Structural properties and phase transformations under pressure of XTe compounds (X = Be,Mg, and Ca): The role of the exchange–correlation potential

We have performed first principles total energy calculations to investigate the structural properties and possible phase transitions under pressure of IIA–VI compounds: BeTe, MgTe and CaTe. We have considered the following possible structures: rock-salt, nickel arsenide, cesium chloride, zinc-blende, and in some cases wurtzite. Calculations are done using the periodic density functional theory. We employ the full potential linearized augmented plane wave method as implemented in the wien2k code. The exchange and correlation potential energies are treated according to the generalized gradient approximation (GGA) using the Perdew, Burke, Ernzerhof (PBE) parameterization, and the local density approximation (LDA). Our results show that the GGA calculations correctly predict the ground state structure of all three binary compounds: zinc-blende for BeTe, wurtzite/zinc-blende for MgTe, and rock-salt for CaTe. Under pressure, BeTe and MgTe transform to the nickel arsenide structure, while CaTe transforms from rock-salt to cesium chloride. Slightly different results are found using the LDA approximation. We discuss the role of the ionicity in the difference between the LDA and GGA results.     

A core–shell structured BaTiO3 precursor preparation,characterization and potential

A new procedure for the preparation of a core-shell-structured BaTiO(3) precursor (core=TiO(2); shell=BaCO(3)) will be described. The structure of this precursor is characterized by electron microscopy (environmental scanning electron microscopy; energy disperse X-ray spectroscopy), whereas the development of phases during thermal treatment is followed by X-ray powder diffraction.     

Analytical angular solutions for the atom–diatom interaction potential in a basis set of products of two spherical harmonics: two approaches

We present general analytical expressions for the matrix elements of the atom–diatom interaction potential, expanded in terms of Legendre polynomials, in a basis set of products of two spherical harmonics, especially significant to the recently developed adiabatic variational theory for cold molecular collision experiments [J. Chem. Phys. 143, 074114 (2015); J. Phys. Chem. A 121, 2194 (2017)]. We used two approaches in our studies. The first involves the evaluation of the integral containing trigonometric functions with arbitrary powers. The second approach is based on the theorem of addition of spherical harmonics.

     

Correlation consistent, Douglas–Kroll–Hess relativistic basis sets for the 5p and 6p elements

The diatomic carbon molecule has a complex electronic structure with a large number of low-lying electronic excited states. In this work, the potential energy curves (PECs) of the four lowest lying singlet states ( $X^{1} \Sigma^{ + }_{g}$ , $A^{1} \Pi_{u}$ , $B^{1} \Delta_{g}$ , and $B^{\prime1} \Sigma^{ + }_{g}$ ) were obtained by high-level ab initio calculations. Valence electron correlation was accounted for by the correlation energy extrapolation by intrinsic scaling (CEEIS) method. Additional corrections to the PECs included core–valence correlation and relativistic effects. Spin–orbit corrections were found to be insignificant. The impact of using dynamically weighted reference wave functions in conjunction with CEEIS was examined and found to give indistinguishable results from the even weighted method. The PECs showed multiple curve crossings due to the $B^{1} \Delta_{g}$ state as well as an avoided crossing between the two $^{1} \Sigma^{ + }_{g}$ states. Vibrational energy levels were computed for each of the four electronic states, as well as rotational constants and spectroscopic parameters. Comparison between the theoretical and experimental results showed excellent agreement overall. Equilibrium bond distances are reproduced to within 0.05 %. The dissociation energies of the states agree with experiment to within ~0.5 kcal/mol, achieving “chemical accuracy.” Vibrational energy levels show average deviations of ~20 cm^?1 or less. The $B^{1} \Delta_{g}$ state shows the best agreement with a mean absolute deviation of 2.41 cm^?1. Calculated rotational constants exhibit very good agreement with experiment, as do the spectroscopic constants.     

Amphiphilic polymeric particles with core–shell nanostructures: emulsion-based syntheses and potential applications

The design and synthesis of amphiphilic nano- to micro-sized polymeric particles with core–shell nanostructures have attracted more and more attention because of their wide applicability in modern material science and their technological importance in the areas of colloid and interface science. Many synthetic strategies have been developed for the preparation of amphiphilic core–shell particles that consist of hydrophobic polymer cores and hydrophilic polymeric shells. In this review, we focus on emulsion-based approaches and properties of particles produced. These methods are: (1) grafting to functionalized particle that produces a corona-like particle, (2) grafting from reactive seed particle that produces a brush-like particle, (3) copolymerization of reactive macro-monomer with hydrophobic monomer that produces a corona-like particle, (4) emulsion polymerization in the presence of block or comb-like copolymer containing controlled free-radical moiety that produces a multi-layered particle, and (5) redox-initiated graft polymerization of vinyl monomer from a water-soluble polymer containing amino groups that produces a hairy-like particle. Potential applications of some of these particles in drug and gene deliveries, enzyme immobilization, colloidal nanocatalyst, chemical sensing, smart coating, and thermal laser imaging will be discussed.     

Electrical surface charge and potential of hematite/yttrium oxide core–shell colloidal particles

Interest in the synthesis of composite colloidal particles consisting of a core and shell with different compositions stems from the fact that such particles can be useful in processes where the properties of both core (e.g., size and shape homogeneity, ease of preparation in large amounts, magnetic characteristics, etc.) and shell (interfacial properties, porosity, chemical stability, etc.) might be of interest. However, the applicability must be based on a proper characterization of those properties. In this work, colloidal spheres of hematite (α-Fe₂O₃) were used as nuclei of mixed particles where the shell is yttrium oxide. The electrical properties of the aqueous interface are compared to those of the pure oxides by means of potentiometric titration of their surface charge and potential against pH, as a function of indifferent electrolyte concentration. It is found that the mixed particles efficiently mimic yttrium oxide, since the behavior of their surface electrical characteristics closely resembles that of the latter compound. Differences are found, however, that can be ascribed to an incomplete or porous coverage, but such divergences are of little significance when an overall comparison is carried out. Received: 30 January 2001 Accepted: 11 July 2001     

Study of benznidazole–cyclodextrin inclusion complexes, cytotoxicity and trypanocidal activity

The current chemotherapy for Chagas disease is still based on benznidazole, which has low solubility, but complexation with cyclodextrins provides a way of increasing the solubility. The objective of this work was to characterize the inclusion complexes formed between benznidazole (BNZ) and randomly 2-methyled-β-cyclodextrin (RM-β-CD) in aqueous solution and study cytotoxicity and trypanocidal. BNZ:RM-β-CD solution complex systems were prepared and characterized using the phase solubility diagram, nuclear magnetic resonance and a photostability assays, also to investigate the in vitro trypanocidal activity with epimastigote forms of Trypanossoma cruzi and the study of cytotoxicity against mammal cells. The phase-solubility diagram displayed an A _L-type feature, providing evidence of the formation of soluble inclusion complexes. The continuous variation method showed the existence of a complex with 1:1 stoichiometry. Toxicity assays demonstrated that inclusion complexes were able to reduce the toxic effects caused by benznidazole alone and that this did not interfere with the trypanocidal activity of the benznidazole. The use of inclusion complexes benznidazole:cyclodextrin is thus a promising alternative for the development of a safe and stable liquid formulation and a new option for the treatment of Chagas disease.     

On the relation between basis set convergence and electron correlation: a critical test for modern ab initio quantum chemistry on a ��mindless�� data set

The correlation of the only two error sources in the solution of the electronic Schrödinger equation is addressed: the basis set convergence (incompleteness) error (BSIE) and the electron correlation effect. The electron correlation effect and basis set incompleteness error are found to be correlated for all of the molecules in Grimme??s ??mindless?? data set (MB08-165). One can use an extrapolation to the HF or MP2 complete basis set (CBS) limit to see with which type of quantum chemical problem (??simple?? and ??hard??) the researcher is dealing. The origin of the slow convergence of the partial wave expansion can be the Kato cusp condition for electron?Celectron coalescence. Such an extrapolation is possible for many large molecular systems and would give the researcher an idea about the expected electron correlation level that would lead to the desired theoretical accuracy. In other words, it is possible to use not only the CBS energy value itself but the speed with which it is reached to get extra information about the molecular system under study.     

Responsive core–shell microgels: Synthesis,characterization, and possible applications

Core–shell microgels are of increasing interest as smart carriers of catalysts, as sensors, or as building blocks for colloidal superstructures. In the context of colloidal assemblies, photonic applications are probably the most promising ones. This progress report presents and discusses the most recent results in this area focusing on the last 2–3 years, and also gives some background information. In addition, potential perspectives of this area will be outlined. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1073–1083     

Stability of XSO2 (X=F,Cl, and Br) radical: impact of the basis set on X–S bonding energy in ab initio and DFT calculations

XSO₂ is a labile intermediate, which can be formed upon pyrolysis of SO₂X₂ or upon interaction between halogen atom and sulphur dioxide in the troposphere. Detailed theoretical investigation demonstrated that even high correlated ab initio methods reproduce incorrectly BDE(X–SO₂), unless very extended basis set is used. Thus, the bond energies derived previously using small basis sets may not be correct. At the same time, B3LYP with medium size basis sets produced results close to those obtained from Gaussian and CBS model chemistries. The source of discrepancies between ab initio and DFT methods is analysed and the implication of our results in the oxidation of tropospheric SO₂ is discussed.     

The basis set convergence of the Hartree–Fock energy for H3 +, Li2 and N2

 By using completely optimized basis functions it is shown that the convergence of the Hartree–Fock energy for the H₃ ⁺, Li₂ and N₂ molecules is significantly better described by exponential behavior than by inverse power dependence. This is the case both with respect to the number of basis functions of a given type and with respect to the highest angular momentum function included. The Hartree–Fock limit for H₃ ⁺ is estimated to be −1.300372125 hartree. Received: 14 February 2000 / Accepted: 12 April 2000 / Published online: 18 August 2000     

Influence of basis set and electron correlation on calculated barriers to 1,2-hydrogen shifts. The oxoniomethylene cation: A new CH3O+ isomer?

Ab initio molecular orbital theory has been used to study the intramolecular rearrangement of ethylidene to ethylene and of the oxoniomethylene cation to the hydroxymethyl cation. Optimized geometries of stable isomers and transition structures have been determined using gradient procedures and the 4-31G basis set. Improved energy comparisons have been obtained with the double-zeta plus polarization 6-3IG⁺⁺ basis set with electron correlation incorporated at the levels of second- (MP2) and third- (MP3) order Møller—Plesset perturbation theory.     

Assembly of peptide–thiophene conjugates: the influence of peptide content and location

Biomolecule-directed self-assembly of π-conjugated oligomers has attracted great attention in the past decade. In this contribution, two conjugates composed of quaterthiophene and tetrapeptide (Gly-Val-Gly-Val) were synthesised, namely peptide–thiophene–peptide (PTP) and thiophene–peptide–thiophene (TPT), to investigate the influence of peptide content ratio and its location in the molecular structures on the nanostructures and properties of the assemblies. Both conjugates formed organogels consisting of left-handed twisted nanostructures; however, anti-parallel β-sheets were observed in PTP while parallel β-sheets were obtained for TPT, although in both cases oligothiophenes adopted an H-like stacking mode. Obvious solvent-induced supramolecular chirality inversion from the oligothiophene segment was observed for PTP while such phenomenon was not clear for TPT. PTP and TPT gels also showed different stabilities towards temperature increase, as evidenced by variable-temperature circular dichroism study. From the data, it is suggested that the rational design of the location and ratio of peptide plays a key role in constructing materials with determined properties based on peptide–thiophene conjugates.     

Quantum chemical studies of azoles 5. Electrophilic substitution in tetrazole and 1, 2, 4-triazole: the influence of basis set on calculated thermodynamic parameters of the elimination—addition mechanism without preliminary formation of N-protonated azolium salts

Thermodynamic parameters of electrophilic substitution reactions of 1H-tetrazole and 1H-1, 2, 4-triazole proceeding by the addition–elimination and elimination–addition mechanisms were calculated by the DFT/B3LYP/6-31G(2df, p) method using proton as model electrophile and compared. The results obtained substantiate that the elimination–addition mechanism may not involve preliminary formation of N-protonated azolium salts, as was shown earlier in our DFT/B3LYP/6-31G(d, p) calculations.