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Showing papers on "Homolysis published in 2003"


Book
04 Apr 2003
TL;DR: In this article, the authors present a model for the formation of catalytic catalytic reactivity in the context of catalytical catalytic decomposition of organic compounds. But the model is not suitable for the analysis of the catalytic reaction in the presence of a cage effect.
Abstract: Preface. Symbols and Abbreviations. PART I: INITIATORS. Chapter 1. Mechanism of Decomposition of Initiators. 1.1 Introduction. 1.2 Nonconcerted Unimolecular Decomposition. 1.3 Concerted Fragmentation of Initiators. 1.4 Anchimerically Assisted Decomposition of Peroxides. 1.5 Decay of Initiators to Free Radicals and Molecular Products. 1.6 Chain Decomposition of Initiators. References. Chapter 2. Cage Effect. 2.1 Introduction. 2.2 Experimental Evidence for the Cage Effect. 2.3 Mechanistic Schemes of Cage Effect. 2.4 Cage Effect in Solid Polymers. References. Chapter 3. Methods of Study of Initiator Decomposition and Free Radical Generation. 3.1 Kinetic Decay of Initiator (KDI). 3.2 Kinetic Product Formation (KPF). 3.3 Acceptors of Free Radicals (AFR). 3.4 Kinetic Chain Initiated Reaction (KIR). 3.5 Chemiluminescence (CL) Method. References. Chapter 4. Dialkyl Peroxides and Hydroperoxides. 4.1 Dialkyl Peroxides. 4.2 Hydroperoxides and Peracids. References. Chapter 5. Diacyl Peroxides, Peroxy Esters, Polyatomic, and Organometallic Peroxides. 5.1 Diacyl Peroxides. 5.2 Peroxy Esters. 5.3 Decomposition of Polyatomic Peroxides. 5.4 Organometallic Peroxides. References. Chapter 6. Organic Polyoxides. 6.1 Dialkyl Trioxides. 6.2 Hydrotrioxides. 6.3 Tetroxides. References. Chapter 7. Azo Compounds. 7.1 Synthesis and Structure of Azo Compounds. 7.2 Thermochemistry of Azo Compounds. 7.3 Decomposition of Azo Compounds. References. Chapter 8. Compounds with Weak C-C, N-N, C-N and N-O Bonds. 8.1 Polyphenylhydrocarbons. 8.2 Substituted Hydrazines. 8.3 Alkoxyamines. 8.4 Nitro Compounds. 8.5 Nitrates and Nitrites. 8.6 Disulfides and Polysulfides. 8.7 Organometallic Compounds. References. PART II: BIMOLECULAR REACTIONS OF FREE RADICAL GENERATION. Chapter 9. Parabolic Model of Bimolecular Homolytic Reaction. 9.1 Introduction. 9.2 Principles for the Parabolic Model of Bimolecular Homolytic Reaction. 9.3 Parameters of Bimolecular Homolytic Reaction in the Parabolic Model. References. Chapter 10. Bimolecular and Trimolecular Reactions of Free Radical Generation by Dioxygen. 10.1 Reaction of Dioxygen with C-H Bonds of Organic Compounds. 10.2 Reaction of Dioxygen with the Double Bond of Olefins. 10.3 Trimolecular Reaction of Radical Initiation by Dioxygen. References. Chapter 11. Bimolecular Reactions of Free Radical Generation by Ozone. 11.1 Initiation of Radicals by Ozone Reactions. 11.2 Chain Reactions of Ozone Decomposition. References. Chapter 12. Bimolecular Reactions of Hydroperoxides with Free Radical Generation. 12.1 Bimolecular Decomposition of Hydroperoxides. 12.2 Bimolecular Reactions of Hydroperoxides with a pi-Bond to Olefins. 12.3 Bimolecular Reactions of Hydroperoxides with C-H, N-H, and O-H Bonds of Organic Compounds. 12.4 Acid Catalysis for Homolytic Reactions of Hydroperoxides. 12.5 Reaction of Peroxides with Amines. References. Chapter 13. Free Radical Generation by Olefins. 13.1 Reactions of Retrodisproportionation. 13.2 Chain Initiation in Thermal Radical Polymerization. References. Chapter 14. Free Radicals Generation by Haloid Molecules and Nitrogen Dioxide. 14.1 Reactions of Fluorine Compounds. 14.2 Reactions of Dichlorine and Other Chlorine Compounds. 14.3 Initiation by Nitrogen Dioxide. References. Chapter 15. Free Radical Generation by Reactions of Ions with Molecules. 15.1 Decomposition of Hydrogen Peroxide Catalyzed by Transition Metal Ions. 15.2 Catalysis by Ions and Complexes of Transition Metals in Liquid-Phase Oxidation of Organic Compounds. 15.3 Reactions of Free Radicals with Transition Metal Ions. 15.4 Oxidation of Transition Metal Ions by Dioxygen. 15.5 Oxidation of Organic Compounds by Transition Metal Ions. 15.6 Reduction of Peroxides by Radical Anions. References. PART III: REACTIONS OF FREE RADICALS. Chapter 16. Isomerization and Decomposition of Free Radicals. 16.1 Intramolecular Abstraction of Hydrogen Atom. 16.2 Cyclization of Free Radicals. 16.3 Decyclization of Cyclic Radicals. 16.4 Fragmentation of Free Radicals. References. Chapter 17. Free Radical Abstraction Reactions. 17.1 Classification of Radical Abstraction Reactions. 17.2 Enthalpy of Reaction. 17.3 Force Constants of Reacting Bonds. 17.4 Triplet Repulsion. 17.5 Electron Affinity of Atoms in Reaction Center. 17.6 Repulsion of Atoms Forming the Reaction Center. 17.7 Influence of pi-Bonds in the Vicinity of the Reaction Center. 17.8 Steric Effect. 17.9 Polar Effect in Radical Reactions. 17.10 Effect of Multidipole Interaction. 17.11 Solvating Effect. References. Chapter 18. Free Radical Reactions of Hydrogen Transfer and Substitution. 18.1 Reactions of Hydrogen Atom Transfer from a Free Radical to a Molecule. 18.2 Free Radical Substitution Reactions. 18.3 Reaction of Peroxides with Ketyl Radicals. References. Chapter 19. Free Radical Addition. 19.1 Enthalpy and Entropy of Free Radical Addition. 19.2 Empirical Correlation Equations. 19.3 Quantum Chemical Calculations for the Activation Energy. 19.4 Parabolic Model of Radical Addition. 19.5 Contribution of Enthalpy for an Addition Reaction to Its Activation Energy. 19.6 Force Constants of Reacting Bonds. 19.7 Triplet Repulsion in the Transition State of Addition Reactions. 19.8 Influence of Neighboring pi-Bonds on the Activation Energy of Radical Addition. 19.9 Role of the Radius of the Atom Bearing the Free Valence. 19.10 Interaction of Two Polar Groups. 19.11 Multidipole Interaction in Addition Reactions. 19.12 Steric Hindrance. 19.13 Addition of Alkyl Radicals to Dioxygen. References. Chapter 20. Recombination and Disproportionation of Free Radicals. 20.1 Alkyl Radicals. 20.2 Macroradicals. 20.3 Peroxyl Radicals. References. Index.

178 citations


Journal ArticleDOI
TL;DR: In this paper, the formation of active intermediates from the Fenton-like reagent (a mixture of iron(III) ions and hydrogen peroxide) in aqueous solution was investigated using static DFT calculations and Car−Parrinello molecular dynamics simulations.
Abstract: The formation of active intermediates from the Fenton-like reagent (a mixture of iron(III) ions and hydrogen peroxide) in aqueous solution has been investigated using static DFT calculations and Car−Parrinello molecular dynamics simulations. We show the spontaneous formation of the iron(III) hydroperoxo intermediate in a first step. The Fenton-like reaction thus proceeds very differently compared to Fenton's reagent (i.e., the FeII/H2O2 mixture), for which we have recently shown that the first step is the spontaneous O−O lysis of hydrogen peroxide when coordinated to iron(II) in water. For the second step in the reaction mechanism of the Fenton-like reagent, we compare the possibilities of homolysis and heterolysis of the O−O bond and the Fe−O bond of the produced [(H2O)5FeIIIOOH]2+ intermediate. We find that concomitant hydrolysis of the reacting species plays a crucial role and, taking this into account, that O−O homolysis ([(H2O)4(OH)FeIIIOOH]+ → [(H2O)4(OH)FeIVO]+ + OH•) in vacuo is a likely second st...

175 citations


Journal ArticleDOI
TL;DR: This review states that cytochrome P450 enzymes catalyze a number of oxidations in nature including the difficult hydroxylations of unactivated positions in an alkyl group, and computational work has suggested that iron-oxo can react through multiple spin states, a low-spin ensemble that reacts by insertion of oxygen, and a high- spin ensemble that react by hydrogen atom abstraction to give a radical.

157 citations


Journal ArticleDOI
TL;DR: Non-heme manganese and iron complexes with terminal hydroxo or oxo ligands are proposed to mediate the transfer of hydrogen atoms in metalloproteins and these complexes have similar primary and secondary coordination spheres, which are enforced by [H(3)1](3-).
Abstract: Non-heme manganese and iron complexes with terminal hydroxo or oxo ligands are proposed to mediate the transfer of hydrogen atoms in metalloproteins. To investigate this process in synthetic systems, the monomeric complexes [MIII/IIH31(OH)]-/2- and [MIIIH31(O)]2- have been prepared, where MIII/II = Mn and Fe and [H31]3- is the tripodal ligand, tris[(N‘-tert-butylureaylato)-N-ethyl)]aminato. These complexes have similar primary and secondary coordination spheres, which are enforced by [H31]3-. The homolytic bond dissociation energies (BDEsO-H) for the MIII/II−OH complexes were determined, using experimentally obtained values for the pKa(M−OH) and E1/2 measured in DMSO. This thermodynamic analysis gave BDEsO-H of 77(4) kcal/mol for [MnIIH31(O−H)]2- and 66(4) kcal/mol for [FeIIH31(O−H)]2-. For the MIII−OH complexes, [MnIIIH31(OH)]- and [FeIIIH31(OH)]-, BDEsO-H of 110(4) and 115(4) kcal/mol were obtained. These BDEsO-H were verified with reactivity studies with substrates having known X−H bond energies (X = C...

142 citations


Journal ArticleDOI
Xiao Qing Zhu1, Hai Rong Li1, Qian Li1, Teng Ai1, Jin Yong Lu1, Yuan Yang1, Jin-Pei Cheng1 
TL;DR: Heterolytic and homolytic bond dissociation energies of the C4-H bonds in ten NADH models and their radical cations in acetonitrile were evaluated by titration calorimetry and electrochemistry, suggesting that the hydride is released more easily than the corresponding hydrogen atom from BNAH and vice versa for AcrH(2), and that there are two almost equal possibilities for the hydraide and the hydrogen atom transfers from HEH.
Abstract: Heterolytic and homolytic bond dissociation energies of the C4-H bonds in ten NADH models (seven 1,4-dihydronicotinamide derivatives, two Hantzsch 1,4-dihydropyridine derivatives, and 9,10-dihydroacridine) and their radical cations in acetonitrile were evaluated by titration calorimetry and electrochemistry, according to the four thermodynamic cycles constructed from the reactions of the NADH models with N,N,N',N'-tetramethyl-p-phenylenediamine radical cation perchlorate in acetonitrile (note: C9-H bond rather than C4-H bond for 9,10-dihydroacridine; however, unless specified, the C9-H bond will be described as a C4-H bond for convenience). The results show that the energetic scales of the heterolytic and homolytic bond dissociation energies of the C4-H bonds cover ranges of 64.2-81.1 and 67.9-73.7 kcal mol(-1) for the neutral NADH models, respectively, and the energetic scales of the heterolytic and homolytic bond dissociation energies of the (C4-H)(.+) bonds cover ranges of 4.1-9.7 and 31.4-43.5 kcal mol(-1) for the radical cations of the NADH models, respectively. Detailed comparison of the two sets of C4-H bond dissociation energies in 1-benzyl-1,4-dihydronicotinamide (BNAH), Hantzsch 1,4-dihydropyridine (HEH), and 9,10-dihydroacridine (AcrH(2)) (as the three most typical NADH models) shows that for BNAH and AcrH(2), the heterolytic C4-H bond dissociation energies are smaller (by 3.62 kcal mol(-1)) and larger (by 7.4 kcal mol(-1)), respectively, than the corresponding homolytic C4-H bond dissociation energy. However, for HEH, the heterolytic C4-H bond dissociation energy (69.3 kcal mol(-1)) is very close to the corresponding homolytic C4-H bond dissociation energy (69.4 kcal mol(-1)). These results suggests that the hydride is released more easily than the corresponding hydrogen atom from BNAH and vice versa for AcrH(2), and that there are two almost equal possibilities for the hydride and the hydrogen atom transfers from HEH. Examination of the two sets of the (C4-H)(.+) bond dissociation energies shows that the homolytic (C4-H)(.+) bond dissociation energies are much larger than the corresponding heterolytic (C4-H)(.+) bond dissociation energies for the ten NADH models by 23.3-34.4 kcal mol(-1); this suggests that if the hydride transfer from the NADH models is initiated by a one-electron transfer, the proton transfer should be more likely to take place than the corresponding hydrogen atom transfer in the second step. In addition, some elusive structural information about the reaction intermediates of the NADH models was obtained by using Hammett-type linear free-energy analysis.

126 citations


Journal ArticleDOI
TL;DR: The anaerobic microbial and photochemical degradation pathways of 4,4'-dibromodiphenyl ether (BDE15) were examined and photochemically induced reductive debromination was found to occur via homolytic C-Br bond cleavage, with no evidence of C-O Bond cleavage or products arising from heterolytic bonded cleavage.

111 citations


Journal ArticleDOI
TL;DR: The highly hindered silyloxy-substituted TEMPO alkoxyamines turned out to be excellent mediator/initiators for the controlled acrylate polymerization known to date.
Abstract: The synthesis of new 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) styryl derivatives as mediators for the living free-radical polymerization is described. Two of the alpha-methyl groups at the 2- and 6-position of the parent TEMPO styryl alkoxyamine have been replaced by hydroxymethyl and silyloxymethyl groups. To further increase the steric hindrance around the alkoxyamine oxygen atom, the remaining two methyl groups have been substituted with larger ethyl groups. Styrene polymerizations using hydroxy-substituted TEMPO derivatives are fast, but are not well-controlled. As previously shown for other OH-substituted alkoxyamines, intramolecular H-bonding leads to an acceleration of the C-O bond homolysis and, hence, to an acceleration of the polymerization process. However, the OH groups also increase the alkoxyamine decomposition rate constant. The kinetics of the C-O bond homolysis have been determined using EPR spectroscopy. Decomposition studies have been conducted with the aid of 1H NMR spectroscopy. In contrast to the OH-substituted alkoxyamines, highly hindered silyloxy-substituted TEMPO alkoxyamines turned out to be excellent mediator/initiators for the controlled styrene polymerization. Polystyrene with M(n) of up to 80 000 g/mol and narrow polydispersities (PDI) has been prepared using the new alkoxyamines. Reactions have been conducted at 105 degrees C; however, even at 90 degrees C controlled but slow polymerizations can be achieved. Furthermore, and more importantly, poly(n-butyl acrylates) with narrow PDIs (<1.15) have been prepared at 105 degrees C with the new alkoxyamines. Controlled acrylate polymerization can be conducted at temperatures as low as 90 degrees C. The silylated alkoxyamines presented belong to the most efficient initiator/mediators for the controlled acrylate polymerization known to date. The effect of the addition of free nitroxide on the acrylate polymerization is discussed. Moreover, the synthesis of diblock copolymers with narrow PDIs is described.

106 citations


Journal ArticleDOI
TL;DR: In this article, the homolytic dissociation enthalpies of various bonds (C−H, N−H and O−H) have been computed by using five density functional methods (B3LYP, MPW1PW91, B3PW 91, B 3PW86, and MPW 1P86).
Abstract: The homolytic dissociation enthalpies of various bonds (C−H, N−H, O−H, S−H, X−H, C−C, C−N, C−O, C−S, and C−halogen) have been computed by using five density functional methods (B3LYP, MPW1PW91, B3PW91, B3P86, and MPW1P86). The quality of these methods is comprehensively evaluated on the basis of the available experimental bond dissociation enthalpies, and it is found that the MPW1P86 has the best agreement, while B3LYP performs the largest deviations. Large deviations also are found at the sophisticated CCSD(T) level of theory. The restricted open-shell method underestimates the radical stability.

103 citations


Journal ArticleDOI
TL;DR: Pauling's original electronegativity equation describes quite accurately homolytic bond dissociation enthalpies of common covalent bonds, including highly polar ones, with an average deviation of +/-1.5 kcal mol(-1) from literature values for 117 such bonds.
Abstract: Contrary to other recent reports, Pauling's original electronegativity equation, applied as Pauling specified, describes quite accurately homolytic bond dissociation enthalpies of common covalent bonds, including highly polar ones, with an average deviation of +/-1.5 kcal mol(-1) from literature values for 117 such bonds. Dissociation enthalpies are presented for more than 250 bonds, including 79 for which experimental values are not available. Some previous evaluations of accuracy gave misleadingly poor results by applying the equation to cases for which it was not derived and for which it should not reproduce experimental values. Properly interpreted, the results of the equation provide new and quantitative insights into many facets of chemistry such as radical stabilities, factors influencing reactivity in electrophilic aromatic substitutions, the magnitude of steric effects, conjugative stabilization in unsaturated systems, rotational barriers, molecular and electronic structure, and aspects of autoxidation. A new corollary of the original equation expands its applicability and provides a rationale for previously observed empirical correlations. The equation raises doubts about a new bonding theory. Hydrogen is unique in that its electronegativity is not constant.

100 citations


Journal ArticleDOI
TL;DR: The 5-pyrimidinol structure should serve as a useful template for the rational design of novel air-stable radical scavengers and chain-breaking antioxidants that are more effective than phenols.
Abstract: Six substituted 5-pyrimidinols were synthesized, and the thermochemistry and kinetics of their reactions with free radicals were studied and compared to those of equivalently substituted phenols. To assess their potential as hydrogen-atom donors to free radicals, we measured their O-H bond dissociation enthalpies (BDEs) using the radical equilibration electron paramagnetic resonance technique. This revealed that the O-H BDEs in 5-pyrimidinols are, on average, about 2.5 kcal mol(-1) higher than those in equivalently substituted phenols. The results are in good agreement with theoretical predictions, and confirm that substituent effects on the O-H BDE of 5-pyrimidinol are essentially the same as those on the Obond;H BDE in phenol. The kinetics of the reactions of these compounds with peroxyl radicals has been studied by their inhibition of the AIBN-initiated autoxidation of styrene, and with alkyl and alkoxyl radicals by competition kinetics. Despite their larger O-H BDEs, 5-pyrimidinols appear to transfer their phenolic hydrogen-atom to peroxyl radicals as quickly as equivalently substituted phenols, while their reactivity toward alkyl radicals far exceeds that of the corresponding phenols. We suggest that this rate enhancement, which is large in the case of alkyl radical reactions, small in the case of peroxyl radical reactions, and nonexistent in the case of alkoxyl radical reactions, is due to polar effects in the transition states of these atom-transfer reactions. This hypothesis is supported by additional experimental and theoretical results. Despite this higher reactivity of 5-pyrimidinols towards radicals compared to phenols, electrochemical measurements indicate that they are more stable to one-electron oxidation than equivalently substituted phenols. For example, the 5-pyrimidinol analogues of 2,4,6-trimethylphenol and butylated hydroxytoluene (BHT) were found to have oxidation potentials approximately 400 mV higher than their phenolic counterparts, but reacted roughly one order of magnitude faster with alkyl radicals and at about the same rate with peroxyl radicals. The 5-pyrimidinol structure should, therefore, serve as a useful template for the rational design of novel air-stable radical scavengers and chain-breaking antioxidants that are more effective than phenols.

91 citations



Journal ArticleDOI
TL;DR: The rate constants for alkoxyamines carrying the styryl (PhEt) group as leaving alkyl radical in terms of polar inductive/field (sigmaL) and steric (Es) effects of the nitroxide substituents are shown to increase with the increasing electron-donating capacities, the steric demand, and the intramolecular bonding capabilities of the substituent.
Abstract: Alkoxyamines and persistent nitroxyl radicals are important regulators of living radical polymerizations. Because polymerization times decrease with the increasing rate of the homolytic C−O bond cleavage between the polymer chain and the nitroxide moiety, the factors influencing the homolysis rate are of considerable interest. Here, we present an analysis of the cleavage rate constants for 28 alkoxyamines carrying the styryl (PhEt) group as leaving alkyl radical in terms of polar inductive/field (σL) and steric (Es) effects of the nitroxide substituents, using the Taft−Ingold equation, i.e., log(k/k0) = ρLσL + δEs. The rate constants are shown to increase with the increasing electron-donating capacities, the steric demand, and the intramolecular (hydrogen) bonding capabilities of the substituents. A good correlation, (R2 = 0.95, 23 data) log kd = −3.07σL − 0.88Es − 5.88, is obtained, which should facilitate the design of new nitroxyl radicals and alkoxyamine regulators.

Journal ArticleDOI
TL;DR: This work provides the first absolute rate constants for these reactions with model peptides and indicates that these reactions may be controlled by conformation and dynamic flexibility around the (alpha)C-H bonds.
Abstract: Thiyl radicals are important intermediates in biological oxidative stress and enzymatic reactions, for example, the ribonucleotide reductases. On the basis of the homolytic bond dissociation energies (BDEs) only, the αC−H bonds of peptides and proteins would present suitable targets for hydrogen abstraction by thiyl radicals. However, additional parameters such as polar and conformational effects may control such hydrogen-transfer processes. To evaluate the potential of thiyl radicals for hydrogen abstraction from αC−H bonds, we provide the first absolute rate constants for these reactions with model peptides. Thiyl radicals react with αC−H bonds with rate constants between 1.7 × 103 M-1 s-1 (N-acetylproline amide) and 4 × 105 M-1 s-1 (sarcosine anhydride). However, the correlation of rate constants with BDEs is poor. Rather, these reactions may be controlled by conformation and dynamic flexibility around the αC−H bonds.


Journal ArticleDOI
TL;DR: Observations resolve major disputes over experimental data existing in the literature; despite extensive investigation of these reactions, no verifiable experimental evidence has been advanced that contradicts the homolysis model.
Abstract: Peroxynitrite decay in weakly alkaline media occurs by two concurrent sets of pathways which are distinguished by their reaction products. One set leads to net isomerization to NO(3)(-) and the other set to net decomposition to O(2) plus NO(2)(-). At sufficiently high peroxynitrite concentrations, the decay half-time becomes concentration-independent and approaches a limiting value predicted by a mechanism in which reaction is initiated by unimolecular homolysis of the peroxo O-O bond, i.e., the following reaction: ONOOH --> (*)OH + (*)NO(2). This dynamical behavior excludes alternative postulated mechanisms that ascribe decomposition to bond rearrangement within bimolecular adducts. Nitrate and nitrite product distributions measured at very low peroxynitrite concentrations also correspond to predictions of the homolysis model, contrary to a recent report from another laboratory. Additionally, (1) the rate constant for the reaction ONOO(-) --> (*)NO + (*)O(2)(-), which is critical to the kinetic model, has been confirmed, (2) the apparent volume of activation for ONOOH decay (DeltaV() = 9.7 +/- 1.4 cm(3)/mol) has been shown to be independent of the concentration of added nitrite and identical to most other reported values, and (3) complex patterns of inhibition of O(2) formation by radical scavengers, which are impossible to rationalize by alternative proposed reaction schemes, are shown to be quantitatively in accord with the homolysis model. These observations resolve major disputes over experimental data existing in the literature; despite extensive investigation of these reactions, no verifiable experimental evidence has been advanced that contradicts the homolysis model.

Journal ArticleDOI
TL;DR: In this article, the heterolytic and homolytic C4-H bond dissociation energies of coenzyme couple NADH/NAD+ in aqueous solution were estimated according to the reaction of NADH with N,N,N',N'-tetramethyl-p-phenylenediamine radical cation perchlorate (TMPA*+) in the presence of N.
Abstract: The heterolytic and homolytic C4-H bond dissociation energies of NADH and its radical cation (NADH*+) in aqueous solution were estimated according to the reaction of NADH with N,N,N',N'-tetramethyl-p-phenylenediamine radical cation perchlorate (TMPA*+) in aqueous solution. The results show that the values of the heterolytic and homolytic C4-H bond dissociation energies of NADH in aqueous solution are 53.6 and 79.3 kcal/mol, respectively; the values of the heterolytic and homolytic C4-H bond dissociation energies of NADH*+*+ in aqueous solution are 5.1 and 36.3 kcal/mol, respectively, which, to our knowledge, is first reported. This energetic information disclosed in the present work should be believed to furnish hints to the understanding of the mechanisms for the redox interconversions of coenzyme couple NADH/NAD+ in vivo.

Journal ArticleDOI
TL;DR: Density functional calculations have been used to investigate C-C, C-N and C-O bond forming reactions via reductive elimination from Group 10 cis-M(PH3)2(CH3)(X) species and suggest that C(sp3)-N reductive Elimination should be feasible from Ni and Pd systems.
Abstract: Density functional calculations have been used to investigate C–C, C–N and C–O bond forming reactions via reductive elimination from Group 10 cis-M(PH3)2(CH3)(X) species (X=CH3, NH2, OH). Both direct reaction from the four-coordinate species and a three-coordinate mechanism involving initial PH3 loss have been considered. For the four-coordinate pathway the ease of reductive elimination to give M(PH3)2 and CH3–X follows the trend M=Pd Pd

Journal ArticleDOI
TL;DR: In this paper, the ability of conventional electron correlation (MP2 and QCISD) and density functional theory (B3LYP and B3P86) methods to provide accurate and reliable optimized structures, and homolytic S−N bond dissociation energies (BDEs), for a range of S-nitrosothiols (RSNOs) has been investigated.
Abstract: The ability of conventional electron correlation (MP2 and QCISD) and density functional theory (B3LYP and B3P86) methods to provide accurate and reliable optimized structures, and homolytic S−N bond dissociation energies (BDEs), for a range of S-nitrosothiols (RSNOs) has been investigated. It is found that, in general, for any given method the 6-311+G(2df,p) or larger basis set must be used to obtain reliable structures. With a suitably large basis set, the different methods generally give optimized structures in close agreement with each other. However, the B3LYP method consistently overestimates the RS−NO bond length. The trends observed are found to be due in part to the fact that the RS−NO bond does not possess considerable double-bond character as previously suggested, but rather is a long single S−N bond, with the −NO moiety possessing considerable multiple-bond character. The B3P86/6-311+G(2df,p) method consistently gives BDEs in best agreement with values obtained with higher accuracy methods, e.g...

Journal ArticleDOI
TL;DR: In this article, quantum chemical calculations at the DFT (BP86/TZP) and ab initio (CCSD(T)/III+) levels of the title compounds were reported.
Abstract: We report on quantum chemical calculations at the DFT (BP86/TZP) and ab initio (CCSD(T)/III+) levels of the title compounds. The geometries, vibrational spectra, heats of formation, and homolytic a...

Journal ArticleDOI
TL;DR: The conditions determining the efficiency and mode of dehalogenation have been defined and are significant for devising synthetic methods via photogenerated phenyl cations and for rationalizing the photodegradation of halogenated aromatic pollutants and the phototoxic effect of some fluorinated drugs.
Abstract: The photochemistry of 4-haloanilines and 4-halo-N,N-dimethylanilines has been studied in apolar, polar aprotic, and protic solvents. Photophysical and flash photolysis experiments show that the reaction proceeds in any case from the triplet state. It is rather unreactive in apolar media, the highest value being Phi = 0.05 for the iodoanilines in cyclohexane. Changing the solvent has little effect for iodoanilines and for the poorly reacting bromo analogue, while it leads to a variation of over 2 orders of magnitude in the quantum yield for the chloro and fluoro derivatives. The triplets have been characterized at the UB3LYP/6-31G(d) level of theory, evidencing a deformation and an elongation (except for C-F) of the C-X bond. Homolytic fragmentation is in every case endothermic, but calculations in acetonitrile solution show that heterolytic cleavage of C-Cl and C-Br is exothermic. Experimentally, the occurrence of heterolytic fragmentation has been monitored through selective trapping of the resulting phenyl cation by allyltrimethylsilane. Heterolytic dechlorination occurs efficiently in polar media (e.g., Phi = 0.77 in MeCN), while debromination remains ineffective due to the short lifetime of the triplet. Heterolytic defluorination is efficient only in protic solvents (Phi = 0.48 in MeOH), in accord with calculations showing that in the presence of an ancillary molecule of water fragmentation is exothermic due to the formation of the strong H-F bond. The energy profile for both homo- and heterolytic dissociation paths has been mapped along the reaction coordinates in the gas phase and in acetonitrile. The conditions determining the efficiency and mode of dehalogenation have been defined. This is significant for devising synthetic methods via photogenerated phenyl cations and for rationalizing the photodegradation of halogenated aromatic pollutants and the phototoxic effect of some fluorinated drugs.

Journal ArticleDOI
TL;DR: In this paper, the extent of diradical character of a recently reported "localized singlet Diradical that is indefinitely stable at room temperature" (R4P2B2R‘2) is assessed by electronic structure calculation of orbital occupation numbers compared to other well-studied diradicoid systems.
Abstract: The extent of diradical character of a recently reported “localized singlet diradical that is indefinitely stable at room temperature” (R4P2B2R‘2) is assessed by electronic structure calculation of orbital occupation numbers compared to other well-studied diradicaloid systems Our study shows that it has significantly less diradical character (and much more bonding character) than other typical organic diradicals How this molecule, R4P2B2R‘2, attains this stability (bonding character) despite the long (260 A) B−B distance is satisfactorily explained using a simplified model compound, H4P2B2H2, and frontier orbital mixing ideas Increasing bond length usually makes a molecule more diradicaloid For example, as the H−H bond stretches in a homolytic single-bond breaking process, H2 becomes more and more diradical-like, eventually becoming a pure diradical at complete separation A counter-example is presented in this paper in which the molecule with a longer B−B “bond” distance (260 A), H4P2B2H2, represen

Journal ArticleDOI
TL;DR: It is noted that for simple hydrocarbons which give localized carbanions upon deprotonation there is an apparent linear correlation between any two of the following three quantities: deltaH degrees (acid), BDE, and EA.
Abstract: The gas-phase acidity of 3,3-dimethylcyclopropene (1) has been measured by bracketing and equilibrium techniques. Consistent with simple hybridization arguments, our value (deltaH degrees (acid) = 382.7 +/- 1.3 kcal mol(-)(1)) is indistinguishable from that for methylacetylene (i.e., deltadeltaH degrees (acid)(1 - CH(3)Ctbd1;CH) = 1.6 +/- 2.5 kcal mol(-)(1)). The electron affinity of 3,3-dimethylcyclopropenyl radical (1r) was also determined (EA = 37.6 +/- 3.5 kcal mol(-)(1)), and these quantities were combined in a thermodynamic cycle to afford the homolytic C-H bond dissociation energy. To our surprise, the latter quantity (107 +/- 4 kcal mol(-)(1)) is the same as that for methane, which cannot be explained in terms of the s-character in the C-H bonds. An orbital explanation (delocalization) is proposed to account for the extra stability of 1r. All of the results are supplemented with G3 and B3LYP computations, and both approaches are in good accord with the experimental values. We also note that for simple hydrocarbons which give localized carbanions upon deprotonation there is an apparent linear correlation between any two of the following three quantities: deltaH degrees (acid), BDE, and EA. This observation could be of considerable value in many diverse areas of chemistry.

Journal ArticleDOI
TL;DR: Comparisons reveal the O-H bond in 1H to be among the strongest of any Mn-hydroxo complex measured thus far, andustrates the transition to two-electron one-proton pcet chemistry in the [Mn(4)O(4)](7+) core that is understood on the basis of free energy consideration.
Abstract: Synthesis, characterization, and reactions of the novel manganese-oxo cubane complex [Mn(4)O(4)(O(2)PPh(2))(6)](ClO(4)), 1+ (ClO(4)(-)), are described. Cation 1+ is composed of the [Mn(4)O(4)](7+) core surrounded by six bidentate phosphinate ligands. The proton-coupled electron transfer (pcet) reactions of phenothiazine (pzH), the cation radical (pzH(.+)(ClO(4)(-)), and the neutral pz* radical with 1+ are reported and compared to Mn(4)O(4)(O(2)PPh(2))(6) (1). Compound 1+ (ClO(4)(-)) reacts with excess pzH via four sequential reduction steps that transfer a total of five electrons and four protons to 1+. This reaction forms the doubly dehydrated manganese cluster Mn(4)O(2)(O(2)PPh(2))(6) (2) and two water molecules derived from the corner oxygen atoms. The first pcet step forms the novel complex Mn(4)O(3)(OH)(O(2)PPh(2))(6) (1H) and 1 equiv of the pz+ cation by net hydride transfer from pzH. Spectroscopic characterization of isolated 1H is reported. Reduction of 1 by pzH or a series of para-substituted phenols also produces 1H via net H atom transfer. A lower limit to the homolytic bond dissociation energy (BDE) (1H --> 1 + H) was estimated to be >94 kcal/mol using solution phase BDEs for pzH and para-substituted phenols. The heterolytic BDE was estimated for the hydride transfer reaction 1H --> 1+ + H(-) (BDE approximately 127 kcal/mol). These comparisons reveal the O-H bond in 1H to be among the strongest of any Mn-hydroxo complex measured thus far. In three successive H atom transfer steps, 1H abstracts three hydrogen atoms from three pzH molecules to form complex 2. Complex 2 is shown to be identical to the "pinned butterfly" cluster produced by the reaction of 1 with pzH (Ruettinger, W. F.; Dismukes, G. C. Inorg. Chem. 2000, 39, 1021-1027). The Mn oxidation states in 2 are formally Mn(4)(2II,2III), and no further reduction occurs in excess pzH. By contrast, outer-sphere electron-only reductants such as cobaltacene reduce both 1+ and 1 to the all Mn(II) oxidation level and cause cluster fragmentation. The reaction of pzH(.+) with 1+ produces 1H and the pz+ cation by net hydrogen atom transfer, and terminates at 1 equiv of pzH(.+) with no further reaction at excess. By contrast, pz* does not react with 1+ at all, indicating that reduction of 1+ by electron transfer to form pz+ does not occur without a proton (pcet to 1+ is thermodynamically required). Experimental free energy changes are shown to account for these pcet reactions and the absence of electron transfer for any of the phenothiazine series. Hydrogen atom abstraction from substrates by 1 versus hydride abstraction by 1(+ )()illustrates the transition to two-electron one-proton pcet chemistry in the [Mn(4)O(4)](7+) core that is understood on the basis of free energy consideration. This transition provides a concrete example of the predicted lowest-energy pathway for the oxidation of two water molecules to H(2)O(2) as an intermediate within the photosynthetic water-oxidizing enzyme (vs sequential one-electron/proton steps). The implications for the mechanism of photosynthetic water splitting are discussed.

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TL;DR: Hydride transfer to 1(+) provides a concrete example of two-electron pcet that is hypothesized for the OH bond cleavage step during catalysis of photosynthetic water oxidation.
Abstract: The kinetics of proton-coupled electron-transfer (pcet) reactions are reported for Mn4O4(O2PPh2)6, 1, and [Mn4O4(O2PPh2)6]+, 1+, with phenothiazine (pzH). Both pcet reactions form 1H, by H transfer to 1 and by hydride transfer to 1+. Surprisingly, the rate constants differ by only 25% despite large differences in the formal charges and driving force. The driving force is proportional to the difference in the bond-dissociation energies (BDE >94 kcal/mol for homolytic, 1H → H + 1, vs. ≈127 kcal/mol for heterolytic, 1H → H− + 1+, dissociation of the O—H bond in 1H). The enthalpy and entropy of activation for the homolytic reaction (ΔH‡ = −1.2 kcal/mol and ΔS‡ = −32 cal/mol⋅K; 25–6.7°C) reveal a low activation barrier and an appreciable entropic penalty in the transition state. The rate-limiting step exhibits no H/D kinetic isotope effect (kH/kD = 0.96) for the first H atom-transfer step and a small kinetic isotope effect (1.4) for the second step (1H + pzH → 1H2 + pz•). These lines of evidence indicate that formation of a reactive precursor complex before atom transfer is rate-limiting (conformational gating), and that little or no N—H bond cleavage occurs in the transition state. H-atom transfer from pzH to alkyl, alkoxyl, and peroxyl radicals reveals that BDEs are not a good predictor of the rates of this reaction. Hydride transfer to 1+ provides a concrete example of two-electron pcet that is hypothesized for the O—H bond cleavage step during catalysis of photosynthetic water oxidation.

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TL;DR: Kinetic-mechanistic studies show that the substrate reactions are bimolecular and occur through the use of two Rh(II) centers in the molecular unit of 1, consistent with a rate-limiting step of C-H bond homolysis through a linear transition state.
Abstract: Carbon−hydrogen bond cleavage reactions of CH3OH and CH4 by a dirhodium(II) diporphyrin complex with a m-xylyl tether (·Rh(m-xylyl)Rh·(1)) are reported. Kinetic-mechanistic studies show that the substrate reactions are bimolecular and occur through the use of two Rh(II) centers in the molecular unit of 1. Second-order rate constants (T = 296 K) for the reactions of 1 with methanol (k(CH3OH) = 1.45 × 10-2 M-1 s-1) and methane (k(CH4) = 0.105 M-1 s-1) show a clear kinetic preference for the methane activation process. The methanol and methane reactions with 1 have large kinetic isotope effects (k(CH3OH)/k(CD3OD) = 9.7 ± 0.8, k(CH4)/k(CD4) = 10.8 ± 1.0, T = 296 K), consistent with a rate-limiting step of C−H bond homolysis through a linear transition state. Activation parameters for reaction of 1 with methanol (ΔH⧧ = 15.6 ± 1.0 kcal mol-1; ΔS⧧ = −14 ± 5 cal K-1 mol-1) and methane (ΔH⧧ = 9.8 ± 0.5 kcal mol-1; ΔS⧧ = −30 ± 3 cal K-1 mol-1) are reported.

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TL;DR: Rho+ (BDE) for X-Z bond homolysis can be quantitatively predicted by using the change in deprotonation enthalpy from HOOC-C6H4-X-Z to HOOC
Abstract: In the study we tried to answer two questions. First, does X-Z homolytic bond dissociation energy (BDE) of Y-C6H4-X-Z obey the Hammett relationship? Second, if it does what factors determine the ma...

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TL;DR: Together, these results are consistent with homolysis becoming completely rate determining in the forward direction in the two mutants and points to the role of Y89 as a molecular wedge in accelerating Co-carbon bond cleavage.
Abstract: The contribution of the active-site residue, Y89, to the trillion-fold acceleration of Co-carbon bond homolysis rate in the methylmalonyl-CoA mutase-catalyzed reaction has been evaluated by site-directed mutagenesis Conversion of Y89 to phenylalanine or alanine results in a 10(3)-fold diminution of k(cat) and suppression of the overall kinetic isotope effect The spectrum of the enzyme under steady-state conditions reveals the presence of AdoCbl but no cob(II)alamin Together, these results are consistent with homolysis becoming completely rate determining in the forward direction in the two mutants and points to the role of Y89 as a molecular wedge in accelerating Co-carbon bond cleavage

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TL;DR: Investigation of effects produced by 26 various phenol and diphenol derivatives, including industrial and natural antioxidants (ionol, bis-phenol 2246, α-tocopherol), on final product yields of radiation-induced free-radical processes involving peroxyl, alkyl and α-hydroxyalkyl radicals has been performed.
Abstract: Investigation of effects produced by 26 various phenol and diphenol derivatives, including industrial and natural antioxidants (ionol, bis-phenol 2246, α-tocopherol), on final product yields of radiation-induced free-radical processes involving peroxyl, alkyl, α-hydroxyalkyl and α,β-dihydroxyalkyl radicals has been performed. Ionol and bis-phenol 2246 have been shown to be more effective than α-tocopherol or diphenol derivatives in suppressing hydrocarbon oxidation processes. At the same time, α-tocopherol and its water-soluble analogues, as well as diphenol-based substances, are more effective than phenol derivatives in regulating various homolytic processes involving carbon-centered radicals. This fact can be accounted for by taking into consideration the contribution to formation of the final product set and the respective yields made by semiquinone radicals and compounds with quinoid structure arising in the course of homolytic transformations in systems containing diphenol derivatives.

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TL;DR: Two new indene compounds were synthesized by the reaction of C9H6RLi with ClCH2SiMe2Cl, followed by treatment with lithium pyrrolidinide C4H8NLi.