Theoretical investigations on hydrogen peroxide decomposition in aquo (original) (raw)
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Catalytic Oxidation of Water with High-Spin Iron(IV)−Oxo Species: Role of the Water Solvent
We use density functional theory (DFT) and ab initio molecular dynamics to study the conversion of H 2 O into H 2 O 2 in water solution by the Fe IV O 2+ group under room-temperature and-pressure conditions. We compute the free energy of formation of an O(water)−O(oxo) bond using thermodynamic integration with explicit solvent and we examine the subsequent generation of H 2 O 2 by proton transfer. We show that the O−O bond formation follows the standard reactivity pattern observed in hydroxylation reactions catalyzed by high-spin (S = 2) iron(IV)− oxo species, which is initiated by the transfer of one electron from the highest occupied molecular orbital of the moiety attacking the Fe IV O 2+ group, either a −C−H bonding orbital (hydroxylation) or a lone pair of a water molecule (water oxidation). The highly electrophilic character exhibited by the Fe IV O 2+ ion, which is related to the presence of an acceptor 3σ* orbital at low energy with a large contribution on the O end of the Fe IV O 2+ ion, is the crucial factor promoting the electron transfer. The electron transfer occurs at an O(water)− O(oxo) distance of ca. 1.6 Å, and the free energy required to favorably orient a solvent H 2 O molecule for the O(oxo) attack and to bring it to the transition state amounts to only 35 kJ mol −1. The ensuing exoergonic O−O bond formation is accompanied by the progressive weakening of one of the O−H bonds of the attacking H 2 O assisted by a second solvent molecule and leads to the formation of an incipient Fe 2+ −[O−O−H] − [H 3 O + ] group. Simultaneously, three additional solvent molecules correlate their motion and form a hydrogen-bonded string, which closes to form a loop within 5 ps. The migration of the H + ion in this loop via a Grotthuss mechanism leads to the eventual protonation of the [O−O−H] − moiety, its progressive removal from the Fe 2+ coordination sphere, and the formation of free H 2 O 2 in solution.
Catalytic decomposition of hydrogen peroxide in alkaline solutions
Electrochemistry Communications, 1999
This paper describes a kinetic model for the decomposition of hydrogen peroxide by ferric ion in homogeneous aqueous solution (pH < 3). The reaction was investigated experimentally at 25.0°C and I ) 0.1 M (HClO 4 /NaClO 4 ), in a completely mixed batch reactor and under a wide range of experimental conditions (1 e pH e 3; 0.2 mM e [H 2 O 2 ] 0 e 1 M; 50 μM e [Fe(III)] 0 e 1 mM; 1 e [H 2 O 2 ] 0 / [Fe(III)] 0 e 5000). The results of this study demonstrated that the rate of decomposition of hydrogen peroxide by Fe-(III) could be predicted very accurately by a kinetic model which takes into account the rapid formation and the slower decomposition of Fe(III)-hydroperoxy complexes (Fe III -(HO 2 ) 2+ and Fe III (OH)(HO 2 ) + ). The rate constant for the unimolecular decomposition of the Fe(III)-hydroperoxy complexes was determined to be 2.7 × 10 -3 s -1 . The use of the kinetic model allows a better understanding of the effects of operational parameters (i.e., pH and [H 2 O 2 ] 0 / [Fe(III)] 0 ) on the complex kinetics of decomposition of H 2 O 2 by Fe(III).
Inorganic Chemistry, 1978
The decomposition of hydrogen peroxide catalyzed by configurationally different iron(II1) complex ions, Le., cis-Fe(bmen)X,"+ and trans-Fe(tetpy)Xznt (where bmen = N,N'-bis(2-methylpyridyl)ethylenediamine, tetpy = 2,2':6',2":6",2"'-tetrapyridyl and X = H20 or OH-, depending on pH), has been investigated. The order of the Fe(bmen)X2"+-catalyzed reaction is unity with respect to both Hz02 and the metal compound, within the whole range of pH explored (3.5-7.3). In contrast, within the pH range 6-8.5, the reaction carried out on quaterpyridineiron(IT1) derivative may be described by mixed total second-and third-order kinetics, the higher degree mechanism being predominant at high substrate concentration. At fixed concentration of complex ions, saturation phenomena are observed in all cases on increasing the concentration of H202. As expected, the Lineweaver-Burk plot of the Fe(bmen)X2"+-catalyzed reaction is linear whereas it is nonlinear (at high substrate concentration) when the trans complex ions are used. The steady-state rate laws were derived and the calculated reaction velocities were found to agree satisfactorily with those experimentally determined. At 25 "C, the rate constant for the irreversible decomposition of the Michaelis complex, which represents the rate-limiting step, is 3.2 f 0.2 and 4.8 f 0.3 sd for the F e (b m e~~) X~~~ and Fe(tetpy)X2"+ ions, respectively. The energy of activation of this step is 10.6 f 1.6 and 7.5 f 1.2 kcal/mol and the entropy of activation (298 K) is-23 zk 5 and-32 f 4 eu, respectively. Implications of the role played by the different topologies of the complex ions in the kinetics of the catalysis as well as in the activation parameters are discussed.
Ferrous Iron Reduction of Superoxide, A Proton-Coupled Electron-Transfer Four-Point Test
Journal of Physical Chemistry A, 2009
Nelsen's four-point method of separating oxidants and reductants has been tested to evaluate its applicability to proton-coupled electron-transfer reactions. An efficient computational method was developed to determine rate-limiting steps in complex, multistep redox reactions. Geochemical redox reactions are rarely single-step, and by identifying the rate-limiting steps, computational time can be greatly reduced. The reaction of superoxide and ferrous oxide was selected as a test case for its simplicity and its importance in environmental radical generation chemistry (Fenton's reaction). Two approaches, one quantum mechanical and the other semiempirical, were compared. In both approaches, hybrid density functional theory (DFT) was used with the B3LYP/6-31+G(d,p) basis set and a polarized continuum model of the solvent to minimize the structures and determine the energies. In the quantum mechanical case, DFT was used to determine both the Gibbs free energies and the values for the intrinsic component of the reorganization energy of possible combinations of reactants and products. In the latter, experimental ∆G f values were combined with calculated intrinsic reorganization energy values. The computational results matched the relative difference in rate barriers between the reduction of superoxide by ferrous iron above and below pH 4.8. In the acidic pH range, the proton is coupled to the electron transfer, whereas in the neutral case, the proton initiates the electron transfer.
International Journal of Chemical Kinetics, 2000
Hydrogen peroxide was discovered in 1818 and has been used in bleaching for over a century [1]. H 2 O 2 on its own is a relatively weak oxidant under mild conditions: It can achieve some oxidations unaided, but for the majority of applications it requires activation in one way or another. Some activation methods, e.g., Fenton's reagent, are almost as old . However, by far the bulk of useful chemistry has been discovered in the last 50 years, and many catalytic methods are much more recent.
Fenton-like Chemistry in Water: Oxidation Catalysis by Fe(III) and H2O2
Journal of Physical Chemistry A, 2003
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 Fe II /H 2 O 2 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 [(H 2 O) 5 Fe III OOH] 2+ intermediate. We find that concomitant hydrolysis of the reacting species plays a crucial role and, taking this into account, that O-O homolysis ([(H 2 O) 4 (OH)Fe III-OOH] + f [(H 2 O) 4 (OH)Fe IV O] + + OH •) in vacuo is a likely second step with ∆E 0k ‡) 26 kcal/mol. However, proper inclusion of the solvent effects is important, in particular, for the heterolysis reactions, in which case the large endothermicy of the charge separations can be compensated by the hydration energies from the ion solvation. In this work, we also calculate the free energy barrier for the O-O homolysis of the iron(III) hydroperoxo intermediate in aqueous solution at T) 300 K, using the method of constrained molecular dynamics and thermodynamic integration, resulting in ∆A 300K ‡) 21 kcal/mol. Analysis of the vibrational spectra of the high-spin (S) 5/2) and low-spin (S) 1/2) Fe(III)OOH intermediates confirm the, in the literature, suggested effect of the spin state on the Fe-O and O-O bond strengths. In fact, we predict that with ligands inducing a low-spin iron(III) hydroperoxo intermediate, the barrier for the O-O homolysis will be even significantly lower.
H2O2-Dependent Fe-Catalyzed Oxidations: Control of the Active Species
Angewandte Chemie International Edition, 2001
The reaction of ferrous ion with hydroperoxides has been investigated for more than a century (Fenton chemistry), and yet the mechanism has still not been satisfactorily rationalized. The nature of the reactive species has always been a matter of debate, oscillating between hydroxyl or alkoxyl Experimental Section General protocol: The 2-deoxythioglycoside (0.1 mmol), alcohol (1.5 equiv), and DTBMP (4.5 equiv) were dissolved in CH 2 Cl 2 (1.0 mL) under argon, using light-protected glassware. After the mixture had been stirred for 1.5 h at 23 8C over molecular sieves (4 ,`5 microns, freshly activated), powdered AgPF 6 (3 ± 4 equiv) was added at O 8C. When the reaction was complete (0.5 ± 2 h), pyridine (50 equiv) was added, and the mixture was stirred for a further 0.5 h. Filtration (celite pad, diethyl ether/ n-hexane (1:4)), concentration, and chromatographic purification provided the 2-deoxyglycoside.
Archives of Biochemistry and Biophysics, 1983
Utilizing an electron paramagnetic resonance (EPR) spin-trapping technique it was demonstrated that the di- and triphosphate nucleotides of adenosine, cytidine, thymidine, and guanosine in the presence of Fe(II) catalyze hydroxyl free radical formation from H2O2. The triphosphate nucleotides in general were about 20% more effective than the diphosphate nucleotides. The amount of OH produced from H2O2 as a function of nucleotide level tended to increase in a sigmoidal fashion beginning at a nucleotide/Fe(II) ratio of 2 but then rose rapidly up to a ratio of 5 at which point the increase became more gradual. The monophosphate nucleotides did not cause an increase in the amount of hydroxyl free radical produced from H2O2 over the low level obtained in the buffer system only. The cations, Mg2+ and Ca2+, even at much higher than physiological levels and much higher than the level of added Fe(II), did not cause a substantial diminution of the Fe(II)-nucleotide-catalyzed breakdown of H2O2 to yield OH. A study of the time course of the effectiveness of Fe(II)-nucleotide-mediated OH formation from H2O2 demonstrated that Fe(II) in the presence of nucleotides remained in an effective catalytic state with a halftime of about 160 s whereas in the absence of the nucleotides the halftime was 7.5 s. All observations indicate that Fe(II) ligates with di- and triphosphate nucleotides and remains in the ferrous state which is then capable of catalyzing OH formation from H2O2; but with time, oxidation of the metal ion to the ferric state occurs, which either ligated to the nucleotide or to buffer ions, is ineffective in H2O2 catalysis to yield OH. Iron-nucleotide complexes may be of importance in mediating oxygen free radical damage to biological systems. The observations presented here indicate that hydroxyl free radicals will be produced when H2O2 is present with ferrous-nucleotide complexes.