Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 2. Reduction potentials

This study utilizes photoelectron spectroscopy (PES) combined with theoretical methods to determine the electronic structure contributions to the large reduction potential difference between [FeCl(4)](2)(-)(,1)(-) and [Fe(SR)(4)](2)(-)(,1)(-) (DeltaE(0) approximately 1 V). Valence PES data confirm that this effect results from electronic structure differences because there is a similarly large shift in the onset of valence ionization between the two reduced species (DeltaI(vert) = 1.4 +/- 0.3 eV). Specific electronic contributions to DeltaI(vert) have been investigated and defined. Ligand field effects, which are often considered to be of great importance, contribute very little to DeltaI(vert) (DeltaE(LF) < -0.05 eV). By contrast, electronic relaxation, a factor that is often neglected in the analysis of chemical reactivity, strongly affects the valence ionization energies of both species. The larger electronic relaxation in the tetrathiolate allows it to more effectively stabilize the oxidized state and lowers its I(vert) relative to that of the chloride (DeltaE(rlx) = 0.2 eV). The largest contribution to the difference in redox potentials is the much lower effective charge () of the tetrathiolate in the reduced state, which results in a large difference in the energy of the Fe 3d manifold between the two redox couples (DeltaE(Fe)( )(3d) = 1.2 eV). This difference derives from the significantly higher covalency of the iron-thiolate bond, which decreases and significantly lowers its redox potential.     

Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 1. Photoelectron spectroscopic determination of electronic relaxation

Electronic relaxation, the change in molecular electronic structure as a response to oxidation, is investigated in [FeX(4)](2)(-)(,1)(-) (X = Cl, SR) model complexes. Photoelectron spectroscopy, in conjunction with density functional methods, is used to define and evaluate the core and valence electronic relaxation upon ionization of [FeX(4)](2)(-). The presence of intense yet formally forbidden charge-transfer satellite peaks in the PES data is a direct reflection of electronic relaxation. The phenomenon is evaluated as a function of charge redistribution at the metal center (Deltaq(rlx)) resulting from changes in the electronic structure. This charge redistribution is calculated from experimental core and valence PES data using a valence bond configuration interaction (VBCI) model. It is found that electronic relaxation is very large for both core (Fe 2p) and valence (Fe 3d) ionization processes and that it is greater in [Fe(SR)(4)](2)(-) than in [FeCl(4)](2)(-). Similar results are obtained from DFT calculations. The results suggest that, although the lowest-energy valence ionization (from the redox-active molecular orbital) is metal-based, electronic relaxation causes a dramatic redistribution of electron density ( approximately 0.7ē) from the ligands to the metal center corresponding to a generalized increase in covalency over all M-L bonds. The more covalent tetrathiolate achieves a larger Deltaq(rlx) because the LMCT states responsible for relaxation are significantly lower in energy than those in the tetrachloride. The large observed electronic relaxation can make significant contributions to the thermodynamics and kinetics of electron transfer in inorganic systems.     

Electronic structure and reactivity of 3-hydroxyquinoline

The reactivity indexes of the neutral, dipolar, cationic, and anionic forms of 3-hydroxy-quinoline were calculated by the simple MO LCAO method using dynamic and statistical approximations. The predicted (on the basis of the localization energies) charge distributions, boundary densities, free valence indexes, and orientations of electrophilic substituents for the cationic and anionic forms of 3-hydroxyquinoline are in good agreement with the experimental data. The orientations of nucleophilic and radical substituents for the four forms of 3-hydroxyquinoline are predicted. The reactivity indexes of the neutral form of 3-hydroxyquinoline were calculated by means of the Pariser-Parr-Pople method.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 191–196, February, 1972.     

Electronic structure of a low-spin heme/Cu peroxide complex: spin-state and spin-topology contributions to reactivity

This study details the electronic structure of the heme–peroxo–copper adduct {[(F8)Fe(DCHIm)]-O2-[Cu(AN)]}+ (LS(AN)) in which O2(2–) bridges the metals in a μ-1,2 or “end-on” configuration. LS(AN) is generated by addition of coordinating base to the parent complex {[(F8)Fe]-O2-[Cu(AN)]}+ (HS(AN)) in which the O2(2–) bridges the metals in an μ-η2:η2 or “side-on” mode. In addition to the structural change of the O2(2–) bridging geometry, coordination of the base changes the spin state of the heme fragment (from S = 5/2 in HS(AN) to S = 1/2 in LS(AN)) that results in an antiferromagnetically coupled diamagnetic ground state in LS(AN). The strong ligand field of the porphyrin modulates the high-spin to low-spin effect on Fe–peroxo bonding relative to nonheme complexes, which is important in the O–O bond cleavage process. On the basis of DFT calculations, the ground state of LS(AN) is dependent on the Fe–O–O–Cu dihedral angle, wherein acute angles (<~150°) yield an antiferromagnetically coupled electronic structure while more obtuse angles yield a ferromagnetic ground state. LS(AN) is diamagnetic and thus has an antiferromagnetically coupled ground state with a calculated Fe–O–O–Cu dihedral angle of 137°. The nature of the bonding in LS(AN) and the frontier molecular orbitals which lead to this magneto-structural correlation provide insight into possible spin topology contributions to O–O bond cleavage by cytochrome c oxidase.     

Cation-modulated electron-transfer channel: H-atom transfer vs proton-coupled electron transfer with a variable electron-transfer channel in acylamide units

The mechanism of proton transfer (PT)/electron transfer (ET) in acylamide units was explored theoretically using density functional theory in a representative model (a cyclic coupling mode between formamide and the N-dehydrogenated formamidic radical, FF). In FF, PT/ET normally occurs via a seven-center cyclic proton-coupled electron transfer (PCET) mechanism with a N-->N PT and an O-->O ET. However, when different hydrated metal ions are bound to the two oxygen sites of FF, the PT/ET mechanism may significantly change. In addition to their inhibition of PT/ET rate, the hydrated metal ions can effectively regulate the FF PT/ET cooperative mechanism to produce a single pathway hydrogen atom transfer (HAT) or a flexible proton coupled electron transfer (PCET) mechanism by changing the ET channel. The regulation essentially originates from the change in the O...O bond strength in the transition state, subject to the binding ability of the hydrated metal ions. In general, the high valent metal ions and those with large binding energies can promote HAT, and the low valent metal ions and those with small binding energies favor PCET. Hydration may reduce the Lewis acidity of cations, and thus favor PCET. Good correlations among the binding energies, barrier heights, spin density distributions, O...O contacts, and hydrated metal ion properties have been found, which can be used to interpret the transition in the PT/ET mechanism. These findings regarding the modulation of the PT/ET pathway via hydrated metal ions may provide useful information for a greater understanding of PT/ET cooperative mechanisms, and a possible method for switching conductance in nanoelectronic devices.     

Quadricyclanes. Part II: Electronic structure and chemical reactivity

The electronic structure of quadricyclane and 3-methylidenequadricyclane obtained by photoelectron spectroscopy, is used as a basis for the discussion of cycloadditions to these systems. The electronic structure of 3-heteroquadricyclanes, arrived at by theoretical calculations, agrees well with that expected from the above measured systems. A surprising outcome is that the orbital most responsible for the observed 2,4-cycloadditions to these heterosystems in not the HOMO but the third highest orbital which lies well below the former. This strongly suggests that these 2,4-cycloadditions proceed not in a concerted fashion but presumably involve as rate-determining step the formation of a resonance-stabilized zwitterionic intermediate. The nature of this intermeiate is discussed and the feasability of its formation investigated on the basis of thermochemical considerations.     

Electronic structure and reactivity of organofluorine compounds. 5. Electronic structure of CF3-substituted fluoroethylenes from MNDO and STO-3G calculations

It was established by MNDO and STO-3G quantum-chemical calculations of higher perfluorinated olefins that substitution of the fluorine atoms in C₂F₄ by CF₃ groups leads to stabilization of the -MO and destabilization of the -MO of the molecule. The CF₃ group in perfluorinated olefins acts simultaneously as electron donor to the system and electron acceptor to the system. The effective charge on the CF₃ group acquires a small positive value, i.e., according to the sum of the and effects the allylic CF₃ group in perfluorinated olefins is an electron donor.A. N. Nesmeyanov Institute of Heteroorganic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1334–1340, June, 1992.     

The contributions to the electronic factor in bridge-assisted electron transfer processes

We present a new formulation of the various contributions to the electronic factor of bridge-assisted electron transfer rates, which explicitly takes into account the many-electron character of the wavefunctions and the overlap integrals between adjacent orbitals. Aside from superexchange terms, this formulation focuses on an important new multiple-exchange contribution.     

Photoinduced electron transfer within ion pairs. Direct measure of the intramolecular electron-transfer quenching constant

The electron-transfer quenching of the luminescence emission of Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine) by CoSiW₁₁O₃₉H₂O⁶⁻ has been studied by steady-state and pulsed techniques. Both static and dynamic quenching were observed. The luminescence emission was found to decrease with time according to two distinct exponential decays. A kinetic analysis of the system shows that the faster decay corresponds to the intramolecular electron transfer rate constant within the =Ru(bpy)₃²⁺ -polytungstate ion pair and that the slower decay is related to the dynamic quenching.     

Basic reactivity of CaO: investigating active sites under operating conditions

A set of CaO samples was prepared from thermal decomposition of several precursors, leading to very different surface properties. During storage, CaO samples rehydrated quickly but reversibly. Before characterization, the samples were pre-treated at 1023 K under nitrogen flow to obtain CaO as the active phase. Although this pre-treatment led to almost the same specific surface areas for all samples, their basic reactivity levels toward 2-methylbut-3-yn-2-ol conversion were different from one preparation to another. In contrast with the properties of MgO pre-treated at the same temperature, the basic reactivity of CaO correlates neither with the concentration of surface defects (exposing ions in low coordination) determined by photoluminescence nor with the deprotonation ability toward methanol. In order to identify the active sites on CaO pre-treated under nitrogen in the temperature range 673 K-1023 K, OH groups were quantified with (1)H NMR: the higher the surface density of OH groups, the higher the basic reactivity. Even after pre-treatment at 1023 K, after which only a few hydroxyls remain, the basic reactivity is governed by the remaining hydroxylation of the surface. The higher reactivity of OH groups of CaO compared to those of Ca(OH)(2) and MgO is discussed.     

Electronic and spectroscopic studies of the non-heme reduced binuclear iron sites of two ribonucleotide reductase variants: comparison to reduced methane monooxygenase and contributions to O2 reactivity

Circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD have been used to probe the biferrous active site of two variants of ribonucleotide reductase. The aspartate to glutamate substitution (R2-D84E) at the binuclear iron site modifies the endogenous ligand set of ribonucleotide reductase to match that of the binuclear center in the hydroxylase component of methane monooxygenase (MMOH). The crystal structure of chemically reduced R2-D84E suggests that the active-site structure parallels that of MMOH. However, CD, MCD, and VTVH MCD data combined with spin-Hamiltonian analysis of reduced R2-D84E indicate a different coordination environment relative to reduced MMOH, with no mu-(1,1)(eta(1),eta(2)) carboxylate bridge. To further understand the variations in geometry of the active site, which lead to differences in reactivity, density functional theory (DFT) calculations have been carried out to identify active-site structures for R2-wt and R2-D84E consistent with these spectroscopic data. The effects of varying the ligand set, positions of bound and free waters, and additional protein constraints on the geometry and energy of the binuclear site of both R2-wt and variant R2s are also explored to identify the contributions to their structural differences and their relation to reduced MMOH.     

Electronic structure and reactivity of ylidic systems : V. Ylid structure information form NMR data

Bond systems in triphenylphosphorus ylids, methylenetrimethylphosphorane and their tricarbonylnickel salts are discussed by interpretation of ¹³C NMR results.     

Metals in biology: Low-lying states of iron-oxygen and iron-sulfur complexes modeling electron transfer in redox reactions

Energy surfaces of low-lying states of planar complexes [Fe(C₂H₂X₂)₂]ⁿ with X=O and S and total charges n varying between + 3 and − 2 have been investigated by quantum chemical ab initio MO-SCF calculations of double zeta quality. Through studies of such properties as geometry, electron density distribution and molecular orbital energies it has been concluded that some of the states can be referred to as Fe (III) and others as Fe (II) states. The lowest states are found to be those with n = − 1 and 0 for X = O and n = − 1 and − 2 for X = S. For both the oxygen and the sulfur complexes Fe (III) and Fe (II) states are very close in energy. Supposing other electron acceptors and donors to be available, possible schemes for redox reactions between metal and ligand are suggested.     

Some contributions to the characterization of active sites in Mg-supported Ziegler–Natta catalysts

The contribution of different MgO supports to the coordination polymerization of ethylene was studied by x-ray diffractometry and infrared (IR) and electron spin resonance (ESR) spectroscopy of the supports and their products after treatment with TiCl₄. It was concluded that TiCl₄ was bonded on the surface OH groups of MgO mainly in inactive form, whereas the majority of the active sites was associated with the coordinatively unsaturated O^2? ions.     

Kinetics of photoinduced electron transfer in polythiophene-porphyrin-fullerene molecular films

Photoinduced electron transfer (ET) processes were studied by the time-resolved Maxwell displacement charge (TRMDC) method in bilayer structures consisting of an electron donor-acceptor and conductive polymer monolayers, porphyrin-fullerene dyad and polyhexylthiophene, respectively, both layers prepared by the Langmuir-Blodgett (LB) method. The charge separation involves two fast steps: an intramolecular ET in the dyad molecule followed by an interlayer ET from the polymer to the formed porphyrin radical cation. These fast vertical intra- and interlayer processes could not be time-resolved by the TRMDC method. The lifetime of the charge separated state in the system was extended to hundreds of milliseconds by lateral electron and hole transfers in fullerene and polymer sublayers. The kinetics of the system was described by a model involving two long-living energetically different complete charge separated states. The data analysis indicates that the charge separation has a recombination time of 0.5 s. This is a promising result for possible applications.