formation) is successful in predicting the toxicity of some complex phenols, which are predicted poorly using the phenoxyl radical argument. We also discuss the toxicities of some estrogens in light of the quinone mechanism.

Using elementary reactions to model growth processes of polyaromatic hydrocarbons under pyrolysis conditions of light feedstocks

Density functional theory results are presented for elementary steps leading to coke growth within a steam cracking unit. The discussed pathway starts from toluene and ultimately, 1-methylnaphthalene is formed. In order to find the rate determining step for coke formation, the pseudo first-order rate coefficients of the various steps are compared taking into account the concentrations of diverse coke precursors. The influence of the polyaromatic environment is studied for a large set of methylated polycyclic aromatic molecules, by means of carbon–hydrogen bond dissociation enthalpy values. Subsequent hydrogen abstraction reactions at the methylated polyaromatics, by a methyl radical, are also examined. The abstraction is found to preferentially occur at the larger systems and is in general faster compared to abstractions at the analogous non-methylated species.

Effect of ortho-SR Groups on O−H Bond Strength and H-Atom Donating Ability of Phenols: A Possible Role for the Tyr-Cys Link in Galactose Oxidase Active Site?

Rotation about the Ar-S bond in ortho-(alkylthio)phenols strongly affects the bond dissociation enthalpy (BDE) and the reactivity of the OH group. Newly synthesized sulfur containing heterocycles 3 and 4, where the -SR group is almost coplanar with the phenolic ring, are characterized by unusually low BDE(O-H) values (79.6 and 79.2 kcal/mol, respectively) and by much higher reactivities toward peroxyl radicals than the ortho-methylthio derivative 1 (82.0 kcal/mol). The importance of the intramolecular hydrogen bond (IHB) in determining the BDE(O-H) was demonstrated by FT-IR experiments, which showed that in heterocycles 3 and 4 the IHB between the phenolic OH group and the S atom is much weaker than that present in 1. Since the IHB can be formed only if the -SR group adopts an out-of-plane geometry, this interaction is possible only in the methylthio derivative 1 and not in 3 and 4. The additive contribution to the phenolic BDE(O-H) of the -SR substituent therefore varies from -3.1 to +2.8 kcal/mol for the in-plane and out-of-plane conformations, respectively. These results may be relevant to understanding the role of the tyrosine-cysteine link in the active site of galactose oxidase, an important enzyme that catalyzes the two-electron aerobic oxidation of primary alcohols to aldehydes. The switching of the ortho -SR substituent between perpendicular and planar conformations may account for the catalytic efficiency of this enzyme.

Energetics of the Allyl Group

Aiming to improve our understanding of the stability of radicals containing the allylic moiety, carbon-hydrogen bond dissociation enthalpies (BDEs) in propene, isobutene, 1-butene, (E)-2-butene, 3-metylbut-1-ene, (E)-2-pentene, (E)-1,3-pentadiene, 1,4-pentadiene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene have been determined by quantum chemistry calculations. The BDEs in cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene have also been obtained by time-resolved photoacoustic calorimetry. The theoretical study involved a DFT method as well as ab initio complete basis-set approaches, including the composite CBS-Q and CBS-QB3 procedures, and basis-set extrapolated coupled-cluster calculations (CCSD(T)). By taking the C(sp3)-H BDE in propene as a reference, we have concluded that one methyl group bonded to C3 in propene (i.e., 1-butene) leads to a decrease of 12 kJ mol(-1) and that a second methyl group bonded to C3 (3-methylbut-1-ene) further decreases the BDE by 8 kJ mol(-1). When the methyl group is bonded to C2 in propene (isobutene), an increase of 7 kJ mol(-1) is observed. Finally, a methyl group bonded to C1 in propene (2-butene) has essentially no effect (-1 kJ mol(-1)). While this trend can be rationalized in terms of stabilization of the corresponding radical (through hyperconjugation and pi-delocalization), the BDE values observed for the dienes can only be understood by considering the thermodynamic stabilities of the parent compounds.

Kinetic and Thermochemical Study of the Antioxidant Activity of Sulfur‐Containing Analogues of Vitamin E

Sulfur-containing analogues of vitamin E (thiachromanols), either linked or not to a catechol moiety, were synthesized and their hydrogen-atom donating ability evaluated. The determination of the O--H bond dissociation enthalpy (BDE) of the alpha-tocopherol analogue 4 by the electron paramagnetic resonance (EPR) equilibration technique provided a value of 78.9 kcal mol(-1), that is, approximately 1.8 kcal mol(-1) higher than that of alpha-tocopherol. The kinetic rate constants for the reaction with peroxyl radicals (kinh), measured by inhibited autoxidation studies, showed that thiachromanols react 2.5 times slower than the corresponding tocopherols, in agreement with the higher BDE value. This behavior was explained, on the basis of crystallographic analyses and DFT calculations, in terms of a change in the molecular geometry caused by insertion of a sulfur atom into the framework of vitamin E. This behavior implies a greater deviation of the condensed ring from coplanarity with the aromatic ring, thus giving rise to a decrease in the conjugative stabilization of the phenoxyl radical and consequently to an increase in the O--H bond strength. Although less reactive than tocopherols, thiachromanols may, however, act as bimodal antioxidants as a result of the hydroperoxide decomposing ability of the sulfur atom.

Accurate bond dissociation enthalpies of popular antioxidants predicted by the ONIOM-G3B3 method

Radical-scavenging antioxidants play vital roles in the prevention of oxidative damage caused by free radicals, which is involved in many important chemical and biological processes. Using the ONIOM-G3B3 method, the bond dissociation enthalpies (BDEs) of coenzyme Q, flavonoids, olives, curcumins, indolinonic hydroxylamines, phenothiazines, edaravones and antioxidants used as food additives are predicted in the present study. On the basis of the computed BDE values, discussions were then made about their antioxidant activities, structure–activity relationships, and radical scavenging mechanisms. This work may be useful to clarify the radical scavenging mechanism of antioxidants and to design novel antioxidants.

Theoretical Insights, in the Liquid Phase, Into the Antioxidant Mechanism-Related Parameters in the 2-Monosubstituted Phenols

The paper describes a DFT/B3LYP study, in the liquid phase, [using the PCM continuum model] on the O-H bond dissociation enthalpy (BDE) and ionization energy (IE) parameter values of the 2-monosubstituted phenols (2-X-ArOH), related to the H-atom transfer (HAT) and single-electron transfer (SET) mechanisms. The solvent and substituent effects on the conformers, the BDEs, and the IEs were studied using four electron-donating (EDG) and five electron-withdrawing (EWG) groups, in seven different solvents. In both the EDG- and/or EWG-substituted species of the parent compounds, radicals, and/or cation radicals, the most stable conformer is varied, depending on the medium and the substitution. The EWG-substituents increase IEs, resulting in a weaker antioxidant activity than the EDG ones; the effect appears stronger on the IEs than on BDEs. However, although the liquid-phase IEs, which are related to solution-phase oxidation potentials, decrease with the polarity and/or the hydrogen-bonding ability of the solvent, the opposite holds true for the BDEs, exhibiting a weaker effect. The gas-phase-calculated IE for benzene is among the most accurate ones in the field, compared to the experiment, that for phenol being the most accurate. In addition, calculated IEs for the 2-X-ArOH are in close agreement with the very few existing experimental ones. It is shown that the oxidation potentials are (a) highly correlated with the gas-phase ones, and (b) strongly solvent dependent. The stabilization/destabilization of the cation radical (SPC) contribution, in all media, is the decisive factor in the DeltaIE calculation. The reasonable correlations found between the DeltaBDE and DeltaIE could account well for the assumption of the simultaneous action of both mechanisms in the 2-X-ArOH, in both the gas and the liquid phase. It seems, however, that the presence of a particular solvent by itself is not sufficient enough for the HAT to SET transition. The involvement of specific ED and/or EW groups in the 2-X-ArOH seems also necessary. It appears that our theoretical approach is not only generally applicable to the set of substituents important to antioxidant activity but also useful in (a) the rational design of phenolic antioxidants and (b) affording accurate BDE and IE parameter values related to both possible antioxidant mechanisms.

Electronic and Hydrogen Bonding Effects on the Chain-Breaking Activity of Sulfur-Containing Phenolic Antioxidants

A kinetic and thermodynamic investigation of phenols para-substituted with thiyl (SR), sulfinyl (SOR), and sulfonyl (SO(2)R) groups and ortho-substituted with thiyl groups is reported. The effect of the sulfur substituents on the O-H bond dissociation enthalpy values, BDE(O-H), was measured by means of the EPR radical equilibration technique and the reactivity toward peroxyl radicals, k(inh), of these phenolic antioxidants was determined by inhibited autoxidation studies. An inverse correlation between these two parameters was found. A p-SMe substituent decreased the BDE(O-H) value to a lesser extent than a p-OMe group (-3.6 vs -4.4 kcal/mol), whereas the effect of the same groups in an ortho position showed an opposite trend (-0.85 vs -0.2 kcal/mol). The latter result is explained in terms of the different strength of the intramolecular hydrogen bond between the OH proton and the sulfur or oxygen substituents in ortho derivatives. ESI-MS analysis of the products formed by reacting the sulfides with peroxyl radicals from the azoinitiator AIBN revealed the formation of a complex mixture of products, which may play an important role in determining the overall antioxidant activity of the parent compounds.

DFT/B3LYP study of O–H bond dissociation enthalpies of para and meta substituted phenols: Correlation with the phenolic C–O bond length

In this paper, the study of phenol and 30 compounds representing various para- and meta-substituted phenols is presented. Molecules and their radical structures were studied using DFT/B3LYP method in order to calculate the O–H bond dissociation enthalpies (BDEs). Calculated BDEs were compared with available experimental data. BDE values approximated from the total electronic energies are in very good agreement with data from solution measurements. Calculated gas-phase BDEs are lower than BDEs approximated from the total electronic energies about 26–30 kJ mol −1 and this difference does not depend on the substituent. DFT describes the effect of substituents on BDE satisfactorily, though ΔBDEs are in slightly narrower range than experimental values. Dependence of BDE values on Hammett constants of the substituents is linear. We found, that instead of phenolic O–H bond properties, BDE can be successfully correlated with the neighboring C–O bond length or with its shortening after hydrogen atom abstraction. In the case of substituents in para position, BDE linearly depends on the partial charge on phenolic group oxygen, too.

Synthesis and Antioxidant Profile of all-rac-α-Selenotocopherol

all-rac-alpha-Selenotocopherol (6c) has been synthesized in 11 steps in 6.6% total yield. Key steps include chloromethylation to approach the persubstituted aromatic 9b and cyclization of alcohol precursor 10 by radical homolytic substitution at selenium to form the selenotocopherol heterocycle. Determination of the OH bond dissociation enthalpy (BDE) of 6c by electron paramagnetic resonance (EPR) equilibration techniques gave a value of 78.1 +/- 0.3 kcal mol(-1), approximately 1 kcal mol(-1) higher than that of alpha-tocopherol. Kinetic studies performed by measuring oxygen uptake of the induced oxidation of styrene in the presence of an antioxidant showed that selenotocopherol (6c) was a slightly poorer inhibitor than alpha-tocopherol, in agreement with the BDE values. In contrast to simpler selenotocopherol analogues, 6c was not regenerable in the presence of a stoichiometric coreductant in a two-phase lipid peroxidation model.