Nuclear magnetic resonance restricted diffusion between parallel planes in a cosine magnetic field: an exactly solvable model

We propose a theoretical and numerical analysis of restricted diffusion between parallel planes in a cosine magnetic field. The specific choice of this spatial profile as proportional to an eigenfunction of the Laplace operator in this confining geometry considerably simplifies the underlying mathematics. In particular, exact and explicit relations for several moments of the total phase accumulated by diffusing spins are derived. These relations are shown to provide good approximations for the typical case of a linear magnetic field gradient, for which the theoretical analysis was in general limited to the second moment. We study the structure and the properties of the higher order moments which are responsible for the breakdown of the "Gaussian phase approximation" (GPA) at intense magnetic fields. The limits of applicability of the GPA for nonlinear magnetic fields and the transition to the localization regime are discussed. In particular, a diagram of different restricted diffusion regimes is presented.     

Phase behavior of a simple dipolar fluid under shear flow in an electric field

Nonequilibrium molecular dynamics simulations are performed on a dense simple dipolar fluid under a planar Couette shear flow. Shear generates heat, which is removed by thermostatting terms added to the equations of motion of the fluid particles. The spatial structure of simple fluids at high shear rates is known to depend strongly on the thermostatting mechanism chosen. Kinetic thermostats are either biased or unbiased: biased thermostats neglect the existence of secondary flows that appear at high shear rates superimposed upon the linear velocity profile of the fluid. Simulations that employ a biased thermostat produce a string phase where particles align in strings with hexagonal symmetry along the direction of the flow. This phase is known to be a simulation artifact of biased thermostatting, and has not been observed by experiments on colloidal suspensions under shear flow. In this paper, we investigate the possibility of using a suitably directed electric field, which is coupled to the dipole moments of the fluid particles, to stabilize the string phase. We explore several thermostatting mechanisms where either the kinetic or configurational fluid degrees of freedom are thermostated. Some of these mechanisms do not yield a string phase, but rather a shear-thickening phase; in this case, we find the influence of the dipolar interactions and external field on the packing structure, and in turn their influence on the shear viscosity at the onset of this shear-thickening regime.     

Modeling fluid diffusion using the lattice density functional theory approach: counterdiffusion in an external field

The dependence of the diffusivity on temperature, pressure, and composition is not understood well; consequently, data is preferred significantly over correlations in most practical situations. Even in dilute gases, the contributions of attractions and repulsions to the diffusivity are difficult to understand on a molecular level without performing simulations. We have developed a Lattice Density Functional Theory (LDFT) approach for modeling diffusion to supplement existing methods that are very rigorous but computationally demanding. The LDFT approach is analogous to the van der Waals equation in how it accounts for molecular interactions in that it has first-order corrections to ideal behavior; it is an extension of the Equilibrium LDFT for adsorption and phase behavior. In this work, the LDFT approach is presented and demonstrated by modeling the problem of color counterdiffusion in an externally-applied potential field. This potential field, in combination with the intermolecular potential function, creates a diffusion regime in which repulsions cause oscillations in the density profile. Using the LDFT approach, the oscillations were described and attributed to nearest-neighbor and next nearest-neighbor interactions. The LDFT approach gives qualitative and quantitative agreement with dual control-volume Grand Canonical Molecular Dynamics simulations.     

Molecular interactions in a homogeneous electric field: the (HF)2 complex

Summary The interaction energy of two HF molecules, subjected to a static external field, is analyzed. The analysis aims at the elaboration of simple expressions able to reproduce environmental and substitution effects on noncovalent molecular interactions.     

On the possibility of aligning paramagnetic molecules or ions in a magnetic field

Strong magnetic fields can hybridize low rotational states of paramagnetic molecules or molecular ions whose electronic angular momentum is coupled to the molecular axis. The hybridization creates pendular states in which the molecular axis is confined to librate over a limited angular range about the field direction. In this way substantial spatial alignment associated with large Zeeman shifts can be attained for many ground-state radicals or ions and electronically excited states of diatomic or linear molecules. The magnetic hybridization is analogous to that recently demonstrated for polar molecules in electric fields. The magnetic version can only provide ensemble alignment rather than orientation, but offers complementary chemical scope by virtue of its applicability to nonpolar molecules and ions.     

Semiclassical model for the absorption or emission of a photon by a charged particle in an intense static electric field

Quantum simulations made using Floquet methods show that a charged particle can exchange energy with an oscillating potential barrier in discrete quanta , where is the frequency of oscillation. However, this exchange is classically forbidden because no other mass is included in the model, so that energy and momentum could not both be conserved in the absorption or emission of a photon. We define a semiclassical mechanism for these inelastic processes in which a photon may be absorbed by a charged particle moving against an intense static electric field, or emitted when the particle moves with this field. In this model, the particle has an energy loss Q in photon absorption, and an energy gain Q in photon emission. Then the particle travels a short distance at constant momentum until the energy increment Q is made up by the interaction with the static electric field, after which the particle resumes classical motion with the initial energy plus or minus exactly one quantum. We use the energy–time uncertainty relation to determine the minimum value for the static electric field that is required for this process, and this value is typical of the experimental conditions for laser-assisted scanning tunneling microscopy and laser-assisted field emission where the exchange of quanta is found to occur.     

The alignment of cybotactic groups in a nematic mesophase by an electric field

The anomalous electric field alignment of the nematic mesophase of 4,4′-dimethoxyazoxybenzene, as inferred from the electron resonance spectrum of dissolved vanadyl acetyl acetonate, is discussed. A model, based on the alignment of cybotactic groups with their planes parallel to the electric field, is able to account for the major features of the angular dependence of the spectrum. The analysis shows that, at 125°C, the molecular axes are inclined at least 13° to the plane of the cybotactic group.     

Modulating magnetic anisotropy in Ln(iii) single-ion magnets using an external electric field

Single-molecule magnets have potential uses in several nanotechnology applications, including high-density information storage devices, the realisation of which lies in enhancing the barrier height for magnetisation reversal (U_eff). However, Ln(iii) single-ion magnets (SIMs) that have been reported recently reveal that the maximum value of U_eff values that can be obtained by modulating the ligand fields has already been achieved. Here, we have explored, using a combination of DFT and ab initio CASSCF calculations, a unique way to enhance the magnetisation reversal barrier using an oriented external electric field in three well-known Ln(iii) single-ion magnets: [Dy(Py)₅(O^tBu)₂]⁺ (1), [Er{N(SiMe₃)₂}₃Cl]⁻ (2) and [Dy(Cp^Me3)Cl] (3). Our study reveals that, for apt molecules, if the appropriate direction and values of the electric fields are chosen, the barrier height can be enhanced by twice that of the limit set by the ligand field. The application of an electric field along the equatorial direction was found to be suitable for oblate shaped Dy(iii) complexes and an electric field along the axial direction was found to enhance the barrier height for a prolate Er(iii) complex. For complexes 2 and 3, the external electric field was able to magnify the barrier height to 2–3 times that of the original complexes. However, a moderate enhancement was noticed after application of the external electric field in the case of complex 1. This novel non-chemical fine-tuning approach to modulate magnetic anisotropy is expected to yield a new generation of SIMs.

Using a combination of theoretical tools, we show that the application of an external electric field in a certain direction can boost the axiality beyond that set by the ligands, opening up a new avenue for the generation of novel SIMs.     

Quantum-mechanical investigation of the properties of the molecules and ions of acetals in a strong electric field

Theoretical and Experimental Chemistry -     

An oscillating electric field with thermal noise increases the rotational diffusion and drives rotation in a dipole

Here we consider a dipole in a viscous medium under the influence of an oscillating electric field and thermal noise. Because of the very low Reynolds numbers involved in molecular processes, we considered overdamped Langevin dynamics. As a consequence the inertia term becomes negligible. We observed a great increase in the rotational diffusion and also net rotation for some values of the parameters.     

ZnSe:Mn/ZnSe纳米晶体中的Mn~(2+)离子扩散

以ZnSe:Mn/ZnSe纳米晶体为基础,分析了Mn~(2+)离子在纳米晶体中的扩散行为.通过电子顺磁共振(EPR)谱参数确定Mn~(2+)离子处在纳米晶体内部.考虑到合成过程,确定Mn~(2+)离子处于纳米晶体的核壳界面.通过荧光光谱来揭示不同退火温度和退火时间下的纳米晶体的光学性质.不同退火条件情况下纳米晶体中各元素的含量由电感耦合等离子体(ICP)发射光谱得到.随着退火温度的增加和退火时间的延长,相同物质的量的纳米晶体中Mn元素含量逐渐变小直到为零,Zn和Se的含量几乎不变.相同退火温度不同退火时间下的电子透射显微镜照片说明退火对纳米晶体的形貌和粒径几乎没有影响.     

Converting the nanodomains of a diblock-copolymer thin film from spheres to cylinders with an external electric field

We investigate the ability of an applied electric field to convert the morphology of a diblock-copolymer thin film from a monolayer of spherical domains embedded in the matrix to cylindrical domains that penetrate through the matrix. As expected, the applied field increases the relative stability of cylindrical domains, while simultaneously reducing the energy barrier that impedes the transition to cylinders. The effectiveness of the field is enhanced by a large dielectric contrast between the two block-copolymer components, particularly when the low-dielectric contrast component forms the matrix. Furthermore, the energy barrier is minimized by selecting sphere-forming diblock copolymers that are as compositionally symmetric as possible. Our calculations, which are the most quantitatively reliable to date, are performed using a numerically precise spectral algorithm based on self-consistent-field theory supplemented with an exact treatment for linear dielectric materials.     

Reactions of metal(II) ions with sugar α-amino acids: nickel(II) complexes containing an α-amino acid derived from galactose or mannose

Summary The reaction of Ni^II ions with 2-(benzylamino)-2-deoxy-d-glycero-l-gluco. heptonic acid (BnGa:C₁₄H₂₁NO₇) or 2-(benzylamino)-2-deoxy-d-glycero-d-talo heptonic acid (BnMa:C₁₄H₂₁NO₇) in water yields complexes of formulae [Ni(BnGa)₂(H₂O)₂] and [Ni(BnMa)₂]·2H₂O, respectively, which were characterized by elemental analysis, spectral techniques (u.v.-vis.-n.i.r. and i.r.), magnetic susceptibility measurements, thermal measurements (t.g. and d.s.c.) and X-ray powder diffraction. Both complexes are octahedral with two positions of the coordination sphere occupied by nitrogen atoms. Moreover, in [Ni(BnMa)₂]·2H₂O four oxygen atoms of bridging carboxylate groups are coordinated to the metal ion, while in [Ni(BnGa)₂(H₂O)₂] only an oxygen atom of each carboxylate group is bound to Ni^II. In this case, the coordination is completed via two water molecules.     

The electric field dependence of DNA mobilities in agarose gels: a reinvestigation

The electric field dependence of the electrophoretic mobility of linear DNA fragments in agarose gels was reinvestigated in order to correct the observed mobilities for the different temperatures actually present in the gel during electrophoresis in different electric field gradients. When corrected to a common temperature, the electrophoretic mobilities of DNA fragments less than or equal to 1 kilobase pairs (kbp) in size were independent of electric field strength at all field strengths from 0.6 to 4.6 V/cm if the gels contained less than or equal to 1.4% agarose. The mobilities of larger DNA fragments increased approximately linearly with electric field strength. If the agarose concentration was higher than 2%, the mobilities of all DNA fragments increased with increasing electric field strength. The electric field dependence of the mobility was larger in gels cast and run in Tris-borate buffer (TBE) than in gels cast and run in Tris-acetate buffer (TAE), and was more pronounced in gels without ethidium bromide incorporated in the matrix. Ferguson plots were constructed for the various DNA fragments, both with and without extrapolating the temperature-corrected mobilities to zero electric field strength. Linear Ferguson plots were obtained for all fragments less than or equal to 12 kbp in size in agarose gels less than or equal to 1.4% in concentration if the mobilities were first extrapolated to zero electric field strength. Concave upward curvature of the Ferguson plots was observed for DNA fragments greater than or equal to 2 kbp in size at finite electric field strengths. Convex downward curvature of the Ferguson plots was observed for DNA fragments greater than or equal to 1 kbp in size in agarose gels greater than or equal to 2% in concentration. The mobilities of the various DNA fragments, extrapolated to zero agarose concentration and zero electric field strength, decreased with increasing DNA molecular weight; extrapolating to zero molecular weight gave an "intrinsic" DNA mobility of 2.7 x 10(-4) cm2/Vs at 20 degrees C. The pore sizes of LE agarose gels cast and run in TAE and TBE buffers were estimated from the mobility of the DNA fragments.(ABSTRACT TRUNCATED AT 400 WORDS)