Impact of substituents on the luminescent properties and thermostability of Zn(II) 8-hydroxyquinolinates: insight from experimental and theoretical approach

Four unique 2-arylethenyl-8-hydroxyquinoline ligands (B1–B4) and their corresponding Zn(II) complexes (C1–C4) were synthesized and characterized by ¹H NMR, ESI-MS, FTIR, and elemental analysis. The aggregation behavior of Zinc salt and ligands in solution was investigated by several techniques, including ¹H NMR, UV–vis, and photoluminescence (PL). The electronic nature of arylethenyl substituents affects the absorption wavelength, the emission color, fluorescence lifetime, fluorescence quantum yield, and thermostability of Zn(II) complexes. Luminescent properties of the Zn(II) complexes correspond to the electron-withdrawing/-donating character of the arylethenyl substituents. Photophysical analyses combined with density functional theory (DFT) calculations established that the introduction of strong electron-withdrawing group (CN) decreased the HOMO–LUMO energy gap, and the introduction of electron-donating group (tert-butyl) enlarged the HOMO–LUMO energy gap.     

In-depth quantum chemical investigation of electro-optical and charge-transport properties of trans-3-(3,4-dimethoxyphenyl)-2-(4-nitrophenyl)prop-2-enenitrile

The structural, electro-optical and charge-transport properties of compound trans-3-(3,4-dimethoxyphenyl)-2-(4-nitrophenyl)prop-2-enenitrile (DMNPN) were studied using quantum chemical methods. The neutral, cation and anion molecular geometries were optimized in the ground state using density functional theory (DFT) at the restricted and unrestricted B3LYP/6-31G** level of theory. The excited state geometries were optimized by applying time-dependent DFT at the TD-B3LYP/6-31G** level of theory. The absorption and fluorescence wavelengths were calculated at the TD-CAM-B3LYP/6-31G** and TD-LC-BLYP/6-31G** levels of theory. The distribution pattern of the charge densities on the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are discussed. Intramolecular charge transfer was observed from the dimethoxyphenyl to (nitrophenyl)prop-2-enenitrile moieties. The detailed charge-transport behavior of the DMNPN molecule is investigated based on its ionization potential, electron affinity, hole and electron reorganization energies, hole and electron-transfer integrals, and hole and electron intrinsic mobilities. The total/partial densities of states and structure–property relationship are discussed in detail. The higher computed hole intrinsic mobility than electron intrinsic mobility reveals that DMNPN is an efficient hole-transport material.     

Structural and electronic properties of stable Aun (n = 2–13) clusters: A density functional study

All-electron scalar relativistic calculations have been performed to investigate the electronic structures of neutral gold clusters (Au_n, n = 2–13) in the gas phase using density functional theory with the generalized gradient approximation. Full geometry optimizations of topologically different clusters and clusters belonging to different symmetry groups have been carried out. Binding energies, ionization potentials, electron affinities, and chemical hardness values are calculated and they are found to be comparable with the available experimental and theoretical results. The most stable structure of each of the cluster has a two-dimensional planar configuration. A three dimensional distorted Y shaped structure (4b) for Au₄, a tri-capped triangle (6b), a chair (6f), and a see-saw structure (6j) for Au₆, an eclipsed sandwich structure (7g) for Au₇, a condensed trigonal bipyramid (9e) and a boat shaped structure (9f) for Au₉, a staggered sandwich (11c) and an eclipsed sandwich structure (11d) for Au₁₁, a ladderane structure (12d) for Au₁₂, and a staggered (13d) and a distorted sandwich structure (13e) for Au₁₃ are characterized for the first time in this work.     

A quantum chemical study of the mechanism of manganese catalase

The catalytic mechanism of manganese catalase has been studied using the Becke's three parameter hybrid method with the Lee, Yang and Parr correlation functional. On the basis of available experimental information on the geometric and electronic structure of the active manganese dimer complex, different possibilities were investigated. The mechanism finally suggested consists of eight steps. In the first steps, the first hydrogen peroxide becomes bound and its O–O bond is activated. This occurs in a spin-forbidden process found to be common in many biological processes where the O–O bond is cleaved, and two general rules are formulated for the requirements for a low activation energy in this type of reaction. As the O–O bond is cleaved a hydroxyl radical is initially formed in the overall rate-limiting step of the catalytic cycle. This radical is then immediately and irreversibly quenched in a strongly exothermic step. In the subsequent steps, the second hydrogen peroxide becomes bound and its two O–H bonds are broken, leading to the formation of an O₂ molecule, which is released. Parallels between the reversal of the present O–O cleavage mechanism in manganese catalase and the recently suggested O–O bond formation in photosystem II are drawn. Received: 12 July 2000 / Accepted: 12 July 2000 / Published online: 21 December 2000     

Photoinduced intramolecular charge transfer (ICT) reaction in trans-methyl p-(dimethylamino) cinnamate: A combined fluorescence measurement and quantum chemical calculations

The photophysical behaviour of trans-methyl p-(dimethylamino) cinnamate (t-MDMAC) donor–acceptor system has been investigated by steady-state absorption and emission spectroscopy and quantum chemical calculations. The molecule t-MDMAC shows an emission from the locally excited state in non-polar solvents. In addition to weak local emission, a strong solvent dependent red shifted fluorescence in polar aprotic solvents is attributed to highly polar intramolecular charge transfer state. However, the formation of hydrogen-bonded clusters with polar protic solvents has been suggested from a linear correlation between the observed red shifted fluorescence band maxima with hydrogen bonding parameters (). Calculations by ab initio and density functional theory show that the lone pair electron at nitrogen center is out of plane of the benzene ring in the global minimum ground state structure. In the gas phase, a potential energy surface along the twist coordinate at the donor (–NMe₂) and acceptor (–CH = CHCOOMe) sites shows stabilization of S₁ state and destabilization S₂ and S₀ states. A similar potential energy calculation along the twist coordinate in acetonitrile solvent using non-equilibrium polarized continuum model also shows more stabilization of S₁ state relative to other states and supports solvent dependent red shifted emission properties. In all types of calculations it is found that the nitrogen lone pair is delocalized over the benzene ring in the global minimum ground state and is localized on the nitrogen centre at the 90° twisted configuration. The S₁ energy state stabilization along the twist coordinate at the donor site and localized nitrogen lone pair at the perpendicular configuration support well the observed dual fluorescence in terms of proposed twisted intramolecular charge transfer (TICT) model.     

An ab initio quantum chemical comparative study of possible additive rules and linear relations in parent and extended sulfur diimide families

In this study, it has been demonstrated that there are additive rules corresponding to ab initio derived total electronic energies between members of triple sets of some extended sulfur diimides and their mono- and bi-derivatives. It has been shown that the additive rules are insensitive to the combination of methods and basis sets used to derive the total electronic energies. This insensitivity to the level of calculation is demonstrated to be the case for some linear alkanes also. It has been found that the total electronic energies of certain members of extended sulfur diimide sets ((ZZ)_k and (EE)_k conformers) follow a linear relation although chemical accuracy may be achieved only by excluding the smallest members of these sets. The details of this deviation have been employed to quantify the “Z-effect” proposed previously by the same authors.     

O deficiency in the rutile TiO2 (110) surface: Ab initio quantum chemical investigation of the electronic properties

The (110) surface of rutile TiO₂ (110) has been modeled using a density functional theory (DFT) plane‐wave pseudo‐potential method (CASTEP). In this study, 6 and 9 atomic‐layer slabs have been examined. The stoichiometric surface converges to a low‐spin solution in both cases with a density of states (DOS) similar to that for the bulk. O deficiencies are introduced by the removal of neutral O atoms thus leaving a neutral model with a surfeit of 2 e^? per vacancy. This results in the partial filling of the Ti t_2g conduction band orbitals and a compensatory shift in the Fermi level. The reduced surface converges to a high‐spin solution in all cases, with the excess spin located within the previously unoccupied Ti t_2g orbitals. Removal of the bridging surface O atoms results in an excess spin of 2 electrons per unit cell with approximately one‐half that for removal of in‐plane surface O atoms and subsurface O atoms. The removal of O atoms from the surface leads to an increase of the band gap, with the largest increase due to the removal of in‐plane 3‐fold coordinated surface O atoms, and the smallest one due to the removal of subsurface O atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

A study of NMR chemical shielding in 5-coordinate phosphorus compounds (phosphoranes)

Calculations of the phosphorus NMR chemical shielding in 5-coordinate phosphorus compounds have been carried out using the gauge-including-atomic-orbital (GIAO) 6-311+G(nd,p) basis set at both scaled density functional theory (sDFT) and estimated infinite order Møller–Plesset (EMPI) approaches. Results are generally in accord with previous studies on 3-coordinate phosphorus compounds but fail badly for compounds containing multiple chlorine atoms and indicate a need for a relativistic treatment of these species. We observe that some compounds with reported experimental ³¹P NMR chemical shifts far downfield of the calculated values are in fact in the range known from experiment and calculation to be in that expected for phosphonium ions; the reported structures need to be reconsidered.     

Structural,mechanical, electronic,optical properties and effective masses of CuMO2 (M = Sc,Y, La) compounds: First-principles calculations

The structural, elastic, mechanical, electronic, optical properties and effective masses of CuM_IIIBO₂ (M_IIIB = Sc, Y, La) compounds have been investigated by the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory under local density approximation. The equilibrium structural parameters are in good agreement with previous experimental and theoretical data. To our knowledge, there are no available data of elastic constants for comparison. The bulk, shear and Young's modulus, ratio of B/G, Poisson's ratio and Lamé's constants of CuM_IIIBO₂ have been studied. The electronic structures of CuM_IIIBO₂ are consistent with other calculations. The population analysis, charge densities and effective masses have been shown and analyzed. The imaginary and real parts of the dielectric function, refractive index and extinction coefficient of CuM_IIIBO₂ are calculated. The interband transitions to absorption of CuM_IIIBO₂ have been analyzed.     

First-principles study of structural and vibrational properties of crystalline silver azide under high pressure

A detailed first-principles study of the structural and vibrational properties of crystalline silver azide under hydrostatic pressure of 0–500 GPa has been performed with density functional theory in the generalized gradient approximation. The crystal structure is relaxed to allow ionic configurations, cell shape, and volume to change without any symmetry constraints. It is found that the silver azide crystal remains orthorhombic structure with Ibam space group for pressures up to 7 GPa, where there is a transition to an I4/mcm tetragonal symmetry. The lattice parameter and electronic structure are investigated as functions of pressure. The calculated vibrational frequencies at ambient pressure are in agreement with available experimental data. We also discuss the pressure-induced frequency shifts for the internal and lattice modes of silver azide crystal upon compression.     

Mechanical properties of diboron-porphyrin sheet under strain: A density functional theory study

The mechanical properties of a new two-dimensional semiconductor material named diboron-porphyrin (DP) are studied based on density functional theory. The behavior of DP monolayer under uniaxial and biaxial loadings as well as shear stress is investigated. The in-plane stiffness, Poisson's ratio, bulk and shear moduli of the DP monolayer are found to be close to those of a graphene sheet. It can be concluded that the DP monolayer has stiffness close to the graphene sheet. The difference in magnitude of in-plane stiffness and Poisson's ratio along x- and y-directions shows slightly anisotropic mechanical properties of DP monolayer. It is also observed that DP monolayers can bear high tensile strains before failure. The high stability and hardly deformable structure of DP monolayer are due to its high planar packing density which is comparable with graphene sheet. The fantastic mechanical properties of DP show these materials are desirable for application in nanomechanical devices.     

Nonaqueous capillary electrophoresis and quantum chemical calculations applied to investigation of acid–base and electromigration properties of azahelicenes

Nonaqueous capillary electrophoresis (NACE) using methanol (MeOH) as a solvent of the BGEs and quantum mechanical density functional theory (DFT) have been applied to determine the thermodynamic acidity (ionization) constants (pK_a) of mono- and diaza[5]helicenes, mono- and diaza[6]helicenes, and their dibenzo derivatives in MeOH and water. First, the mixed acidity constants, , of ionogenic pyridinium groups of azahelicenes and their derivatives in MeOH were obtained by nonlinear regression analysis of pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by NACE in a large series of methanolic BGEs within a wide conventional pH range (pH_MeOH 1.6–12.0) and at ambient temperature (21–26°C) in a home-made CE device. Prior to mixed acidity constant calculation, the effective mobilities were corrected to reference temperature (25°C) and constant ionic strength (25 mM). Then, the mixed acidity constants were recalculated to the thermodynamic acidity constants pK_a,MeOH by the Debye–Hückel theory of nonideality of electrolyte solutions. Finally, from the methanolic thermodynamic pK_a,MeOH values, the aqueous thermodynamic constants were estimated using the empirical relations between methanolic and aqueous acidity constants derived for structurally related pyridine derivatives. Depending on the number and position of the nitrogen atoms in their molecules, the analyzed azahelicenes were found to be weak to moderate bases with methanolic pK_a,MeOH in the range 2.01–8.75 and with aqueous in the range 1.67–8.28. The thermodynamic pK_a,MeOH obtained by the DFT calculations were in a good agreement with those determined experimentally by NACE.     

Development of a model for the rational design of molecular imprinted polymer: Computational approach for combined molecular dynamics/quantum mechanics calculations

A new rational approach for the preparation of molecularly imprinted polymer (MIP) based on the combination of molecular dynamics (MD) simulations and quantum mechanics (QM) calculations is described in this work. Before performing molecular modeling, a virtual library of functional monomers was created containing forty frequently used monomers. The MD simulations were first conducted to screen the top three monomers from virtual library in each porogen-acetonitrile, chloroform and carbon tetrachloride. QM simulations were then performed with an aim to select the optimum monomer and progen solvent in which the QM simulations were carried out; the monomers giving the highest binding energies were chosen as the candidate to prepare MIP in its corresponding solvent. The acetochlor, a widely used herbicide, was chosen as the target analyte. According to the theoretical calculation results, the MIP with acetochlor as template was prepared by emulsion polymerization method using N,N-methylene bisacrylamide (MBAAM) as functional monomer and divinylbenzene (DVB) as cross-linker in chloroform. The synthesized MIP was then tested by equilibrium-adsorption method, and the MIP demonstrated high removal efficiency to the acetochlor. Mulliken charge distribution and ¹H NMR spectroscopy of the synthesized MIP provided insight on the nature of recognition during the imprinting process probing the governing interactions for selective binding site formation at a molecular level. We think the computer simulation method first proposed in this paper is a novel and reliable method for the design and synthesis of MIP.     

Spectroscopic and inclusion properties of G-series chemical warfare agents and their simulants: a DFT study

A computational protocol to predict the infrared spectra of chemical warfare agents (CWAs) tabun (GA), sarin (GB), soman (GD) and cyclosarin (GF) has been developed. Sarin was used to benchmark the method through gas phase simulations. DFT calculations using the EDF2 functional and diffuse 6-311++G** basis set was found to give the closest match to experimental infrared spectra. Using the same functional the 6-31G (2df, 2p) basis set was found to be superior when hydrated sarin was modelled. GA, GB, GD and GF, together with 11 commonly used simulants, were modelled in the gas and hydrated states. Complexes of GB and a number of CWA mimics with α-cyclodextrin were modelled to give insight into their different modes of inclusion.     

First-principles study of structural,elastic, electronic and vibrational properties of BiCoO3

We used density functional theory (DFT) to study the structural, elastic, electronic, and lattice dynamical properties of tetragonal BiCoO₃ applying the “norm-conserving” pseudopotentials within the local spin density approximation (LSDA). The calculated equilibrium lattice parameters and atomic displacements are in agreement with the available experimental and theoretical results. Moreover, the structural stability of tetragonal BiCoO₃ were confirmed by the calculated elastic constants. In addition, the elastic properties of polycrystalline aggregates including bulk, shear and Young's moduli, and Poisson's ratio are also determined. The electronic band structure, total and partial density of states (DOS and PDOS) with ferromagnetic spin configuration are obtained. The results show that tetragonal BiCoO₃ has an indirect band gap with both up- and down-spin configurations and its bonding behavior is of covalent nature. We compute Born effective charge (BEC) which is found to be quite anisotropic of Bi, Co and O atoms. The infrared and Raman active phonon mode frequencies at the Г point are found. The phonon dispersion curves exhibit imaginary frequencies which lead from the high-symmetry tetragonal phase to low-symmetry rhombohedral phase in BiCoO₃. The six independent elastic constants, including bulk, shear and Young's moduli, and Poisson's ratio, complete BEC tensor and phonon dispersion relations in tetragonal BiCoO₃ are predicted for the first time. Results of the calculations are compared with the existing experimental and theoretical data.     

First-principles study of the phonon vibrational spectra and thermal properties of hexagonal MoS2

The phonon spectra and thermal properties of the hexagonal MoS₂ are investigated by using first-principles calculations within the density functional theory (DFT). Finite displacement method is used to calculate the phonon vibrational spectra and phonon density of states. The vibrational modes at the Gamma point are analyzed by using group theory. The temperature and pressure dependence of its thermal quantities such as the thermal expansion, the heat capacity at constant volume, the Gibbs energy and entropy are obtained based on the quasi-harmonic approximation (QHA). Our results show that both the thermal expansion coefficient α and the heat capacity C_V increase with T³ at low temperatures and gradually turn almost linear as the temperature increases. It is found that the entropy is sensitive to the temperature while the Gibbs free energy is more sensitive to the pressure change.