Considerable Charge Transfer Research Articles

Density functional calculations of the electronic structure and optical properties of the ternary carbidesAl4SiC4andAl4Si2C5

The electronic structure and spectroscopic properties of two ternary aluminum silicon carbide ceramics ${\text{Al}}_{4}{\text{SiC}}_{4}$ and ${\text{Al}}_{4}{\text{Si}}_{2}{\text{C}}_{5}$ are studied by density functional theory calculations based on the orthogonalized linear combination of atomic orbitals method. Both crystals are shown to be small gap semiconductors with indirect band gaps of 1.05 and 1.02 eV, respectively. The calculated hole and electron effective mass and the interband optical properties, in the form of the complex dielectric function, show a high degree of anisotropy which can be traced to the unique structures of these two crystals. The calculated refractive indices are consistent with the values proposed in the literature. Mulliken effective charge and bond order calculations show that these crystals have a high degree of covalency with considerable charge transfer from Al and Si to the C atoms. The x-ray absorption near-edge-structure for all crystallographically nonequivalent sites ($K$ and $L$ edges) is calculated and compared with those of cubic SiC. It is shown that the site-averaged $\text{Si-}K$ and $\text{Si-}{L}_{3}$ edges, and also the $\text{C-}K$ edges are slightly different and broader than those of cubic SiC. Potential applications of these ternary ceramics are also discussed.

Impedance measurements in catalysis: charge transfer in titania supported noble metal catalysts

Hydrogen treatment of Pt/TiO2 leads to an exceptional increase in electric conductivity even at moderate temperature presumably due to the formation of donor electrons during a superficial reduction process. Interestingly, after cooling to lower temperatures that are typically applied for catalytic reactions the observed high conductivity was largely preserved. The enhanced charge carrier density in titania is assumed to cause a considerable charge transfer between support and metal. In this work we show that this charge transfer is correlated with typical catalytic parameters (e.g. hydrogen adsorption capacity and catalytic activity for a hydrocarbon skeleton reaction). Based on these observations we suggest and discuss a modified charge transfer mechanism in addition to the decoration mechanism as a possible reason for the SMSI effect occurring during hydrogen treatment already at moderate temperatures.

DNA-wrapped carbon nanotubes

The stability and electronic properties of complexes of single-walled carbon nanotubeswrapped by homopolymeric single-stranded DNA molecules (CNT@DNA) are consideredusing a quantum mechanical density-functional tight-binding method (DFTB). Aphenomenological model of the CNT@DNA formation energy depending on the nanotuberadii is developed, which shows that the decoration of a CNT by a few DNA chains leads toa high water solubility of CNT@DNA. Pyrimidine-based DNAs are found to be moreeffective to wrap the CNTs than other DNAs. The densities of states of the CNT@DNAcomplexes are close to the superposition of those of the ‘free’ components with someadditional states below the Fermi level. The band gap in a hybrid CNT@DNA system isdetermined by the competition between the Fermi levels of the ‘free’ DNA and CNT. Ina few specific cases a considerable charge transfer from the DNA to the CNTwas observed, combined with an additional gain in the CNT@DNA formationenergy.

Theoretical study of F −–(H 2) n and Cl −–(H 2) n ( n = 1–8) anion complexes

In the present study, the structural and bonding properties of F −–(H 2) n and Cl −–(H 2) n ( n = 1–8) anion complexes have been investigated by quantum chemical calculations at the MP2/aug-cc-pVTZ level. The computations gave different global minimum structures for most of the F −–(H 2) n and Cl −–(H 2) n complexes. The strength of the F −–H 2 interaction depends strongly, while that of the Cl −–H 2 one only marginally on the number of H 2 ligands. Our NBO analysis revealed a considerable charge transfer from the halide ion to the σ ∗(H 2) orbital.

Modeling dioxygen binding to the non‐heme iron‐containing enzymes

AbstractThe structures and properties of the complexes formed upon binding the oxygen molecule to the iron sites in non‐heme 2‐oxoglutarate‐dependent enzymes are characterized by QM(CASSCF)/MM and density functional theory (DFT) calculations. Molecular models for the calculations are constructed following the crystal structure of hypoxia‐inducible factor asparaginyl hydroxylase (FIH‐1). DFT calculations for the 37‐atomic cluster have been carried out at the B3LYP(LANL2DZdp) level. The flexible effective fragment potential method is used as a combined quantum mechanical–molecular mechanical (QM/MM) technique to characterize the fragment of the enzymatic system, including 1,758 atoms in the MM part and 27 atoms in the QM part. In these calculations, the CASSCF(LANL2DZdp) approach is applied in the QM subsystem, and AMBER force field parameters are used in the MM subsystem. With both approaches, equilibrium geometry configurations have been located for different spin states of the system. In DFT calculations, the order of the states is as follows: septet, triplet (+7.7 kcal/mol), quintet (+10.7 kcal/mol). Geometry configurations correspond to the end‐on structures with no evidences of electron transfer from Fe(II) to molecular oxygen. In contrast, QM(CASSCF)/MM calculations predict the quintet state as the lowest one, while the septet structure has slightly (<2 kcal/mol) higher energy, and the triplet state is considerably more energetic. In QM/MM calculations, in both quintet and septet states, the electronic configurations show considerable electron charge transfer from iron to oxygen, and the oxidation state of iron in the metal binding site can be characterized as Fe(III). © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Calcium/Poly(9,9-dioctylfluorene) Interaction: A Theoretical Study

The geometric and electronic structures of poly(9,9-dioctylfluorene) (PFO) oligomer interacting with Ca atoms were studied using Møller-Plesset perturbation theory. A weak interaction with little charge transfer and with a relatively long Ca-C distance (about 4.0 Angstrom) was found when only one Ca atom was attached to a PFO unit. However, when two Ca atoms were adsorbed at a PFO unit, a strong interaction with a shorter Ca-C distance (about 2.67 Angstrom) took place with considerable charge transfer from the Ca atom to the PFO, resulting in significant deformation in the backbone of the PFO oligomer. In the latter case, the frontier orbitals of the PFO were modified. However, the deformed PFO and its modified frontier orbitals could be recovered when oxygen was added, which is in good agreement with experimental observation.

The effect of solvent polarity on the photophysical properties of 4-cyano-(4′-methylthio)diphenylacetylene: A prototypic donor–acceptor system

The photophysical properties of the target compound are extremely sensitive to changes in solvent polarity since the lowest-energy excited states possess considerable charge-transfer character. Excitation results in a greatly increased dipole moment, with the resultant excited singlet state retaining a lifetime of ca. 1 ns in all solvents. Radiative decay involves coupling between the lowest-energy excited singlet state and both the ground state and an upper excited singlet state. The level of coupling to the upper singlet decreases in non-polar solvents, presumably due to symmetry factors. The radiative rate constant decreases smoothly with increasing solvent polarity function as the molecule acquires an ever increasing dipolar character. Non-radiative decay includes both intersystem crossing and internal conversion, but the former process dominates in polar solvents. The excited singlet state lifetime is very weakly dependent upon temperature in the solid state. However, in polar solutions where the Stokes' shift decreases with decreasing temperature, there is clear evidence for an activated process. This is believed to involve coupling to the upper-lying singlet excited state.

Variations of quasi-electrostatic fields and ionosphere potential above lightning discharge at equatorial latitudes

Lightning discharges by thunderstorms cause generation of electromagnetic pulses and of quasi-electrostatic fields (QESF) in the atmosphere above, which occur in different time-scales. QESF penetrate into the mesosphere and the lower ionosphere where they are big enough to generate considerable electric charge transfer there and, in some cases, to cause red sprites. These processes may have an important contribution to the global atmospheric electric circuit. Significant transient variations of the ionospheric potential above the thunderstorm take place as well. QESF depend on the atmospheric conductivity and in the ionosphere they are affected also by its anisotropy determined by geomagnetic field orientation. QESF after a lightning discharge are investigated theoretically in this work in the case of equatorial latitudes (by horizontal geomagnetic field), where thunderstorms are important contributors to the global circuit. Results for DC electric fields in the lower equatorial ionosphere above a thundercloud obtained by earlier models demonstrate some specific features of the spatial distribution of these fields, which appear due to geomagnetic field orientation. Thus, the electric fields can be shifted by tens or more kilometers to east of the cloud charge region; also their horizontal scale is much bigger than in the case of middle latitudes. Here, a presence of similar specific features of quasi-electrostatic field distributions and ionospheric potential variations caused by a lightning stroke is studied. A situation when no secondary ionization is generated is considered. A model based on the Maxwell equations for potential electric fields is proposed. Computations of QESF in the middle atmosphere and of the ionospheric potential variations are provided as dependent on conductivity and its anisotropy in D-region. The obtained results for the ionosphere show that the electric fields in the equatorial lower ionosphere are comparable to these formed in the case of middle latitudes. However, their horizontal scales are much bigger and depend on conductivity profiles. Similar features are valid also for the ionospheric potential variations and for their horizontal scales.

Investigation of vacancy in C54 TiSi 2 using ab initio method

Investigated by ab initio plane-wave ultrasoft pseudopotential method based on generalized gradient approximation (GGA), it has been found that the formation energies of the vacancies in C54 TiSi 2 largely depend on the atomic chemical potentials of Ti and Si. In Si-rich limit, the formation energies of the Si and Ti vacancy are 2.39 eV and 2.40 eV while they are 1.53 eV and 4.07 eV in Ti-rich limit, respectively. The introduction of Si or Ti vacancy only slightly changes total density of states (DOS) and it also causes no considerable charge transfer.

Electronic structure calculations on alloys using the polymorphous coherent-potential approximation

We present self-consistent calculations of the electronic density of states of disordered copper-palladium and silver-palladium alloys using the polymorphous coherent-potential approximation and the Korringa-Kohn-Rostoker coherent-potential approximation. We find that the agreement between the theoretical partial density of states of palladium $d$ bands in copper-rich copper-palladium alloys and experiment is significantly improved when the polymorphous coherent-potential approximation is used. The densities of states of silver-palladium alloys calculated with the two versions of the coherent-potential approximation are identical and agree with experiment. This indicates that the improved treatment of Coulomb effects in the polymorphous coherent-potential approximation is necessary only for alloys such as copper palladium that have considerable charge transfer.

X-ray absorption study of the electronic structure of Li-excess spinel Li1+xTi2−xO4 (0⩽x⩽0.33)

Li-excess spinel oxide, Li1+xTi2−xO4 (0⩽x⩽0.33), was investigated by x-ray absorption spectra (XAS) technique. XAS results revealed that both oxygen and titanium ions affect electronic structural changes in the system. Early transition metal compounds are considered to have smaller d–d Coulomb correlation energy (Udd) than p–d Charge-transfer energy (Δ), indicating that electronic transfer for charge compensation occurs mainly on transition metal. However, present XAS results show that considerable charge-transfer character due to strong hybridization between titanium 3d and oxygen 2p orbitals exists in this early transition metal oxide system.

Spatial electron distribution of CO adsorbed on Ni(100) and Ni(111) surfaces probed by metastable impact electron spectroscopy

Electron emission spectra obtained by thermal collisions of He*(2 3S) metastable atoms with CO on Ni(100) in the c(2×2) structure and on Ni(111) in the c(4×2) structure were measured to probe directly the spatial electron distribution. For a systematic comparison, the metastable spectra of free CO, condensed CO on Ni(111), and gaseous Cr(CO)6 were also measured under the same beam conditions. Our data showed that the relative ionization cross sections for the CO 4σ-, 1π-, and 5σ-derived states depend drastically on the molecular orientation of CO with respect to the metastable beam, reflecting the local electron density of CO in the impact region. Moreover, it was found that the 4σ- and 5σ- derived states of CO at hollow sites on Ni(111) are strongly modified in space by mixing with each other, where considerable charge transfer occurs from the C site to the O site in the 5σ-derived state and in the opposite way in the 4σ-derived state. In contrast, such a strong charge redistribution was not seen in the cases of terminal CO on Ni(100) and Cr(CO)6. These findings were in good accordance with the crystal orbital overlap population obtained by density functional theory through a generalized gradient approximation.

Local Electron Distribution of CO Adsorbed on Ni(100) and Ni(111) Surfaces Studied by Metastable Atom Electron Spectroscopy.

Electron emission spectra caused by thermal collisions of He* (23S) metastable atoms with Ni(100)c(2×2)-CO and Ni(111)c(4×2)-CO were measured to probe the local electron distribution. Our data showed that the 4σ- and 5σ-derived states of CO at hollow sites on Ni(111) are strongly modified in space by mixing with each other, where considerable charge transfer occurs from the C atom to the O atom in the 5σ-derived state and in the opposite way in the 4σ-derived state. In contrast, such a heavy charge redistribution was not seen in the case of terminal CO on Ni(100). These findings were in good accordance with the crystal orbital overlap population obtained by density functional theory within the generalized gradient approximation.

Effect of Terminal Groups, Polyene Chain Length, and Solvent on the First Excited Singlet States of Carotenoids

The effect of terminal groups, polyene chain length, and solvent on the first excited singlet states (S1) of carotenoids was studied by steady-state and transient optical absorption spectroscopy, and AM1 semiempirical molecular orbital calculations. The carotenoids studied were ethyl 8‘-apo-β-caroten-8‘-oate (I), ethyl 6‘-apo-β-caroten-6‘-oate (II), ethyl 4‘-apo-β-caroten-4‘-oate (III), 8‘-apo-β-caroten-8‘-nitrile (IV), 6‘-apo-β-caroten-6‘-nitrile (V), 4‘-apo-β-caroten-4‘-nitrile (VI), 8‘-apo-β-caroten-8‘-al (VII), and 6‘-apo-β-caroten-6‘-al (VIII). Solvents were 3-methylpentane (3-MP) and MeCN. The effect of solvent on the S1 absorption maxima is similar to that on the ground state (S0) absorption maxima, which suggests that both effects stem from the same type of interaction, i.e., the dispersive interaction between carotenoids and solvents. Carotenoids with terminal CHO groups have S1 absorption maxima at longer wavelenths than those with terminal CN or CO2Et groups. The S1 absorption maxima are red-shifted with increasing polyene chain length. In the nonpolar solvent 3-MP, the S1 lifetimes of carotenoids depend mainly on the polyene chain length. With a one CC bond increase, the S1 lifetime decreases by a factor of ca. 2 (ca. 24 ps for I, IV, and VII; 12 ps for II, V, and VIII; and 7 ps for III and VI). Terminal groups have little effect on the S1 lifetimes in 3-MP. However, in the polar solvent MeCN, carotenoids with terminal CHO groups have decreased S1 lifetimes (ca. 8 ps for VII and 6 ps for VIII), while carotenoids with terminal CN and CO2Et groups have essentially unchanged S1 lifetimes. This observation, along with the data of β-carotene and 7‘-apo-7‘,7‘-dicyano-β-carotene, suggests that polar solvents could decrease the S1 lifetimes of carotenoids, when, and only when, there is considerable charge transfer character in their excited states.

Bonding in alpha-quartz (SiO2): A view of the unoccupied states

High-resolution core-loss and low-loss spectra of α-quartz were acquired by electron energyloss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra contain the Si L 1 , L 2,3 , K, and O K core-loss edges, and the surface and bulk low-loss spectra. The core-loss edges represent the atom-projected partial densities of states of the excited atoms and provide information on the unoccupied s, p, and d states as a function of energy above the edge onset. The band structure and total density of states were calculated for α-quartz using a self-consistent pseudopotential method. Projected local densities of Si and O s, p, and d states (LDOS) were calculated and compared with the EELS core-loss edges. These LDOS successfully reproduce the dominant Si and O core-loss edge shapes up to ca. 15 eV above the conduction-band onset. In addition, the calculations provide evidence for considerable charge transfer from Si to O and suggest a marked ionicity of the Si-O bond. The experimental and calculated data indicate that O 2p-Si d π-type bonding is minimal. The low-loss spectra exhibit four peaks that are assigned to transitions from maxima in the valence-band density of states to the conduction band. A band gap of 9.65 eV is measured from the low-loss spectrum. The structures of the surface low-loss spectrum are reproduced by the joint density of states derived from the band-structure calculation. This study provides a detailed description of the unoccupied DOS of α-quartz by comparing the core-loss edges and low-loss spectrum, on a relative energy scale and relating the spectral features to the atom- and angular-momentum-resolved components of a pseudopotential band-structure calculation.

Does density functional theory contribute to the understanding of excited states of unsaturated organic compounds?

A comparative study has been performed on the electronic spectra of a number of unsaturated organic molecules, using on the one hand density functional linear response theory and on the other multiconfigurational second-order perturbation theory, in order to establish the accuracy that the density functional based methods can give for excitation energies and energy surfaces for excited states. The following molecules are included in the study: tetrazine; the five-membered ring systems cyclopentadiene, furan, pyrrole, and thiophene; acetone; and a dipeptide. The results show that DFT valence excited states have errors that vary between 0 and 1 eV, while Rydberg states are accurate to about 0.2eV in most cases. The use of an asymptotically corrected exchange-correlation potential was essential for the latter result. However, transitions which involve a considerable charge transfer have much larger errors. The results show in some cases a surprisingly strong interaction between valence and Rydberg excited states, with considerable effects both on energies and computed intensities.

Excited charge transfer states in donor–acceptor indole derivatives

Solvent-dependent electronic structure of the selected donor (D)–acceptor (A) derivatives of 5-methylindole and 5-methoxyindole containing benzonitrile as an electron acceptor in the fluorescent charge transfer (CT) states has been investigated. The mechanism of the radiative charge recombination 1 CT→ S 0 is discussed in terms of the Mulliken–Murrell model of the CT complexes and the Marcus theory of photoinduced electron transfer (ET). Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra point to a considerable CT character of the emitting singlet states of all the compounds studied. The determination of the electronic transition dipole moments corresponding to the 1 CT→ S 0 fluorescence and the band-shape analysis of the corresponding spectra leads to the quantities relevant for the charge recombination in the Marcus inverted region. It is shown that some of the photophysical properties of the compounds can be predicted in terms of a simple model from the properties of individual chromophores.

Quantum Chemical Calculations of the Electronic States and Fluorescence Properties of Carbazolyl- and Carbazolylmethylene-Substituted Diacetylenes

The carbazolyldiacetylenes show very different fluorescence behaviors depending on how the carbazolyl (Cz) moiety is connected to the diacetylenic backbone. In fact, when Cz is attached to the backbone through one or more CH2 spacer(s) fluorescence properties are observed which are closely related to those of the fluorescent Cz group. On the contrary, almost no fluorescence is observed when Cz is directly attached to the backbone. In this paper, these phenomena are studied quantum chemically and an explanation for the observed facts is provided. The relevant excited states are studied in detail, showing that in the lowest singlet excited state a considerable intramolecular charge transfer occurs from Cz to the backbone when they are directly bonded.

Nucleotides enhance the fluorescence of trans-4-( p- N, N-dimethylaminostyryl)- N-vinylbenzylpyridinium chloride

The newly synthesized trans-4-( p- N, N-dimethylaminostyryl)- N-vinylbenzylpyridinium chloride ( 1) displays strong solvatochromic behavior resulting from a large increase of ∼18 D in its dipole moment upon excitation. This indicates that its excited state has considerable charge transfer character, suggesting that it may be subject to solute-induced fluorescence changes. The fluorescence quantum yield of 1 is enhanced dramatically (12-fold) in the presence of a buffered aqueous solution of cAMP, a purine nucleotide. In contrast, the pyrimidine nucleotides, CMP and UMP, have virtually no effect on the fluorescence of 1. cGMP induced an increase in the quantum yield of 1 similar to that induced by cAMP, whereas other purine nucleotides (AMP, ADP and ATP) and adenosine induced changes of somewhat lesser magnitude. Association constants for 1 with these analytes ranged from 13.8 M −1 for cAMP to 0.15 M −1 for adenine. We conclude that the changes in fluorescence of 1 in the presence of nucleotides require the presence of a purine base, and are enhanced by ribose and phosphate moieties. The environmentally sensitive fluorescence of 1 suggests that such compounds may be useful as chemosensors for purine nucleotides.

Anomalous lattice expansion of the electrodeposited Ag bilayer on Pt(111)

A 2.2% lateral and 8.5% vertical lattice expansion, as compared to bulk Ag(111), has been found in a high resolution surface X-ray scattering study for the underpotentially deposited Ag bilayer on Pt(111). The larger-than-bulk lattice constants are unusual for an incommensurate adlayer. It presumably arises from a considerable charge transfer from Ag to Pt, which results in an additional repulsion among the adatoms.