Synthesis and spectral properties of autocomplexes of the nitroquinoline series

A number of new NH-bridged nitroquinoline derivatives with intramolecular charge transfer (autocomplexes) were synthesized starting from 8-chloro-5,7-dinitroquinoline and aromatic N,O-heterocyclic amines characterized by different donor powers and substitution patterns. The charge transfer in their molecules may occur via both direct polar conjugation through the bond sequence including the bridging nitrogen atom and through space between spatially close molecular fragments (contact charge transfer).     

Incorporation of a QM/MM buffer zone in the variational double self-consistent field method

The explicit polarization (X-Pol) potential is an electronic-structure-based polarization force field, designed for molecular dynamics simulations and modeling of biopolymers. In this approach, molecular polarization and charge transfer effects are explicitly treated by a combined quantum mechanical and molecular mechanical (QM/MM) scheme, and the wave function of the entire system is variationally optimized by a double self-consistent field (DSCF) method. In the present article, we introduce a QM buffer zone for a smooth transition from a QM region to an MM region. Instead of using the Mulliken charge approximation for all QM/MM interactions, the Coulombic interactions between the adjacent fragments are determined directly by electronic structure theory. The present method is designed to accelerate the speed of convergence of the total energy and charge density of the system.     

Evaluation of the molecular polarizability using the IPPP-CLOPPA-INDO/S method. Application to molecules of biological interest

The IPPP-CLOPPA-INDO/S method is introduced to investigate the static molecular polarizability in macromolecules. As an example of application, the polarizability of phospholipidic compounds, with and without the presence of water molecules has been estimated. The IPPP technique was employed to calculate the polarizability of the polar head and the hydrocarbon chains separately to analyze the feasibility of evaluating the total polarizability of the molecule by addition of these two projected results. INDO/S dipole moments of different fragments of the complex molecule were obtained by means of localized molecular orbitals in order to evaluate the charge transfer in the system.     

PNA versus DNA: effects of structural fluctuations on electronic structure and hole-transport mechanisms

The effects of structural fluctuations on charge transfer in double-stranded DNA and peptide nucleic acid (PNA) are investigated. A palindromic sequence with two guanine bases that play the roles of hole donor and acceptor, separated by a bridge of two adenine bases, was analyzed using combined molecular dynamics (MD) and quantum-chemical methods. Surprisingly, electronic structure calculations on individual MD snapshots show significant frontier orbital electronic population on the bridge in approximately 10% of the structures. Electron-density delocalization to the bridge is found to be gated by fluctuations of the covalent conjugated bond structure of the aromatic rings of the nucleic bases. It is concluded, therefore, that both thermal hopping and superexchange should contribute significantly to charge transfer even in short DNA/PNA fragments. PNA is found to be more flexible than DNA, and this flexibility is predicted to produce larger rates of charge transfer.     

QM/MM investigation of the degradation mechanism of the electron-transporting layer

The mechanism of charge transfer among tris(8-hydroxyquinolinate)aluminum (Alq₃) molecules in the electron-transporting layer (ETL) under amorphous conditions was theoretically investigated using both quantum mechanical/molecular mechanical (QM/MM) calculations and molecular dynamics (MD) simulations. The rate constant of the electron transfer was estimated for the equilibrated structure taken from the QM/MM MD simulations, based on the hopping model and Marcus theory. It was found that the coordination of a (LiF)₄ cluster in ETL drastically lowers the energy of the lowest unoccupied molecular orbital in the Alq₃ molecule. The small rate constant, namely the slow charge mobility, in ETL is believed to be causally related to the low-lying delocalized unoccupied molecular orbital of Alq₃ coordinated by the (LiF)₄ cluster. The results suggest that their interaction has a considerable influence on efficiency and is attributed in part to ETL degradation in organic light-emitting diodes.     

On the Low-Lying Electronically Excited States of Azobenzene Dimers: Transition Density Matrix Analysis

Azobenzene-containing molecules may associate with each other in systems such as self-assembled monolayers or micelles. The interaction between azobenzene units leads to a formation of exciton states in these molecular assemblies. Apart from local excitations of monomers, the electronic transitions to the exciton states may involve charge transfer excitations. Here, we perform quantum chemical calculations and apply transition density matrix analysis to quantify local and charge transfer contributions to the lowest electronic transitions in azobenzene dimers of various arrangements. We find that the transitions to the lowest exciton states of the considered dimers are dominated by local excitations, but charge transfer contributions become sizable for some of the lowest ππ* electronic transitions in stacked and slip-stacked dimers at short intermolecular distances. In addition, we assess different ways to partition the transition density matrix between fragments. In particular, we find that the inclusion of the atomic orbital overlap has a pronounced effect on quantifying charge transfer contributions if a large basis set is used.     

六烷氧基取代的苯并菲和六氮杂苯并菲盘状液晶中的电荷转移

用电子转移的半经典模型在量子化学B3LYP/6-31G(d)水平(对单体)和B3LYP/STO-3G水平(对二聚物)对六烷氧基取代的苯并菲和六烷氧基取代的六氮杂苯并菲组成的盘状晶体系的电荷转移性质进行了研究,发现在用量子化学方法研究电荷转移反应时,不能简单地用氢氧基代替长链烷氧取代基。由于在电荷转移反应中,要考虑参与反应的分子之间前线轨道的细微差别,所以将长链取代基用氢氧基取代,可能得不到定性正确的结果。     

Influence of charge state and amino acid composition on hydrogen transfer in electron capture dissociation of peptides

Although conventional N-Cα bond cleavage in electron capture dissociation (ECD) of multiply-charged peptides generates a complementary c′ and z′ fragment pair, the N-Cα cleavage followed by hydrogen transfer from c′ to z′ fragments produces other fragments, namely c′ and z′. In this study, the influence of charge state and amino acid composition on hydrogen transfer in ECD is described using sets of peptides. Hydrogen transferred ionic species such as c′ and z′ were observed in ECD spectra of doubly-protonated peptides, while the triply-protonated form did not demonstrate hydrogen transfer. The extent of hydrogen transfer in ECD of doubly-protonated peptides was dependent on constituent amino acids. The ECD of doubly-protonated peptides possessing numerous basic sites showed extensive hydrogen transfer compared with ECD of less basic peptides. The extent of hydrogen transfer is discussed from the viewpoints of the structure of peptide ions, the possibility of internal hydrogen bonding and intermediate lifetime of complex [c′+z′].     

Ab initio electronic analysis of the hydride transfer in the [CH3-H-CH3]+ system

Summary The electronic aspects of the hydride transfer process between CH₄ and CH ₃ ⁺ fragments, are studied theoretically withab initio molecular orbital methods, subject to the constraint of maintaining a fix distance between both fragments. Mulliken and Natural population analyses are performed to gain an insight into the hydride character of the atom being transferred. From these analyses, charge migrating diagrams are depicted to obtain a more visual information. Further analysis is performed from the contour maps of the electronic charge density, together with the analysis of its gradient and laplacian. Basis set and electronic correlation effects are also discussed. Finally, the effect of applying a uniform electric field is assessed.     

环聚炔苯和环聚炔吡啶组成的盘状液晶中的电荷转移

用电子转移的半经典模型在量子化学B3LYP/6-31G(d)水平(对单体)和B3LYP/STO-3G水平(对二聚物)对环聚炔苯和环聚炔吡啶组成的盘状液晶体系的电荷转移性质进行了研究. 盘状液晶体系的电荷转移速率主要依赖于重组能和电荷转移矩阵元, 重组能越小, 电荷转移矩阵元越大, 则电荷转移速率常数越大. 计算结果表明, 这些大环化合物比目前广泛研究和应用的苯并菲衍生物组成的液晶有较小的重组能, 所以有更好的电荷转移性质. 计算结果对有效地设计和合成高效的光导材料和载流子输送材料是有帮助的.     

羟基取代苯并菲化合物分子的结构与电荷传输性质研究

在量子化学B3LYP/6-31G**水平上对苯并菲, 氟及羟基取代苯并菲化合物分子及其分子离子的结构进行理论研究, 得到稳定构型. 在此基础上, 计算二聚物的单点能随旋转角度和盘间距离的变化关系, 得到能量最低点. 根据电子转移的半经典模型计算了苯并菲及氟和羟基取代苯并菲化合物分子的电荷转移常数, 氟和羟基的引入使正电荷转移速率常数明显减小, 即不利于正电荷的传输, 对负电荷的传输速率常数影响不大.     

Charge transfer processes in the course of metal-ion electrochemical intercalation

Charge transfer in the course of the electrochemical ion intercalation is typically understood as the transfer of an alkali metal ion across the intercalating material/electrolyte interface. The activation energy of this step determines the rate capability of intercalation-based energy storage devices, which calls for the investigation of the origin of the charge transfer limitations in various intercalation systems. The major focus of the experimental studies in this area is on the experimental determination of the charge transfer rates under different experimental conditions, while molecular modeling approaches allow to unveil the mechanistic aspects of the intercalation processes.     

Theoretical analysis of the photoinduced electron transfer in porphyrin—quinone cyclophanes

Quantum calculations of the INDO-S/CI type are performed on a porphyrin—quinone cyclophane complex to determine the molecular parameters which govern the photoinduced charge separation, such as energy locations of the excited state and the lowest charge transfer states as well as the electron transfer couplings. From the molecular dynamics studies, the structural arrangement of the quinone relative to the porphyrin and the reorganization energies of the solvents are extracted. The results are cast in analytic form, which shows explicitly the polarity dependence of the solvent-induced intramolecular electron transfer process. The theory is used to interpret experimental results in a companion paper.     

The role of potassium in the catalytic synthesis of ammonia

A model study on the promotor action of K in the synthesis of ammonia on iron catalysts was performed by studying the influence of preadsorbed potassium on the rate of dissociative nitrogen chemisorption (which is the rate-limiting step) on a Fe(100) single crystal surface. It was found that a concentration of 1.5 × 10¹⁴ K atoms/cm² increases this rate by a factor of about 300 at 430 K and removes also the activation energy for this process. This effect is attributed to the increased adsorption energy of molecular N₂ (11.5 kcal/mole) which is related with the pronounced transfer of electronic charge from K to the Fe surface.     

How the surrounding water changes the electronic and magnetic properties of DNA

We study the influence of humidity on the transport and magnetic properties of DNA within the quantum chemistry methods. Strong influence of water molecules on these properties, observed in this study, opens up opportunities for application of DNA in molecular electronics. Interaction of the nucleobases with water molecules leads to breaking of some of the pi bonds and appearance of unbound pi electrons. These unbound electrons contribute significantly to the charge transfer at room temperature by up to 10(3) times, but at low temperature the efficiency of charge transfer is determined by the spin interaction of two unbound electrons located on the intrastrand nucleobases. The charge exchange between the nucleobases is allowed only when the spins of unbound electrons are antiparallel. Therefore, the conductance of DNA molecule can be controlled by a magnetic field. That effect has potentials for applications in developing nanoscale spintronic devices based on the DNA molecule, where efficiency of spin interaction will be determined by the DNA sequence.     

The nature of chemical bonding in nitramide

The spatial and electronic structure studies of nitramide NH₂NO₂ suggest that the change in its molecular geometry upon transition from the gas phase to the condensed state is caused by an increase in the contribution of conjugation between functional groups. According to the analysis of the Bader atomic charges, the effects of such conjugation are to a considerable extent governed by intramolecular charge transfer from the amino to the nitro group. From estimation of the contribution of conjugation to the charge transfer it follows that conjugation remains in the isolated molecule. The influence of hydrogen bonding on the increase in the contribution of conjugation and the corresponding charge redistribution in the molecule was considered. Despite the presence of conjugation between functional groups, the planar configuration of the molecule in the crystal is not realized and the crystallographic twofold axis corresponds to superposition of two molecular configurations with C _s symmetry.     

Single configuration interaction study on conjugated betainic chromophores based on DFT optimized geometries

The structure of a series of heterocyclic betaines was calculated by methods of density functional theory (DFT). The charge distribution and bond characteristics of these compounds were analyzed by Weinhold’s natural bond orbital analysis (NBO) and by natural resonance theory (NRT). In order to probe the aromatic character of the ring fragments, Schleyer’s nucleus-independent chemical shifts (NICSs) were calculated by GIAO-RHF. Ab initio single configuration interaction calculations (SCI) correctly predict intense π→π* transitions at low energies, but the transition energies of the color bands are overestimated. Torsion around the interfragmental bond increases the charge separation between the molecular fragments and the dipole moment. The molecular fragments become increasingly aromatic. The absorption wavelengths increase on torsion while the oscillator strengths decrease.     

The effects of polyelectrolytes on the inhibition and aggregation of calcium oxalate crystallization

The influence of polyelectrolytes with different architecture on spontaneous batch crystallization of calcium oxalate was investigated. A series of acidic acrylate block copolymers were been made, by radical polymerization, with defined molecular weight and structure. Radical polymerization of acrylic acid (AA) was carried out in the presence of α‐thiopolyethylene glycol monomethylether as a chain transfer agent to produce poly(ethylene glycolblockacrylic acid) copolymers. Poly(ethylene glycol) (PEG) block length in the copolymers was controlled by using three different molecular weight chain transfer agents (M_n = 350, 750 and 2000 g/mol). The presence of copolymers inhibited the crystal growth of calcium oxalate possibly through adsorption onto the active growth sites for crystal growth due to the charge and hydrophilic effects. Copyright © 2006 John Wiley & Sons, Ltd.