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PubMed2003

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.

Journal Article↗
PubMed2003

Electron donor-acceptor dyads and triads based on tris(bipyridine)ruthenium(II) and benzoquinone: synthesis, characterization, and photoinduced electron transfer reactions.

Two electron donor-acceptor triads based on a benzoquinone acceptor linked to a light absorbing [Ru(bpy)(3)](2+) complex have been synthesized. In triad 6 (denoted Ru(II)-BQ-Co(III)), a [Co(bpy)(3)](3+) complex, a potential secondary acceptor, was linked to the quinone. In the other triad, 8 (denoted PTZ-Ru(II)-BQ), a phenothiazine donor was linked to the ruthenium moiety. The corresponding dyads Ru(II)-BQ (4) and PTZ-Ru(II) (9) were prepared for comparison. Upon light excitation in the visible band of the ruthenium moiety, electron transfer to the quinone occurred with a rate constant k(f) = 5 x 10(9) s(-)(1) (tau(f) = 200 ps) in all the quinone containing complexes. Recombination to the ground state followed, with a rate constant k(b) approximately 4.5 x 10(8) s(-)(1) (tau(b) approximately 2.2 ns), for both Ru(II)-BQ and Ru(II)-BQ-Co(III) with no indication of a charge shift to generate the reduced Co(II) moiety. In the PTZ-Ru(II)-BQ triad, however, the initial charge separation was followed by a rapid (k > 5 x 10(9) s(-)(1)) electron transfer from the phenothiazine moiety to give the fairly long-lived PTZ(*)(+)-Ru(II)-BQ(*)(-) state (tau = 80 ns) in unusually high yield for a [Ru(bpy)(3)](2+)-based triad (> 90%), that lies at DeltaG degrees = 1.32 eV relative to the ground state. Unfortunately, this triad turned out to be rather photolabile. Interestingly, coupling between the oxidized PTZ(*)(+) and the BQ(*)(-) moieties seemed to occur. This discouraged further extension to incorporate more redox active units. Finally, in the dyad PTZ-Ru(II) a reversible, near isoergonic electron transfer was observed on excitation. Thus, a quasiequilibrium was established with an observed time constant of 7 ns, with ca. 82% of the population in the PTZ-Ru(II) state and 18% in the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state. These states decayed in parallel with an observed lifetime of 90 ns. The initial electron transfer to form the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state was thus faster than what would have been inferred from the Ru(II) emission decay (tau = 90 ns). This result suggests that reports for related PTZ-Ru(II) and PTZ-Ru(II)-acceptor complexes in the literature might need to be reconsidered.

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PubMed2004

Pseudooctahedral complexes of vanadium(III): electronic structure investigation by magnetic and electronic spectroscopy.

A variety of physical methods has been used to probe the non-Kramers, S = 1, V(III) ion in two types of pseudooctahedral complexes: V(acac)(3), where acac = anion of 2,4-pentanedione, and VX(3)(thf)(3), where thf = tetrahydrofuran and X = Cl and Br. These methods include tunable frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 95-700 GHz and fields up to 25 T) in conjunction with electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies. Variable-temperature magnetic susceptibility and field-dependent magnetization measurements were also performed. All measurements were conducted on complexes in the solid state (powder or mull samples). The field versus sub-THz wave quantum energy dependence of observed HFEPR resonances yielded the following spin Hamiltonian parameters for V(acac)(3): D = +7.470(1) cm(-1); E = +1.916(1) cm(-1); g(x) = 1.833(4); g(y) = 1.72(2); g(z) = 2.03(2). For VCl(3)(thf)(3), HFEPR detected a single zero-field transition at 15.8 cm(-1) (474 GHz), which was insufficient to determine the complete set of spin Hamiltonian parameters. For VBr(3)(thf)(3), however, a particularly rich data set was obtained using tunable-frequency HFEPR, and analysis of this data set gave the folowing: D = -16.162(6) cm(-1); E = -3.694(4) cm(-1); g(x) = 1.86(1); g(y) = 1.90(1); g(z) = 1.710(4). Analysis of the VTVH-MCD data gave spin Hamiltonian parameters in good agreement with those determined by HFEPR for both V(acac)(3) and VBr(3)(thf)(3) and in rough agreement with the estimate for VCl(3)(thf)(3) (D approximately 10 cm(-1), |E/D| approximately 0.18), together with the finding that the value of D is negative for both thf complexes. The electronic structures of these V(III) complexes are discussed in terms of their molecular structures and the electronic transitions observed by electronic absorption and MCD spectroscopies.

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PubMed2005

Electron exchange between alpha-Keggin tungstoaluminates and a well-defined cluster-anion probe for studies in electron transfer.

Fully oxidized alpha-AlIIIW12O40(5-) (1ox), and one-electron-reduced alpha-AlIIIW12O40(6-) (1red), are well-behaved (stable and free of ion pairing) over a wide range of pH and ionic-strength values at room temperature in water. Having established this, 27Al NMR spectroscopy is used to measure rates of electron exchange between 1ox (27Al NMR: 72.2 ppm relative to Al(H2O)63+; nu(1/2) = 0.77 Hz) and 1red (74.1 ppm; nu(1/2) = 0.76 Hz). Bimolecular rate constants, k, are obtained from line broadening in 27Al NMR signals as ionic strength, mu, is increased by addition of NaCl at the slow-exchange limit of the NMR time scale. The dependence of k on is plotted using the extended Debye-Hückel equation: log k = log k0 + 2alphaz1z2mu(1/2)/(1 + betarnu(1/2)), where z1 and z2 are the charges of 1ox and 1red, alpha and beta are constants, and r, the distance of closest contact, is fixed at 1.12 nm, the crystallographic diameter of a Keggin anion. Although not derived for highly charged ions, this equation gives a straight line (R2 = 0.996), whose slope gives a charge product, z1z2, of 29 +/- 2, statistically identical to the theoretical value of 30. Extrapolation to mu = 0 gives a rate constant k11 of (6.5 +/- 1.5) x 10(-3) M(-1) s(-1), more than 7 orders of magnitude smaller than the rate constant [(1.1 +/- 0.2) x 10(5) M(-1) s(-1)] determined by 31P NMR for self-exchange between P(V)W12O40(3-) and its one-electron-reduced form, P(V)W12O40(4-). Sutin's semiclassical model reveals that this dramatic difference arises from the large negative charges of 1ox and 1red. These results, including independent verification of k11, recommend 1red as a well-behaved electron donor for investigating outer-sphere electron transfer to molecules or nanostructures in water, while addressing a larger issue, the prediction of collision rates between uniformly charged nanospheres, for which 1ox and 1red provide a working model.

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PubMed1996

Electronic Structure of Bis(2,4-pentanedionato-O,O')oxovanadium(IV). A Photoelectron Spectroscopy, Electronic Spectroscopy, and ab Initio Molecular Orbital Study.

The electronic structure of the title VO(acac)(2) complex has been investigated using effective core potential configuration interaction ab initio calculations, UV-photoelectron spectroscopy, and electronic spectroscopy. The metal-ligand bonding with the equatorial acac(-) ligands is dominated by sigma interactions involving the filled ligand orbitals and the empty orbitals of the d(1) vanadium(IV) ion. The oxovanadium interactions involve a larger metal-d participation thus resulting in a strong V-O bonding having partial triple-bond character. Additional three-orbital-four-electron stabilizing interactions involving the filled acac(-) MOs and the oxovanadium orbitals further reinforce both the axial and equatorial bonds. The unpaired metal-d electron is completely localized in the nonbonding d(x)()()2(-)(y)()()2 orbital. The low ionization energy of the photoelectron spectrum has been fully assigned on the basis of combined DeltaSCF and configuration interaction calculations. The same theoretical approach has, in addition, provided a good fitting of frequencies associated with "d-d" and charge transfer electronic transitions.

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PubMed2000

Resonance Raman and semiempirical electronic structure studies of an odd-electron dinickel tetraiminoethylenedimacrocycle complex.

Resonance Raman studies of Ni2TIED3+ (TIED = tetraiminoethylenedimacrocycle) reveal that many modes couple to the intense electronic transition centered at 725 nm, a feature that is nominally similar to the intense delocalized intervalence absorption bands observed in the same region for Fe2(TIED)L4(5+) and Ru2(TIED)L4(5+) (L is any of several axial ligands). Time-dependent spectral modeling of the Raman and absorption spectra for the nickel compound was undertaken to understand the electronic transition. We were unable to model the Raman and absorption spectra successfully with a single electronic transition, suggesting that the absorption band is made up of two overlapping transitions. Semiempirical electronic structure calculations corroborate the suggestion. Additionally, these calculations indicate that the transitions are in fact ligand-localized transitions, with little metal involvement and no charge-transfer character. Furthermore, the ground-state electronic structure is best described as an identical pair of NiII centers bridged by a radical anion rather than a three-site mixed-valence assembly. Previous EPR studies (McAuley and Xu, Inorg. Chem. 1992, 31, 5549) had indicated primarily ligand character for the radical. The assignments are consistent with the resonance Raman results where the dominant modes coupled to the transitions are assigned as totally symmetric bridge vibrations.

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PubMed2001

Energy distribution and redistribution and chemical reactivity. The generalized delta overlap-density method for ground state and electron transfer reactions: a new quantitative counterpart of electron-pushing.

A new approach to prediction of organic reactions and understanding of the electron flow involved in the reaction mechanisms is presented. The method developed permits comparison of electronic structures of species different in multiplicity, charge, and geometry based on use of spin- and charge-independent entities-"overlap corrected density matrices". The method utilizes the basis orbitals of one molecule A (e.g. reactant) in the computation of a second molecule, B, derived from the first by an approach to product. This then provides two Overlap-Density Matrices with a common set of basis (e.g. hybrid) orbitals. Subtraction of Overlap-Density Matrix B from Matrix A affords the Delta Overlap-Density Matrix. Each element of the Delta Overlap-Density matrix gives the change in electron population of a bond or of a single hybrid orbital. Molecule B may differ from A by the addition or loss of an electron, by stretching of a bond, by electronic excitation, or by some other perturbation. The Delta Overlap-Density matrices afford a detailed description of the reaction process and provide predictions of overall reactions including such subtleties as regiochemistry.

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PubMed2001

Dioxygen binding to deoxyhemocyanin: electronic structure and mechanism of the spin-forbidden two-electron reduction of o(2).

Spectroscopically calibrated DFT is used to investigate the reaction coordinate of O(2) binding to Hemocyanin (Hc). A reaction path is calculated in which O(2) approaches the binuclear copper site with increasing metal-ligand overlap, which switches the coordination mode from end-on eta(1)-eta(1), to mu-eta(1):eta(2), then to butterfly, and finally to the planar [Cu(2)(mu-eta(2):eta(2)O(2))] structure. Analysis of the electronic structures during O(2) binding reveals that simultaneous two-electron transfer (ET) takes place. At early stages of O(2) binding the energy difference between the triplet and the singlet state is reduced by charge transfer (CT), which delocalizes the unpaired electrons and thus lowers the exchange stabilization onto the separated copper centers. The electron spins on the copper(II) ions are initially ferromagnetically coupled due to close to orthogonal magnetic orbital pathways through the dioxygen bridging ligand, and a change in the structure of the Cu(2)O(2) core turns on the superexchange coupling between the coppers. This favors the singlet state over the triplet state enabling intersystem crossing. Comparison with mononuclear model complexes indicates that the protein matrix holds the two copper(I) centers in close proximity, which enthalpically and entropically favors O(2) binding due to destabilization of the reduced binuclear site. This also allows regulation of the enthalpy by the change of the Cu--Cu distance in deoxyHc, which provides an explanation for the O(2) binding cooperativity in Hc. These results are compared to our earlier studies of Hemerythrin (Hr) and a common theme emerges where the spin forbiddeness of O(2) binding is overcome through delocalization of unpaired electrons onto the metal centers and the superexchange coupling of the metal centers via a ligand bridge.

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PubMed2003

Mapping the influence of molecular structure on rates of electron transfer using direct measurements of the electron spin-spin exchange interaction.

The spin-spin exchange interaction, 2J, in a radical ion pair produced by a photoinduced electron transfer reaction can provide a direct measure of the electronic coupling matrix element, V, for the subsequent charge recombination reaction. We have developed a series of dyad and triad donor-acceptor molecules in which 2J is measured directly as a function of incremental changes in their structures. In the dyads the chromophoric electron donors 4-(N-pyrrolidinyl)- and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, 5ANI and 6ANI, respectively, and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked to the meta positions of a phenyl spacer to yield 5ANI-Ph-NI and 6ANI-Ph-NI. In the triads the same structure is used, except that the piperidine in 6ANI is replaced by a piperazine in which a para-X-phenyl, where X = H, F, Cl, MeO, and Me(2)N, is attached to the N' nitrogen to form a para-X-aniline (XAn) donor to give XAn-6ANI-Ph-NI. Photoexcitation yields the respective 5ANI(+)-Ph-NI(-), 6ANI(+)-Ph-NI(-), and XAn(+)-6ANI-Ph-NI(-) singlet radical ion pair states, which undergo subsequent radical pair intersystem crossing followed by charge recombination to yield (3)NI. The radical ion pair distances within the dyads are about 11-12 A, whereas those in the triads are about approximately 16-19 A. The degree of delocalization of charge (and spin) density onto the aniline, and therefore the average distance between the radical ion pairs, is modulated by the para substituent. The (3)NI yields monitored spectroscopically exhibit resonances as a function of magnetic field, which directly yield 2J for the radical ion pairs. A plot of ln 2J versus r(DA), the distance between the centroids of the spin distributions of the two radicals that comprise the pair, yields a slope of -0.5 +/- 0.1. Since both 2J and k(CR), the rate of radical ion pair recombination, are directly proportional to V(2), the observed distance dependence of 2J shows directly that the recombination rates in these molecules obey an exponential distance dependence with beta = 0.5 +/- 0.1 A(-)(1). This technique is very sensitive to small changes in the electronic interaction between the two radicals and can be used to probe subtle structural differences between radical ion pairs produced from photoinduced electron transfer reactions.

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PubMed2003

Selective oxygenation of ring-substituted toluenes with electron-donating and -withdrawing substituents by molecular oxygen via photoinduced electron transfer.

A ring-substituted toluene with an electron-withdrawing substituent, p-tolunitrile, is oxygenated by molecular oxygen to yield the corresponding aldehyde with tetrafluoro-p-dicyanobenzene as a photocatalyst under photoirradiation with an Hg lamp (lambda > 300 nm). The oxygenation of a ring-substituted toluene with an electron-donating substituent, p-xylene, by molecular oxygen is also achieved with 10-methyl-9-phenylacridinium ion as a photocatalyst under visible light irradiation, yielding p-tolualdehyde exclusively as the final oxygenated product. Both the oxygenation reactions are initiated by photoinduced electron transfer from the ring-substituted toluene to the singlet excited state of the photocatalyst. The reason for the high selectivity in the photocatalytic oxygenation of various toluene derivatives by molecular oxygen is discussed on the basis of the photoinduced electron transfer mechanism that does not involve the autoxidation process (radical chain reactions). The reactive intermediates in the photocatalytic cycle are successfully detected as the transient absorption spectra and the electron spin resonance spectra.

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PubMed2004

Effects of spiroconjugation on the calculated singlet-triplet energy gap in 2,2-dialkoxycyclopentane-1,3-diyls and on the experimental electronic absorption spectra of singlet 1,3-diphenyl derivatives. Assignment of the lowest-energy electronic transition of singlet cyclopentane-1,3-diyls.

The effect of a 2,2-ethylene-ketal functionality on the singlet-triplet energy gap (Delta E(ST)) and on the first electronic transition in singlet cyclopentane-1,3-diyls (1) has been investigated. UDFT calculations predict a significant increase in the preference for a singlet ground state in the diradical with the cyclic ketal at C2 (1g; Delta E(ST) = -6.6 kcal/mol in C(2) symmetry and -7.6 kcal/mol in C(2v) symmetry), compared to the 2,2-dihydroxy- and 2,2-dimethoxy-disubstituted diradicals (1d, Delta E(ST) = -3.6 kcal/mol in C(2) symmetry, and 1e, Delta E(ST) = -3.4 kcal/mol in C(2) symmetry). Spiroconjugation is shown to be responsible for the larger calculated value of absolute value Delta E(ST) in 1g, relative to 1d and 1e. A strong correlation between the calculated values of Delta E(ST) and the computed electronic excitation energies of the singlet diradicals is found for diradicals 1d, 1e, and 1g and for 2,2-difluorocyclopentane-1,3-diyl (1c). A similar correlation between Delta E(ST) and lambda(calcd) is predicted for the corresponding 1,3-diphenylcyclopentane-1,3-diyls 3, and the predicted blue shift in the spectrum of 3g, relative to 3e, has been confirmed by experimental comparisons of the electronic absorption spectra of the annelated derivatives 2c, 2e, and 2g in a glass at 77 K. The wavelength of the first absorption band in the singlet diradicals decreases in the order 2e (lambda(onset) = 650 nm) > 2g (lambda(onset) = 590 nm) > 2c (lambda(onset) = 580 nm). The combination of these computational and experimental results provides a sound basis for reassignment of the first electronic absorption band in singlet diradicals 2c, 2e, and 2g to the excitation of an electron from the HOMO to the LUMO of these 2,2-disubstituted derivatives of cyclopentane-1,3-diyl.

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PubMed2004

Donor-acceptor (electronic) coupling in the precursor complex to organic electron transfer: intermolecular and intramolecular self-exchange between phenothiazine redox centers.

Intermolecular electron transfer (ET) between the free phenothiazine donor (PH) and its cation radical (PH*+) proceeds via the [1:1] precursor complex (PH)(2)*+ which is transiently observed for the first time by its diagnostic (charge-resonance) absorption band in the near-IR region. Similar intervalence (optical) transitions are also observed in mixed-valence cation radicals with the generic representation: P(br)P*+, in which two phenothiazine redox centers are interlinked by p-phenylene, o-xylylene, and o-phenylene (br) bridges. Mulliken-Hush analysis of the intervalence (charge-resonance) bands afford reliable values of the electronic coupling element H(IV) based on the separation parameters for (P/P*+) centers estimated from some X-ray structures of the intermolecular (PH)(2)*+ and the intramolecular P(br)P*+ systems. The values of H(IV), together with the reorganization energies lambda derived from the intervalence transitions, yield activation barriers DeltaG(ET)() and first-order rate constants k(ET) for electron-transfer based on the Marcus-Hush (two-state) formalism. Such theoretically based values of the intrinsic barrier and ET rate constants agree with the experimental activation barrier (E(a)) and the self-exchange rate constant (k(SE)) independently determined by ESR line broadening measurements. This convergence validates the use of the two-state model to adequately evaluate the critical electronic coupling elements between (P/P*+) redox centers in both (a) intermolecular ET via the precursor complex and (b) intramolecular ET within bridged mixed-valence cation radicals. Important to intermolecular ET mechanism is the intervention of the strongly coupled precursor complex since it leads to electron-transfer rates of self-exchange that are 2 orders of magnitude faster (and activation barrier that is substantially lower) than otherwise predicted solely on the basis of Marcus reorganization energy.

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PubMed2005

Some electronic correlation effects in the topological analysis of the Laplacian of the electronic charge density in C-n-butonium cations.

In this work, we present a topological study of the Laplacian of the electronic density using a 6-311++G basis set, at Hartree-Fock (HF) and second-order Møller-Plesset (MP2) (full-electron and frozen-core) levels of theory, for the carbocations 2-C-n-butonium generated upon the insertion of a proton into the secondary C-C bond during the protonation of n-butane. The charge concentration, CC, critical points of the Laplacian distribution at each valence shell, VS, of carbon atoms, and the charge concentration closer to hydrogen atoms are studied. Also, the bonding critical points of the electronic density are analyzed. We analyze some effects that Coulomb correlation has on topological features of the electronic distribution. It is shown that they are mainly reflected in a decreasing of the charge concentrations at the VS and in a contraction of the VS to the nuclei. They are more pronounced over C-C bonds than in C-H bonds. The sensitivity of some parameters derived from this topological analysis to the correlation effect of core electrons and subtle effects related to hyperconjugative interactions are shown. Some consequences of different schemes (double and triple split-valence basis set with diffuse and polarization functions) in the definition of subtle VS charge concentrations at 3c-2e bond paths are presented. It is also demonstrated here how the facts that allow us to understand the MP2 stability order found in the carbocationic species 2-C-n-butonium > 1-C-n-butonium > 2-H-n-butonium > 1-H-n-butonium are similarly depicted at correlated and uncorrelated levels of calculation.

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PubMed2005

Ultrafast electron dynamics at metal interfaces: intraband relaxation of image state electrons as friction.

Two-photon photoemission of image potential states above monolayers of p-xylene/Ag(111) shows that electrons with different momenta have very different rise and decay rates as a function of parallel momentum. The dynamics are due to energy and momentum loss (intraband relaxation), which we model as a stochastic process isomorphic to the overdamped motion of a harmonic oscillator. The method extracts a friction coefficient from the data which can be explained by electron-electron scattering in a formalism based on the Lindhard dielectric function. One-electron excitations (interband transistions) dominate the dissipation mechanism, with a smaller contribution from collective electronic excitations (plasmons).

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PubMed2005

Electron density distributions calculated for the nickel sulfides millerite, vaesite, and heazlewoodite and nickel metal: a case for the importance of ni-ni bond paths for electron transport.

Bond paths and the bond critical point properties (the electron density (rho) and the Hessian of rho at the bond critical points (bcp's)) have been calculated for the bonded interactions comprising the nickel sulfide minerals millerite, NiS, vaesite, NiS(2), and heazlewoodite, Ni(3)S(2), and Ni metal. The experimental Ni-S bond lengths decrease linearly as the magnitudes of the properties each increases in value. Bond paths exist between the Ni atoms in heazlewoodite and millerite for the Ni-Ni separations that match the shortest separation in Ni metal, an indicator that the Ni atoms are bonded. The bcp properties of the bonded interactions in Ni metal are virtually the same as those in heazlewoodite and millerite. Ni-Ni bond paths are absent in vaesite where the Ni-Ni separations are 60% greater than those in Ni metal. The bcp properties for the Ni-Ni bonded interactions scatter along protractions of the Ni-S bond length-bcp property trends, suggesting that the two bonded interactions have similar characteristics. Ni-Ni bond paths radiate throughout Ni metal and the metallic heazlewoodite structures as continuous networks whereas the Ni-Ni paths in millerite, a p,d-metal displaying ionic and covalent features, are restricted to isolated Ni(3) rings. Electron transport in Ni metal and heazlewoodite is pictured as occurring along the bond paths, which behave as networks of atomic size wires that radiate in a contiguous circuit throughout the two structures. Unlike heazlewoodite, the electron transport in millerite is pictured as involving a cooperative hopping of the d-orbital electrons from the Ni(3) rings comprising Ni(3)S(9) clusters to Ni(3) rings in adjacent clusters via the p-orbitals on the interconnecting S atoms. Vaesite, an insulator at low temperatures and a doped semiconductor at higher temperatures, lacks Ni-Ni bond paths. The net charges conferred on the Ni and S atoms are about a quarter of their nominal charges for the atoms in millerite and vaesite with the net charge on Ni increasing with increasing Ni-S bond length. Reduced net charges are observed on the Ni atoms in heazlewoodite and are related to its Ni-Ni metal bonded interactions and to the greater covalent character of its bonds. Local energy density and bond critical point properties of the electron density distributions indicate that the Ni-S and Ni-Ni bonded interactions are intermediate in character between ionic and covalent.

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PubMed2003

Photoinduced electron transfer and electron-mediating systems from aromatic amines to triplet states of C60 and C70 in the presence of a viologen dication.

Photoinduced electron transfer between fullerenes (C60 and C70) and various aromatic amines (AA's) in the absence and presence of a viologen dication has been studied by the transient absorption method in the visible and near-IR regions. Electron-transfer takes place from AA's to the triplet states of fullerenes (3C60* and 3C70*) giving the anion radicals of fullerenes (C60*- and C70*-) and the radical cations of AA's (AA*+). The rate constants and efficiencies of electron transfer are quite high, because of the high electron-donor abilities of AA's as their low oxidation potentials indicate. The absorption bands of AA*+ appeared also in the near-IR region indicating that the radical-cation center (hole) delocalizes over the entire region of each AA. On addition of an octylviologen dication (OV2+) to C60/C70-AA systems, the electron-mediating process from C60*- and C70*- to OV2+ occurs yielding the viologen radical cation (OV*+) with longer lifetime.

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PubMed2003

Associative return electron transfer. A bond-coupled electron transfer in the photoreactions of cyclopropylamines.

The dynamics of the geminate radical-ion pairs formed by electron transfer to the excited states of cyanoanthracenes from 2-phenylcyclopropylamines are dominated by exothermic bond cleavage of the amine radical cations. Quantitative studies of product formation as a function of the energetics of the photochemical and corresponding thermal reactions provide support for a novel mechanism in which return electron transfer in the geminate pair occurs in concert with bond formation from the ring-opened radical cations. This bond-coupled electron transfer process is referred to as an associative return electron transfer reaction. The important features of the associative electron transfer process that explain the experimental observations are described in terms of potential energy surfaces and competition between adiabatic and non-adiabatic deactivation paths.

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PubMed2002

On the role of electron-ion recombination in low vacuum scanning electron microscopy.

Here we demonstrate the effects of electron-ion recombination on imaging signals utilized in low vacuum scanning electron microscopes (SEMs). The presented results show that, under normal operating conditions, recombination of ionized gas molecules with secondary electrons (SEs) suppresses a significant fraction of emitted electrons. If the ion flux (and hence the spatial dependence of the SE-ion recombination rate) is laterally inhomogeneous across the imaged region of a specimen, contrast in SE images can be influenced and in some cases (under conditions of high detector field strength and long ionic mean free path) dominated by variations in the recombination rate. Consequently, SE images of features such as topographic asperities can exhibit edge-darkening, leading to inversion of some topographic contrast. Recognition of the extent and nature of electron-ion recombination is required for a correct understanding of processes occurring in variable pressure SEMs and, subsequently, for models of image formation.

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