首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到18条相似文献,搜索用时 15 毫秒
1.
The use of well‐defined macromolecular assemblies with tunable interactions represents the key for exploring the regime of soft‐material behavior between hard spheres and polymer coils. Colloidal stars are ideal choices for such a formidable task and especially for shedding light on the formation and properties of dynamically arrested states. In this brief review, we demonstrate the rich variety of kinetic frustration phenomena that can be encountered with such ultrasoft particles. We address two specific examples in particular: the reversible vitrification upon heating and the melting of star gels upon the addition of linear polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2931–2941, 2004  相似文献   

2.
The interaction of cypermethrin with β-cyclodextrin was investigated using different (coprecipitation, suspension, kneading and ‘melting in solution’) complexation methods and qualifying the resulted complexes by UV-spectrophotometry, thermal methods (TG, DTG and DSC) and X-ray powder diffraction. The total guest content of complexes can be measured by UV-spectrophotometry in aqueous ethanol solution, while the uncomplexed guest fraction of samples can be determined by DSC based on a previous calibration curve, which was found between the melting enthalpy change of cypermethrin and the guest content of physical mixture samples. The combination of both analytical methods enables the determination of really complexed guest content.  相似文献   

3.
The role played by the law of the conservation of momentum in the primary processes of charge carrier generation is considered. External magnetic field is shown to be able to influence the rate constants governing spin dependent processes. Magnetic field effect and Reaction Yield Detected Magnetic Resonance on the photoconductivity of organic solids give important information on the mechanism of primary steps of charge photogeneration.  相似文献   

4.
5.
Neutron scattering is a valuable tool not only in studying the structure but also the dynamics of nanoscale soft matter systems. By combining different neutron scattering methods, it is possible to observe energy changes from millielectronvolt to nanoelectronvolt. A unique advantage of neutron scattering is the possibility to highlight different parts of a system simply by changing its isotopic composition, allowing to study specific parts of complex systems. This review tries to give an overview of the possibilities of neutron scattering methods together with some recent examples, mostly from neutron spin-echo spectroscopy, which is the method allowing to measure the smallest changes in energy.  相似文献   

6.
We generate inherent structures, local potential-energy minima, of the "k-space overlap potential" in two-dimensional many-particle systems using a cooling and quenching simulation technique. The ground states associated with the k-space overlap potential are stealthy (i.e., completely suppress single scattering of radiation for a range of wavelengths) and hyperuniform (i.e., infinite wavelength density fluctuations vanish). However, we show via quantitative metrics that the inherent structures exhibit a range of stealthiness and hyperuniformity depending on the fraction of degrees of freedom χ that are constrained. Inherent structures in two dimensions typically contain five-particle rings, wavy grain boundaries, and vacancy-interstitial defects. The structural and thermodynamic properties of the inherent structures are relatively insensitive to the temperature from which they are sampled, signifying that the energy landscape is relatively flat along the directions sampled, with wide shallow local minima and devoid of deep wells. Using the nudged-elastic-band algorithm, we construct paths from ground-state configurations to inherent structures and identify the transition points between them. In addition, we use point patterns generated from a random sequential addition (RSA) of hard disks, which are nearly stealthy, and examine the particle rearrangements necessary to make the configurations absolutely stealthy. We introduce a configurational proximity metric to show that only small local, but collective, particle rearrangements are needed to drive initial RSA configurations to stealthy disordered ground states. These results lead to a more complete understanding of the unusual behaviors exhibited by the family of "collective-coordinate" potentials to which the k-space overlap potential belongs.  相似文献   

7.
8.
The collective motion of synthetic active colloids is an emerging area of research in soft matter physics and is important both as a platform for fundamental studies ranging from non-equilibrium statistical mechanics to the basic principles of self-organization, emergent phenomena, and assembly underlying life, as well as applications in biomedicine and metamaterials. The potentially transformative nature of the field over the next decade and beyond is a topic of critical research importance. Electrokinetic active colloids represent an extremely flexible platform for the investigation and modulation of collective behavior in active matter. Here, we review progress in the past five years in electrokinetic active systems and related topics in active matter with important fundamental research and applicative potential to be investigated using electrokinetic systems.  相似文献   

9.
We present a detailed analysis of the application of density functional theory (DFT) methods to the study of structural properties of molecular and supramolecular systems, using as a paradigmatic example three para-phenylene-based systems: isolated biphenyl, single chain poly-para-phenylene, and crystalline biphenyl. We use different functionals for the exchange correlation potential, the local density (LDA), and generalized gradient approximations (GGA), and also different basis sets expansions, localized, plane waves (PW), and mixed (localized plus PW), within the reciprocal space formulation for the hamiltonian. We find that regardless of the choice of basis functions, the GGA calculations yield larger interring distances and torsion angles than LDA. For the same XC approximation, the agreement between calculations with different basis functions lies within 1% (LDA) or 0.5% (GGA) for distances, and while PW and mixed basis calculations agree within 1 degrees for torsion angles, the localized basis results show larger angles by approximately 8 degrees and a nonmonotonic dependence on basis size, with differences within 6 degrees. The most prominent features, namely the torsion between rings for isolated molecule and infinite chain, and planarity for the molecule in crystalline environment, are well reproduced by all DFT calculations.  相似文献   

10.
Recent developments in the field of ion mobility spectrometry provide new possibilities to explore and understand gas-phase ion chemistry. In this study, hyphenated trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) was applied to investigate analyte ion mobility as function of adduct ion formation for twelve pharmaceutically relevant molecules, and for tetrahydrocannabinol (THC) and its isomer cannabidiol (CBD). Samples were introduced by direct infusion and ions were generated with positive electrospray ionization (ESI+) observing protonated and sodiated ions. Measurements were performed with and without addition of cesium-, lithium-, silver- and sodium ions to the samples. For the tested compounds, metal adduct ions with the same m/z but with different mobility and collision cross section (CCSs) were observed, indicating different molecular conformations. Formation of analyte dimers was also observed, which could be associated with molecular geometry of the compounds. By optimizing the range and speed of the electric field gradient and ramp, respectively, the separation of THC and CBD was achieved by employing the adduct formation. This study demonstrates that the favorable resolution of TIMS combined with the ability to detect weakly bound counter ions is a valuable means for rapid detection, separation and structural assignment of molecular isomers and analyte conformations.  相似文献   

11.
Tetradeca-2,6-O-methyl-beta-cyclodextrin, per-2,6-OMe-beta-CD, is an unusual beta-cyclodextrin, beta-CD, derivative because it has a negative thermal solubility coefficient in aqueous solution. This report describes two new crystal structures for hydrates of per-2,6-OMe-beta-CD crystallized at different temperatures: per-2,6-OMe-beta-CD.1.08H(2)O crystallized at 50 degrees C and per-2,6-OMe-beta-CD.14.7H(2)O crystallized at 4 degrees C. The crystal structure for per-2,6-OMe-beta-CD.1.08H(2)O reveals conformational flexibility in the methylated beta-CD molecule; two adjacent glucose residues are disordered as a result of their adopting different tilts with respect to the O(4) plane characterizing the molecule. When one conformation is present there is a water molecule in the lattice (0.5 population parameter); said water is displaced by the methyl of a methoxy substituent when the other conformer is present.  相似文献   

12.
It remains a big challenge to develop solid-state stimuli-responsive materials for time-dependent information encryption and inkless erasable printing with long retention times. Herein, a 2D Cu2I2-based MOF with photoresponsive spiropyran(SP) groups orderly installed on its skeleton is developed. The structural isomerization from SP to colored merocyanine(MC) form can be triggered by removing the CH3CN guests. Besides, the degree of structural isomerization and t...  相似文献   

13.
The local structures and spin Hamiltonian parameters (g factors and the hyperfine structure constants) of the Rh4+(4d5) and Ir4+(5d5) centers in rhombohedral BaTiO3 are theoretically investigated from the formulas of these parameters for a nd5 (n = 4 and 5) ion with low spin (S = 1/2) in a trigonally distorted octahedron. From the calculations, the impurity ions are found not to occupy exactly the host Ti4+ site in BaTiO3 but to suffer a slight inward shift (0.13 Å) towards the center of the oxygen octahedron along the C3 axis, yielding much smaller trigonal distortion as compared with that of the host Ti4+ site. The theoretical spin Hamiltonian parameters based on the above impurity axial shifts are in good agreement with the observed values.  相似文献   

14.
The presumably "small molecule structure" of the organopalladium complex rac-PdCl(C(6)H(3)-4-OMe-CHMeNH(2))(py) crystallizes with 16 molecules in the asymmetric unit, a highly unusual situation according to the CSD database. The symmetrically independent residues differ with respect to soft conformational degrees of freedom. Packing in the acetonitrile solvate of the same compound is unexceptional.  相似文献   

15.
Determination of the NMR anisotropic magnetic shielding parameters from magic angle spinning, MAS, powder samples containing half‐integer spin quadrupolar nuclei is achieved by analysis of the difference spectrum obtained with and without application of a hyperbolic secant pulse. Application of a hyperbolic secant pulse to any spinning sideband associated with the central transition, mI = 1/2 to mI = ? 1/2, results in ‘saturation’ of the entire central transition manifold. Similarly, if one spinning sideband associated with the mI = 3/2 to mI = 1/2 and mI = ? 1/2 to mI = ? 3/2 satellite transitions is perturbed, the entire satellite manifold associated with these transitions is ‘saturated’ while the central transition is enhanced by population transfer. Three ‘difference spectrum’ techniques are employed to selectively yield the spinning sidebands associated predominantly from the central transition. The success of these difference techniques is first demonstrated by examining 51V NMR spectra of three metavanadate salts and 59Co NMR spectra of Co(acac)3. The vanadium and cobalt chemical shift tensors in these compounds have spans between 400 and 1400 ppm. Because the hyperbolic secant techniques proposed here yielded results that are in good agreement with earlier reports, they have been applied to characterize the 51V chemical shift tensor of the dimer of bis(N, N‐dimethylhydroxamido)‐hydroxooxovanadate, {V(O)(ONMe2)2}2O, whose chemical shift tensor has not been previously reported. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

16.
The spin Hamiltonian parameters (g factors, hyperfine structure constants and superhyperfine parameters) and local structures for Ag2+ centers in AgCl and KCl crystals are theoretically studied using the high-order perturbation formulas for a tetragonally elongated 4d 9 cluster. The impurity centers undergo relative elongations (≈0.05 Å and 0.23 Å for Ag2+ in AgCl and KCl, respectively) along the C 4 axis owing to the Jahn–Teller effect. All the calculated spin Hamiltonian parameters show good agreement with the experimental data, and the ligand contributions to the spin Hamiltonian parameters are important and should be taken into account. The unpaired spin densities in the superhyperfine parameters are determined from molecular orbital coefficients based on the cluster approach, instead of being taken as the adjustable parameters in the previous treatments. Increasing tetragonal elongation from AgCl to KCl is attributed to a decrease in chemical bonding (or lower force constant) with increasing Ag2+–Cl distance.  相似文献   

17.
The heptanuclear complex [Mo(iv)(CN)(2)(CN-CuL)(6)](8+) switches from a paramagnetic dark state corresponding to six spin-1/2 Cu(ii) ions to a predominantly high spin S = 3 state, on prolonged irradiation with 406 nm laser radiation at low temperature. The system returns to a paramagnetic state on warming to room temperature. The temperature dependence of the chi(M)T vs. T curve depends upon duration of irradiation. An earlier microscopic model showed that the excitation cross sections in different spin manifolds are similar in magnitude and that photomagnetism is not due to preferential excitation to the S = 3 state. In this paper, we attribute photomagnetism to a long lived S = 3 charge transfer excited state for which there appears to be sufficient experimental evidence. Based on this postulate, we model the photomagnetism by employing a kinetic model which includes internal conversions and intersystem crossings. The key feature of the model is the assumption of the existence of two kinds of S = 3 states: one of which has no direct pathway for internal conversion and the other characterized by slow kinetics for internal conversion to the low-energy states. The trapped S = 3 state can decay via a thermally activated barrier to the other S = 3 state. The experimental chi(M)T vs. T for two different irradiation times are fitted using Arrhenius dependence of the rate constants in the model.  相似文献   

18.
The spin Hamiltonian parameters (zero-field splitting D, g factors g parallel, g perpendicular and hyperfine structure constants A parallel, A perpendicular) for M2+ (M=Co, Mn, V and Ni) ions in CsMgCl3 are studied by using the perturbation formulas of the spin Hamiltonian parameters for 3dn (n=7, 5, 3, 8) ions in trigonal symmetry based on the cluster approach. In these formulas, the contributions to the spin Hamiltonian parameters from the admixture of d orbitals of the central ions with the p orbitals of the ligands and from the trigonal distortion are included and the parameters related to these effects can be obtained from the optical spectra and the local structures of the studied systems. Based on the studies, it is found that the local trigonal distortion angle beta in the M2+ impurity center is unlike that betaH (approximately 51.71 degrees) in the host CsMgCl3. The spin Hamiltonian parameters for these divalent ions in CsMgCl3 are also satisfactorily explained by using the local angle beta. The validity of the results is discussed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号