Electron Transfer Transition State Research Articles

Significant electron transfer in heme catalysis: The case of chlorite dismutase

Electron transfer (ET) is significant in heme catalysis but usually difficult to be characterized. In the present DFT calculations, a transition state for ET has been optimized during the decomposition of peracetic acid (PAA) catalyzed by chlorite dismutase (Cld), using an active-site model based on the X-ray crystal structure of Cld. The Cld-catalyzed PAA reaction is revealed to proceed via a homolytic OO bond cleavage to transiently form compound II (Cpd II) and acetate radical (OAc), and a subsequent fast ET from Cpd II porphyrin to OAc leading to compound I (Cpd I) and acetate anion. The second step of ET is rate-limiting. The results highlight the importance of ET in heme chemistry and imply that the mechanisms of heme enzymes may be masked by fast ET even if a key intermediate has been detected (like Cpd I in this case). The comparison with the Cld-catalyzed chlorite reaction further indicates that a heme enzyme may employ different mechanisms for different substrates.

Convergent Iterative Constrained Variation Algorithm for Calculation of Electron-Transfer Transition States

We present an efficient mathematical framework to determine the potential-dependent transition states of electron transfer reactions by quantum calculations. This approach makes it more feasible to study heterogeneous electron transfer processes with the theory of local reaction center for electron transfer. It is shown that the new formulation regenerates previously published results obtained by the constrained variation method. Our solution algorithm replaces the constrained optimization problem defined in a multidimensional space by a single equation in terms of only one variable that is solved for in each iteration. This method leads to fast convergence, reliability, and robustness of the located transition states for more complex systems with a larger number of degrees of freedom, especially for smooth energy surfaces.

Theoretical studies to understand surface chemistry on carbon anodes for lithium-ion batteries: how does vinylene carbonate play its role as an electrolyte additive?

To elucidate the role of vinylene carbonate (VC) as a solvent additive in organic polar solutions for lithium-ion batteries, reductive decompositions for vinylene carbonate (VC) and ethylene carbonate (EC) molecules have been comprehensively investigated both in the gas phase and in solution by means of density functional theory calculations. The salt and solvent effects are incorporated with the clusters (EC)nLi+(VC) (n = 0-3), and further corrections that account for bulk solvent effects are added using the polarized continuum model (PCM). The electron affinities of (EC)nLi+(VC) (n = 0-3) monotonically decrease when the number of EC molecules increases; a sharp decrease of about 20.0 kcal/mol is found from n = 0 to 1 and a more gentle variation for n > 1. For (EC)nLi+(VC) (n = 1-3), the reduction of VC brings about more stable ion-pair intermediates than those due to reduction of the EC molecule by 3.1, 6.1, and 5.3 kcal/mol, respectively. This finding qualitatively agrees with the experimental fact that the reduction potential of VC in the presence of Li salt is more negative than that of EC. The calculated reduction potentials corresponding to radical anion formation are close to the experimental potentials determined with cyclic voltammetry on a gold electrode surface (-2.67, -3.19 eV on the physical scale for VC and EC respectively vs experimental values -2.96 and -2.94 eV). Regarding the decomposition mechanisms, the VC and EC moieties undergo homolytic ring opening from their respective reduction intermediates, and the energy barrier of VC is about one time higher than that of EC (e.g., 20.1 vs 8.8 kcal/mol for (EC)2Li+(VC)); both are weakly affected by the explicit solvent molecules and by a bulk solvent represented by a continuum model. Alternatively, starting from the VC-reduction intermediate, the ring opening of the EC moiety via an intramolecular electron-transfer transition state has also been located; its barrier lies between those of EC and VC (e.g., 17.2 kcal/mol for (EC)2Li+(VC)). On the basis of these results, we suggest the following explanation about the role that VC may play as additive in EC-based lithium-ion battery electrolytes; VC is initially reduced to a more stable intermediate than that from EC reduction. One possibility then is that the reduced VC decomposes to form a radical anion via a barrier of about 20 kcal/mol, which undergoes a series of reactions to give rise to more active film-forming products than those resulting from EC reduction, such as lithium divinylene dicarbonate, Li-C carbides, lithium vinylene dicarbonate, R-O-Li compound, and even oligomers with repeated vinylene and carbonate-vinylene units. Another possibility starting from the VC-reduction intermediate is that the ring opening occurs on the unreduced EC moiety instead of being on the reduced VC, via an intramolecular electron transfer transition state, the energy barrier of which is lower than that of the former, in which VC just helps the intermediate formation and is not consumed. The factors that determine the additive functioning mechanism are briefly discussed, and consequently a general rule for the selection of electrolyte additive is proposed.

Estimation of electron transfer parameters from AM1 calculations.

The utility of AM1 calculations for estimation of the electron-transfer parameters lambda'(v) (the enthalpy part of the Marcus internal reorganization energy) and H(ab) (the electronic coupling between the charge-bearing units) is considered for some charge-localized intervalence bis(hydrazine) radical cations, for which these parameters have been experimentally determined from optical measurements. The Koopmans estimate of lambda'(v) that employs the orbital separation for the neutral compound at the radical cation geometry is far from that calculated from the enthalpies of the species involved (eq 1) and is not correct. The eq 1 lambda'(v) enthalpies estimated by AM1 are reasonably good for compounds with only alkyl substituents but are overestimated by 33-59% for aryl-substituted hydrazines. The Koopmans estimate of H(ab) as half the orbital separation for the neutral species at the transition state geometry requires adjustment for the twist angles to those of the relaxed ground state to produce useful H(ab) values. Symmetry breaking occurs for the electron-transfer transition states of the compounds with saturated bridges, and the Koopmans estimate predicts H(ab) values that are slightly less than half as large as the optical measurements.

Competition between SN2 and Single Electron Transfer Reactions as a Function of Steric Hindrance Illustrated by the Model System AlkylCl + NO-

The SN2 reaction is a good example of the dichotomy and connection between electron-pair transfer chemistry and single electron transfer (ET) chemistry. Based on experimental stereochemical and kinetic data and on theoretical considerations, the dichotomy may be envisioned in two ways. One is competition between two distinct pathways, implying the existence of two distinct transition states on the potential energy hypersurface representing the reacting system, each connected to the SN2 and ET products, respectively. The other considers a single transition state which could competitively give rise to both products. In both cases, steric hindrance is expected to favor the formation of the ET over the SN2 products. An ab initio quantum chemical analysis of the model systems RCl + NO-, with R = methyl, ethyl, isopropyl, and tert-butyl, taking account of electron correlation (at the MP2 level) and of solvation, shows the existence of distinct SN2 and ET transition states. As steric hindrance increases, the SN2...

Kinetics and Mechanism of the Outer-Sphere Oxidation of Horse-Heart Cytochrome c by an Anionic Chromium(v) Complex – Kinetic Evidence for Precursor Formation and a Late Electron-Transfer Transition State

Kinetics and Mechanism of the Outer-Sphere Oxidation of Horse-Heart Cytochrome c by an Anionic Chromium(v) Complex – Kinetic Evidence for Precursor Formation and a Late Electron-Transfer Transition State

Kinetics and Mechanism of the Outer-Sphere Oxidation of Horse-Heart Cytochromec by an Anionic Chromium(v) Complex – Kinetic Evidence for Precursor Formation and a Late Electron-Transfer Transition State

The irreversible outer-sphere electron-transfer reaction between trans-bis(2-ethyl-2-hydroxybutanoato(2–))oxochromate(v) and cytochrome cII was investigated as a function of pH, concentration, temperature and pressure. The plot of the observed pseudo-first order rate constant as a function of the CrV concentration shows a clear trend towards saturation at higher CrV concentrations, from which the precursor formation constant and the electron-transfer rate constant could be separated (K = 37 ± 5 M−1 and kET = 1510 ± 180 s−1 at pH 4.8 and 279 K). In the low CrV concentration range the second-order electron-transfer rate contants were measured as a function of temperature (ΔH# = 20.9 ± 0.6 kJ mol−1; ΔS# = –79.9 ± 2.1 J K−1 mol−1; ΔG# (279 K) = 43.2 kJ mol−1). High-pressure experiments were performed at two different pH values. The kinetic (stopped-flow) and thermodynamic (electrochemical) measurements as a function of pressure enabled the construction of a volume profile for the system at 279 K. The activation volumes for the redox process are –9.2 ± 0.2 (pH 5.0) and –11.1 ± 0.8 cm3 mol−1 (pH 4.7), and the overall reaction volumes were estimated to be –7 ± 2 (pH 5.0) and –10 ± 2 cm3 mol−1 (pH 4.7) . The transition state of the redox reaction lies to a large extent on the product side and can be described as “late”. The results are discussed in comparison to earlier measurements using cobalt and ruthenium complexes as reaction partners for cytochrome c.

Towards the Definition of the Maximum Allowable Tightness of an Electron Transfer Transition State in the Reactions of Radical Anions and Alkyl Halides

A mechanistic continuum with a sharp changeover zone from electron transfer (ET) to substitution (SUB) reactions has been found in the analysis of the reactions of formyl radical anions with methyl halides (see below). The minimum C – C distance at which the transition state for ET collapses to that of SUB could be defined.

Dynamics Calculations on Bis(hydrazine) and Bis(hydrazyl) Radical Cation Intramolecular Electron Transfer

Bis(hydrazine) [22/HEX/22] exists principally in the unsymmetrical double nitrogen inversion form at 230 K. UHF/AM1 dynamics calculations starting from the radical cation of the above material and syn and anti [B/TET/B]+, each constrained to have equivalent dinitrogen units so they represent the electron transfer transition states, indicate that many modes (10 or more) contribute to reaching the electron transfer transition state, and that the frequencies involved are predictable from those of the relaxed radical cations. The results were analyzed to give average barrier-crossing frequencies activated at the transition state for electron transfer (〈hνin〉) of 827, 1403, and 1293 cm-1, respectively. A V of 5.0 kcal/mol was obtained from the experimental data for [22/HEX/22]+ when used with the first-order adiabatic energy surface expression (text eq 4), and of 4.7 at 〈hνin〉 = 827 cm-1 using Jortner's single averaged vibronic coupling treatment (text eq 6). These V values are between that estimated using Hus...

Structured Electron Transfer Transition State. Valence Bond Configuration Mixing Analysis and ab Initio Calculations of the Reactions of Formaldehyde Radical Anion with Methyl Chloride

Ab initio computations are performed on the competing electron transfer (ET) and substitution (SUB) pathways between formaldehyde radical anion and methyl chloride. The results are followed by a valence bond configuration mixing (VBCM) analysis. A variety of computational procedures starting from UHF and ROHF, with four different basis sets, on to higher levels up to UQCISD optimization, establish the existence of two transition states (TSs). One having a general (OH2C---CH3---Cl)•- structure corresponds to the ET pathway, and the other with the general (CH2O---C---Cl)•- structure corresponds to the O-alkylation (SUB(O)) pathway. No TS could be located for the direct C-alkylation pathway, and the C-alkylated product is formed in a stepwise manner. The ET-TS is bonded and has a distinct stereochemistry compared to the analogous SUB(O)-TS. A detailed path-following study, starting from the ET-TS in the direction of products, shows that the mode with the negative force constant (the reaction vector) along the reaction coordinate changes its character from a C---C approach to a C---C recoil. The recoil is triggered by the flapping mode of the two hydrogens of the formyl group which opposes the C---C approach and directs the final destiny of the path toward dissociative ET. A systematic computational study establishes that the ROHF/6-31G* level is a consistent level for geometry optimization, while a single point calculation UCCSD(T)/6-31+G*//ROHF/6-31G* is sufficient for obtaining reliable energetics. The points along the IRC path at UHF and UMP2 levels are highly spin contaminated and lead to erroneous conclusions when using MW (mass-weighted) coordinates. The mechanistic details and the TS stereochemistries established for the molecular calculations in vacuum persist when the calculations are repeated with the SCRF method based on the simple reaction field solvent model. Orbital selection rules are derived from the VBCM analysis to understand the structural features of ET- and SUB(O)-TSs and also the absence of a TS for the direct C-alkylation pathway. Probes are proposed to test the bonded nature of the ET-TS and its distinct stereochemistry compared with the SUB(O)-TS.

Statistical rate theory of gas-phase electron transfer. The ferrocene/ferrocenium self-exchange reaction

The ferrocene/ferrocenium self-exchange electron-transfer reaction in the gas phase at 350 K is modelled by using statistical rate theory. The RRKM formulation of unimolecular rate theory is used to calculate the relative rates of the dissociation of a chemically activated ferrocene/ferrocenium precursor complex and the electron self-exchange process that produces the successor complex. Relative rates are used to calculate reaction efficiencies as a function of the estimated properties of the activated complexes for the two pathways. The RRKM model results are consistent with the high observed efficiency of the ferrocene reaction, but the results are sensitive to the assumed properties of the electron-transfer transition state. Particular attention is given to the effect of intermolecular degrees of freedom involving the interactions between the ferrocene and ferrocenium ion in the activated complex for electron transfer. These degrees of freedom must be modelled as either relatively unhindered internal rotations or low frequency vibrations (≲50 cm −1) to account for the observed reaction efficiency.

Electron transfer in reactions of ketones with organolithium reagents. A carbon-14 kinetic isotope effect probe

Kinetic isotope effects have been determined for reactions of ketones labeled with carbon-14 at the carbonyl carbon with MeLi and Me/sub 2/CuLi in diethyl ether at 0/sup 0/C. Observed isotope effects were as follows: (C/sub 6/H/sub 5/)/sub 2/C double bonds O + MeLi, /sup 12/k//sup 14/k = 1.000 +/- 0.002; (C/sub 6/H/sub 5/)/sub 2/C double bonds O + Me/sub 2/CuLi, 1.029 +/- 0.005; 2,4,6-Me/sub 3/C/sub 6/H/sub 2/COC/sub 6/H/sub 5/ + MeLi, 1.023 +/- 0.004. The relative reactivities of ortho-, meta-, and para-substituted benzophenones with these reagents were also determined by the competition experiments. These results are consistent with an electron-transfer step which is followed by a carbon-carbon bond-forming step that is or is not rate determining depending on the structure of ketones and reagents. The reaction of benzophenone with MeLi proceeds via rate-determining electron transfer; the change in nucleophile from MeLi to Me/sub 2/CuLi shifts the rate-determining step from electron transfer to recombination; the change in ketone from benzophenone to 2,4,6-trimethylbenzophenone also shifts the rate-determining step from electron transfer to recombination because the latter step becomes slower for the more hindered ketone. The extent of the geometrical change of the substrate at the electron-transfer transition state of the reaction ofmore » benzophenone with MeLi was estimated to be small on the basis of the magnitude of the KIE and the rho value of the Hammett correlation.« less

Redox reactions of platinum-(II) and -(IV) complexes: influence of the σ-donor ability of non-labile 2,2′-bipyridyl and substituted 1,10-phenanthroline ligands on the rates of reaction

The reduction of [PtCl4(N–N)] by [NBu4]I and the oxidation of [PtCl2(N–N)] by [AuCl4]– in the presence of chloride ions have been kinetically investigated in acetonitrile [N–N = 1,10-phenanthroline (phen), 5Me-, 5NO2-, 5,6Me2-, and 3,4,7,8Me4-phen, or 2,2′-bipyridyl(bipy)]. The rate law for the reduction of [PtCl4(N–N)] has the form: Rare =k1[PtCl4(N–N)]+k2[PtCl4(N–N)][I–]. The first-order rate term is ascribed to a dissociative Pt–Cl rate-determining bond rupture, followed by a fast redox step. An inner-sphere redox mechanism is thought to be responsible for the second-order rate term. The dissociative path is only slightly affected by changes in N–N, whereas the second-order term is strongly influenced by the σ-donor ability of the uncharged ligands, the relation log k2=–0.62 PKa+ 2.05 being found at 25 °C. The rate law for the oxidation of [PtCl2,(N–N)] by [AuCl4]– has the form: Rate =k3[PtCl2(N–N)][AuCl4–][Cl–]. The rates are not sensitive to changes in N–N, indicating that the energetics of the interactions occurring in the activated complexare on the whole nearly constant throughout the set of ligands emproved. Atentative explanation of this behaviour takes into consideration the opposite effects caused by changing N–N on the formation free energy of the precursor inner-sphere intermediate, and on the free energy required to form the transition state for electron transfer from such an intermediate, both effects contributing to the free energy of activation of the redox reaction.

Contribution of ionic effects to electron transfer reactions

This paper reports an initial effort to combine the theory of ionic conductivity, due to Friedman, with the theory of simple outer sphere electron transfer reactions. In this work the ions, which do not participate in the electron transfer transitions, are considered individually. The solvent is considered, as elsewhere, as a dielectric continuum. The calculations are carried out for extreme ionic dilution, a Debye–Huckel limit. The purpose of this work is to attempt to provide a more detailed account of the effect of ions present in the solution on the reaction rate. The effect which arises manifests itself through the reaction rate constant expression. Specifically, we find that as a result of the interaction between the electron transfer system and the ions in solution, transport coefficients associated with ionic mobilities in solution appear in the rate constant expression. The nature of the ionic effect is similar to the effect found when one considers either damping of the electron transfer transition state or damping of the energy states of the polarization modes of the dielectric continuum.