Charge Transfer Region Research Articles

VARIETY OF STRUCTURES OF BINUCLEAR CHIRAL SCHIFF BASE CE(III)/PR(III)/LU(III)-NI(II)/CU(II)/ZN(II) COMPLEXE

We have prepared and characterized several 3d-4f (Ni(II), Cu(II) and Zn(II) transition metal ions and Ce(III), Pr(III), and Lu(III) lanthanide metal ions) complexes incorpolating chiral Schiff base ligands (abbreviated as CeNi , CeCu , CeZn , PrNi , PrCu , PrZn , LuNi , LuCu , and LuZn , respectively). Solid-state CD spectra and diffuse reflectance electronic spectra exhibited reasonable shift by combination of 3d and 4f ions at charge-transfer region. Magnetic features (antiferromagnetic interactions, if any) also exhibited substitution of 3d-4f ions of siamagnetic or paramagnetic spin numbers. Interestingly, determined crystal structures of some of them exhibited different structural chacters about coordination numbers, crystalline solvents, and dimetic (or bridging) features nevertheless of 3d-4f metal substituion in identical organic ligands.

ChemInform Abstract: Charge-Transfer Spectroscopy and Photochemistry of Alkylamine Cobalt( III) Complexes

Abstract Spectroscopic and photochemical studies are reported for Co(NH2R)63+, Co(NH2R)5OH23+, and Co(NH2R)5X2+ (where R is methyl, ethyl, or n-propyl, and X is Cl− or Br−). These complexes absorb strongly in the ultraviolet region owing to the presence of one or more ligand-to-metal charge transfer (LMCT) band(s). Irradiation into this charge transfer region results in redox decomposition of the complex with the release of Co2+, alkylamine, and a radical. Photoredox quantum yields measured in fluid solution are generally high and dependent upon excitation wavelength, oxygen concentration, temperature, and solution viscosity. In contrast to their well-studied ammine analogues, the alkylamine complexes (1) possess a N → Co charge transfer band at wavelengths well above 200 nm, (2) undergo redox decomposition from the X → Co and N → Co charge transfer excited states, and (3) avoid an intermolecular decomposition pathway involving photooxidation of the solvent. Reasons for these differences between the two families of complexes are discussed.

Binding Modes of New Bis-Ru(II) Complexes to DNA: Effect of the Length of the Linker

Bis-[dipyrido[3,2-<TEX>$\alpha$</TEX>:2',3'-c]phenazine)<TEX>$_2$</TEX>(1,10-phenanthroline)<TEX>$_2Ru_2$</TEX>]<TEX>$^{2+}$</TEX> complexes (bis-Ru(II) complexes) tethered by linkers of various lengths were synthesized and their binding properties to DNA investigated by normal absorption and linear dichroism spectra, and fluorescence techniques in this study. Upon binding to DNA, the bis-Ru(II) complex with the longest linker (1,3-bis-(4-pyridyl)-propane), exhibited a negative <TEX>$LD^r$</TEX> signal whose intensity was as large as that in the DNA absorption region, followed by a complicate <TEX>$LD^r$</TEX> signal in the metal-to-ligand charge transfer region. The luminescence intensity of this bis-Ru(II) complex was enhanced. The observed <TEX>$LD^r$</TEX> and luminescence results resembled that of the [Ru(1,10-phenanthroline)<TEX>$_2$</TEX> dipyrido[3,2-<TEX>$\alpha$</TEX>:2',3'-c]phenazine]<TEX>$^{2+}$</TEX> complex, whose dipyrido[3,2-<TEX>$\alpha$</TEX>:2',3'-c]phenazine (dppz) ligand has been known to intercalate between DNA bases. Hence, it is conclusive that both dppz ligands of the bis-Ru(II) complex intercalate. The binding stoichiometry, however, was a single intercalated dppz per ~ 2.3 bases, which violates the "nearest binding site exclusion" model for intercalation. The length between the two Ru(II) complexes may be barely long enough to accommodate one DNA base between the two dppz ligands, but not for two DNA bases. When the linker was shorter (4,4'-bipyridine or 1,2-bis-(4-pyridyl)-ethane), the magnitude of the LD in the dppz absorption region, as well as the luminescence intensity of both bis-Ru(II) complexes, was half that of the bis-Ru(II) complex bearing a long linker. This observation can be elucidated by a model whereby one of the dppz ligands intercalates while the other is exposed to the aqueous environment.

Optical conductivity and x-ray absorption spectra of the Mott-Hubbard compoundsRVO3(R=Sr, Ca, La, and Y)

Core-level and valence-band optical spectra provide important information on highly correlated systems. The former corresponds to transitions from the core level to the conduction band, and it is usually related to the unoccupied electronic structure. The later corresponds to transitions from the valence band to the conduction band, and it is sometimes described in terms of the joint density of states. These spectra are usually treated separately due to the differences in the experimental and theoretical methods. We present here a combined description of the core-level and valence-level optical spectra of Mott-Hubbard compounds. In particular, we studied the $\text{O}\text{ }1s$ x-ray absorption and the optical-conductivity spectra of ${\text{SrVO}}_{3}{\text{-CaVO}}_{3}{\text{-LaVO}}_{3}{\text{-YVO}}_{3}$. The experimental data were analyzed using an extended $p\text{\ensuremath{-}}d$ cluster model solved by an exact diagonalization method. The results show that correlation effects alone cannot account for the experimental structures, and that crystal-field effects and exchange interactions are necessary to explain the spectra. We also show that there is a correspondence between the features in the charge-transfer region of both spectra.

DNA-Binding and Physical Studies of Pt(4′-NR2-trpy)CN+ Systems (trpy = 2,2′:6′,2′′-Terpyridine)

This paper focuses on DNA-binding interactions exhibited by Pt(dma-T)CN(+), where dma-T denotes 4'-dimethylamino-2,2':6',2''-terpyridine, and includes complementary studies of the corresponding pyrr-T complex, where pyrr-T denotes 4'-(N-pyrrolidinyl)-2,2':6',2''-terpyridine. The chromophores are useful for understanding the interesting and rather intricate DNA-binding interactions exhibited by these and related systems. One reason is that the terpyridine ligands employed provide intense visible absorption and enhanced photoluminescence signals. Incorporating cyanide as a coligand further aids analysis by suppressing covalent binding. Physical methods utilized include X-ray crystallography for structures of the individual inorganic complexes. Viscometry as well as spectral studies of the absorbance, emission, and circular dichroism (CD) yield information about interactions with a variety of DNA hosts. Although there is no sign of covalent binding under the conditions used, most hosts exhibit two phases of uptake. Under conditions of high loading (low base-pair-to-platinum ratios), the dma-T complex preferentially binds externally and aggregates on the surface of the host, except for the comparatively rigid host [poly(dG-dC)]2. Characteristic signs of the aggregated form include a bisignate CD signal in the charge-transfer region of the spectrum and strongly bathochromically shifted emission. When excess DNA is present, however, the complex shifts to intercalative binding, preferentially next to G[triple bond]C base pairs if available. Once the complex internalizes into DNA it becomes virtually immune to quenching by O2 or solvent, and the emission lifetime extends to 11 micros when [poly(dI-dC)]2 is the host. On the other hand, the host itself becomes a potent quenching agent when G[triple bond]C base pairs are present because of the reducing strength of guanine residues.

Modeling Distributed Charge-Transfer Processes in SOFC Membrane Electrode Assemblies

A model is developed to represent chemistry and transport in porous mixed ionic-and-electronic conducting composite electrode structures in solid oxide fuel cells (SOFC). The model considers the coupled behavior of a full membrane electrode assembly (MEA, i.e., cathode, electrolyte, and anode), which is an important advance compared to earlier models that consider only a single electrode structure. Within each electrode the model represents parallel conduction of electrons and ions, as well as porous-media, chemically reacting gas transport. The model predicts electric-potential distributions in both phases. Charge-transfer chemistry is handled via a modified Butler–Volmer formalism, which depends on the local electric-potential difference between phases. Heterogeneous chemistry (e.g., reforming or partial oxidation) is handled via a detailed surface-reaction mechanism. For typical composite-electrode structures the charge-transfer region extends about 10– from the dense electrolyte. The results show cell performance depends upon particle sizes within the porous electrodes. Smaller particles generally improve cell performance as a result of expanded three-phase-boundary length. However, smaller particle sizes impede gas transport. Cell performance can be optimized as a function of functional-layer thickness and particle sizes. A distributed charge-transfer formulation is especially important in advanced thin-film electrode structures (e.g., segmented-in-series architectures) where the entire MEA is only a few tens of micrometers thick. The model is formulated as continuum differential equations, which are solved computationally on a discrete mesh network. The paper illustrates the model with examples comparing alternative MEA structures.

Modeling Distributed Charge-Transfer Processes in Membrane Electrode Assemblies with Mixed-Conducting Composite Electrodes

A model is developed to represent chemistry and transport in a membrane- electrode assembly with porous mixed ionic-and-electronic conducting composite electrodes. Reactive porous media transport within electrodes is represented with a Dusty-Gas model. Charge-transfer chemistry is handled via a modified Butler- Volmer formalism, which depends on the local electric-potential differences. Heterogeneous chemistry (e.g., reforming or partial oxidation) is handled via a detailed surface-reaction mechanism. For typical composite- electrode structures the charge-transfer region extends about 10 to 20 microns from the dense electrolyte. The model is formulated as continuum differential equations, which are solved computationally on a discrete mesh network.

Rapid-Freeze-Quench Magnetic Circular Dichroism of Intermediate X in Ribonucleotide Reductase: New Structural Insight

To elucidate the electronic structure of intermediate X in the oxygen activation reaction of the R2 subunit of ribonucleotide reductase, a protocol has been developed to perform magnetic circular dichroism (MCD) on a rapid-freeze-quench, strain free optical sample. RFQ-MCD data have been collected on intermediate X in the double mutant of R2, Y122/Y356F. While X has been reported to exhibit a broad absorption band at 365 nm, there are at least 10 electronic transitions observed at low-temperature MCD. From C0/D0 ratios, the transitions of X can be divided into three regions: 16 000-22 000 cm-1 region involving spin-allowed ligand field transitions of the Fe(IV), 23 000-24 000 cm-1 region of spin-forbidden, spin-flip transitions on the Fe(IV), and the charge transfer (CT) region from 26 000 to 32 000 cm-1. The C0/D0 ratios from d --> d and CT transitions strongly support significant Fe(IV) character coupled into the paramagnetic center. Ligand field (spin-allowed d --> d region) analysis allows the bis-mu-oxo and mu-oxo plus other monoanionic bridge possibilities for the structure of intermediate X to be distinguished, providing new insight into the molecular mechanism of the cluster formation in R2.

Zinc-mediated Amino Acid Discrimination in Cysteinyl-tRNA Synthetase

Escherichia coli cysteinyl-tRNA synthetase (CysRS) achieves a high level of amino acid specificity without an editing reaction. The crystal structure of CysRS bound to substrate cysteine suggested that direct thiol coordination to a tightly bound zinc ion at the base of the active site is the primary determinant of selectivity against non-cognate amino acids. This hypothesis has now been supported by spectroscopic studies of cobalt-substituted CysRS. Binding of cysteine, but not non-cognate amino acids, induces high absorption in the ligand-to-metal charge transfer region, providing evidence for formation of a metal–thiolate bond. In addition, mutations in the zinc ligands alter the absorption spectrum without reducing the discrimination against non-cognate amino acids. These results argue strongly for a major role for the zinc ion in amino acid discrimination by CysRS, where the tight zinc–thiolate interaction and the strict structural geometry of the metal ion are sufficient to reject serine by more than 20,000-fold at the binding step.

Charge Transfer Luminescence in Ce-Doped Fluorides

New emission bands were found in Ce3☎ doped SrF2 and BaF2 crystals under excitation into the charge transfer region. The bands at 4.85 eV in BaF2 and 4.35 eV SrF2 were found in all crystals with Ce-concentration from 0.001 to 1 mol.%, most prominent in 0.01% samples. Decay of luminescence was simple exponential in BaF2, decay time decreased from 1.3 ms at 65 K to 0.41 μs at 523 K. The decay curve in SrF2 shows t−0.3 dependence at room temperature. All experimental results were naturally explained by assumption that new emission belongs to radiative charge transfer recombination in Ce2☎—Fio pairs with different distances between them.

Y2O2S : Eu3+nanocrystals—synthesis and luminescent properties

Luminescent Y2O2S : Eu3+ nanoceramics having an average particle size of 20 nm has been prepared through a gel–polymer thermolysis process employing a urea–formaldehyde resin. In this nanocrystalline system, large blue shifts (in relation to the bulk system) of ∼5000 cm−1 and ∼2000 cm−1 in the photoluminescence excitation bands corresponding to the excitonic region and the Eu3+–ligand charge transfer region respectively have been observed. These may be explained by considering possible size dependent changes in optical electronegativity and quantum confinement effects occurring in this large bandgap semiconductor system.

Resonantsoft x-ray Raman scattering of NiO

Resonant soft x-ray Raman scattering measurements on NiO havebeen made at photon energies across the Ni 2p absorptionedges. The details of the spectral features are identified asRaman scattering due to d-d and charge-transfer excitations.The spectra are interpreted within the single-impurity Andersonmodel, including multiplets, crystal-field and charge-transfereffects. At threshold excitation, the spectral featuresconsists of triplet-triplet and triplet-singlet transitions ofthe 3d8 configuration. For excitation energiescorresponding to the charge-transfer region in the Ni 2px-ray absorption spectrum of NiO, the emission spectra areinstead dominated by charge-transfer transitions to the3d9\\underline L-1 final state. Comparisons of the finalstates with other spectroscopical techniques are also made.

Zinc-thiolate intermediate in catalysis of methyl group transfer in Methanosarcina barkeri.

Methyl group transfer reactions are essential in methane-forming pathways in all methanogens. The involvement of zinc in catalysis of methyl group transfer was studied for the methyltransferase enzyme MT2-A important for methanogenesis in Methanosarcina barkeri growing on methylamines. Zinc was shown to be required for MT2-A activity and was tightly bound by the enzyme with an apparent stability constant of 10(13.7) at pH 7.2. Oxidation was a factor influencing activity and metal stoichiometry of purified MT2-A preparations. Methods were developed to produce inactive apo MT2-A and to restore full activity with stoichiometric reincorporation of Zn(2+). Reconstitution with Co(2+) yielded an enzyme with 16-fold higher specific activity. Cysteine thiolate coordination in Co(2+)-MT2-A was indicated by high absorptivity in the 300-400 nm charge transfer region, consistent with more than one thiolate ligand at the metal center. Approximate tetrahedral geometry was indicated by strong d-d transition absorbance centered at 622 nm. EXAFS analyses of Zn(2+)-MT2-A revealed 2S + 2N/O coordination with evidence for involvement of histidine. Interaction with the substrate CoM (2-mercaptoethanesulfonic acid) resulted in replacement of the second N/O group with S, indicating direct coordination of the CoM thiolate. UV-visible spectroscopy of Co(2+)-MT2-A in the presence of CoM also showed formation of an additional metal-thiolate bond. Binding of CoM over the range of pH 6.2-7.7 obeyed a model in which metal-thiolate formation occurs separately from H(+) release from the enzyme-substrate complex. Proton release to the solvent takes place from a group with apparent pK(a) of 6.4, and no evidence for metal-thiolate protonation was found. It was determined that substrate metal-thiolate bond formation occurs with a Delta G degrees ' of -6.7 kcal/mol and is a major thermodynamic driving force in the overall process of methyl group transfer.

Negative-ion conversion of fluorine atoms in grazing scattering from a LiF(001) surface: A coupled cluster approach

The ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ion formation from fluorine atoms in grazing scattering from a LiF(001) surface is studied. The coupled cluster treatment of the LiF target allows one to take into account the possible effect of the finite width of the valence band of the crystal on the negative-ion formation. The finite width of the valence band implies that the hole created in the crystal by electron transfer to the projectile can a priori migrate out of the charge transfer region thus promoting the negative-ion formation. We find that while for the perfect crystal the hole diffusion is rather fast, in the case of a collision, it is temporarily suppressed by the attractive Coulomb interaction between the hole and the negative ion in the final state of the charge transfer reaction. As a result, the charge transfer has a ``localized'' character and corresponds to binary-type electronic transitions between the projectile and the closest lattice sites along the trajectory.

NO (v‘ ‘ = 0) Rotational Distributions from the Photodissociation of Organometallic Nitrosyls in the Charge Transfer Region

The photodissociations of two metal nitrosyl compounds, Co(CO)3NO and Mn(CO)4NO, were studied at 450 nm, in the metal-to-nitrosyl charge-transfer region for both compounds. The rotational distributions of free NO in the v‘ ‘ = 0 vibrational state were determined by laser-induced fluorescence excitation. A Surprisal Analysis shows that the NO population distributions display a dynamical bias toward lower rotation in both compounds. The similarity between the NO rotational distributions is surprising, considering that Co(CO)3NO was expected to dissociate through a bent transition state and Mn(CO)4NO was not. The spin−orbit populations of the NO products (2Π1/2 and 2Π3/2) are equally populated in both compounds, indicating that the electronic surfaces that produce these two states are strongly coupled at the transition state. An upper limit of the M−NO bond dissociation energy is estimated at 52 kcal/mol from the rotational assignments.

A Small Signal Capacitance Model for A Metallic Electrochemical Electrode in the Charge Transfer Region

A Small Signal Capacitance Model for A Metallic Electrochemical Electrode in the Charge Transfer Region

Chargeexchange in slow H+ + D(1s) collisions

We present the elastic and charge exchange cross sections for slow (meV)H+ + D(1s) collisions based upon a recently formulated adiabaticrepresentation. This representation avoids many of the usual`translational factor' issues, and those that remain have no significanteffect for scattering in the n = 1 manifold. The cross sections areconverged at the 1% level. The various Feshbach and shape resonancesthat form in the isotopic gap and in the charge transfer region areidentified and compared with previous work. An exact analyticalexpression for the ratio of H+ + D(1s) and D+ + H(1s) chargeexchange cross sections suitable for astrophysical simulations ispresented.

Discriminatory influence of Glu-376-->Asp mutation in medium-chain acyl-CoA dehydrogenase on the binding of selected CoA-ligands: spectroscopic, thermodynamic, kinetic, and model building studies.

We investigated the influence of Glu-376-->Asp (E376D) mutation on the UV/visible spectral, thermodynamic, and kinetic properties for the interaction of structurally different types of CoA-ligands (viz., octenoyl-CoA, acetoacetyl-CoA, and indoleacryloyl-CoA) to human liver medium-chain acyl-CoA dehydrogenase (MCAD). Whereas the E376D mutation had minimal/negligible effect on the above properties for the binding of octenoyl-CoA to the enzyme, it had pronounced effects (albeit in opposite directions) for the binding of acetoacetyl-CoA and indoleacryloyl-CoA to the enzyme. In the case of acetoacetyl-CoA, the spectrum of the enzyme-ligand complex (in the charge-transfer region; lambdamax = 545 nm) was 1.8-fold more pronounced, and the DeltaH degrees value for the binding of acetoacetyl-CoA to the enzyme was 5.6 kcal/mol more favorable with wild-type as compared to the E376D mutant enzyme. The kinetic data revealed that the above effects were related to an increase in the dissociation "off-rate" of acetoacetyl-CoA from the enzyme-acetoacetyl-CoA complex. In contrast, in the case of IACoA, the resultant UV/visible spectrum of the enzyme-IACoA complex (lambdamax = 416 nm) was 2.7-fold less pronounced, and the DeltaH degrees value of the enzyme-IACoA complex was 6.4 kcal/mol less favorable with the wild-type than the E376D mutant enzyme. The latter effects were supported by the fact that the above mutation impaired the dissociation "off-rate" of IACoA from the enzyme-IACoA complex by 5.7-fold. Molecular model building studies revealed that the discriminatory influence of the E376D mutation on the spectral, thermodynamic, and kinetic properties of the enzyme-ligand complexes is due to ligand-specific changes in the spatial relationship between the FAD and CoA-ligands at the enzyme site. Arguments are presented that the "void" created by excision of a methylene group from Glu-376 (upon Glu-376-->Asp mutation) is adjusted differently upon interaction with structurally different types of CoA-ligands.

Nature of optical transitions in the charge-transfer region of ZnS:Co and ZnSe:Co.

The cobalt center in ZnS:Co and ZnSe:Co has long been considered as a possible candidate for the observation of so-called bound-electron levels. The optical transitions in the vicinity of the charge-transfer transition from the ${\mathit{d}}^{6}$/${\mathit{d}}^{7}$ charge-transfer level to the conduction band are therefore investigated by their magneto-optical properties. These clearly reveal the identical origin of the observed transitions in the two materials. With the aid of the known charge-transfer energies the transitions are identified as internal ${\mathit{d}}^{7}$\ensuremath{\rightarrow}${\mathit{d}}^{7}$* excitations. Possible assignments are discussed within the framework of ligand-field theory, which also delivers an explanation of why similar internal transitions were not yet observed in other II-VI or even III-V compound semiconductors doped with cobalt. A uniform picture of the cobalt center in these semiconductor host materials is thus derived. \textcopyright{} 1996 The American Physical Society.

Optical spectroscopic and reverse-phase HPLC analyses of Hg(II) binding to phytochelatins

Optical spectroscopy and reverse-phase HPLC were used to investigate the binding of Hg(II) to plant metal-binding peptides (phytochelatins) with the structure (gammaGlu-Cys)2Gly, (gammaGlu-Cys)3Gly and (gammaGlu-Cys)4Gly. Glutathione-mediated transfer of Hg(II) into phytochelatins and the transfer of the metal ion from one phytochelatin to another was also studied using reverse-phase HPLC. The saturation of Hg(II)-induced bands in the UV/visible and CD spectra of (gammaGlu-Cys)2Gly suggested the formation of a single Hg(II)-binding species of this peptide with a stoichiometry of one metal ion per peptide molecule. The separation of apo-(gammaGlu-Cys)2Gly from its Hg(II) derivative on a C18 reverse-phase column also indicated the same metal-binding stoichiometry. The UV/visible spectra of both (gammaGlu-Cys)3Gly and (gammaGlu-Cys)4Gly at pH 7.4 showed distinct shoulders in the ligand-to-metal charge-transfer region at 280-290 mm. Two distinct Hg(II)-binding species, occurring at metal-binding stoichiometries of around 1.25 and 2.0 Hg(II) ions per peptide molecule, were observed for (gammaGlu-Cys)3Gly. These species exhibited specific spectral features in the charge-transfer region and were separable by HPLC. Similarly, two main Hg(II)-binding species of (gammaGlu-Cys)4Gly were observed by UV/visible and CD spectroscopy at metal-binding stoichiometries of around 1.25 and 2.5 respectively. Only a single peak of Hg(II)-(gammaGlu-Cys)4Gly complexes was resolved under the conditions used for HPLC. The overall Hg(II)-binding stoichiometries of phytochelatins were similar at pH 2.0 and at pH 7.4, indicating that pH did not influence the final Hg(II)-binding capacity of these peptides. The reverse-phase HPLC assays indicated a rapid transfer of Hg(II) from glutathione to phytochelatins. These assays also demonstrated a facile transfer of the metal ion from shorter- to longer-chain phytochelatins. The strength of Hg(II) binding to glutathione and phytochelatins followed the order: gammaGlu-Cys-Gly<(gammaGlu-Cys)2Gly<(gammaGlu-Cy s)3Gly<(gamma Glu-Cys)4Gly.