Kinetic solvent effects on proton and hydrogen atom transfers from phenols. Similarities and differences

Bimolecular rate constants for proton transfer from six phenols to the anthracene radical anion have been determined in up to eight solvents using electrochemical techniques. Effects of hydrogen bonding on measured rate constants were explored over as wide a range of phenolic hydrogen-bond donor (HBD) and solvent hydrogen-bond acceptor (HBA) activities as practical. The phenols' values ranged from 0.261 (2-MeO-phenol) to 0.728 (3,5-Cl(2)-phenol), and the solvents' values from 0.44 (MeCN) to 1.00 (HMPA), where and are Abraham's parameters describing relative HBD and HBA activities (J. Chem. Soc., Perkin Trans. 2 1989, 699; 1990, 521). Rate constants for H-atom transfer (HAT) in HBA solvents, k(S), are extremely well correlated via log k(S) = log k(0) - 8.3 , where k(0) is the rate constant in a non-HBA solvent (Snelgrove et al. J. Am. Chem. Soc. 2001, 123, 469). The same equation describes the general features of proton transfers (k(S) decreases as increases, slopes of plots of log k(S) against increase as increases). However, in some solvents, k(S) values deviate systematically from the least-squares log k(S) versus correlation line (e.g., in THF and MeCN, k(S) is always smaller and larger, respectively, than "expected"). These deviations are attributed to variations in the solvents' anion solvating abilities (THF and MeCN are poor and good anion solvators, respectively). Values of log k(S) for proton transfer, but not for HAT, give better correlations with Taft et al.'s (J. Org. Chem. 1983, 48, 2877) beta scale of solvent HBA activities than with . The beta scale, therefore, does not solely reflect solvents' HBA activities but also contains contributions from anion solvation.     

Abnormal solvent effects on hydrogen atom abstractions. 1. The reactions of phenols with 2,2-diphenyl-1-picrylhydrazyl (dpph*) in alcohols

Rate constants, k(ArOH/dpph*)(S), for hydrogen atom abstraction from 13 hindered and nonhindered phenols by the diphenylpicrylhydrazyl radical, dpph*, have been determined in n-heptane and a number of alcoholic and nonalcoholic, hydrogen-bond accepting solvents. Abnormally enhanced k(ArOH/dpph*)(S) values of have been observed in alcohols. It is proposed that this is due to partial ionization of the phenols and a very fast electron transfer from phenoxide anion to dpph*. The popular assessment of the antioxidant activities of phenols with dpph* in alcohol solvents will generally lead to an overestimation of their activities.     

Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols

The kinetics and mechanism of proton-coupled electron transfer (PCET) from a series of phenols to a laser flash generated [Ru(bpy)(3)](3+) oxidant in aqueous solution was investigated. The reaction followed a concerted electron-proton transfer mechanism (CEP), both for the substituted phenols with an intramolecular hydrogen bond to a carboxylate group and for those where the proton was directly transferred to water. Without internal hydrogen bonds the concerted mechanism gave a characteristic pH-dependent rate for the phenol form that followed a Marcus free energy dependence, first reported for an intramolecular PCET in Sj?din, M. et al. J. Am. Chem. Soc. 2000, 122, 3932-3962 and now demonstrated also for a bimolecular oxidation of unsubstituted phenol. With internal hydrogen bonds instead, the rate was no longer pH-dependent, because the proton was transferred to the carboxylate base. The results suggest that while a concerted reaction has a relatively high reorganization energy (lambda), this may be significantly reduced by the hydrogen bonds, allowing for a lower barrier reaction path. It is further suggested that this is a general mechanism by which proton-coupled electron transfer in radical enzymes and model complexes may be promoted by hydrogen bonding. This is different from, and possibly in addition to, the generally suggested effect of hydrogen bonds on PCET in enhancing the proton vibrational wave function overlap between the reactant and donor states. In addition we demonstrate how the mechanism for phenol oxidation changes from a stepwise electron transfer-proton transfer with a stronger oxidant to a CEP with a weaker oxidant, for the same series of phenols. The hydrogen bonded CEP reaction may thus allow for a low energy barrier path that can operate efficiently at low driving forces, which is ideal for PCET reactions in biological systems.     

Isotope effects upon hydrogen atom loss from molecular ions

The isotope effect (T_H/T_D) upon the kinetic energy release and the isotope effect (k_H/k_D) upon ion abundance for unimolecular H· loss from molecular ions has been determined for several compounds. It is suggested that the isotope effect upon abundance might provide a convenient method of estimating the relative life-time of ions which fragment in the metastable region for different instruments or different experimental conditions. The value of k_H/k_D varies from <2 to >1000 for different molecular ions and this variation is apparently largely due to the rate of increase of the reaction rate with internal energy in the threshold region. The magnitude of the isotope effect is thus related to the entropy of activation. The isotope effect upon energy release was found to be slightly less than unity in almost every case studied; this included both reactions in which the reverse activation energy is very small and those in which it is appreciable.     

Ligand effects on hydrogen atom transfer from hydrocarbons to three-coordinate iron imides

A new β-diketiminate ligand with 2,4,6-tri(phenyl)phenyl N-substituents provides protective bulk around the metal without exposing any weak C-H bonds. This ligand improves the stability of reactive iron(III) imido complexes with Fe═NAd and Fe═NMes functional groups (Ad = 1-adamantyl; Mes = mesityl). The new ligand gives iron(III) imido complexes that are significantly more reactive toward 1,4-cyclohexadiene than the previously reported 2,6-diisopropylphenyl diketiminate variants. Analysis of X-ray crystal structures implicates Fe═N-C bending, a longer Fe═N bond, and greater access to the metal as potential reasons for the increase in C-H bond activation rates.     

Theoretical study of the thermodynamics and kinetics of hydrogen abstractions from hydrocarbons

Thermochemical and kinetic data were calculated at four cost-effective levels of theory for a set consisting of five hydrogen abstraction reactions between hydrocarbons for which experimental data are available. The selection of a reliable, yet cost-effective method to study this type of reactions for a broad range of applications was done on the basis of comparison with experimental data or with results obtained from computationally demanding high level of theory calculations. For this benchmark study two composite methods (CBS-QB3 and G3B3) and two density functional theory (DFT) methods, MPW1PW91/6-311G(2d,d,p) and BMK/6-311G(2d,d,p), were selected. All four methods succeeded well in describing the thermochemical properties of the five studied hydrogen abstraction reactions. High-level Weizmann-1 (W1) calculations indicated that CBS-QB3 succeeds in predicting the most accurate reaction barrier for the hydrogen abstraction of methane by methyl but tends to underestimate the reaction barriers for reactions where spin contamination is observed in the transition state. Experimental rate coefficients were most accurately predicted with CBS-QB3. Therefore, CBS-QB3 was selected to investigate the influence of both the 1D hindered internal rotor treatment about the forming bond (1D-HR) and tunneling on the rate coefficients for a set of 21 hydrogen abstraction reactions. Three zero curvature tunneling (ZCT) methods were evaluated (Wigner, Skodje & Truhlar, Eckart). As the computationally more demanding centrifugal dominant small curvature semiclassical (CD-SCS) tunneling method did not yield significantly better agreement with experiment compared to the ZCT methods, CD-SCS tunneling contributions were only assessed for the hydrogen abstractions by methyl from methane and ethane. The best agreement with experimental rate coefficients was found when Eckart tunneling and 1D-HR corrections were applied. A mean deviation of a factor 6 on the rate coefficients is found for the complete set of 21 reactions at temperatures ranging from 298 to 1000 K. Tunneling corrections play a critical role in obtaining accurate rate coefficients, especially at lower temperatures, whereas the hindered rotor treatment only improves the agreement with experiment in the high-temperature range.     

Studies of site selective hydrogen atom abstractions by Cl atoms from isobutane and propane by laser flash photolysis/IR diode laser spectroscopy

The kinetics of chlorine atom abstractions from normal and selectively deuterated propane and isobutane have been measured at room temperature and 195 K using a laser flash photolysis system, and following the course of the reaction via IR diode laser absorption measurements of HCl product. In conjunction with the kinetic measurements, a comparison of the HCl signal heights from pairs of measurements on normal and selectively deuterated systems has allowed the determination of the branching fractions of the reactions at the primary, secondary (propane) and tertiary (isobutane) positions. The kinetic data (all in units of cm(3) molecule(-1) s(-1)) for the reaction of Cl atoms with propane ((1.22 +/- 0.02) x10(-10), 195 K; (1.22 +/- 0.03) x10(-10) 298 K) and isobutane ((1.52 +/- 0.02) x10(-10), 195 K; (1.25 +/- 0.04) x10(-10), 298 K) are generally in good agreement with literature data. No data are available for comparison with our measurements for the reactions of Cl atoms with CH(3)CD(2)CH(3) ((1.02 +/- 0.03) x10(-10), 195 K; (1.09 +/- 0.02) x10(-10), 298 K) or (CH(3))(3)CD ((1.32 +/- 0.03) x10(-10), 195 K; (1.12 +/- 0.04) x10(-10), 298 K). Rate coefficients at 195 K for the reactions of Cl atoms with ethane ((5.04 +/- 0.08) x10(-11) and n-butane ((2.19 +/- 0.03) x10(-10)) were also measured. The branching fractions for abstraction at the primary position increased with temperature for both propane ((40 +/- 3)% at 195 K to (48 +/- 3)% at 298 K) and isobutane ((49 +/- 4)% at 195 K to (62 +/- 5)% at 298 K). The direct measurements from this study are in good agreement with most calculations based on structure activity relationships.     

Hydrogen abstraction by chlorine atom from amino acids: remarkable influence of polar effects on regioselectivity

Quantum chemistry computations have been used to investigate hydrogen-atom abstraction by chlorine atom from protonated and N-acetylated amino acids. The results are consistent with the decreased reactivity at the backbone α-carbon and adjacent side-chain positions that is observed experimentally. The individual effects of NH(3)(+), COOH, and NHAc substituents have been examined and reveal important insights. The NH(3)(+) group in isolation is found to be deactivating at the α-position, while the acetamido group is activating. For the COOH group, polar effects lead to a contrathermodynamic deactivation of the thermodynamically most favorable α-abstraction. In the N-acetylamino acid, the α-position is deactivated by the combined inductive effect of the substituents and the presence of an early transition structure, again overriding the greater thermodynamic stability of the α-centered radical product. Deactivation of the α-, β-, and γ-positions results in a peculiar stability for amino acids and peptides and their derivatives with respect to radical degradation.     

Exploring excited-state hydrogen atom transfer along an ammonia wire cluster: competitive reaction paths and vibrational mode selectivity

The excited-state hydrogen-atom transfer (ESHAT) reaction of the 7-hydroxyquinoline(NH(3))(3) cluster involves a crossing from the initially excited (1)pipi(*) to a (1)pisigma(*) state. The nonadiabatic coupling between these states induces homolytic dissociation of the O-H bond and H-atom transfer to the closest NH(3) molecule, forming a biradical structure denoted HT1, followed by two more Grotthus-type translocation steps along the ammonia wire. We investigate this reaction at the configuration interaction singles level, using a basis set with diffuse orbitals. Intrinsic reaction coordinate calculations of the enol-->HT1 step predict that the H-atom transfer is preceded and followed by extensive twisting and bending of the ammonia wire, as well as large O-H...NH(3) hydrogen bond contraction and expansion. The calculations also predict an excited-state proton transfer path involving synchronous proton motions; however, it lies 20-25 kcal/mol above the ESHAT path. Higher singlet and triplet potential curves are calculated along the ESHAT reaction coordinate: Two singlet-triplet curve crossings occur within the HT1 product well and intersystem crossing to these T(n) states branches the reaction back to the enol reactant side, decreasing the ESHAT yield. In fact, a product yield of approximately 40% 7-ketoquinoline.(NH(3))(3) is experimentally observed. The vibrational mode selectivity of the enol-->HT1 reaction step [C. Manca, C. Tanner, S. Coussan, A. Bach, and S. Leutwyler, J. Chem. Phys. 121, 2578 (2004)] is shown to be due to the large sensitivity of the diffuse pisigma(*) state to vibrational displacements along the intermolecular coordinates.     

Formation of pure hydrogen from methane

The methods for production of pure hydrogen from methane are summarized. One method is methane decomposition to hydrogen and carbon nanofibers. Ni-based catalysts with high activity and long life for the methane decomposition were developed. The other method is based on the redox of iron oxides, i.e., Fe₃O₄ is reduced with methane to iron metals and, subsequently, iron metals are oxidized with water vapor to form hydrogen. Iron oxide mediators that could be reduced with methane and subsequently be oxidized with water vapor at low temperatures were designed.     

Formation of pure hydrogen from methane

The methods for production of pure hydrogen from methane are summarized. One method is methane decomposition to hydrogen and carbon nanofibers. Ni-based catalysts with high activity and long life for the methane decomposition were developed. The other method is based on the redox of iron oxides, i.e., Fe₃O₄ is reduced with methane to iron metals and, subsequently, iron metals are oxidized with water vapor to form hydrogen. Iron oxide mediators that could be reduced with methane and subsequently be oxidized with water vapor at low temperatures were designed.     

Investigation of methylene group hydrogen atom reactivity in some azolidines

UV specttoscopy is used to investigate the kinetics of formation of arylidenerhodanines from rhodanine and various aromatic aldehydes whose benzene ring substitutents differ in nature and position. Hammett's equation is used to determine quantitatively the reactivities of the substituted aldehydes. The order of effect of substituent on reaction velocity is p-NO₂ > p-Cl > H > p-N(CH₃)₂, corresponding to a positive value of ρ in the Hammett equation. The position sequence is p-NO₂ > m-NO₂ > o-NO₂, and for chloro derivatives m-Cl > p-Cl, in agreement with views on transmission of inductive and conjugation effects through a benzene ring to a reaction center.     

Chemistry of the t-butoxyl radical: evidence that most hydrogen abstractions from carbon are entropy-controlled

Absolute rate constants and Arrhenius parameters for hydrogen abstractions (from carbon) by the t-butoxyl radical ((t) BuO.) are reported for several hydrocarbons and tertiary amines in solution. Combined with data already in the literature, an analysis of all the available data reveals that most hydrogen abstractions (from carbon) by (t) BuO. are entropy controlled (i.e., TdeltaS > deltaH, in solution at room temperature). For substrates with C-H bond dissociation energies (BDEs) > 92 kcal/mol, the activation energy for hydrogen abstraction decreases with decreasing BDE in accord with the Evans-Polanyi equation, with alpha approximately 0.3. For substrates with C-H BDEs in the range from 79 to 92 kcal/mol, the activation energy does not vary significantly with C-H BDE. The implications of these results in the context of the use of (t) BuO. as a chemical model for reactive oxygen-centered radicals is discussed.     

Solvent effects on chemiluminescence from the hydrogen peroxide-lucigenin reaction: Kinetics of light emission in mixed polar solvents

Summary The effect of reaction media composition on reaction kinetics was studied for the reaction of lucigenin (10,10-dimethyl-9,9-biacridinium nitrate) with hydrogen peroxide and alkali. Chemiluminescent emission as well as lucigenin disappearance were recorded in mixtures of water with the co-solvents methanol, ethanol, 1-propanol, dimethylsulfoxide, and dimethylformamide. The kinetic results (base and peroxide concentration influence on the reaction rate and the relative chemiluminescence yield) are very similar in all the reaction media, suggesting that the fundamental step in the disappearance of lucigenin and in light emission decay is HO ₂ ^– addition to lucigenin. Lucigenin can also disappear through dark reactions with OH^– or H₂O₂. The co-solvent acts as a catalyst for the reaction with HO ₂ ^– and increases both the initial chemiluminescence intensity and the decay rate constant.

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Lösungsmitteleffekte bei der Chemilumineszenz der Wasserstoffperoxid-Lucigenin Reaktion. Kinetik der Lichtemission in gemischten polaren Lösungsmitteln

Zusammenfassung Es wurde der Einfluß der Zusammensetzung des Reaktionsmediums auf die Kinetik der Reaktion von Lucigenin (10,10-dimethyl-9,9-biacridiniumnitrat) mit Wasserstoffperoxid und Alkali untersucht. Die Emission der Chemilumineszenz und das Verschwinden von Lucigenin wurde in Mischungen von Wasser mit den Kosolventien Methanol, Ethanol, 1-Propanol, Dimethylsufoxid und Dimethylformamid gemessen. Die kinetischen Resultate (Einfluß der Basen- und Peroxid-Konzentration auf die Reaktionsgeschwindigkeit und die relative Chemilumineszenzausbeute) sind für alle Reaktionsmedien sehr ähnlich; das legt den Schluß nahe, daß der grundlegende Schritt im Verbrauch des Lucigenin unter Lichtemission die Addition von HO ₂ ^– an Lucigenin ist. Lucigenin kann auch über Dunkelreaktionen mit OH^– oder H₂O₂ verschwinden. Das Kosolvens agiert als Katalysator für die Rekation mit HO ₂ ^– und erhöht sowohl die anfängliche Chemilumineszenzintensität als auch die Zerfallsgeschwindigkeitskonstante.