Copper(II) complexes with imidazol-4-yl derivatives of 2-imidazoline nitroxides

Cu(hfac)₂LH, Cu(hfac)₂L¹H, Cu(hfac)(CF₃COO)LH, Cu(LH)₂(NO₃)₂, and Cu(L¹H)₂(NO₃)₂, where LH and L¹H are nitronyl- and iminonitroxides, respectively, containing an imidazol-4-yl substituent in the side chain have been synthesized. In the solid state, the molecules are linked by intermolecular NH⋯O hydrogen bonds, leading to the formation of dimers, bands, or polymer layers. The antiferromagnetic exchange interactions between the odd electrons of the paramagnetic centers are concentrated in the Cu(II)-coordinated nitroxide exchange clusters. The energies of these interactions are between −154 and −350 cm⁻¹.     

Interaction of a lithiated nitronyl nitroxide with polyfluorinated 1,4-naphthoquinones

A 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl lithium derivative was found to react with 2-methoxypentafluoro-1,4-naphthoquinone to form a product of addition at the carbonyl function: radical 2-(3,5,6,7,8-pentafluoro-1-hydroxy-2-methoxy-4-oxo-1,4-dihydronaphthalen-1-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl. The yield of the addition product increased with temperature and reached 84% at 0?°C. The reaction of the lithium derivative with hexafluoro-1,4-naphthoquinone gave rise to a product of addition at both carbonyl groups, namely, nitronyl nitroxide diradical 2,3,5,6,7,8-hexafluoro-1,4-bis(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazole-2-yl)-1,4-dihydronaphthalene-1,4-diol in a 16% yield. The structures of both mono- and diradical were solved by X-ray diffraction analysis, which revealed formation of an intramolecular H-bond between the OH group and nitroxide oxygen. According to electron paramagnetic resonance (EPR) spectroscopy, the obtained mono- and dinitroxide are prone to spontaneous deoxygenation in a toluene solution to give corresponding iminonitroxides. In water, they are much more stable.     

Quantum chemical interpretation of different types of exchange magnetic interactions in dinuclear copper azomethine chelates

B3LYP/LANL2DZ calculations of two series of isomeric dinuclear copper chelates based on the tridentate dibasic salicylaldimines from aromatic and heterocyclic amines were performed. According to calculations, the complexes with N,N-or S,S-donor atoms in the exchange fragment are mainly characterized by ferromagnetic exchange, whereas the formation of O, O-bridging fragments leads to antiferromagnetic spin-spin interaction.     

Extension of the LAP functional to include parallel spin correlation

The kinetic energy density-dependent correlation functional LAP1 is extended to include parallel-spin correlation beyond the exchange level. Two exchange–correlation schemes are considered, combining the new correlation functional (LAP3) with the GGA exchange of Becke and the GGA exchange of Perdew. Extensive tests on molecules and hydrogen-bonded systems are presented and discussed elucidating the role of parallel–spin correlation in different cases. Its inclusion in the LAP functional leads, on average, to a slight improvement of the calculated binding energies and equilibrium geometries of molecules. Particularly high sensitivity of the energy results on the relative share of parallel-spin correlation is observed for aromatic molecules and for systems involving weak hydrogen bonds. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64 : 427–446, 1997     

A morphological interpretation of water chemical exchange and mobility in cellulose materials derived from proton NMR T2 relaxation

Proton T₂ relaxation times of water in cellulosic fibres have been interpreted using a 3-term average model. Motional and chemical exchange contributions to relaxation show opposing temperature behaviour, enabling the use of Arrhenius analysis to determine proton exchange rates and water rotational correlation times. Both parameters vary dramatically with extent of hydration, with chemical exchange dominating relaxation at saturated water contents. Interpretations are based on a morphological model with two types of accessible cellulose, at void surfaces and internally within the cellulose phase. In native cellulose fibres, the presence of crystalline fibrils with low internal accessibility leads to rapid proton exchange at low moisture contents. Regenerated cellulose fibres typically have lower crystallinity and higher internal accessibility, which results in slower exchange as result of migration of water between void and internal environments. Exchange behaviour in regenerated fibres is highly dependent on structural organisation, which depends on the manufacturing process.     

On the test of different atomic exchange functionals

The yet unknown exchange-correlation energy functional and the lack of a systematic way to find such a functional has led to propose different expressions for exchange, on the one hand, and correlation on the other, by several authors. A common factor in the design of exchange functionals is to reproduce different kinds of energies rather than the important atomic shell structure. We present a strategy for testing the behavior of exchange functionals based on the reproduction of atomic shell structure. A variety of well-known exchange functionals are tested on He (closed shell), Li (open shell), and Be (closed subshell). Important differences are found among them. Finally, we propose an exchange energy functional that includes a Becke-88-type expression and, in addition, a nongradient correction term that leads to improved shell structure behavior. © 1995 John Wiley & Sons, Inc.     

On the influence of ion exchange on the local structure of the titanosilicate ETS-10

The effect of ion exchange with different monovalent cations (NH(4)(+), K(+), Na(+) and Cs(+)) on the local structure of the titanosilicate ETS-10 has been studied by (29)Si MAS NMR and Raman spectroscopy. Although X-ray diffraction shows no significant influence of ion exchange on the long range order, ammonium exchange is found to result in substantial damage to the local structure. Ion exchange experiments with alkali cations under significantly more acidic conditions clearly show that the structural damage brought about by ammonium exchange is not caused by the low pH of the exchange solution. The exchange with potassium and caesium ions also leads to significant changes in the (29)Si NMR and Raman spectra. However, these changes can largely be reversed by sodium back-exchange.     

A generalized gradient approximation for exchange derived from the model potential of van Leeuwen and Baerends

The common way to obtain energies from Kohn-Sham exchange potentials is by using the Levy-Perdew virial relation. For potentials that are not functional derivatives (i.e., nearly all model exchange potentials in existence), this approach leads to energy expressions that lack translational and rotational invariance. We propose a method for constructing potential-based energy functionals that are free from these artifacts. It relies on the same line-integration technique that gives rise to the Levy-Perdew relation, but uses density scaling instead of coordinate scaling. The method is applicable to any exchange or correlation potential that depends on the density explicitly, and correctly recovers the parent energy functional from a functional derivative. To illustrate our approach we develop a properly invariant generalized gradient approximation for exchange starting from the model potential of van Leeuwen and Baerends.     

Direct determination of the anisotropy and exchange splittings in the dimeric single-molecule magnet [Mn4O3Cl4(O2CEt)3(py)3]2.8MeCN by inelastic neutron scattering

Energy splittings resulting from anisotropy and exchange interactions in the dimer of single-molecule magnets [Mn4O3Cl4(O2CEt)3(py)3]2.8MeCN are determined for both an undeuterated and a partially deuterated sample using inelastic neutron scattering. The antiferromagnetic (AF) exchange coupling between the two Mn4 subunits strongly depends on their separation. The Cl...Cl distance between the two subunits can be modified either by exchanging the solvent of crystallization or by deuteration of the C-H...Cl hydrogen bonds. The exchange of acetonitrile for n-hexane leads to a five times greater shortening of the Cl...Cl separation than does full deuteration of all the hydrogen bonds. As a result, the AF exchange coupling constants between the subunits are 0.0073(4) and 0.0103(9) meV in the samples with acetonitrile and n-hexane solvent molecules, respectively, in the crystal structure. On the other hand, the effect of C-H...Cl deuteration on the AF exchange coupling is not detectable within the experimental accuracy of INS.     

Grignard reagents: alkoxide-directed iodine-magnesium exchange at sp3 centers

Sequential addition of i-PrMgCl and BuLi to sp3 hybridized iodoalcohols triggers a facile iodine-metal exchange. Intercepting the resulting cyclic Grignard reagents with a slight excess of an electrophile leads to a diverse range of substituted alcohols. The iodine-magnesium exchange strategy is effective with 3-carbon iodoalcohols bearing alkyl substitutents on the carbinol or adjacent carbons and with the chain-extended homolog 4-iodobutan-1-ol.     

Transport properties of vanadium ions in cation exchange membranes:: Determination of diffusion coefficients using a dialysis cell

Diffusion coefficients of vanadium ions in cation exchange membranes are of interest because they allow to calculate the ion exchange across the membrane in an all vanadium redox flow battery which leads to undesired cross contamination and energy losses in the battery system. Diffusion coefficients of V²⁺, V³⁺, VO²⁺ and VO⁺₂ ions in CMS, CMV and CMX cation exchange membranes have been determined by measuring the ion exchange fluxes of these ions with H₃O⁺ ions using a dialysis cell. The experimental data are evaluated on the basis of integrated flux equations which require also ion exchange sorption equilibria obtained already in previous work. The lowest diffusion coefficients are observed in the CMS membrane for all vanadium ions. This membrane turns out to be the most suitable one for being applied in a vanadium battery since it is expected to prevent most effectively cross contamination of vanadium ions.     

Monte Carlo models for nanoparticle formation in two microemulsion systems

The process of formation of nanoparticles obtained by mixing two micellized, aqueous solutions has been simulated using the Monte Carlo technique. The model includes the phenomena of finite reaction, nucleation, and growth via intermicellar exchange. This exploratory study examines the characteristic particle size distributions (PSDs) that result from using combinations of different initial reactant distributions (Poissonian and geometric) and different types of intermicellar exchange protocols (random, cooperative, and binomial). It is observed that the PSDs obtained using an initial Poissonian distribution of reactants and random exchange rules are similar to reported experimental results for CdS nanoparticles. The effect of exchange efficiency and reaction rate has also been studied. It is seen that a high exchange efficiency leads to relatively larger particle sizes. Also, a slow reaction rate has been shown to lead to the formation of larger nanoparticles.     

Tetracarbonyl cobaltate ion supported on anion exchangers as a new heterogenized homogeneous catalyst: synthesis of acids and esters from organic halides

Catalysts consisting of tetracarbonyl cobaltate anion supported on commercially available anion exchange resins are readily prepared. Carbonylation of organic halides using this heterogenized catalyst leads either to acids or esters depending on the conditions. Some features of this catalytic system and the possible reaction mechanism are discussed.     

Asymptotic properties of the exchange energy density and the exchange potential of finite systems: relevance for generalized gradient approximations

It is shown that generalized gradient approximations (GGAs) for exchange only, due to their very limited form, quite generally can not simultaneously reproduce both the asymptotic forms of the exchange energy density and the exchange potential of finite systems. Furthermore, mechanisms making GGAs formally approach at least one of these asymptotic forms do not improve the corresponding quantity in the relevant part of the asymptotic regime of atoms. By constructing a GGA which leads to superior atomic exchange energies compared to all GGAs heretofore but does not reproduce the asymptotic form of the exact exchange energy density it is demonstrated that this property is not important for obtaining extremely accurate atomic exchange energies. We conclude that GGAs by their very concept are not suited to reproduce these asymptotic properties of finite systems. As a byproduct of our discussion we present a particularly simple and direct proof of the well known asymptotic structure of the exchange potential of finite spherical systems.     

Thermal,electric and spin transport in superconductor/ferromagnetic-insulator structures

A ferromagnetic insulator (FI) attached to a conventional superconductor (S) changes drastically the properties of the latter. Specifically, the exchange field at the FI/S interface leads to a splitting of the superconducting density of states. If S is a superconducting film, thinner than the superconducting coherence length, the modification of the density of states occurs over the whole sample. The coexistence of the exchange splitting and superconducting correlations in S/FI structures leads to striking transport phenomena that are of interest for applications in thermoelectricity, superconducting spintronics and radiation sensors. Here we review the most recent progress in understanding the transport properties of FI/S structures by presenting a complete theoretical framework based on the quasiclassical kinetic equations. We discuss the coupling between the electronic degrees of freedom, charge, spin and energy, under non-equilibrium conditions and its manifestation in thermoelectricity and spin-dependent transport.     

Preparation and reactions of some 2,2-difunctional 1,1-dibromocyclopropanes

The synthesis of 2,2-dibromocyclopropane-1,1-dicarboxylic acids is described. Reaction of substituted 1,1-dibromo-2-acyloxymethylcyclopropanes with methyl lithium at low temperature leads to a bromine-lithium exchange and then either formal protonation to give the corresponding monobromocyclopropanes or intramolecular cyclisation to give a substituted 3-oxabicyclo[3.1.0]hexane. Oxidative ring opening of these compounds leads stereoselectively to 1,1,2,2-tetrasubstituted cyclopropanes with four functionalities on the ring.     

Pyridine-dinucleotide models IV; stereoselective hydride transfer at a bridged pyridinium-ion

Two achiral bridged 1,4-dihydro-3,5-biscarboxamido pyridines are described. In one of these the bridging induces significant diastereotopy of the C-4 protons. This not only leads to magnetic anisochrony of these protons, but also allows for highly stereoselective (- 90%) substitution of one of them by deuterium via hydride exchange with a simple deuterated dihydropyridine.     

Sequential Anion and Cation Exchange Reactions for Complete Material Transformations of Nanoparticles with Morphological Retention

Ion exchange reactions of colloidal nanocrystals provide access to complex products that are synthetically challenging using traditional hot‐injection methods. However, such reactions typically achieve only partial material transformations by employing either cation or anion exchange processes. It is now shown that anion and cation exchange reactions can be coupled together and applied sequentially in one integrated pathway that leads to complete material transformations of nanocrystal templates. Although the product nanocrystals do not contain any of the original constituent elements, the original morphology is retained, thereby fully decoupling morphology and composition control. The sequential anion/cation exchange process was applied to pseudo‐spherical CdO nanocrystals and ZnO tetrapods, producing fully transformed and shape‐controlled nanocrystals of copper and silver sulfides and selenides. Furthermore, hollow core–shell tetrapod ZnS@CdS heterostructures were readily accessible.