Nonsingular two/one-component relativistic Hamiltonians accurate through arbitrary high order in α2

A series of nonsingular two-component relativistic Hamiltonians is derived from the Dirac Hamiltonian by first performing the free-particle Foldy–Wouthuysen transformation and then a block-diagonalizing transformation. The latter is defined in terms of operators which can be determined iteratively through arbitrary order in α, leading to transformed Hamiltonians with the two-component block accurate through α^2k, k=1, 2, 3,… . These Hamiltonians give relativistic energies which differ from Dirac's energies only in terms higher than α^2k. Their relation to other nonsingular methods of relativistic quantum chemistry (the Douglas–Kroll method, the regular Hamiltonian schemes) is discussed. By removing the spin-dependent operators, the derived Hamiltonians can be written in spin-free one-component form. The computational effort involved is essentially the same as in the case of the Douglas–Kroll scheme and amounts to relatively easy modification of the core Hamiltonian. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65 : 225–239, 1997     

6‐TIPS‐β‐Cyclodextrin‐Modified Fe3O4 for Facile Enantioseparation of 1‐(1‐Naphthyl)ethylamine

A new type of chiral magnetic nanoparticle was prepared from covalently linked magnetic nanoparticles (Fe₃O₄) and heptakis‐(6‐O‐triisopropylsilyl)‐β‐cyclodextrin (6‐TIPS‐β‐CD). The resulting selectors (TIPS‐β‐CD‐MNPs) combined the good magnetic properties Fe₃O₄ and efficient chiral recognition ability of 6‐TIPS‐β‐CD. The enantioselectivity of TIPS‐β‐CD‐MNPs towards 1‐(1‐naphthyl)ethylamine was six times higher than that of the parent β‐CD modified Fe₃O₄ particles.     

MP2, DFT‐D,and PCM study of the HMB–TCNE complex: Thermodynamics,electric properties,and solvent effects

Geometry, thermodynamic, and electric properties of the π‐EDA complex between hexamethylbenzene (HMB) and tetracyanoethylene (TCNE) are investigated at the MP2/6‐31G* and, partly, DFT‐D/6‐31G* levels. Solvent effects on the properties are evaluated using the PCM model. Fully optimized HMB–TCNE geometry in gas phase is a stacking complex with an interplanar distance 2.87 × 10^?10 m and the corresponding BSSE corrected interaction energy is ?51.3 kJ mol^?1. As expected, the interplanar distance is much shorter in comparison with HF and DFT results. However the crystal structures of both (HMB)₂–TCNE and HMB–TCNE complexes have interplanar distances somewhat larger (3.18 and 3.28 × 10^?10 m, respectively) than our MP2 gas phase value. Our estimate of the distance in CCl₄ on the basis of PCM solvent effect study is also larger (3.06–3.16 × 10^?10 m). The calculated enthalpy, entropy, Gibbs energy, and equilibrium constant of HMB–TCNE complex formation in gas phase are: ΔH⁰ = ?61.59 kJ mol^?1, ΔS = ?143 J mol^?1 K^?1, ΔG⁰ = ?18.97 kJ mol^?1, and K = 2,100 dm³ mol^?1. Experimental data, however, measured in CCl₄ are significantly lower: ΔH⁰ = ?34 kJ mol^?1, ΔS = ?70.4 J mol^?1 K^?1, ΔG⁰ = ?13.01 kJ mol^?1, and K = 190 dm³ mol^?1. The differences are caused by solvation effects which stabilize more the isolated components than the complex. The total solvent destabilization of Gibbs energy of the complex relatively to that of components is equal to 5.9 kJ mol^?1 which is very close to our PCM value 6.5 kJ mol^?1. MP2/6‐31G* dipole moment and polarizabilities are in reasonable agreement with experiment (3.56 D versus 2.8 D for dipole moment). The difference here is due to solvent effect which enlarges interplanar distance and thus decreases dipole moment value. The MP2/6‐31G* study supplemented by DFT‐D parameterization for enthalpy calculation, and by the PCM approach to include solvent effect seems to be proper tools to elucidate the properties of π‐EDA complexes. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Construction of Giant‐Spin Hamiltonians from Many‐Spin Hamiltonians by Third‐Order Perturbation Theory and Application to an Fe3Cr Single‐Molecule Magnet

A general giant‐spin Hamiltonian (GSH) describing an effective spin multiplet of an exchange‐coupled metal cluster with dominant Heisenberg interactions was derived from a many‐spin Hamiltonian (MSH) by treating anisotropic interactions at the third order of perturbation theory. Going beyond the existing second‐order perturbation treatment allows irreducible tensor operators of rank six (or corresponding Stevens operator equivalents) in the GSH to be obtained. Such terms were found to be of crucial importance for the fitting of high‐field EPR spectra of a number of single‐molecule magnets (SMMs). Also, recent magnetization measurements on trigonal and tetragonal SMMs have found the inclusion of such high‐rank axial and transverse terms to be necessary to account for experimental data in terms of giant‐spin models. While mixing of spin multiplets by local zero‐field splitting interactions was identified as the major origin of these contributions to the GSH, a direct and efficient microscopic explanation had been lacking. The third‐order approach developed in this work is used to illustrate the mapping of an MSH onto a GSH for an trigonal Fe₃Cr complex that was recently investigated by high‐field EPR spectroscopy. Comparisons between MSH and GSH consider the simulation of EPR data with both Hamiltonians, as well as locations of diabolical points (conical intersections) in magnetic‐field space. The results question the ability of present high‐field EPR techniques to determine high‐rank zero‐field splitting terms uniquely, and lead to a revision of the experimental GSH parameters of the Fe₃Cr SMM. Indeed, a bidirectional mapping between MSH and GSH effectively constrains the number of free parameters in the GSH. This notion may in the future facilitate spectral fitting for highly symmetric SMMs.     

Microstructure and mechanical properties of isotactic polypropylene composite with two‐scale reinforcement

Isotactic polypropylene (iPP) composite with two‐scale reinforcement structure, i.e. nanoscale shish–kebab structure and micron‐scale glass fiber (GF) with orientation, was fabricated by an oscillatory shear injection molding (OSIM) technology. The oscillatory shear flow provided by the OSIM gave rise to a high fraction of shish–kebab structures in the iPP composite, characterized by X‐ray scattering technique. On the other hand, the oscillatory shear flow oriented GFs in the iPP composite, which was revealed by scanning electron microscopy measurement. The iPP composite with this two‐scale reinforcement structure exhibited simultaneously remarkably enhanced tensile strength and impact strength. Fracture mechanism of this iPP composite was also proposed. Copyright © 2012 John Wiley & Sons, Ltd.     

On the performance of long‐range‐corrected density functional theory and reduced‐size polarized LPol‐n basis sets in computations of electric dipole (hyper)polarizabilities of π‐conjugated molecules

Static longitudinal electric dipole (hyper)polarizabilities are calculated for six medium‐sized π‐conjugated organic molecules using recently developed LPol‐n basis set family to assess their performance. Dunning's correlation‐consistent basis sets of triple‐ζ quality combined with MP2 method and supported by CCSD(T)/aug‐cc‐pVDZ results are used to obtain the reference values of analyzed properties. The same reference is used to analyze (hyper)polarizabilities predicted by selected exchange‐correlation functionals, particularly those asymptotically corrected. © 2012 Wiley Periodicals, Inc.     

Characterization and electric field dependence of N,N′‐bis(9H‐fluoren‐9‐ylidene)benzene‐1, 4‐diamine thin film/substrate interface

N,N′‐bis(9H‐fluoren‐9‐ylidene)benzene‐1,4‐diamine deposited onto highly oriented pyrolytic graphite (HOPG) was investigated by contact angle measurement(CAM), Raman spectroscopy and tunneling spectroscopy. The results of CAM and Raman spectra have confirmed that organic layers had deposited on substrate. Tunneling spectra obtained in the scanning tunneling microscopy measurement system were reported as a function of electrode potential. The tunneling current data were acquired at different electrode–electrode separations and depicted significant trend under the action of electric field. Under weak electric fields, the electrode–electrode separation has little effect on the potential of conductance peak. However, with the shrinkage of electrode–electrode separation, the electron transport model obeys the Ohmic law. Copyright © 2010 John Wiley & Sons, Ltd.     

Thomas–Fermi approximation for the quasi‐two‐dimensional electron gas

To take into account static correlation effects in the quasi‐two‐dimensional electron gas a screened Coulombic interaction between particles is studied. The Thomas–Fermi approximation is used and the potential screening appears as a function of the Wigner–Seitz density parameter r_s and the effective width t of the system. With the self‐consistent field theory applied to the modified deformable jellium, the ground‐state energy per particle and the conditions for electron localization are obtained in terms of the interparticle distance and the screening parameter μ. A critical minimum characteristic width t_c is obtained; below t_c no long‐range order is obtained. For larger widths a stable localized state is predicted at finite densities. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 269–276, 2001     

Magnetic Properties of a Single‐Molecule Lanthanide–Transition‐Metal Compound Containing 52 Gadolinium and 56 Nickel Atoms

Monodisperse metal clusters provide a unique platform for investigating magnetic exchange within molecular magnets. Herein, the core–shell structure of the monodisperse molecule magnet of [Gd₅₂Ni₅₆(IDA)₄₈(OH)₁₅₄(H₂O)₃₈]@SiO₂ ( 1 a @SiO₂) was prepared by encapsulating one high‐nuclearity lanthanide–transition‐metal compound of [Gd₅₂Ni₅₆(IDA)₄₈(OH)₁₅₄(H₂O)₃₈]?(NO₃)₁₈?164 H₂O ( 1 ) (IDA=iminodiacetate) into one silica nanosphere through a facile one‐pot microemulsion method. 1 a @SiO₂ was characterized using transmission electron microscopy, N₂ adsorption–desorption isotherms, and inductively coupled plasma‐atomic emission spectrometry. Magnetic investigation of 1 and 1 a revealed J₁=0.25 cm^?1, J₂=?0.060 cm^?1, J₃=?0.22 cm^?1, J₄=?8.63 cm^?1, g=1.95, and z J=?2.0×10^?3 cm^?1 for 1 , and J₁=0.26 cm^?1, J₂=?0.065 cm^?1, J₃=?0.23 cm^?1, J₄=?8.40 cm^?1 g=1.99, and z J=0.000 cm^?1 for 1 a @SiO₂. The z J=0 in 1 a @SiO₂ suggests that weak antiferromagnetic coupling between the compounds is shielded by silica nanospheres.     

Thermal‐ and Light‐Induced Spin Crossover in a Guest‐Dependent Dinuclear Iron(II) System

We previously reported the dinuclear material [Fe^II₂(ddpp)₂(NCS)₄] ? 4 CH₂Cl₂ ( 1? 4 CH₂Cl₂; ddpp=2,5‐di(2′,2′′‐dipyridylamino)pyridine) and its partially desolvated analogue ( 1? CH₂Cl₂), which undergo two‐ and one‐step spin‐crossover (SCO) transitions, respectively. Here, we manipulate the type and degree of solvation in this system and find that either a one‐ or two‐step spin transition can be specifically targeted. The chloroform clathrate 1? 4 CHCl₃ undergoes a relatively abrupt one‐step SCO, in which the two equivalent Fe^II sites within the dinuclear molecule crossover simultaneously. Partial desolvation of 1? 4 CHCl₃ to form 1? 3 CHCl₃ and 1? CHCl₃ occurs through single‐crystal‐to‐single‐crystal processes (monoclinic C2/c to P2₁/n to P2₁/n) in which the two equivalent Fe^II sites become inequivalent sites within the dinuclear molecule of each phase. Both 1? 3 CHCl₃ and 1? CHCl₃ undergo one‐step spin transitions, with the former having a significantly higher SCO temperature than 1? 4 CHCl₃ and the latter, and each has a broader SCO transition than 1? 4 CHCl₃, attributable to the overlap of two SCO steps in each case. Further magnetic manipulation can be carried out on these materials through reversibly resolvating the partially desolvated material with chloroform to produce the original one‐step SCO, or with dichloromethane to produce a two‐step SCO reminiscent of that seen for 1? 4 CH₂Cl₂. Furthermore, we investigate the light‐induced excited spin state trapping (LIESST) effect on 1? 4 CH₂Cl₂ and 1? CH₂Cl₂ and observe partial LIESST activity for the former and no activity for the latter.     

Infrared laser synthesis and properties of magnetic nano‐iron–polyoxocarbosilane composites

Nano‐magnetic, thermally stable iron‐based composites were obtained by a one‐step procedure consisting of continuous‐wave infrared laser‐induced and ethylene‐sensitized co‐pyrolysis of gaseous iron pentacarbonyl and hexamethyldisiloxane in argon. The simultaneously occurring formation of iron from iron pentacarbonyl and that of organosilicon polymer from hexamethyldisiloxane yield iron nanoparticles surrounded by an organosilicon polymer shell. The particles were characterized by spectral analyses, electron microscopy, thermal gravimetry and magnetic measurements. They become superficially oxidized in the atmosphere. Their composition, thermal behaviour and magnetic properties depend on the flow rates of the precursors and the total pressure of the procedure. Magnetization curves, exchange bias H_ex at T = 5 K and AC susceptibility were studied in the temperature range 5–400 K. The values of H_ex verified the observed degree of the particle surface oxidation. The system of the iron nanoparticles is in a ferromagnetic blocked state and the temperature dependence of the coercivity and susceptibility is in accord with the transmission electron microscopy data. Copyright © 2005 John Wiley & Sons, Ltd.     

Polystyrene‐Core–Silica‐Shell Hybrid Particles Containing Gold and Magnetic Nanoparticles

Polystyrene‐core–silica‐shell hybrid particles were synthesized by combining the self‐assembly of nanoparticles and the polymer with a silica coating strategy. The core–shell hybrid particles are composed of gold‐nanoparticle‐decorated polystyrene (PS‐AuNP) colloids as the core and silica particles as the shell. PS‐AuNP colloids were generated by the self‐assembly of the PS‐grafted AuNPs. The silica coating improved the thermal stability and dispersibility of the AuNPs. By removing the “free” PS of the core, hollow particles with a hydrophobic cage having a AuNP corona and an inert silica shell were obtained. Also, Fe₃O₄ nanoparticles were encapsulated in the core, which resulted in magnetic core–shell hybrid particles by the same strategy. These particles have potential applications in biomolecular separation and high‐temperature catalysis and as nanoreactors.     

Preparation of Si–C–N–Fe magnetic ceramics from iron‐containing polysilazane

A new type of hyperbranched polysilazane containing iron (PSZI) compound was synthesized by the polycondensation of silazane lithium salts with FeCl₃, and the structure of the PSZIs was investigated by IR, NMR and elemental analyses. The PSZIs were pyrolyzed under nitrogen, argon or NH₃, and magnetic ceramics could be obtained. The ceramic yields of the PSZIs were higher than those of their corresponding silazanes, and the PSZIs or silazanes with reactive groups containing Si? H, ? CH?CH₂ or higher branched structures had higher yields. The magnetism of the ceramics could be controlled by a pyrolytic atmosphere and temperature: the saturation magnetization M_s ranged from 20 to 100 emu g^?1 and coercivity H_c ranged from 463 to 50 Oe. The transformation of the magnetic loop of the PSZIs pyrolyzed at different temperatures under NH₃ was quite different from those under nitrogen. It was shown by X‐ray diffraction measurements that the magnetic crystalline form could exist as Fe₄N, Fe(0) or Fe₃N depending on temperature under NH₃, but under a nitrogen atmosphere Fe(0) was nearly the only magnetic crystalline form from 600 to 1100 °C. By dipping or spin‐coating of the PSZI solution, then through pyrolysis under nitrogen, argon or NH₃, thin uniform magnetic ceramic films could be fabricated on the substrates. Copyright © 2003 John Wiley & Sons, Ltd.     

Selected‐Control Fabrication of Multifunctional Fluorescent–Magnetic Core–Shell and Yolk–Shell Hybrid Nanostructures

The selected‐control preparation of uniform core–shell and yolk–shell architectures, which combine the multiple functions of a superparamagnetic iron oxide (SPIO) core and europium‐doped yttrium oxide (Y₂O₃:Eu) shell in a single material with tunable fluorescence and magnetic properties, has been successfully achieved by controlling the heat‐treatment conditions. Furthermore, the shell thickness and interior cavity of SPIO@Y₂O₃:Eu core–shell and yolk–shell nanostructures can be precisely tuned. Importantly, as‐prepared SPIO@Y₂O₃:Eu yolk–shell nanocapsules (NCs) modified with amino groups as cancer‐cell fluorescence imaging agents are also demonstrated. To the best of our knowledge, this is the first report on the selected‐control fabrication of uniform SPIO@Y₂O₃:Eu core–shell nanoparticles and yolk–shell NCs. The combined magnetic manipulation and optical monitoring of magnetic–fluorescent SPIO@Y₂O₃:Eu yolk–shell NCs will open up many exciting opportunities in dual imaging for targeted delivery and thermal therapy.     

Fabrication of glucose‐derived carbon‐decorated magnetic microspheres for extraction of bisphenols from water and tea drinks

Glucose‐derived carbon‐decorated magnetic microspheres were synthesized by an easy hydrothermal carbonization method and used as a high‐efficiency adsorbent to extract bisphenols in water and tea drinks. The as‐prepared carbon‐decorated magnetic microspheres had a well‐defined core–shell structure with a shell thickness of about 5 nm. The microspheres possessed high saturation magnetization at 60.8 emu/g and excellent chemical stability in aqueous solution. The experimental parameters affecting the extraction efficiency, including extraction time, pH, adsorbent dosage, desorption solvents, desorption time, and solution volume were evaluated. Electrostatic and π–π interactions were the major driving forces during extraction. Overall, a new magnetic solid‐phase extraction method of determining bisphenols was developed on the basis of as‐prepared magnetic microspheres. The method had a wide linear range, low limits of detection (0.03–0.10 µg/L), and high recoveries (85.4–104.6%).     

Palladium supported on modified magnetic nanoparticles: a phosphine‐free and heterogeneous catalyst for Suzuki and Stille reactions

An efficient magnetic nanoparticle‐supported palladium (Fe₃O₄/SiO₂‐PAP‐Pd) catalyst is reported for the Suzuki cross‐coupling and Stille reactions. This method provides a novel and much improved modification of the Suzuki and Stille coupling reactions in terms of phosphine‐free catalyst, short reaction time, clean reaction and small quantity of catalyst. Another important feature of this method is that the catalyst can be easily recovered from the reaction mixture and reused with no loss of its catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

Design and preparation of magnetic mesoporous melamine–formaldehyde resin: A novel material for pre‐concentration and determination of silver

Using commercially available melamine and formaldehyde as the starting materials, a magnetic mesoporous melamine–formaldehyde resin (MMF@Fe₃O₄) possessing large surface area was prepared via a simple method and could be used as an efficient adsorbent for magnetic solid‐phase extraction. Compared with the traditional synthetic methods of MMF@Fe₃O₄, this approach is easily operated under mild conditions, is time‐saving and environmentally friendly, and can produce the material in high yields. The as‐prepared MMF@Fe₃O₄ possesses good adsorption capacity and selectivity for silver ions. The affecting factors such as pH, amount of MMF@Fe₃O₄, extraction time, desorption solvent, eluent concentration and sample volume were systematically investigated and optimized. Under the optimized conditions, the material exhibited a good response to silver ions at concentrations in the range 2.0–200 μg l^?1 with good linearity (r² = 0.9982), while the limit of detection was found to be 0.12 μg l^?1. The material was successfully applied to the determination of silver in a variety of water samples.     

Tuning the Spin‐Crossover Behaviour of a Hydrogen‐Accepting Porous Coordination Polymer by Hydrogen‐Donating Guests

A Hoffman‐like coordination polymer with appreciable porosity and uncoordinated pyridyl groups, namely, [Fe(2,5‐bpp){Au(CN)₂}₂] ? x Solv (2,5‐bpp=2,5‐bis(pyrid‐4‐yl)pyridine; Solv=solvent), was synthesised and characterised. A series of fascinating spin‐crossover behaviours with abrupt, stepwise and hysteretic features were obtained by exchange with a range of protic solvents (ethanol, n‐propanol, isopropyl alcohol, sec‐butanol and isobutanol). Guest–host hydrogen‐bonding interactions involving the H‐accepting site of the framework are primarily responsible for the pronounced cooperativity of these spin‐crossover behaviours. Meanwhile, the tunable critical temperatures over a range of about 130 K are presumably attributable to a certain degree of competition between internal pressure and local electronic influences of solvents.