Proton Affinity Values Research Articles

Antioxidant Activity of Quercetin and Its Glucosides from Propolis: A Theoretical Study

Among the multiple components of propolis, flavonoids contribute greatly to the antioxidant activities of propolis. Flavonoids mainly exist in the form of sugar-conjugated derivatives. Quercetin glycosides represent the predominant flavonoid fraction in propolis. In this work, density functional theory (DFT) calculations were applied to analyze the antioxidative properties of quercetin and its glucosides in the gas and in the liquid phase (ethanol, water). Three main antioxidant mechanisms, hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET) were used to analyze the antioxidative capacity of the investigated compounds. Solvent effects dominantly affect SET-PT and SPLET. Thus, the thermodynamically preferred mechanism can be altered. HAT and SPLET are the thermodynamically dominant mechanisms in gas and solvent phases, respectively. Therefore, in the gas phase, the sequence of the antioxidative capacity is similar with the bond dissociation enthalpy values: quercetin > quercetin-5-O-glucoside > quercetin-7-O-glucoside > quercetin-3-O-glucoside > quercetin-3′-O-glucoside > quercetin-4′-O-glucoside. While, in the solvent phases, the sequence is similar with the proton affinity values: quercetin-4′-O-glucoside > quercetin-5-O-glucoside > quercetin > quercetin-3-O-glucoside > quercetin-7-O-glucoside > quercetin-3′-O-glucoside. OH groups in B-ring and C-ring contribute mainly to the antioxidative activities of quercetin and glucosides compared with A-ring.

Electronic and Ligand Properties of Skeletally Substituted Cyclic (Alkyl)(Amino)Carbenes (CAACs) and Their Reactivity towards Small Molecule Activation: A Theoretical Study.

Quantum chemical calculations have been carried out to understand the ligand properties of skeletally modified cyclic alkyl amino carbenes. The stability of these carbenes has been assessed from an evaluation of their singlet-triplet and stabilization energy values. Ylide substituted carbenes are found to be more stable than non-ylidic ones in their optimized singlet state. Nucleophilicity and electrophilicity indices values were evaluated in order to further investigate the reactivity of these carbenes. The calculated values of proton affinities and the degree of gallium pyramidalization in the carbene-GaCl3 adducts correlate well with the σ-basicity of these carbenes. The reactivity of non-ylidic carbenes toward the activation of both H2 and NH3 are calculated to be more favourable compared to that of parent CAAC. On the other hand, ylide anchored carbenes are found to be unsuccessful toward the activation of both H2 and NH3 .

Revealing Germylene Compounds to Attain Superbasicity with Sigma Donor Substituents: A Density Functional Theory Study.

Compounds of GeII are shown for the first time to function as superbases. Two B(N=PiPr3 )2 groups attached to a germanium(II) center show a gas-phase proton affinity of 296.2 kcal mol-1 , which is close to the range of a hyperbase as revealed by B3LYP-D3/6-31G(2d,p) level of theory. These DFT calculations showed better agreement of geometrical parameters for the reported stable germylene compound 1 than previously reported calculations. A systematic study with different substitutions of GeII revealed that such a system can achieve basicity close to a hyperbase. The stabilities of these superbases were examined with dimerization energy and singlet-triplet state energy difference (ΔES-T ). Furthermore, the calculated gas-phase proton affinity values also show good correlation with the most negative valued point (Vmin ) in electron-rich regions from the molecular electrostatic potential. The high PA values of compounds were also supported by ionization potential, electron affinity, absolute electronegativity, and absolute hardness calculations. The energetics for the reaction with BH3 and AlMe3 further suggest that the lone pair of GeII can act as a Lewis base and display higher donor-acceptor bond strengths.

MOLECULAR COMPLEXES OF SULPHUR DIOXIDE WITH N,O-CONTAINING ORGANIC BASES (REVIEW)

The literature data on the synthesis, stoichiometry, structure and relative stability of molecular complexes of sulphur dioxide with N,O-containing organic bases have been systematized and generalized. It was shown that the yield of the reaction product of sulfur dioxide with organic bases (such as amines) are strongly influenced by the conditions of synthesis: the nature of the solvent (basicity, polarity), the temperature and SO2:L ratio in the reaction medium. The stoichiometry of SO2*nL molecular complexes depends on ligand denticity, as well as its ability to H-bonding. The reaction of the sulfur oxide (IV) with organic bases can give S←N and S←O complexes. With the increase of the value of base proton affinity the decrease ΔrSN values has been marked. Characteristic parameter Δr SN = r SN – a1(rS+ rN) (where rSNis the S←N donor-acceptor bond length) has been determined by microwave spectroscopy and X-ray analysis, rSand rNwere the tabulated values of the homopolar covalent radii of sulphur and nitrogen heteroatoms. The dependence of formation enthalpy of molecular complexes of basic amines and spectral characteristics has been noted; enthalpy-entropy compensation for S←N and S←O complex-es has been stated. Despite the limited experimental data on the thermodynamics of complex formation and the lengths of donor-acceptor bonds for the same compounds it has been found bond S←N strength in SO2 molecular complexes to depend on the intrinsic value of ΔrSN. The contribution of van der Waals forces and charge transfer forces to the formation of molecular complexes of sulphur dioxide has been stated.

Potent 1,2,4‐Triazole‐3‐thione Radical Scavengers Derived from Phenolic Acids: Synthesis, Electrochemistry, and Theoretical Study

AbstractNine 1,2,4‐triazole‐3‐thiones containing phenolic acid moiety have been synthesized and examined by scavenging of stable DPPH (2,2‐diphenyl‐1‐picrylhydrazyl) radical, measurement of reducing capacity, cyclic voltammetry experiments and density functional theory (DFT). The differences in DPPH‐radical scavenging activity of the compounds 4 a‐i are affected by the stability of the corresponding radicals or radical cations and possibility of delocalization of unpaired electron through benzene and triazole ring. Significantly, lower proton affinity (PA) values than bond dissociation enthalpy (BDE) indicate SPLET (sequential proton loss electron transfer) mechanism under these experimental conditions.

Experimental versus Calculated Proton Affinities for Aromatic Carboxylic Acid Anions and Related Phenide Ions.

Herein, we present the comparison of a large set of experimentally measured proton affinity (PA) values for 65 aromatic carboxylate anions with the values calculated by using selected popular DFT (B3LYP, PBE0, and M05-2X) and composite [G3(MP2), G4(MP2)] quantum chemistry methods. The root-mean-square error (RMSE) values for the chosen methods are RMSEPBE0 =1.7, RMSEB3LYP =4.6, RMSEM05-2X =6.6, RMSEG3MP2 =6.3, RMSEG4MP2 =4.5 kJ mol(-1) . In the second part of the study, 82 PA values for substituted phenide ions and a few heteroaromatic anions were calculated. Again, very good agreement between the calculated and experimental values has been observed: RMSEPBE0 =1.9, RMSEB3LYP =4.5, RMSEM05-2X =6.3, RMSEG3MP2 =4.9, RMSEG4MP2 =5.5 kJ mol(-1) . Our results show that, for medium-sized carboxylate anions, all tested methods give reliable results and, surprisingly, much more computationally demanding composite methods do not perform significantly better than the time-efficient DFT methods.

Gas-phase reactions of methyl thiocyanate with aliphatic carbanions - A mass spectrometry and computational study.

Methyl thiocyanate, like other organic thiocyanates, is a molecule with many electrophilic reactive sites and it has many synthetic applications. For better understanding of the intrinsic reactivity of alkyl thiocyanates against nucleophiles it was important to study gas-phase reactions of methyl thiocyanate with carbanions differing by structure and proton affinity values. All experiments were performed using a modified API 365 triple quadrupole mass spectrometer equipped with a TurboIonSpray electrospray ionization (ESI) source. Carbanions were generated in the ESI source by decarboxylation of the respective carboxylic acid anions. Methyl thiocyanate was delivered as a vapor with nitrogen used as a collision gas to the collision cell where the reactions take place. Mass spectra recorded for the gas-phase reactions of five aliphatic carbanions with methyl thiocyanate showed a variety of product ions formed via different reaction mechanisms, depending on the structure and proton affinity of the carbanion. The pathways considered are: SN 2 nucleophilic substitution, cyanophilic reaction, thiophilic reaction and proton transfer, followed in some instances by subsequent transformations. The proposed reaction pathways are supported by density functional theory (DFT) calculations. Our preliminary experiments showed that mass spectrometry together with quantum chemical calculations is a good tool for studying gas-phase reactions of alkyl thiocyanates with carbanions. In the gas phase all four theoretically possible products can be observed and their formation can be rationalized by the results of the modelling of the reaction energy profiles. Copyright © 2016 John Wiley & Sons, Ltd.

Intermolecular Reactions of a Foiled Carbene with Carbonyl Compounds: The Effects of Trishomocyclopropyl Stabilization.

endo-Tricyclo[3.2.1.0(2,4)]oct-8-ylidene is a foiled carbene reaction intermediate. It was generated by thermolyzing Δ(3)-1,3,4-oxadiazoline precursors dissolved in benzaldehyde and acetophenone. The products appear to stem from direct insertion of the carbene's divalent C atom into the α-bonds of the carbonyl compounds; however, this is only superficial. The strict stereochemistry observed is due to the topologies of the reaction intermediates of the proposed two-step mechanism. Bimolecular nucleophilic addition generates bent 1,3-zwitterions. The neutral reaction intermediates undergo pinacolic rearrangements to form the observed adducts. Product ratios reflect the migratory aptitudes of the carbonyl compounds' α-substituents. The carbene reaction was modeled using DFT. The singlet carbene's bicoordinate C atom bends 31° toward the endo-fused cyclopropane bond, elongating it to r = 1.69 Å. The resulting trishomocyclopropyl HOMO{-1} is a three-center two-electron bond responsible for the electron-deficient carbene's nucleophilicity. Its calculated properties are consistent with this assertion: (1) singlet-triplet (ΔE(S-T)) energy gap of -25 kcal/mol, (2) gas-phase proton affinity (PA) value of 272 kcal/mol, (3) hard and soft acid and base (HSAB) ΔN value of -0.2 in its initial reaction with the carbonyl compounds, and (4) negative frontier orbital interaction values ΔΔE(PhC(O)H) = -4.38 eV and ΔΔE(PhC(O)Me) = -3.97 eV.

Synthesis, Structure, and Reactivities of Iminosulfane- and Phosphane-Stabilized Carbones Exhibiting Four-Electron Donor Ability.

Iminosulfane(phosphane)carbon(0) derivatives (iSPCs; Ar3 P→C←SPh2 (NMe); Ar=Ph (1), 4-MeOC6 H4 (2), 4-(Me2 N)C6 H4 (3)) have been successfully synthesized and the molecular structure of 3 characterized. Carbone 3 is the first thermally and hydrolytically stable carbone stabilized by phosphorus and sulfur ligands. DFT calculations reveal the electronic structures of 1-3, which have two lone pairs of electrons at the carbon center. First and second proton affinity values are theoretically calculated to be in the range of 286.8-301.1 and 189.6-208.3 kcal mol(-1) , respectively. Cyclic voltammetry measurements reveal that the HOMO energy levels follow the order of 3>2>1 and the HOMO of 3 is at a higher energy than those of bis(chalcogenane)carbon(0) (BChCs). The reactivities of these lone pairs of electrons are demonstrated by the C-diaurated and C-proton-aurated complexes. These results are the first experimental evidence of phosphorus- and sulfur-stabilized carbones behaving as four-electron donors. In addition, the reaction of hydrochloric salts of the carbones with Ag2 O gives the corresponding Ag(I) complexes. The resulting silver(I) carbone complexes can be used as carbone transfer agents. This synthetic protocol can also be used for moisture-sensitive carbone species.

Can Nitriles Be Stronger Bases Than Proton Sponges in the Gas Phase? A Computational Analysis

DFT calculations have been performed for a series of push-pull nitriles [(R2N)n(X═Y)iC≡N, where i = 0, 1, or 2, n = 1, 2, or 3, R2N = H2N, Me2N, or C4H8N, X = CH, N, or P, Y = CH or N]. The possible protonation N-sites (N-cyano, N-imino, and N-amino) have been examined and their proton affinities (PA) estimated. For all compounds in the series, even for those containing the guanidino, phosphazeno, and diphosphazeno pushing groups, the N-cyano atom is the favored site of protonation. The n-π conjugation strongly decreases the PA value of the pushing amino group in favor of the pulling cyano one. Nitriles with the phosphazeno groups [(R2N)3P═N-P(R2N)2═N and (R2N)3P═N] exhibit the strongest basicity in the series. Some of them (with PA > 1000 kJ mol(-1)) are stronger bases than DMAN, the so-called "proton sponge". Nitriles bearing the guanidino group [(R2N)2C═N] are less basic than those with the phosphazeno group [(R2N)3P═N] but more basic than those with the formamidino group (R2N-CH═N) containing the same substituent R. The N-imino atoms, present in the transmitter group (X═N, X = CH, N, or P), display PA values lower than those of the N-cyano site by more than 30 kJ mol(-1). When proceeding from the unsubstituted derivatives (R = H) to the methylated ones (R = Me), the Me groups at the N-amino atom increase the PA value of the N-cyano site for Me2N-X═Y-C≡N (X, Y = CH or N) by ca. 30-60 kJ mol(-1). For the guanidino and phosphazeno derivatives containing two and three amino groups, respectively, this effect is not additive. The four Me groups for (Me2N)2C═N-C≡N and the six Me groups for (Me2N)3P═N-C≡N increase the PA(N-cyano) values by only 30-50 kJ mol(-1). The C≡N bond lengths of the neutral forms are well correlated with the PA(N-cyano) values.

Structure of Protonated Threonine Dimers in the Gas Phase: Salt-Bridged or Charge-Solvated?

For homodimers of amino acids, their salt-bridged structures are gradually stabilized as the proton affinity of the component amino acid increases. Threonine has a proton affinity value located in the middle of the list of 20 natural amino acids. Thus, identifying whether the most stable isomer of protonated threonine dimer (Thr(2)H(+)) has a charge-solvated or salt-bridged structure is important and helpful for understanding the structures of other homodimers. By combining infrared photodissociation (IRPD) spectroscopy and theoretical calculations, the structures of Thr(2)H(+) were investigated. Based on calculations at the M062X/6-311++G(d,p)//M062X/6-311++G(d,p) level, the most stable isomer of Thr(2)H(+) was computed to be a charge-solvated structure, with an energy 3.87 kcal/mol lower than the most stable salt-bridged isomer. The predicted infrared spectrum is in good agreement with the experimental spectrum. To evaluate the temperature effect on the distribution of different isomers, the relative concentrations of the six isomers of Thr(2)H(+) were calculated at different temperatures, according to their partition functions and enthalpies. The results show that the isomers are dominated by charge-solvated structures at a temperature of 300 K.

Electronic and Steric Influences of Pendant Amine Groups on the Protonation of Molybdenum Bis(dinitrogen) Complexes.

The synthesis of a series of P(Et)P(NRR(')) (P(Et)P(NRR(')) = Et2PCH2CH2P(CH2NRR')2, R = H, R' = Ph or 2,4-difluorophenyl; R = R' = Ph or (i)Pr) diphosphine ligands containing mono- and disubstituted pendant amine groups and the preparation of their corresponding molybdenum bis(dinitrogen) complexes trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) is described. In situ IR and multinuclear NMR spectroscopic studies monitoring the stepwise addition of triflic acid (HOTf) to trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) complexes in tetrahydrofuran at -40 °C show that the electronic and steric properties of the R and R' groups of the pendant amines influence whether the complexes are protonated at Mo, a pendant amine, a coordinated N2 ligand, or a combination of these sites. For example, complexes containing monoaryl-substituted pendant amines are protonated at Mo and the pendant amine site to generate mono- and dicationic Mo-H species. Protonation of the complex containing less basic diphenyl-substituted pendant amines exclusively generates a monocationic hydrazido (Mo(NNH2)) product, indicating preferential protonation of an N2 ligand. Addition of HOTf to the complex featuring more basic diisopropyl amines primarily produces a monocationic product protonated at a pendant amine site, as well as a trace amount of dicationic Mo(NNH2) product that is additionally protonated at a pendant amine site. In addition, trans-Mo(N2)2(PMePh2)2(depe) (depe = Et2PCH2CH2PEt2) was synthesized to serve as a counterpart lacking pendant amines. Treatment of this complex with HOTf generated a monocationic Mo(NNH2) product. Protonolysis experiments conducted on several complexes in this study afforded trace amounts of NH4(+). Computational analysis of trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) complexes provides further insight into the proton affinity values of the metal center, N2 ligand, and pendant amine sites to rationalize differences in their reactivity profiles.

Theoretical study of the energetics of carboxylic O–H bond cleavage in the para- and meta-substituted benzoic acid derivatives

Abstract Gas phase bond dissociation enthalpies (BDE) of para- and meta-substituted benzoic acids and proton affinities (PA) of their carboxylate anions were investigated using the B3LYP/6-311++G** method for 15 substituents with various electron-donating and electron-withdrawing effects. The employed computational method provided BDE and PA values in fair agreement with experimental data. The substituent effect on BDEs and PAs was analyzed in terms of Hammett constants. Found dependences exhibit satisfactory linearity and enable quick estimation of BDEs and PAs from the Hammett constants.

Comparative antioxidant potency and solvent polarity effects on HAT mechanisms of tocopherols

The antioxidant activity of the four forms of tocopherol has been investigated in gas phase and in different solvents using PBE0/6-31+G(d) level of theory. The three main working mechanisms, homolytic hydrogen atom transfer (HAT), single electron transfer-proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET) have been considered. The O–H bond dissociation free energy (BDFE), ionization potential (IP), proton dissociation free energy (PDFE), proton affinity (PA) and electron transfer free energy (ETFE) parameters have been calculated in gas phase and solvents. The theoretical results show the effect of the substituent position with respect to the phenolic O–H group on the reactivity. The methyl group, in ortho position, lends additional inductive and steric stability to the tocopheroxyl radical and therefore, increases antioxidant activity. α-tocopherol is shown to be the most reactive form in gas phase with the lowest BDFE, IP, and PA values, whereas in all solvents, γ-tocopherol has the lowest PA and PDFE values.The solvent effects are evaluated using an implicit solvation model (IEF-PCM). We have used a good model to calculate the solvation free energy of proton and electron in different media, where proton or electron was attached to one molecule of solvent and the obtained species were optimized using IEF-PCM approach in the same solvent. In gas phase, and non-polar solvents like benzene and toluene, BDFE value is lower than PA and IP, this suggested that HAT would be the most favorable mechanism for explaining the antioxidant activity of tocopherols, whereas SPLET mechanism is thermodynamically preferred in polar solvents like water, DMSO and methanol, where PA value of all forms of tocopherol is considerably lower than BDFE and IP.

Are Benzoic Acids Always More Acidic Than Phenols? The Case ofortho‐,meta‐, andpara‐Hydroxybenzoic Acids

AbstractTo address the title question, the relative intrinsic acidities of phenol and benzoic acid as well as the isomeric family ofortho‐,meta‐, andpara‐hydroxybenzoic acids were compared. Dissociation of the [PhCO2···H···OPh]–proton‐bound dimer showed slightly greater acidity for benzoic acid. Using traveling‐wave ion mobility mass spectrometry (TWIM‐MS) with CO2as the drift gas and post‐TWIM collision‐induced dissociation, the gaseous deprotonated molecules of the isomeric hydroxybenzoic acids were properly separated and characterized. For theparaisomer, an intrinsic gas‐phase acidity order inverse to that in solution was found, as before, that is, the phenol site ofpara‐hydroxybenzoic acid was found to be considerably more acidic than its benzoic acid site, whereas the opposite was observed in solution, regardless of the solvent. However, for theorthoandmetaisomers, the inversion in acidity order upon going from solution to the gas phase was not observed, and gaseous carboxylate anions were still found to predominate over phenoxide anions. Actually, for theorthoisomer, an even greater acidity as estimated from proton affinity values for the benzoic acid site relative to that of the phenol site was predicted, and accordingly, only a single isomer was sampled by TWIM‐MS. Rationales for these contrasting trends based on interfering inductive effects, charge delocalization by resonance, and intraionic H‐bonds that govern the acidity of benzoic acid relative to that of phenols are presented.

Generating and Trapping Metalla‐N‐Heterocyclic Carbenes

AbstractBy means of a combined experimental and theoretical approach, the electronic features and chemical behavior of metalla‐N‐heterocyclic carbenes (MNHCs, N‐heterocyclic carbenes containing a metal atom within the heterocyclic skeleton) have been established and compared with those of classical NHCs. MNHCs are strongly basic (proton affinity and pKa values around 290 kcal mol−1 and 36, respectively) with a narrow singlet–triplet gap (around 23 kcal mol−1). MNHCs can be generated from the corresponding metalla‐imidazolium salts and trapped by addition of transition‐metal complexes affording the corresponding heterodimetallic dicarbene derivatives, which can serve as carbene transfer agents.

Structure and relative stability of neutral, protonated and deprotonated thiazolidine-2-one-4-thione tautomers: DFT calculations

Relative stabilities of tautomers and rotamers of neutral, protonated and deprotonated thiazolidine-2-one-4-thione (TZD) were studied theoretically using the B3LYP/6-311++G(2df,2p)//6-311+G(d,p) method of the Density Functional Theory (DFT) in the gas phase and solution. Gas phase calculated relative energies showed good correlation with those obtained in solution. Proton affinities ( PA ), molecular basicities ( GB ) and deprotonation enthalpies (D H dep) of the title compound were calculated at the same level of theory. Moreover, the isomerization processes leading from the most stable adduct to other stable conformers were investigated. The results reveal that the thione–oxo tautomer is the most stable tautomer in all cases and TZD behaves as sulfur base. Consideration of the solvent causes some reordering of the relative stability of the tautomers and a drastic change in the values of PA , GB and D H dep.

Extremely supercharged proteins in mass spectrometry: profiling the pH of electrospray generated droplets, narrowing charge state distributions, and increasing ion fragmentation.

The effects of 12 acids, 4 solvents, and 8 low-volatility additives that increase analyte charging (i.e., superchargers) on the charge state distributions (CSDs) of protein ions in ESI-MS were investigated. We discovered that (i) relatively low concentrations [5% (v/v)] of 1,2-butylene carbonate (and 4-vinyl-1,3-dioxolan-2-one) can be added to ESI solutions to form higher charge states of cytochrome c and myoglobin ions than by using more traditional additives (e.g., propylene carbonate, sulfolane, or m-nitrobenzyl alcohol) under these conditions and (ii) the width of CSDs narrow as the effectiveness of superchargers increase, which concentrates protein ions into fewer detection channels. The use of strong acids (pKa values < 0) results in essentially no protein supercharging, higher adduction of acid molecules, and wider CSDs for many superchargers and proteins, whereas the use of weak acids (pKa > 0) results in significantly higher protein ion charging, less acid adduction, and narrower CSDs, indicating that protein ion supercharging in ESI can be significantly limited by the binding of conjugate base anions of acids that neutralize charge sites and broaden CSDs. The extent of protein charging as a function of acid identity (HA) does not strongly correlate with gas-phase proton transfer data (i.e., gas-phase basicity and proton affinity values for HA and A(-)), solution-phase protein secondary structures (as determined by circular dichroism spectroscopy), and/or acid molecule volatility data. For protein-denaturing solutions, these data were used to infer that the "effective" pH of ESI generated droplets near the moment of ion formation can be ∼0, which is ca. 1 to 3 pH units lower than the pH of the solutions prior to ESI. Electron capture dissociation (ECD) of [ubiquitin, 17H](17+) resulted in the identification of 223 cleavages, 74 of 75 inter-residue sites, and 92% ECD fragmentation efficiency, which correspond to highest of these values that have been obtained by ECD of a single isolated charge state of ubiquitin.

Cyclopropenimine as a hydrogen bond acceptor—towards the strongest non-phosphorus superbases

Utilizing dialkylamino cyclopropenimines as hydrogen bond acceptors in tri-substituted guanidines and cyclopropenimines, we computationally designed the most basic superbases possessing intramolecular hydrogen bonds (IHB-superbases) so far. The values of proton affinity in the gas phase range between 296.6 and 306 kcal mol−1, with estimated pKa values in acetonitrile between 35.5 and 39.7. The obtained PAs of five new IHB-superbases surpass the basicity of the paradigmatic P4-tBu Schwesinger phosphazene, whereas pKa values come close to that of P4-tBu. None of the designed superbases contain phosphorus, which puts them among few most basic non-phosphorus superbases desiged so far.

Thermodynamical aspect of radical scavenging activity of alizarin and alizarin red S. Theoretical comparative study

In this paper the conformational behavior and thermodynamics of radical scavenging action of alizarin (A) and alizarin red S (ARS) were investigated at the M06-2X/6-311++G(d,p) and M06-2X/cc-pVTZ levels of theory in the gas phase and water solution. For this purpose all BDE (bond dissociation enthalpy), IP (ionization potential), PDE (proton dissociation enthalpy), PA (proton affinity), and ETE (electron transfer enthalpy) values were calculated, implying that all radicals, anions, and radical cations issued from A and ARS were examined. Since it turned out that all thermodynamic parameters for A and ARS are mutually comparable, one can assume that the two compounds are of similar antioxidative capacity, implying that the influence of the sulfonate group is insignificant. It was shown that the hydroxyl group in position 2 of A and ARS is the preferential site for heterolytic and homolytic cleavage in both phases, as the negative charge and spin density in the formed anions and radicals are delocalized over the B ring. In spite of the fact that the IP values of A and ARS are significantly lower in water, SET-PT is not a favored mechanism of both molecules in either environment. On the other hand, HAT is the predominant reaction pathway of A and ARS in gas, whereas SPLET is the preferential mechanism in water.