Structural and electronic properties of zinc blende BxGa1−xN nitrides

First principles total energy calculations were carried out to investigate structural and electronic properties of zinc blende GaN, BN and B_xGa_1?xN solid solutions. We have calculated lattice parameters, bulk modulus, pressure derivative and B_xGa_1?xN band gap energy for zinc blende-type crystals of the compositions x = 0, 0.25, 0.5, 0.75, 1. Discussions will be given in comparison with results obtained with other available theoretical and experimental results.     

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

This article discusses recent progress by a combination of spectroscopy and quantum-chemical calculations in classifying and characterizing organic mixed-valence systems in terms of their localized vs. delocalized character. A recently developed quantum-chemical protocol based on non-standard hybrid functionals and continuum solvent models is evaluated for an extended set of mixed-valence bis-triarylamine radical cations, augmented by unsymmetrical neutral triarylamine-perchlorotriphenylmethyl radicals. It turns out that the protocol is able to provide a successful assignment to class II or class III Robin-Day behavior and gives quite accurate ground- and excited-state properties for the radical cations. The limits of the protocol are probed by the anthracene-bridged system 8, where it is suspected that specific solute-solvent interactions are important and not covered by the continuum solvent model. Intervalence charge-transfer excitation energies for the neutral unsymmetrical radicals are systematically overestimated, but dipole moments and a number of other properties are obtained accurately by the protocol.     

Effect of the position of substitution on the electronic properties of nitrophenyl derivatives of fulleropyrrolidines:Fundamental understanding toward raising LUMO energy of fullerene electron-acceptor

A series of substituted para-, meta- and ortho-nitrophenyl derivatives of fulleropyrrolidine were synthesized to investigate the effects of the position of substitution on electronic properties by using steady-state absorption and fluorescence spectra, combined with DFT calculations. The results confirmed that the position of substitution has little effect on absorption and fluorescence spectra, whereas a significant effect was observed on their LUMO energy levels. The theoretical calculations revealed that the LUMO energy of the ortho-nitrophenyl substituted derivative was increased 0.1 eV above those ofpara- and meta-substitution. The prominent effect ofortho-substitution was attributed to the through-space orbital interaction between spatially closed electron-withdrawing nitro group and fullerene cage. These findings could provide fundamental insights in raising LUMO levels of C6o-based electron acceptor materials and an alternative strategy to increase open circuit voltage Voc in polymer solar cells.     

Structural and electronic properties of γ-Tl2Cu(SO4)2

The compound Tl₂Cu(SO₄)₂, belonging to the dehydrated copper Tutton salts [Cat₂Cu(SO₄)₂, where Cat stands for cation], and especially its glassy γ-modification was investigated. The results of X-ray diffraction analysis, electron paramagnetic resonance, differential thermal analysis, electrical conductivity, and thermostimulated depolarization measurements are presented and discussed. An evident correlation among the results of various experimental techniques was found.     

First principles study of the electronic and magnetic structures and bonding properties of UCoC2 ternary,characteristic of C–C units

The electronic structure of UCoC₂, a di-carbide with the C–C units is examined from ab initio with an assessment of the properties of chemical bonding. The energy–volume equation of state shows large anisotropy effects due to C–C alignment along tetragonal c-axis leading to high linear incompressibility. Relevant features of selective bonding of uranium and cobalt with carbon at two different Wyckoff sites and strong C–C interactions are remarkable. The vibrational frequencies for C⋯C stretching modes indicate closer behavior to aliphatic C–C rather than CC double bond. A ferromagnetic ground state is proposed from the calculations.     

Simulation of the molecular and electronic structure of C60 fullerene π-complex with twelve half-sandwich species FeC5H5

The problem of existence of ⁵--complexes of fullerenes and their derivatives is discussed. The stability of the ⁵--complex C₆₀(FeCp)₁₂ (viz., icosahedral C₆₀ fullerene coated with twelve FeCp groups, each coordinated to its own pentagonal face of the fullerene cage) was first estimated in the framework of density functional approach (PBE approximation). The molecular and electronic structure of a biradical complex of C₆₀ fullerene with ten FeCp groups, C₆₀(FeCp)₁₀ (D _5d symmetry), and a 2⁵--complex of substituted fullerene H₁₀C₆₀, H₁₀C₆₀(FeCp)₂ (D _5d symmetry) in which hydrogen atoms are attached to the C atoms in the -positions relative to the atoms of the polar five-membered cycles, was simulated. According to calculations, the coated complex, I _h-C₆₀(FeCp)₁₂, should be much more stable than the complex of naked fullerene with one or two polarly located FeCp groups and only slightly less stable than the ferrocene molecule FeCp₂. The existence of C₆₀(FeCp)₁₂ and H₁₀C₆₀(FeCp)₂ complexes was suggested.     

Polymer modeling: where has it been and where is it going?

The development of the area of polymer modeling often referred to as molecular modeling has been reviewed from its early beginnings to the present day. Key forces influencing the development include computational power, algorithmic advances and access to computational resources. The desire to apply modeling techniques to predict the properties of increasingly complex polymer-containing systems, taken in conjunction with a number of current limitations discussed in this brief review, is expected to define in part some essential future developments.     

What external perturbations influence the electronic properties of catalase compound I?

We have performed density functional theory calculations on an active-site model of catalase compound I and studied the responses of the catalytic center to external perturbations. Thus, in the gas phase, compound I has close-lying doublet and quartet spin states with three unpaired electrons: two residing in pi(FeO) orbitals and the third on the heme. The addition of a dielectric constant to the model changes the doublet-quartet energy ordering but keeps the same electronic configuration. By contrast, the addition of an external electric field along one of the principal axes of the system can change the doublet-quartet energy splitting by as much as 6 kcal mol(-1) in favor of either the quartet or the doublet spin state. This sensitivity is much stronger than the effect obtained for iron heme models with thiolate or imidazole axial ligands. Moreover, an external electric field is able to change the electronic system from a heme-based radical [Fe=O(Por*+)OTyr-] to a tyrosinate radical [Fe=O(Por)OTyr*]. This again shows that oxo-iron heme systems are chameleonic species that are influenced by external perturbations and change their character and catalytic properties depending on the local environment.     

DFT study of the electronic and charge transfer properties of perfluoroarene–thiophene oligomers

The ground state structures of 5,5″-diperfluorophenyl-2,2′:5′,2″:5″,2‴-quaterthiophene (1), 5,5′-bis{1-[4-(thien-2-yl)perfluorophenyl]}-2,2′-dithiophene (2), 4,4′-bis[5-(2,2′-dithiophenyl)]-perfluorobiphenyl (3), 5,5″-diperfluorophenyl-2,2′:5′,2″-tertthiophene (4), 5,5′-diperfluorophenyl-2,2′-dihiophene (5), and 5,5-diperfluorophenylthiophene (6) have been optimized at the B3LYP/6-31G(d), B3LYP/6-31G(d,p), PBE0/6-31G(d), and PBE0/6-31G(d,p) level of theories. The B3LYP/6-31+G(d) and PBE0/6-31+G(d) level of theories have been applied to investigate the absorption spectra. The PBE0 functional is good to predict the C–S bond lengths while the C–F bond lengths are good envisaged with B3LYP functional. The increment of thiophene rings between two perfluoroarene rings leads to red shift in absorption spectra. The electron affinities are energetically destabilized while energetic stabilization of the radical-cation increases by decreasing the thiophene rings from four to one. The perfluoroarene rings leads to enhance the electron injection.     

Synthesis and photophysical properties of heptamethine cyanine–fullerene C60 dyads with non-quenched fluorescence

Two new heptamethine cyanine dye–fullerene C₆₀ covalently- linked dyads, which absorb in far-red and NIR spectral regions, have been synthesized by esterification click reaction and characterized by physicochemical methods. No significant fluorescence quenching was found due to weak electronic coupling between heptamethine moiety and fullerene core, which was confirmed by photophysical and electrochemical methods. Such dyads can be useful for cell imaging and fluorescence diagnostics of various fullerene derivatives.     

Tailoring the electrochemical charge storage properties of carbonaceous support by redox properties of heteropoly acids: where does the synergy come from?

Journal of Solid State Electrochemistry - The synergistic effects between two Keggin-type heteropoly acids (HPAs) and carbon surface were examined and elucidated. An improved high rate capability...     

Structural requirements for the interaction of combretastatins with tubulin: how important is the trimethoxy unit?

A series of combretastatins possessing both a trimethoxy unit and other substituents on ring A has been synthesised and tested for cytotoxicity and their ability to interact with the protein tubulin. All previous studies have indicated that the trimethoxy unit is essential for interaction with tubulin. The studies herein show that molecules possessing functionalities other than trimethoxy can also interact with tubulin. Importantly a trimethyl substituted agent 52a has shown reduced cytotoxicity, but increased potency in its ability to inhibit the assembly of tubulin.     

Hydronium ion complex of 18-crown-6: where are the protons? A density functional study of static and dynamic properties

A quantum-chemical study employing the BLYP density functional is reported for the complex of H3O+ with 18-crown-6. According to a Car-Parrinello molecular dynamics (CPMD) study at 340 K, the complex is quite flexible, and is characterized by three quasi-linear (two-center) hydrogen-bond interactions for most of the time. On a time scale of 10 ps, frequent inversions of H3O+ are observed, as well as two 120 degrees rotations switching the hydrogen bonds from one set of crown-ether O atoms to the other. These results are consistent with density-functional studies of stationary points on the potential energy surface, which show how the crown "catalyzes" the guest's inversion. Two close-lying minima are characterized, as well as two distinct transition states connecting them, either via H3O+ inversion or rotation, with barriers of 1.0 and 4.6 kcal/mol, respectively, at the BLYP/II'//BLYP/6-31G level. Orbital interactions between lone pairs on ether O atoms and hydronium sigma(OH) antibonding orbitals are important factors for the directionality of the hydrogen bonds.     

Structural and spectroscopic properties of the peroxodiferric intermediate of Ricinus communis soluble Δ9 desaturase

Large-scale quantum and molecular mechanical methods (QM/MM) and QM calculations were carried out on the soluble Δ(9) desaturase (Δ(9)D) to investigate various structural models of the spectroscopically defined peroxodiferric (P) intermediate. This allowed us to formulate a consistent mechanistic picture for the initial stages of the reaction mechanism of Δ(9)D, an important diferrous nonheme iron enzyme that cleaves the C-H bonds in alkane chains resulting in the highly specific insertion of double bonds. The methods (density functional theory (DFT), time-dependent DFT (TD-DFT), QM(DFT)/MM, and TD-DFT with electrostatic embedding) were benchmarked by demonstrating that the known spectroscopic effects and structural perturbation caused by substrate binding to diferrous Δ(9)D can be qualitatively reproduced. We show that structural models whose spectroscopic (absorption, circular dichroism (CD), vibrational and M?ssbauer) characteristics correlate best with experimental data for the P intermediate correspond to the μ-1,2-O(2)(2-) binding mode. Coordination of Glu196 to one of the iron centers (Fe(B)) is demonstrated to be flexible, with the monodentate binding providing better agreement with spectroscopic data, and the bidentate structure being slightly favored energetically (1-10 kJ mol(-1)). Further possible structures, containing an additional proton or water molecule are also evaluated in connection with the possible activation of the P intermediate. Specifically, we suggest that protonation of the peroxide moiety, possibly preceded by water binding in the Fe(A) coordination sphere, could be responsible for the conversion of the P intermediate in Δ(9)D into a form capable of hydrogen abstraction. Finally, results are compared with recent findings on the related ribonucleotide reductase and toluene/methane monooxygenase enzymes.     

A charge iteration-corrected extended-Hückel study of the electronic and spectroscopic properties of conjugated heterocycles

A computationally inexpensive modified extended-Hückel method using coordinates from STO-6G(d,p) geometry optimizations is described and shown to accurately simulate the spectra of conjugated heterocycles. The origins of the natural optical activities of blue indophenines and diheptylindophenines, and several related brown, red, and colorless compounds, were investigated with this method to show how it can be used to examine the changes in spectra and electronic structure that occur with substituent additions and changes. Diheptylindophenines were found to be blue because of absorptions from electronic transitions between spatially congruent π and π* molecular orbitals delocalized across carbon, nitrogen, oxygen, and sulfur atomic orbitals. The effects of conjugated thiophene moieties replacing the isatin moiety β-carbonyls move these absorptions to the blue relative to their locations in n-heptylisatin, and the diheptyl groups move these absorptions slightly to the red relative to indophenines.     

Structure and properties of ionic fullerene complex Co(+)(dppe)2·(C60˙-)·(C6H4Cl2)2: distortion of the ordered fullerene cage of C60˙- radical anions

New ionic complex {Co(+)(dppe)(2)}·(C(60)˙(-))·(C(6)H(4)Cl(2))(2) (1) was obtained by the reduction of a Co(dppe)Br(2) and C(60) mixture by TDAE in o-dichlorobenzene followed by precipitation of crystals by hexane. Optical and EPR spectra of 1 indicated the formation of C(60)˙(-) radical anions and diamagnetic Co(+)(dppe)(2) cations. The structure of 1 solved at 100(2) K involves chains of C(60)˙(-) arranged along the lattice a-axis with a center-to-center distance of 10.271 ?. The chains are separated by bulky Co(+)(dppe)(2) cations and solvent molecules. All components of 1 are well ordered allowing the distortion of the C(60)˙(-) radical anion to be analyzed. An elongation of the C(60)˙(-) sphere by 0.0254(2) was found. It is essentially smaller than those in the salts (Cp*(2)Ni(+))·(C(60)˙(-))·CS(2) and (PPN(+))(2)·(C(60)(2-)) with greater distortion of the fullerene cage. The calculation of the electronic structure of fullerene by the extended Hückel method showed slight splitting of the C(60) LUMO, due to the distortion, by three levels. Two levels are located 180 and 710 cm(-1) higher than the ground level. The averaged 6-6 and 5-6 bonds in C(60)˙(-) with lengths of 1.397(2) and 1.449(2) ? are close to those determined for the C(60)(2-) dianions in (PPN(+))(2)·(C(60)(2-)), but are slightly longer and shorter, respectively, than the length of these bonds in neutral C(60).     

Modeling of lithium-ion batteries is becoming viral: where to go?

Journal of Solid State Electrochemistry - We briefly summarize the most popular theoretical/computational techniques being used to model lithium-ion batteries and suggest some future...     

Ion spectroscopy: Where did it come from; where is it now; and where is it going?

The ASMS conference on ion spectroscopy brought together at Asilomar on October 16–20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation “action spectroscopic” mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, high-resolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions.