Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals

Five bis(tert-butylnitroxide) diradicals connected by a silole (7 a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2(1) with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ray crystal structure determination, electronic spectroscopy, variable-temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6-31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b, 7 c, and 7 d are quite stable both in solution and in the solid state, 7 a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a pi-conjugated Kekulé structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a "Bleaney-Bowers" model that gives values of J = -142.0 cm(-1) for 7 a, -1.8 cm(-1) for 7 b, -1.3 cm(-1) for 7 c, -4.2 cm(-1) for 7 d, and -248.0 cm(-1) for 12. The temperature dependence of the EPR half-field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7 b, 7 c, and 7 d with DeltaE(T-S) values of 3.48, 2.09, and 8 cm(-1), respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12. Similarities between the DeltaE(T-S) and J values (DeltaE(T-S) = -2 J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a "Karplus-Conroy"-type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5-positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon pi system of the silole.     

Tailor-made strong exchange magnetic coupling through very long bridging ligands: theoretical predictions

Computational methods based on density functional theory have been applied to a prospective study of dinuclear transition metal complexes that may show strong exchange coupling interactions through very long bridging ligands. The results indicate that M(III) complexes (being M= Cr, Mn or Fe) with dicyanamidobenzene-type ligands are specially promising for this purpose, since strong ferromagnetic or antiferromagnetic coupling is predicted between paramagnetic metal cations at distances as long as 25 A. The existence of ferromagnetic or antiferromagnetic coupling in the complexes with the different isomers of dicyanamidobenzene can be rationalized in terms of molecular orbitals.     

multilayer contactless dielectrophoresis: theoretical considerations

Dielectrophoresis (DEP), the movement of dielectric particles in a nonuniform electric field, is of particular interest due to its ability to manipulate particles based on their unique electrical properties. Contactless DEP (cDEP) is an extension of traditional and insulator-based DEP topologies. The devices consist of a sample channel and fluid electrode channels filled with a highly conductive media. A thin insulating membrane between the sample channel and the fluid electrode channels serves to isolate the sample from direct contact with metal electrodes. Here we investigate, for the first time, the properties of multilayer devices in which the sample and electrode channels occupy distinct layers. Simulations are conducted using commercially available finite element software and a less computationally demanding numerical approximation is presented and validated. We show that devices can be created that achieve a similar level of electrical performance to other cDEP devices presented in the literature while increasing fluid throughput. We conclude, based on these models, that the ultimate limiting factors in device performance resides in breakdown voltage of the barrier material and the ability to generate high-voltage, high-frequency signals. Finally, we demonstrate trapping of MDA-MB-231 breast cancer cells in a prototype device at a flow rate of 1.0 mL/h when 250 V(RMS) at 600 kHz is applied.     

Rectangular capillary electrophoresis: Some theoretical considerations

Summary A theoretical study of the use of rectangular columns in capillary electrophoresis (CE) is presented. It was done employing some of the most important parameters that normally define the performance of CE separations, i.e. thermal effect, analysis speed, efficiency and sample capacity. Theoretical results from rectangular and cylindrical capillaries are compared in terms of the aforementionated parameters. Also, an estimate of the additional zone broadening that arises from the noninfinite dimension in the y-direction, in which the channel has its largest dimension, is presented.     

Amine exchange in formamidines: an experimental and theoretical study

N-H-containing formamidines combine a reasonably strong association to carboxylic acids to form complexes of well-defined geometries with a simultaneous proton-induced electrophilicity enhancement that allows for the exchange of their amine portion. The N=C(H)-NH fragment, therefore, undergoes "imine-like" exchange with N-containing nucleophiles. Because of the prototropic equilibrium, the N=C(H)-NH fragment may behave as a "bisimine" centred on the same carbon, in which both N-containing fragments can be exchanged. Considering the proton-induced sensitisation of both C-N units and the well-defined formamidine-carboxylic acid complex geometry, it should be possible to use carboxylic acids as templates for the synthesis of defined architectures by dynamic amine exchange within formamidines. This study highlights three exchange regimes based on the nature of the incoming amine (aliphatic amines, aromatic amines and alkoxyamines), as well as exchange rules based on the amine leaving groups. Following this analysis, a proof of concept for carboxylic acid templated macrocycle formation through dynamic exchange is provided.     

Spin exchange interactions and magnetic structures of extended magnetic solids with localized spins: theoretical descriptions on formal, quantitative and qualitative levels

Low-energy excitation energies of a magnetic solid with localized spins are probed by magnetic susceptibility, neutron scattering and Raman scattering measurements, and are analyzed using a spin Hamiltonian with a set of spin exchange parameters. The nature and values of the spin exchange parameters deduced from this analysis depend on what spin exchange paths one includes in the spin Hamiltonian. In this article, we review how spin exchange interactions of magnetic solids with localized spins are described on formal, quantitative and qualitative theoretical levels, investigate antisymmetric and anisotropic interactions for general spin dimers, and discuss the spin exchange interactions and magnetic structures of various extended magnetic solids on the basis of spin dimer analysis. Strongly interacting spin exchange paths of a magnetic solid are determined by the overlap between its magnetic orbitals, so that the strongly interacting spin unit of a magnetic solid does not necessarily have the same geometrical feature as does the arrangement of its magnetic ions or spin-carrying molecules. Therefore, in interpreting results of magnetic susceptibility, inelastic neutron scattering or Raman scattering measurements, it is essential to employ a set of spin exchange parameters chosen on the basis of proper electronic structure considerations. Spin dimer analyses based on extended Hückel tight binding calculations provide a reliable and expedient means to study the relative strengths of superexchange and super-superexchange spin exchange interactions.     

Dielectric spectroscopy in a micromachined flow cytometer: theoretical and practical considerations

We propose a model to determine the influence of different cell properties, such as size, membrane capacitance and cytoplasm conductivity, on the impedance spectrum as measured in a microfabricated cytometer. A dielectric sphere of equivalent complex permittivity is used as a simplified model to describe a biological cell. The measurement takes place between a pair of facing microelectrodes in a microchannel filled with a saline solution. The model incorporates various cell parameters, such as dielectric properties, size and position in the channel. A 3D finite element model is used to evaluate the magnitude of the electric field in the channel and the resultant changes in charge densities at the measurement electrode boundaries as a cell flows past. The charge density is integrated on the electrode surface to determine the displacement current and the channel impedance for the computed frequency range. The complete impedance model combines the finite element model, the electrode-electrolyte interface impedance and stray impedance, which are measured from a real device. The modeled dielectric complex spectra for various cell parameters are discussed and a measurement strategy for cell discrimination with such a system is proposed. We finally discuss the amount of noise and measurement fluctuations of the sensor.     

Role of metal ions in the destruction of TATP: theoretical considerations

The safe decomposition of solid TATP (triacetone triperoxide) explosive is examined theoretically. The route to destruction starts with formation of metal complexes between a metal ion and the TATP molecule. The second step is decomposition of the molecules into stable final products. We examined the structure and stability of both metal ion (including Na(+), Cu(+), Cu(2+), Co(2+), and Zn(2+)) and proton complexes with TATP using quantum chemical calculations at the DFT-PBE0 level of theory. In addition, for each ion complex, we determined the initial steps in the pathway to decomposition together with the associated transition states. We find that the products of decomposition, in particular, acetone, are also stabilized by ion metal complexes. In agreement with experiment, we find the best candidates for metal ion induced decomposition are Cu(2+) and Zn(2+).     

Miniaturisation in pressure and electroendosmotically driven liquid chromatography: Some theoretical considerations

Summary In electrochromatography, electrolyte is driven along a narrow bore chromatographic column at typical HPLC linear velocities by applying a potential gradient of around 50,000 V m⁻¹. Contrasting with pressure driven LC, the tube bore is limited to not more than 200 μm by self heating. Thus miniaturisation is mandatory in electrochromatography whereas it is optional in pressure driven LC unless very small particles (e.g. 1 μm diameter) are used. Because the velocity profile in open tubular electrochromatography is close to that of perfect plug flow, contrasting with the parabolic flow profile in pressure driven LC, open tube electrochromatography provides plate efficiencies limited only by axial diffusion, and comparable to those obtainable in open tubular gas chromatography. Contrasting with pressure driven open tubular LC there is no requirement for the tube bore to be very small (e.g. <10 μm). Although pure electrophoresis is applicable only to ionised solutes, and is not strictly a chromatographic method, neutral species can be chromatographed by partitioning them between two phases which move at different rates, for example by using micellar solutions or colloidal sols as the moving fluid, thereby retaining the high plate efficiency of electrophoresis. With packed tubes electrochromatographic separations of high efficiency can be obtained with 5 μm particles and extremely high efficiencies should be obtainable without sacrifice of eluent velocity by using submicron particles. In principle, electrochromatographic methods can provide a wide range of partitioning methods of extremely high plate efficiency, and will, in future, undoubtedly rival conventional HPLC.     

New theoretical considerations in polymer rheology: elastic breakdown of chain entanglement network

Recent experimental evidence has motivated us to present a set of new theoretical considerations and to provide a rationale for interpreting the intriguing flow phenomena observed in entangled polymer solutions and melts [P. Tapadia and S. Q. Wang, Phys. Rev. Lett. 96, 016001 (2006); 96, 196001 (2006); S. Q. Wang et al., ibid. 97, 187801 (2006)]. Three forces have been recognized to play important roles in controlling the response of a strained entanglement network. During flow, an intermolecular locking force f(iml) arises and causes conformational deformation in each load-bearing strand between entanglements. The chain deformation builds up a retractive force f(retract) within each strand. Chain entanglement prevails in quiescence because a given chain prefers to stay interpenetrating into other chains within its pervaded volume so as to enjoy maximum conformational entropy. Since each strand of length l(ent) has entropy equal to k(B)T, the disentanglement criterion is given by f(retract)>f(ent) approximately k(B)Tl(ent) in the case of interrupted deformation. This condition identifies f(ent) as a cohesive force. Imbalance among these forces causes elastic breakdown of the entanglement network. For example, an entangled polymer yields during continuous deformation when the declining f(iml) cannot sustain the elevated f(retract). This opposite trend of the two forces is at the core of the physics governing a "cohesive" breakdown at the yield point (i.e., the stress overshoot) in startup flow. Identifying the yield point as the point of force imbalance, we can also rationalize the recently observed striking scaling behavior associated with the yield point in continuous deformation of both shear and extension.     

Molecular magnetic properties of heteroporphyrins: a theoretical analysis

B3LYP/6-31G(d) optimization of porphyrin, tetraphenylporphyrin and their 21,23-diheteroatom substituted derivatives with O, S, and Se heteroatoms was performed. A planar macrocycle was found in all cases except 21,23-dioxatetraphenylporphyrin which presents only slight deviations from planarity. A Bader analysis uncovers the presence of S-S and Se-Se interactions in the four corresponding heteroporphyrins, which appreciably distort the original unsubstituted macrocycles. In the minimum energy structures of heterotetraphenylporphyrins the four meso phenyl groups slant alternatively to right or left so that the two pairs of opposite phenyls present a staggered conformation. The pi current induced by a perpendicular magnetic field in porphyrin bifurcates across both types of pyrrole subunits but the presence of O, S and Se heteroatoms in 21,23-diheteroporphyrins causes a diminution of the current density through the inner section of the two heterorings and, consequently, the current path goes then through the outer section of these rings. The NICS values at the ring critical points of the heterorings are much larger (in absolute value) than those at the pyrrole ring critical points but appreciably smaller than that at the ring critical point of a pyrrole molecule. In agreement with experimental data the (1)H NMR present appreciable downfield shifts for the beta H atoms of the heterorings in the 21,23-heterosubstituted molecules.     

Some theoretical considerations on diffusion-controlled one-chain polymerization in two-dimensional systems

A simple model has been developed to investigate the differences between 2D and 3D diffusion-controlled polymerization kinetics. Besides the different analytical structure of 2D and 3D diffusion equation, the model takes into account the dynamics of the polymer chemically reacting end and the restricted motion of the diffusing monomers in a growing polymer network. These effects are strongly related to the dimensionality of the system and may contribute to modulate the polymerization rate. Theoretical considerations suggest different behavior when 2D polymerization takes place in a poor solvent, whereas minor differences should be present in good solvents and 3D systems. © 1994 John Wiley & Sons, Inc.     

Bubble nucleation in polymeric liquids. II. theoretical considerations

A theory based on classical nucleation theory is developed for bubble nucleation in polymer solutions. The theory requires information on solubility, diffusivity, concentration, surface tension, temperature, and degree of supersaturation. The effects of supersaturation and of the presence of large molecules in a liquid mixture on the free energy of bubble formation are included in the theoretical development. A semiempirical equation for the determination of bubble nucleation rate is developed, with the aid of experimental results reported in part I of this series. Using the experimental data, computer simulations of bubble nucleation in polymer solutions are performed. The consumptions of the volatile component in a liquid mixture, due to bubble nucleation and subsequent growth, and the variation of bubble nucleation rate during the expansion process are included in the simulation of the bubble nucleation process.     

Sensitivity improvement in 19F NMR-based screening experiments: theoretical considerations and experimental applications

NMR-based binding and functional screening performed with FAXS (fluorine chemical shift anisotropy and exchange for screening) and 3-FABS (three fluorine atoms for biochemical screening) represents a potential alternative approach to high-throughput screening for the identification of novel potential drug candidates. The major limitation of this method in its current status is its intrinsic low sensitivity that limits the number of tested compounds. One approach for overcoming this problem is the use of a cryogenically cooled (19)F probe that reduces the thermal noise in the receiver circuitry. Sensitivity improvement in the two screening techniques achieved with the novel cryogenic (19)F probe technology permits an increased throughput, detection of weaker binders and inhibitors (relevant in a fragment-based lead discovery program), detection of slow binders, and reduction in protein and substrate consumption. These aspects are analyzed with theoretical simulations and experimental quantitative performance evaluation. Application of 3-FABS combined with the cryogenic (19)F probe technology to rapid screening at very low enzyme concentrations and the current detection limits reached with this approach are also presented.     

Magnetic exchange coupling in bis-nitroxides: a theoretical analysis of the solvent effects

Magnetic properties of nitroxide radicals can be greatly affected by solvent effects. In this study, the change of the magnetic exchange interaction J, coupling the two unpaired electrons of a model solvated antiferromagnetic bis-iminyl-nitroxide molecule (2IN), is rationalized thanks to different geometric and electronic criteria provided by density functional theory calculations. It is shown that for a given geometry, simple tools can be used to analyze with good accuracy the dependence of J with the solvent polarity. Estimates of two important magnetic parameters are given: the magnetic orbitals exchange and the in-site energy gap between ionic and neutral configurations. 2IN can be engaged in different hydrogen-bonds with first shell water molecules, modifying both the 2IN geometry and the electrostatic potential felt by the molecule. In all, the additivity of electrostatic and hydrogen-bond solvent effects is found to be responsible for J variations as large as 50%.     

Mcsoionic purinone analogs. I. Initial theoretical considerations

Thirty mesoionic analogs of purin-6-one, -2-one and -2,6-dione have been formulated based upon reported simple monocyclie mesoionic systems and factors influencing their stability. lluckel molecular orbital calculations were performed for these structures employing the ω, ω′ -technique and the variation of resonance integrals with bond orders. The results provide predictions concerning the properties and stability of the analogs, assist the determination of reasonable synthetic goals, and serve as input for more sophisticated theoretical treatments.     

Amorphous twisted nematic liquid crystal displays: The characteristics and the theoretical considerations

Amorphous twisted nematic liquid crystal display cells have been prepared without rubbing. The textures consisted of twisted nematic multi-domains. Theoretical calculations of the optical properties of such cells have been carried out by the 4 × 4 matrix method. The optical anisotropy of the liquid crystal is optimized to suppress/optimize the background colour. The electro-optical characteristics of the a-TN have been measured. It is shown that the a-TN devices exhibit wide and uniform viewing angle characteristics and possess good gray scale capability. Viewing angles of - 47° to + 45° (up-down) and - 51° to + 46° (left-right) have been achieved. A rise time of 36.5 ms and decay time of 35.5 ms have been measured in an a-TN sample.     

The non-integer cyclovoltammetric electron-exchange numbers of reversible redox reactions of adsorbates: theoretical considerations

Two types of non-integer electron-exchange numbers from uniform and reversible surface-redox reactions without side reactions have been distinguished. The first being the apparent number, n _app, of the apparent faradaic charge corresponding to cyclovoltammetric peak areas above the interpolated baseline, and the second the thermodynamically defined surface-redox valency, n', of Nernstian slopes of cyclovoltammetric peak potentials depending on different solution pH. An analytical expression has been derived for n _app based on a simplified capacitive equivalent circuit and for n' using the potential-dependent free adsorption energies of the reactants involved. It should be pointed out that the different experimental values of n _app and n' refer to the same integer number of electrons per molecule oxidized or reduced. Electronic Publication