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PubMed1997

Nature and electronic structure of the Ni-X dinuclear center of Desulfovibrio gigas hydrogenase. Implications for the enzymatic mechanism.

The recent determination of the X-ray crystal structure of Desulfovibrio gigas hydrogenase has revealed that the active site is a Ni-X dinuclear center [Volbeda, A., Charon, M. H., Piras, C., Hatchikian, E. C., Frey, M., & Fontecilla-Camps, J. C. (1995) Nature 373, 580-587]. This unexpected result calls for a re-examination of the magnetic and redox properties that have been attributed previously to a mononuclear Ni center. We have used a combination of dosimetric and electron paramagnetic resonance (EPR) techniques to investigate the nature and the electronic structure of the Ni-X center in the redox forms of D. gigas hydrogenase giving EPR signals. The metal atom X was first shown to be Fe by accurate metal content analyses. Next, by determining the EPR characteristics of a polycrystal powder, it was shown that the redox form of the enzyme studied in the X-ray crystal experiments was essentially Ni-A. The temperature dependence of the Ni-A, Ni-B, Ni-C, and Ni-L EPR signals was studied over a large temperature range. No deviation from Curie's law could be detected, which places strong constraints upon the magnitude of the possible magnetic interactions between the Ni and Fe centers. When these results and the other available magnetic data are analyzed in the light of the crystal structure, it is concluded that the Fe center is diamagnetic in all the redox states of the enzyme. On the basis of these results, a mechanistic scheme consistent with a large body of experimental data can be proposed for Ni-containing hydrogenases.

Crystallography, X-Ray↗
PubMed2005

Electronic structure and band gaps in cationic heterocyclic oligomers. Multidimensional analysis of the interplay of heteroatoms, substituents, molecular length, and charge on redox and transparency characteristics.

Oxidative doping of extended pi-conjugated polymers and oligomers produces dramatic changes in optical and electrical properties, arising from polaron and soliton-derived midgap states. Despite the great importance of such changes for materials properties, far less is known about the cationic polaron states than about the neutral, semiconducting or insulating, undoped materials. The systematic, multifactor computational analysis of oligoheterocycles such as oligothiophenes, oligofurans, and oligopyrroles presented here affords qualitative and quantitative understanding of the interplay among skeletal substitution pattern, electronic structure, and the effective band gap reduction on p-doping. A simple linear relation is derived for predicting p-doped oligomer and polymer effective band gaps based on those of the neutral oligomers; this relationship confirms the effectiveness of a "fixed band" approximation and explains the counterintuitive increase of the effective band gap on p-doping of many small band gap oligomers. The present analysis also suggests new candidates for transparent conductive polymers and predicts limiting behavior of ionization potential, electron affinity, and other properties for various polyheterocyclic systems. The results yield insight into materials constraints in electrochromic polymers as well as on p- and n-type conductors and semiconductors.

Journal Article↗
PubMed2004

"Lagrange functions": a family of powerful basis sets for real-space order-N electronic structure calculations.

Plane waves have unparalleled simplicity and systematic convergence by a single monotonic parameter, the energy cutoff, but they are limited to speriodic systems and require Fourier transforms that scale as N(2)logN, where N is the number of atoms. Real-space methods for order-N scaling are computationally complex and convergence depends on several variables. Here we introduce and demonstrate "Lagrange functions" as a family of analytical, complete, and orthonormal basis sets that are suitable for efficient, accurate, real-space, order-N electronic structure calculations. Convergence is controlled by a single monotonic parameter, the dimension of the basis set, and computational complexity is lower than that of plane waves.

Journal Article↗
PubMed1980

Magnetic circular dichroism on oxygen complexes of hemoproteins: correlation between magnetic circular dichroism magnitude and electronic structures of oxygen complexes.

Magnetic circular dichroism (MCD) and natural circular dichroism (CD) spectra are reported for horseradish peroxidase Compounds II and III, and kangaroo myoglobin Compound II at pH values of 8.5 and 4.9. These compounds exhibited MCD spectra of apparent Faraday A term both in the Soret and Q regions, except for myoglobin compounds in the Soret region where intrinsic temperature dependence showed large contribution from Faraday C terms. Comparison of these data with the MCD spectra of the dioxygen complexes of hemoglobin (myoglobin) and cytochrome P-450 revealed that the magnitude of the apparent Faraday A term trough at the Q0-0 bands decreased in the order of O2 complexes of hemoglobin (myoglobin) ([theta]M not equal to 16) greater than horseradish peroxidase Compound III ([theta]M not equal to 8) greater than O2 complex of cytochrome P-450 ([theta]M not equal to 4). The [theta]M values of the oxygen complex of cytochrome P-450 is similar to those observed for the compounds II of horseradish peroxidase and kangaroo myoglobin. From these observations it was concluded that the magnitude of MCD, especially the trough depth of the Q0-0 band, has direct correlation to the electronic states of the oxygen complexes of the hemoproteins. The implication of the findings was discussed in terms of the iron electronic structures perturbed by the axial ligation.

Animals↗
PubMed2005

Spectroscopic and electronic structure studies of the trinuclear Cu cluster active site of the multicopper oxidase laccase: nature of its coordination unsaturation.

Laccase is a multicopper oxidase that contains four Cu ions, one type 1 (T1), one type 2 (T2), and a coupled binuclear type 3 Cu pair (T3). The T2 and T3 centers form a trinuclear Cu cluster that is the active site for O2 reduction to H2O. A combination of spectroscopic and DFT studies on a derivative where the T1 Cu has been replaced by a spectroscopically innocent Hg2+ ion has led to a detailed geometric and electronic structure description of the resting trinuclear Cu cluster, complementing crystallographic results. The nature of the T2 Cu ligation has been elucidated; this site is three-coordinate with two histidines and a hydroxide over its functional pH range (stabilized by a large inductive effect, cluster charge, and a hydrogen-bonding network). Both the T2 and T3 Cu centers have open coordination positions oriented toward the center of the cluster. DFT calculations show that the negative protein pocket (four conserved Asp/Glu residues within 12 A) and the dielectric of the protein play important roles in the electrostatic stability and integrity of the highly charged, coordinatively unsaturated trinuclear cupric cluster. These tune the ligand binding properties of the cluster, leading to its high affinity for fluoride and its coordination unsaturation in aqueous media, which play a key role in its O2 reactivity.

Copper↗
PubMed2004

Exploration of the electronic structure of dendrimerlike acetylene-bridged oligothiophenes by correlating Raman spectroscopy, electrochemistry, and theory.

A series of radial thiophene-based structures consisting of a central benzene or thiophene ring surrounded by acetylene-bridged terthienyl arms has been investigated by physical and theoretical methods. Fourier transform Raman spectroscopy of the neutral solids shows that the terthiophene arms are weakly coupled across the core (benzene plus acetylene groups) likely due to cross-conjugation or meta-conjugation effects that may prevent full delocalization. By increasing the number of arms around the central ring, the electronic structure of the molecules seems to be affected only at the core, whereas the outer terthiophene arms remain almost unaltered. Raman spectroelectrochemistry and quantum chemical calculations provide further insight into the charge delocalization of the oxidized species. There is no evidence to suggest that these oxidized forms, obtained upon electrochemical doping of the molecules, show charge delocalization across the core.

Acetylene↗
PubMed2006

Electronic structures of Pt-Co and Pt-Ru alloys for CO-tolerant anode catalysts in polymer electrolyte fuel cells studied by EC-XPS.

CO tolerance at pure Pt, Pt-Co, and Pt-Ru alloys was investigated by X-ray photoelectron spectroscopy combined with an electrochemical cell (EC-XPS) in order to discover a hint for designing higher performance anode catalysts. After the electrochemical stabilization and/or CO adsorption, these electrodes were immediately transferred to the XPS chamber without exposure to air to avoid contamination of the surfaces. It was revealed that alloying with Co or Ru modified the electronic structures of Pt atoms, resulting in a positive core level (CL) shift of Pt 4f(7/2) which could weaken the Pt-CO interaction. For the Pt-Co alloy electrode, the Pt 4f(7/2) CL shift remained after the electrochemical stabilization despite Co dissolution and formation of a Pt skin layer. Changes in surface core level shifts (DeltaSCLSs) induced by CO adsorption were evaluated and related to the CO adsorption energy. The values of DeltaSCLS at these alloys were smaller than that of pure Pt, indicating that Ru and Co are effective elements to weaken the bond strength of Pt-CO.

Adsorption↗
PubMed2006

Electronic structure and redox properties of the open-shell metal-carbide endofullerene Sc3C2@C80: a density functional theory investigation.

Density functional theory calculations have shown that the open-shell metal-carbide endofullerene Sc3C2@C80 has the valence state (Sc3+)3(C2)(3-)@C80(6-). A lot of low-lying isomers differing in geometries and locations of the endohedral [(Sc3+)3(C2)(3-)] cluster have been located, indicating unusual dual intramolecular dynamic behaviors of this endofullerene at room temperature. The electrochemical redox properties of this endofullerene have been elucidated in terms of electronic structure theory. Its redox states are found to follow the general charge-state formula (Sc3+)3C2(3-q)-@C80(6-) (q is the charge of the whole molecule ranging from +1 to -3), demonstrating the high charge flexibility of the endohedral metal-carbide cluster. The structure of the endohedral [(Sc3+)3C2(3-q)-)] cluster varies with the redox processes, shifting from a planar structure (for q = 0 and -1) to a trifoliate structure (for q = +1, -2, -3).

Journal Article↗
PubMed2006

Identification of C5Hx isomers in fuel-rich flames by photoionization mass spectrometry and electronic structure calculations.

The isomeric composition of C(5)H(x) (x = 2-6, 8) flame species is analyzed for rich flames fueled by allene, propyne, cyclopentene, or benzene. Different isomers are identified by their known ionization energies and/or by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine flame-sampling molecular-beam mass spectrometry with photoionization by tunable vacuum-UV synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) electronic structure calculations extrapolated to the complete basis set limit. For C(5)H(3), the comparison reveals the presence of both the H(2)CCCCCH (i-C(5)H(3)) and the HCCCHCCH (n-C(5)H(3)) isomer. The simulations also suggest a modest amount of cyclo-CCHCHCCH-, which is consistent with a minor signal for C(5)H(2) that is apparently due to cyclo-CCHCCCH-. For C(5)H(4), contributions from the CH(2)CCCCH(2) (1,2,3,4-pentatetraene), CH(2)CCHCCH, and CH(3)CCCCH (1,3-pentadiyne) isomers are evident, as is some contribution from CHCCH(2)CCH (1,4-pentadiyne) in the cyclopentene and benzene flames. Signal at m/z = 65 originates mainly from the cyclopentadienyl radical. For C(5)H(6), contributions from cyclopentadiene, CH(3)CCCHCH(2), CH(3)CHCHCCH, and CH(2)CHCH(2)CCH are observed. No signal is observed for C(5)H(7) species. Cyclopentene, CH(2)CHCHCHCH(3) (1,3-pentadiene), CH(3)CCCH(2)CH(3) (2-pentyne), and CH(2)CHCH(2)CHCH(2) (1,4-pentadiene) contribute to the signal at m/z = 68. Newly derived ionization energies for i- and n-C(5)H(3) (8.20 +/- 0.05 and 8.31 +/- 0.05 eV, respectively), CH(2)CCHCCH (9.22 +/- 0.05 eV), and CH(2)CHCH(2)CCH (9.95 +/- 0.05 eV) are within the error bars of the QCISD(T) calculations. The combustion chemistry of the observed C(5)H(x) intermediates and the impact on flame chemistry models are discussed.

Journal Article↗
PubMed1996

Electronic Structure of Rh(2)(&mgr;-CO)(CO)(2)(H(2)PCH(2)PH(2))(2). An Example of a Non-A-Frame Structure.

Calculations based on density functional theory (DFT) and Hartree-Fock configuration interaction (HF-CI) methodology have been carried out to investigate the rhodium-rhodium coupling in Rh(2)(CO)(2)(dppm)(2), 1 (dppm = Ph(2)PCH(2)PPh(2)) and in Rh(2)(&mgr;-CO)(CO)(2)(dppm)(2), 2. DFT geometries, obtained with the Dgauss program, are in good agreement with those determined from X-ray, but HF geometries, calculated using the same basis sets, yield bond distances systematically too long. Calculations indicate that the rhodium atoms in 1 are linked by a single bond. The insertion of a semibridging carbonyl between the two metal atoms leads to a shortening of the rhodium-rhodium distance and also to a noticeable weakening of the metal-metal interaction. Both effects, and also the stabilization of the HOMO of 2, are related to an observed change from square planar to tetrahedral of the ligand environment of the Rh atom proximal to the inserted CO. Both MO analysis and bond characterization from the topology of the charge density confirm the existence of a bonding interaction between the semibridging carbonyl and the distal rhodium atom. The electronic structures of the dicationic complex [Rh(2)(CO)(3)(dppm)(2)](2+) and of the A-frame-like, isoelectronic system Rh(2)Br(2)(&mgr;-CO) (dppm)(2) are also discussed. The electron deformation density is derived from 2 by means of several methodological approaches, namely, HF, HF-CI, DFT, and DFT + gradient corrections. The HF deformation density obtained in the plane containing the metals and the three CO ligands is discussed, as well as the "correlation density" obtained from the difference maps DFT - HF and CI - HF.

Journal Article↗
PubMed2005

Modulation of the electronic structure and the Ni-Fe distance in heterobimetallic models for the active site in [NiFe]hydrogenase.

Reaction of the mononuclear Ni(II) thiolate complexes [Ni(L)] [L, L(1), H2L(1), bis(2-mercaptoethyl)-1,2-dimercaptoethane; L(2), H2L(2), N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)-bis(aminoethyl)sulfide] with [FeCp(CO)2I] gives the dithiolate-bridged heterobimetallic species, [Ni(L(1))FeCp(CO)]PF6, 1, and [Ni(L(2))FeCp]I, 2, respectively. Binding of a Fe(CO)3 fragment via reaction of square-planar [Ni(pdt)(dppe)] (dppe, 1,2-diphenylphosphinoethane; pdt(2-), 1,3-propanedithiolate) with Fe3(CO)12 or [Fe(CO)3(BDA)] (BDA, benzylidene acetone) affords diamagnetic [(dppe)Ni(mu-pdt)Fe(CO)3], 3, in which the Ni(II) center is bound tetrahedrally to two thiolate S-donors and to two P-donors. The complex [(dppe)Ni(mu-pdt)Fe(CO)3], 3, reacts in solution via rearrangement to afford [(OC)Ni(mu-dppe)(mu-pdt)Fe(CO)2], 4, in which one P-donor of dppe is bound to Ni and the other to Fe, and a CO ligand has transferred from Fe to Ni. Additionally, the syntheses of 3 and 4 afford the side products [(dppe)Ni(CO)2] and [(OC)3Fe(pdt)Fe(CO)3] together with the trinuclear species [(dppe)(CO)Fe(mu-CO)(mu-pdt)Fe(mu-pdt)Fe(CO)3], 5. Reaction of [Ni(pdt)(dppe)] with [FeCp(CO)2I] in CH2Cl2 affords two products [(dppe)Ni(mu-pdt)FeCp(CO)]PF6, 6, and [(dppe)Ni(pdt)(mu-I)Ni(dppe)]PF6, 7. The complexes 2, 3, and 4 show Ni-Fe distances of 2.539(4), 2.4666(6), and 2.4777(7) A, respectively, with relatively acute dihedral angles of 79.5-81.8 degrees for the Ni-S2-Fe bridge, thus mimicking the shortened Ni...Fe distance (2.5 A) and the acute dihedral angle of the Ni-S2-Fe moiety observed in certain active forms of [NiFe]hydrogenase. The role of direct Ni-Fe bonding in these complexes is discussed and linked to electronic structure calculations on [(dppe)Ni(pdt)Fe(CO)3], 3, which confirm the presence of a bent Ni(d(z2))-Fe(d(z2)) sigma-bond in a singlet ground state.

Binding Sites↗
PubMed2004

Electronic structure control of the nucleophilicity of transition metal-thiolate complexes: an experimental and theoretical study.

New metal(II)-thiolate complexes supported by the tetradentate ligand 1,5-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8)py(2)) have been synthesized and subjected to physical, spectroscopic, structural, and computational characterization. The X-ray crystal structures of these complexes, [L(8)py(2)M(S-C(6)H(4)-p-CH(3))]BPh(4) (M = Co, Ni, Zn), reveal distorted square-pyramidal divalent metal ions with four equatorial nitrogen donors from L(8)py(2) and axial p-toluenethiolate ligands. The reactions of the complexes with benzyl bromide produce isolable metal(II)-bromide complexes (in the cases of Co and Ni) and the thioether benzyl-p-tolylsulfide. This reaction is characterized by a second-order rate law (nu = k(2)[L(8)py(2)M(SAr)(+)][PhCH(2)Br]) for all complexes (where M = Fe, Co, Ni, or Zn). Of particular significance is the disparity between k(2) for M = Fe and Co versus k(2) for M = Ni and Zn, in that k(2) for M = Ni and Zn is ca. 10 times larger (faster) than k(2) for M = Fe and Co. An Eyring analysis of k(2) for [L(8)py(2)Co(SAr)](+) and [L(8)py(2)Ni(SAr)](+) reveals that the reaction rate differences are not rooted in a change in mechanism, as the reactions of these complexes with benzyl bromide exhibit comparable activation parameters (M = Co: DeltaH() = 45(2) kJ mol(-)(1), DeltaS() = -144(6) J mol(-)(1) K(-)(1); M = Ni: DeltaH() = 43(3) kJ mol(-)(1), DeltaS() = -134(8) J mol(-)(1) K(-)(1)). Electronic structure calculations using density functional theory (DFT) reveal that the enhanced reaction rate for [L(8)py(2)Ni(SAr)](+) is rooted in a four-electron repulsion (or a "filled/filled interaction") between a completely filled nickel(II) d(pi) orbital and one of the two thiolate frontier orbitals, a condition that is absent in the Fe(II) and Co(II) complexes. The comparable reactivity of [L(8)py(2)Zn(SAr)](+) relative to that of [L(8)py(2)Ni(SAr)](+) arises from a highly ionic zinc(II)-thiolate bond that enhances the negative charge density on the thiolate sulfur. DFT calculations on putative thioether-coordinated intermediates reveal that the Co(II)- and Zn(II)-thioethers exhibit weaker M-S bonding than Ni(II). These combined results suggest that while Ni(II) may serve as a competent replacement for Zn(II) in alkyl group transfer enzymes, turnover may be limited by slow product release from the Ni(II) center.

Computer Simulation↗
PubMed2002

Tuning the electronic structure of octahedral iron complexes [FeL(X)] (L = 1-alkyl-4,7-bis(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, X = Cl, CH(3)O, CN, NO). The S = 1/2 <==>3/2 Spin equilibrium of [FeL(Pr)(NO)].

Two new pentadentate, pendent arm macrocyclic ligands of the type 1-alkyl-4,7-bis(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane where alkyl represents an isopropyl, (L(Pr))(2-), or an ethyl group, (L(Et))(2-), have been synthesized. It is shown that they bind strongly to ferric ions generating six-coordinate species of the type [Fe(L(alk))X]. The ground state of these complexes is governed by the nature of the sixth ligand, X: [Fe(III)(L(Et))Cl] (2) possesses an S = 5/2 ground state as do [Fe(III)(L(Et))(OCH(3))] (3) and [Fe(III)(L(Pr))(OCH(3))] (4). In contrast, the cyano complexes [Fe(III)(L(Et))(CN)] (5) and [Fe(III)(L(Pr))(CN)] (6) are low spin ferric species (S = 1/2). The octahedral [FeNO](7) nitrosyl complex [Fe(L(Pr))(NO)] (7) displays spin equilibrium behavior S = 1/2<==>S = (3)/(2) in the solid state. Complexes [Zn(L(Pr))] (1), 4.CH(3)OH, 5.0.5toluene.CH(2)Cl(2), and 7.2.5CH(2)Cl(2) have been structurally characterized by low-temperature (100 K) X-ray crystallography. All iron complexes have been carefully studied by zero- and applied-field Mössbauer spectroscopy. In addition, Sellmann's complexes [Fe(pyS(4))(NO)](0/1+) and [Fe(pyS(4))X] (X = PR(3), CO, SR(2)) have been studied by EPR and Mössbauer spectroscopies and DFT calculations (pyS(4) = 2,6-bis(2-mercaptophenylthiomethyl)pyridine(2-)). It is concluded that the electronic structure of 7 with an S = 1/2 ground state is low spin ferrous (S(Fe) = 0) with a coordinated neutral NO radical (Fe(II)-NO) whereas the S = 3/2 state corresponds to a high spin ferric (S(Fe) = 5/2) antiferromagnetically coupled to an NO(-) anion (S = 1). The S = 1/2<==>S = 3/2 equilibrium is then that of valence tautomers rather than that of a simple high spin<==>low spin crossover.

Journal Article↗
PubMed2003

Valence compounds versus metals. Synthesis, characterization, and electronic structures of cubic Ae(4)Pn(3) phases in the systems Ae = Ca, Sr, Ba, Eu; Pn = As, Sb, Bi.

The isostructural compounds Sr(4)Bi(3), Ba(4)Bi(3), and Ba(4)As( approximately )(2.60) were prepared by direct reactions of the corresponding elements and their structures determined from single-crystal X-ray diffraction data as anti-Th(3)P(4) type in the cubic space group I43d, Z = 4 (a = 10.101(1) A, 10.550(1) A, 9.973 (1) A, respectively). The two bismuth compounds are stoichiometric, and the arsenide refines as Ba(4)As(2.60(2)). Only unrelated phases are obtained for all binary combinations among the title components for either Ca or Sb. The magnetic susceptibility and resistivities of Ba(4)Bi(3) and Eu(4)Bi(3) show that they are good metallic conductors ( approximately 40 microomega.cm at 298 K), whereas Ba(4)As(2.60) exhibits rho(150) > 1000 microomega.cm. The electronic structures of Sr(4)Bi(3), Ba(4)Bi(3), and Ba(4)As(3) were calculated by TB-LMTO-ASA methods. Mixing of cation d states into somewhat disperse valence p bands on Bi results in empty bands at E(F) and metallic behavior, whereas the narrower valence band in the electron-deficient Ba(4)As(3) leads to vacancies in about 11% of the anion sites and a valence compound.

Journal Article↗
PubMed2004

Molecular and electronic structure of four- and five-coordinate cobalt complexes containing two o-phenylenediamine- or two o-aminophenol-type ligands at various oxidation levels: an experimental, density functional, and correlated ab initio study.

The bidentate ligands N-phenyl-o-phenylenediamine, H(2)((2)L(N)IP), or its analogue 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol, ((4)L(N)IP), react with [Co(II)(CH(3)CO(2))(2)]4H(2)O and triethylamine in acetonitrile in the presence of air yielding the square-planar, four-coordinate species [Co((2)L(N))(2)] (1) and [Co((4)L(O))(2)] (4) with an S=1/2 ground state. The corresponding nickel complexes [Ni((4)L(O))(2)] (8) and its cobaltocene reduced form [Co(III)(Cp)(2)][Ni((4)L(O))(2)] (9) have also been synthesized. The five-coordinate species [Co((2)L(N))(2)(tBu-py)] (2) (S=1/2) and its one-electron oxidized forms [Co((2)L(N))(2)(tBu-py)](O(2)CCH(3)) (2 a) or [Co((2)L(N))(2)I] (3) with diamagnetic ground states (S=0) have been prepared, as has the species [Co((4)L(O))(2)(CH(2)CN)] (7). The one-electron reduced form of 4, namely [Co(Cp)(2)][Co((4)L(O))(2)] (5) has been generated through the reduction of 4 with [Co(Cp)(2)]. Complexes 1, 2, 2 a, 3, 4, 5, 7, 8, and 9 have been characterized by X-ray crystallography (100 K). The ligands are non-innocent and may exist as catecholate-like dianions ((2)L(N)IP)(2-), ((4)L(N)IP)(2-) or pi-radical semiquinonate monoanions ((2)L(N)ISQ)(*) (-), ((4)L(N)ISQ)(*) (-) or as neutral benzoquinones ((2) L(N)IBQ)(0), ((4) L(N)IBQ)(0); the spectroscopic oxidation states of the central metal ions vary accordingly. Electronic absorption, magnetic circular dichroism, and EPR spectroscopy, as well as variable temperature magnetic susceptibility measurements have been used to experimentally determine the electronic structures of these complexes. Density functional theoretical (DFT) and correlated ab initio calculation have been performed on the neutral and monoanionic species [Co((1)L(N))(2)](0,-) in order to understand the structural and spectroscopic properties of complexes. It is shown that the corresponding nickel complexes 8 and 9 contain a low-spin nickel(II) ion regardless of the oxidation level of the ligand, whereas for the corresponding cobalt complexes the situation is more complicated. Spectroscopic oxidation states describing a d(6) (Co(III)) or d(7) (Co(II)) electron configuration cannot be unambiguously assigned.

Journal Article↗
PubMed2006

Theoretical insight into electronic structures and spectroscopic properties of [Pt2(pop)4]4-, [Pt2(pcp)4]4-, and related derivatives (pop = P2O5H2(2-) and pcp = P2O4CH4(2-)).

The structures of [Pt2(pop)4]4-, [Pt2(pcp)4]4-, and related species [Pt2(pop)4X2]4- and [Pt2(pop)4]2- in the ground states (pop = P2O5H2(2-), pcp = P2O4CH4(2-), and X = I, Br, and Cl) were optimized using the second-order Møller-Plesset perturbation (MP2) method. It is shown that the Pt-Pt distances decrease in going from [Pt2(pop)4]4- to [Pt2(pop)4X2]4- to [Pt2(pop)4]2-. This is supported by the analyses of their electronic structures. The calculated aqueous absorption spectra at the time-dependent density functional theory (TD-DFT) level agree with experimental observations. The unrestricted MP2 method was employed to optimize the structures of [Pt2(pop)4]4- and [Pt2(pcp)4]4- in the lowest-energy triplet excited states. The Pt-Pt contraction trend is well reproduced in these calculations. For [Pt2(pop)4]4-, the Pt-Pt distance decreases from 2.905 A in the ground state to 2.747 A in the excited state, which is comparable to experimental values of 2.91-2.92 A and 2.64-2.71 A, respectively. On the basis of the excited-state structures of such complexes, TD-DFT predicts the solution emissions at 480 and 496 nm, which is closer to the experimental values of 512 and 510 nm emissions, respectively.

Journal Article↗
PubMed2006

Evolution of electronic structure and spectral evaluation in single-crystal Mn3O4 nanorods.

Single-crystal Mn3O4 nanorods with tetragonal structure have been successfully prepared by a chemical reaction route. Transmission electron microscopy (TEM) and high-resolution TEM studies prove that the single-crystal Mn3O4 nanorod is smooth and straight, and that the geometrical shape is structurally perfect. We investigated the electronic characteristics of Mn3O4 nanorods by various spectral evaluations. The present study confirms that the hybridization between oxygen 2p and manganese 3d orbits plays an important role when considering electronic structures of Mn3O4 nanorods.

Journal Article↗
PubMed2006

Syntheses of [F5TeNH3][AsF6], [F5TeN(H)Xe][AsF6], and F5TeNF2 and characterization by multi-NMR and Raman spectroscopy and by electronic structure calculations: the X-ray crystal structures of alpha- and beta-F5TeNH2, [F5TeNH3][AsF6], and [F5TeN(H)Xe][AsF6].

The salt, [F5TeN(H)Xe][AsF6], has been synthesized in the natural abundance and 99.5% 15N-enriched forms. The F5TeN(H)Xe+ cation has been obtained as the product of the reactions of [F5TeNH3][AsF6] with XeF2 (HF and BrF5 solvents) and F5TeNH2 with [XeF][AsF6] (HF solvent) and characterized in solution by 129Xe, 19F, 125Te, 1H, and 15N NMR spectroscopy at -60 to -30 degrees C. The orange [F5TeN(H)Xe][AsF6] and colorless [F5TeNH3][AsF6] salts were crystallized as a mixture from HF solvent at -35 degrees C and were characterized by Raman spectroscopy at -165 degrees C and by X-ray crystallography. The crystal structure of the low-temperature phase, alpha-F5TeNH2, was obtained by crystallization from liquid SO2 between -50 and -70 degrees C and is fully ordered. The high-temperature phase, beta-F5TeNH2, was obtained by sublimation at room temperature and exhibits a 6-fold disorder. Decomposition of [F5TeN(H)Xe][AsF6] in the solid state was rapid above -30 degrees C. The decomposition of F5TeN(H)Xe+ in HF and BrF5 solution at -33 degrees C proceeded by fluorination at nitrogen to give F5TeNF2 and Xe gas. Electronic structure calculations at the Hartree-Fock and local density-functional theory levels were used to calculate the gas-phase geometries, charges, Mayer bond orders, and Mayer valencies of F5TeNH2, F5TeNH3+, F5TeN(H)Xe+, [F5TeN(H)Xe][AsF6], F5TeNF2, and F5TeN2- and to assign their experimental vibrational frequencies. The F5TeN(H)Xe+ and the ion pair, [F5TeN(H)Xe][AsF6], systems were also calculated at the MP2 and gradient-corrected (B3LYP) levels.

Journal Article↗