Bond Dissociation Enthalpy Values Research Articles

Theoretical study of the substituent effect on the hydrogen atom transfer mechanism of the irigenin derivatives antioxidant action

In this paper, 21 substituents with various electron donating and electron withdrawing characters were placed in available positions of irigenin in order to study their effect on the O – H bond dissociation enthalpy (BDE) via DFT/B3LYP method. Results indicated the substituents in X3 and X4 positions have exerted stronger influence upon BDE values of irigenin derivatives when compared with same substituents in X1 and X2 positions. The results show that intramolecular hydrogen bond effects are able to considerably stabilize the parents and radicals. The natural bond orbital (NBO) analysis results also confirmed the intramolecular hydrogen bond stabilization. The formation of strong intramolecular hydrogen bonds in several radicals results in low BDEs. The 3- OH BDE values for substituents in X2 position have linear dependencies with Hammett constants (Fig. 2 and Eq. (2)). Found dependencies are suitably linear, that can be important for the synthesis of novel antioxidants based on irigenin.

Antioxidant activity, NMR, X-ray, ECD and UV/vis spectra of (+)-terrein: Experimental and theoretical approaches

Fungal metabolite terrein isolated from Aspergillus terreus is endowed with diverse biological and antioxidant activities. To determine the stereochemistry of the isolated terrein, we combined spectroscopic methods (CD and NMR spectra) and theoretical calculations (DFT and TD-DFT methods). Stereochemistry effects on the antioxidant activity of isolated terrein were evaluated by calculating bond dissociation enthalpies (BDEs), ionization potentials (IPs) and spin density delocalization of terrein and isoterrein stereoisomers with B3P86/6-31+G (d,p) method in gas and polarizable continuum model. The results showed a good agreement between experimental data and theoretical calculations which confirmed the (+)-terrein stereochemistry of isolated metabolite. Theoretical calculations showed that the antioxidant activity is relatively influenced by isomeric geometry of the terrein (a variation of 2kcal/mol between BDEs of terrein and isoterrein isomers), while chirality has no influence on the antioxidant activity [0.2kcal/mol difference between BDEs of (+)- and (–)-terrein]. The low antioxidant activity of (+)-terrein with respect to trolox and ascorbic acid was explained by the positive free Gibbs energy of the hydrogen atom transfer (HAT) mechanism and high BDE values of the 2-OH active site.

Characterization and density functional theory study of the antioxidant activity of quercetin and its sugar-containing analogues

Inhibition of free radicals using quercetin, hyperin and rutin is examined to determine their antioxidant effects and the structure–activity relationships of flavonoids. Two species of the free radicals are used, including hydroxyl radical (·OH) and superoxide anion radical (O2 −·). Density functional theory calculations under the level of B3LYP/6-311G (d) have been utilized to explore the structure, molecular properties and antioxidant abilities of the three flavonoids. Bond dissociation enthalpy (BDE) and frontier molecular orbital energy gap are investigated. They are compared with the experiment results assayed by the spectrophotometric. All of the flavonoids show a high activity on inhibiting ·OH and O2 −· radicals. Scavenging activity determined by half maximal inhibitory concentration (IC50) values of the three flavonoids decreases in the order: quercetin > hyperin > rutin. The calculations show that quercetin owns the lowest BDE values, which agree well with the experimental results of antioxidant activity determined by IC50 values.

A comparative study on the antioxidant properties of bractein and cernuoside by the DFT method

Density functional theory was employed to investigate the molecular properties of two flavonoids, bractein and cernuoside, that serve as antioxidants. The B3LYP/6-311G** protocol was used for all computations. Investigations were performed in the gas phase and in two solvents with different polarity (water and benzene); the present work was devoted mainly to the determination of the O–H bond dissociation enthalpies and the ionization potentials of the examined compounds, since these quantities represent the most important parameters on which biological activity can be rationalized. The rotational energy of pyrogallol and catechol moieties together with highest occupied molecular orbital–lowest unoccupied molecular orbital and dipole moment analysis are reported for the two flavonoids. The present analysis also includes the spin density distribution for the radicals formed after H atom removal on each OH site of both flavonoids. The theoretical bond dissociation enthalpy values for these systems follow the same trend in gas and solvent phases. On the basis of computed bond dissociation enthalpy and ionization potential values, the most reactive system that is able to transfer an H-atom and electron transfer mechanism is found to be bractein followed by cernuoside. All these results suggest bractein to be a potential antioxidant similar to quercetin. .

Quantum-chemical study of C— H bond dissociation enthalpies of various small non-aromatic organic molecules

Abstract In this work, C-H bond dissociation enthalpies (BDE) and vertical ionization potentials (IP) for various hydrocarbons and ketones were calculated using four density functional approaches. Calculated BDEs and IPs were correlated with experimental data. The linearity of the corresponding dependences can be considered very good. Comparing two used functionals, B3LYP C-H BDE values are closer to experimental results than PBE0 values for both used basis sets. The 6-31G* basis set employed with both functionals, gives the C-H BDEs closer to the experimental values than the 6-311++G** basis set. Using the obtained linear dependences BDEexp = f (BDEcalc), the experimental values of C-H BDEs for some structurally related compounds can be estimated solely from calculations. As a descriptor of the C-H BDE, the IPs and 13C NMR chemical shifts have been investigated using data obtained from the B3LYP/6-31G* calculations. There is a slight indication of linear correlation between IPs and C-H BDEs in the sets of simple alkanes and alkenes/ cycloalkenes. However, for cycloalkanes and aliphatic carbonyl compounds, no linear correlation was found. In the case of the 13C NMR chemical shifts, the correlation with C-H BDEs can be found for the sets of alkanes and cycloalkanes, but for the other studied molecules, no trends were detected.

THEORETICAL STUDY OF THE SUBSTITUENT EFFECTS ON THE REACTION ENTHALPIES OF THE ANTIOXIDANT MECHANISMS OF STOBADINE DERIVATIVES IN THE GAS-PHASE AND WATER

In this paper the reaction enthalpies of three antioxidant action mechanisms, HAT, SET–PT, and SPLET, for mono-substituted Stobadines were calculated in gas-phase and water. Results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP), and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In comparison to gas-phase, water attenuates the substituent effect on all reaction enthalpies. Results show that IP and BDE values can be successfully correlated with the indolic N–H bond length after electron abstraction, R(N–H+•), and the partial charge on the indolyl radical nitrogen atom, q( N ). Furthermore, calculated IP and PA values for mono-substituted Stobadines show linear dependence on the energy of the highest occupied molecular orbital (E HOMO ) of studied molecules in the two environments. SPLET represents the thermodynamically preferred mechanism in water.

Theoretical investigation of the conformational, electronic and antioxidant properties of azaleatin, isorhamnetin and quercetagetin

Gas phase and solvent phase reaction enthalpies related to the individual steps of three phenolic antioxidants in the view of action mechanisms – hydrogen atom transfer (HAT), single electron transfer–proton transfer (SET–PT) and sequential proton loss electron transfer (SPLET) – have been calculated using DFT/B3LYP/6-311++G** method. The analysis of the theoretical bond dissociation enthalpy (BDE) values for all OH sites of studied compounds clearly shows the importance of ring B, the presence of a catechol moiety in ring A and the 3-OH group in which the C2 = C3 double bond is present. The analysis of structure–acidity and structure–activity relationships for these flavonoids clearly indicates that the hydroxyl group at position 7 to be the most acidic site except for that of azaleatin. The analysis also includes the heat of formation, highest occupied molecular orbital and spin density distribution for the radicals formed after H-removal on each OH site. The calculated ionisation potential, BDE and proton dissociation enthalpy values suggest that one-step H-atom transfer, rather than SPLET or SET–PT, would be the most favourable mechanism for explaining the antioxidant activity of considered molecules in the gas phase. From the thermodynamic point of view, SPLET mechanism represents the most probable process in water and HAT mechanism in represents the most probable process benzene. On the basis of computed reaction enthalpies, the most active system able to transfer a hydrogen atom seems to be the quercetagetin followed by azaleatin and isorhamnetin.

Charge delocalization in Z-isomers of (4α → 6″, 2α → O → 1″)-phenylflavans with R = H, OH and OCH3. Effects on bond dissociation enthalpies and ionization potentials

The (2→O→1″)-bridged 4-phenylflavans comprise an interesting structure, included in natural antioxidants such as simple and dimeric A-type proanthocyanidins, catechins and condensed tannins. This work concerns the analysis of the stereoelectronic effects induced by substitution with R=H, OH and OCH3 in Z-isomers of (4α→6″, 2α→O→1″)-phenylflavans using density functional methods in order to deepen the understanding of the molecular and structural properties of these compounds. A fully relaxed scan procedure was performed. A topological study of the molecular charge density (Bader theory, Atoms in Molecules) and a Natural Bond Orbital (NBO) analysis at the B3LYP/6-311++G** level were carried out. The stereochemistry of the molecules was discussed in detail focusing on the factors related to their antioxidant properties. Bond dissociation enthalpies (BDEs), ionization potentials (IPs) and electron affinities (EAs) were calculated for the lowest energy conformers. The Nuclear Magnetic Resonance (NMR) chemical shifts were also calculated at the B3LYP/6-31G** level, and compared with the earlier reported experimental values, showing that the thermodynamically most stable conformer is also the most stable kinetically. The effects of substituents on chemical shifts were quantified. Through a donor acceptor map a qualitative comparison among the studied compounds is given. The lower (higher) BDE (IP) values found for R=OH (4α→6″, 2α→O→1″)-phenylflavans, were explained herein by specific mechanisms of charge delocalization. These findings highlight the key role played by hyperconjugative interactions in the stereoelectronic effects induced by substitution as an important factor in understanding the associated values of BDEs and IPs.

Computation of the bond dissociation enthalpies and free energies of hydroxylic antioxidants using the ab initio Hartree–Fock method

IntroductionA new method for calculating theoretical bond dissociation enthalpy (BDE) and bond dissociation free energy (BDFE) of hydroxylic antioxidants is forwarded. BDE and BDFE may be understood as activation energies accompanying the formation of transition states, which may undergo downhill homolytic dissociation. The new method does not involve the complete fission of O–H bonds.MethodTheoretical gas phase BDE values were calculated with the ab initio unrestricted Hartree–Fock (UHF) method, as changes in enthalpy between ground singlet states (GS) and triplet dissociative states (DS). Similarly, gas phase BDFEs were estimated from the corresponding changes in Gibbs free energy. The results were then compared with reliable experimental reports.ResultsThe proposed theoretical approach of BDE and BDFE determination was tested using 10 simple phenols, 5 flavonoids, and l-ascorbic acid derivatives. The agreement between our calculated gas phase results and the adopted experimental values were generally within 0.5 kcal mol−1, with a very few exceptions.DiscussionGenerally, steric interactions as well as intramolecular hydrogen bonding involving the dissociating OH group should be minimized in the GS. The DS are both electronically and vibrationally exited transition states. They have one unpaired electron on the carbon atom, which bears the homolytically dissociating OH group and are second order saddle points with a fixed <C–O–H bond angel of 180°.ConclusionIt was concluded that ab initio UHF was well suited for the estimation of gas phase BDE and BDFE. The method presented has a good potential for application across a range of hydroxylic antioxidants. Currently, work is underway to extend its application in other class of antioxidants.

S˭O homolytic bond dissociation enthalpies in sulfones: high-level ab initio and DFT study

The S˭O bond dissociation enthalpies (BDE) of sulfone derivatives were calculated using high-level ab initio methods including G3, G3B3, CBS-Q, CBS-4M and MP2. Based on the comparison of these theoretical values and experimental ones, the performances of a number of density functional theory methods were then assessed. It was found that the KMLYP method gave the lowest root mean square error. We, therefore, used this method to calculate the S˭ O BDEs of a number of substituted sulfones. The electronic effect of the substituents and the remote substituent effect of aryl-substituted sulfones on S˭ O BDEs were investigated. In addition, a molecular orbital analysis of typical molecules was conducted in order to investigate the electronic effect on S˭ O BDEs. We also predicted several S˭ O BDE values of heteroaromatic-substituted sulfones using the KMLYP method.

Computation of the bond dissociation enthalpies and free energies of hydroxylic antioxidants using the ab initio Hartree-Fock method

Introduction A new method for calculating theoretical bond dissociation enthalpy (BDE) and bond dissociation free energy (BDFE) of hydroxylic antioxidants is forwarded. BDE and BDFE may be understood as activation energies accompanying the formation of transition states, which may undergo downhill homolytic dissociation. The new method does not involve the complete fission of O–H bonds. Method Theoretical gas phase BDE values were calculated with the ab initio unrestricted Hartree–Fock (UHF) method, as changes in enthalpy between ground singlet states (GS) and triplet dissociative states (DS). Similarly, gas phase BDFEs were estimated from the corresponding changes in Gibbs free energy. The results were then compared with reliable experimental reports. Results The proposed theoretical approach of BDE and BDFE determination was tested using 10 simple phenols, 5 flavonoids, and l-ascorbic acid derivatives. The agreement between our calculated gas phase results and the adopted experimental values were genera...

DFT/B3LYP study of the substituent effects on the reaction enthalpies of the antioxidant mechanisms of Indole-3-Carbinol derivatives in the gas-phase and water

In this paper the reaction enthalpies of three antioxidant action mechanisms, HAT, SET-PT and SPLET, for mono-substituted Indole-3-Carbinols were calculated in gas-phase and water. Results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP) and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In comparison to gas-phase, water attenuates the substituent effect on all reaction enthalpies. Results show that IP and BDE values can be successfully correlated with the indolic NH bond length after electron abstraction, R(NH+), and the partial charge on the indolyl radical nitrogen atom, q(N). Furthermore, calculated IP and PA values for mono-substituted Indole-3-Carbinols show linear dependence on the energy of the highest occupied molecular orbital (EHOMO) of studied molecules in the two environments. SPLET represents the thermodynamically preferred mechanism in water.

S=O homolytic bond dissociation enthalpies in sulfoxides

The S=O bond dissociation enthalpies (BDE) were calculated using high-level ab initio methods including G3, G3B3, CBS-Q, CBS-4M, CCSD(T), and MP2. Based on the comparison of these theoretical values and experimental ones, the performances of a number of density functional theory (DFT) methods were then assessed. It was found that the B3P86 method gave the lowest root of mean square error. We therefore used this method to calculate the S=O BDEs of a number of substituted sulfoxides. The electronic effect of the substituents and the remote substituents effect of aryl-substituted sulfoxides on S=O BDE were investigated. In addition, a molecular orbital analysis of typical molecules was conducted in order to investigate the electronic effect on S=O BDEs. We also predicted several S=O BDE values of heteroaromatic substituted sulfoxides using the B3P86 method.

Oxidation of sterols: Energetics of C–H and O–H bond cleavage

Abstract Phytosterols, as components of human diet, received much attention because of their cholesterol-lowering and antioxidant properties. We have theoretically studied sterols oxidation in terms of O–H and C–H bond dissociation enthalpies (BDE). In 17 Δ 5 - and Δ 7 -sterols, BDEs were obtained for reported sites of oxidation attack. Obtained results indicate that Δ 7 -sterols are more susceptible to oxidation attack in comparison to Δ 5 -sterols. In sterol nuclei, the lowest BDE was found for C7–H bond in Δ 5 -sterols and for C14–H in Δ 7 -sterols. When Δ 5 -sterol has a C C double bond in the side chain, the lowest BDEs are usually found for C–H bonds in α-positions to this bond. The homolytic cleavage of hydroxyl O–H bond requires larger energy in comparison to the studied C–H bonds. We have shown that the C–H bonds with lowest BDE values actually correspond to the dominant sites of oxidation attack.

A DFT study on the structural, electronic properties and radical scavenging mechanisms of calycosin, glycitein, pratensein and prunetin

Antioxidative properties of naturally occurring isoflavonoids calycosin, glycitein, pratensein, prunetin and their radicals were investigated through density functional theory (DFT) method employing B3LYP/6-311G(d,p) and B3LYP/6-311+G(d,p) methods.The reaction enthalpies related to the individual steps of three phenolic antioxidant action mechanisms–hydrogen atom transfer (HAT), single-electron transfer–proton transfer (SET–PT) and sequential proton loss electron transfer (SPLET) have been computed. The bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), electron transfer enthalpy (ETE) were determined in gas phase and in two solvents. The relative energies of each OH groups, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and spin density distribution for radicals were computed and discussed. From the molecular structure and thermodynamic point of view, it was found that B-ring of isoflavonoids to be the active center and the hydrogen atom transfer (HAT) appeared as a major mechanism which contributes much in antioxidant action in gas phase. The SPLET mechanism will be the thermodynamically preferred pathway in polar solvents. On the basis of the computed IP, BDE and PA values most active system was found to be pratensein followed by calycosin, glycitein and prunetin. The analysis of structure–activity relationships for four isoflavonoids clearly indicates that hydroxyl group at position 7 to be the most acidic site.

A theoretical study of the structure–radical scavenging activity of hydroxychalcones

The molecular structure and radical scavenging activity of six new synthesized hydroxychalcones have been explored by using density functional theory (DFT) with the B3LYP exchange correlation functional. The minimum energy conformations were obtained from the energy scan, then a further geometry optimization was performed at the B3LYP level with 6-31+G∗∗ basis set. For radicals and cations, the geometry optimizations and frequency calculations were also done at the UB3LYP/6-31+G∗∗ level. The homolytic O–H bond dissociation enthalpy (BDE) and the adiabatic ionization potential (IP) were determined both in gas phase and in solvents using PCM model. The geometry structure, radical, electron character and the frontier molecular orbital were analyzed to explore the key factors that influence the radical scavenging activity of the hydroxychalcones. Based on BDE and IP values, it was revealed that compound 3 is expected to be more efficient hydrogen atom and electron donors than others, B-ring of hydroxychalcones is the active center and the hydrogen atom transfer (HAT) appears as a major mechanism in antioxidant action. The calculated results are in good agreement with experimental values.

On the enthalpies of homolytic Se–H bond cleavage in para-substituted benzeneselenols

In this work, the substituent effect on the Se—H bond dissociation enthalpy (BDE) for benzenese- lenol and ten para-substituted benzeneselenols was investigated. The set of various electron-donating and electron-withdrawing substituents was used. The gas-phase bond dissociation enthalpies were calculated using B3LYP/6-311++G ** method. Obtained trends were compared with those found for para-substituted phenols and thiophenols for the same set of substituents. While the BDE = f(sp) dependences for phenols and thiophenols exhibit very good linearity, for benzeneselenols, the linearity is rather insufficient. It was found for oxygen, sulphur and selenium that the larger the atom is, the weaker the substituent induced changes in corresponding BDE values are. It has been also observed that the larger the atom, the smaller corresponding BDEs.

Analysis of the Antioxidant Capacities of Flavonoids under Different Spectrophotometric Assays Using Cyclic Voltammetry and Density Functional Theory

Flavonoids often show inconsistent antioxidant activities (AAs) depending on the assay used. The electrochemical properties of 14 flavonoid standards in cyclic voltammetry [area under anodic wave (Q) and oxidant peak potentials (Epa)] and the structural parameters [bond dissociation enthalpy (BDE) and ionization potential (IP)] were investigated. They were compared with the results of four spectrophotometric assays, namely, diphenyl-1-picrylhydrazyl (DPPH), Folin-Ciocalteu reagent (FCR), ferric reducing ability of plasma (FRAP), Trolox equivalent antioxidant capacity (TEAC), to analyze the chemical reasons for the varying AAs of flavonoids under different assays. Using the cyclic voltammetry method, the AAs of the flavonoids in the DPPH, FCR, and FRAP assays were mainly determined by the ease of charge transferring in the first oxidation step. Meanwhile, the results of TEAC assays were primarily influenced by the amount of charge transfer in the multiple oxidation steps (MOS) of flavonoids. In the theoretical calculation, the BDE values of the selected flavonoids had considerably higher correlations with the results of the DPPH assay (r2=0.89) compared with the other three assays, which indicates that the oxidant-scavenging reaction of the tested flavonoids in the DPPH assay is closer to a hydrogen atom transfer mechanism. Neither the IP values nor BDE values had satisfactory correlation with the AAs of the flavonoids in the TEAC assay (r2=0.57, r2=0.54, respectively), Therefore, complex reaction mechanisms underlie this method and appropriate structural descriptors for reflecting the AAs of flavonoids based on MOS (e.g., TEAC values) need further investigation.

DFT/B3LYP study of the substituent effects on O [sbnd]H bond dissociation enthalpies of chroman derivatives in the gas phase and solvent environment

In this paper, the study of various ortho and meta-substituted chroman-6-ol derivatives is presented. The O H bond dissociation enthalpies (BDE) related to hydrogen atom transfer (HAT) mechanism of the phenolic antioxidants action, for studied molecules were calculated using DFT/B3LYP method. Calculated BDEs for meta-substituted molecules were correlated with Hammett constants and calculated results indicate that dependence of BDE on Hammett constants of the substituents is linear. Comparison of BDEs of the studied molecules with non-substituted chroman-6-ol indicates that electron-withdrawing groups increase BDEs. On the contrary, electron-donating groups reduce BDE values. Furthermore, results for meta-substituted molecules show that calculated BDEs can be correlated with the lengths of phenolic C O bond of the studied molecules successfully. For meta-substituted molecules, the computed results also show that BDEs linearly depend on the partial charge on the phenoxy radical oxygen q(O). In comparison to the gas phase, the effect of studied substituents on the BDEs for meta position was increased in water phase, while in the two ortho positions it was decreased in water phase.

Mechanistic study of the structure–activity relationship for the free radical scavenging activity of baicalein

Density functional theory calculations were performed to evaluate the antioxidant activity of baicalein. The conformational behaviors of both the isolated and the aqueous-solvated species (simulated with the conductor-like polarizable continuum solvation model) were analyzed at the M052X/6-311 + G(d,p) level. The most stable tautomers of various forms of baicalein displayed three IHBs between O4 and OH5, O5 and OH6, and O6 and OH7. The most stable tautomer of the baicalein radical was obtained by dehydrogenating the hydroxyl at C6, while the most stable anion tautomer was obtained by deprotonating the C7 hydroxyl in gaseous and aqueous phases. The expected antioxidant activity of baicalein was explained by its ionization potentials (IPs) and homolytic O-H bond dissociation enthalpies (BDEs), which were obtained via the UM052X optimization level of the corresponding radical species. Heterolytic O-H bond cleavages (proton dissociation enthalpies, PDEs) were also computed. The calculated IP, BDE, and PDE values suggested that one-step H-atom transfer, rather than sequential proton loss-electron transfer or electron transfer-proton transfer, would be the most favorable mechanism for explaining the antioxidant activity of baicalein in the gas phase and in nonpolar solvents. In aqueous solution, the SPLET mechanism was more important.