Micromorphology Characterization of SiO2-Based Composite Thin Films with Immobilized Terbium(III) Complex

The aim of this study was to investigate by atomic force microscopy and multifractal analysis the three-dimensional (3-D) of surface micromorphology of the complex of Tb(III) with the biscoumarin derivative 3,3′-[(4-hydroxyphenyl)methyl)]bis-(4-hydroxy-2H-1-benzopyran-2-one), Tb(C₂₅H₁₅O₇)₃ · 5H₂O immobilized in transparent SiO₂-based films by a simple casting technique. The 3-D surfaces contain irregularities of various orders spread on the surface due to the intrinsic method of surface preparation. We found that the micromorphology of all analyzed samples has multifractal characteristics. The generalized dimension D _q and the singularity spectrum f(α) provided quantitative values that describe the degree of heterogeneity in the 3-D surface geometry at nanometer scale. The results showed that the larger the spectrum width Δα (Δα = α _max  ? α _min ) of the multifractal spectra f(α), the more nonuniform is the surface micromorphology. These results demonstrate that multifractal analysis is a more precise and reliable tool for quantitative characterization of 3-D surface micromorphology.     

A coarse-grained explicit solvent simulation of rheology of colloidal suspensions

We use a simple extension of the dissipative particle dynamics (DPD) model to address the dynamical properties of macrosolutes immersed in complex fluid solvents. In this approach, the solvent particles are still represented as DPD particles, thereby retaining the time and length scale advantages offered by the DPD approach. In contrast, the solute particles are represented as hard particles of the appropriate size. We examine the applicability of this simulation approach to reproduce the correct hydrodynamical characteristics of the mixture. Our results focus on the equilibrium dynamics and the steady-state shear rheological behaviors for a range of volume fractions of the suspension, and demonstrate excellent agreement with many published experimental and theoretical results. Moreover, we are also able to track the glass transition of our suspension and the associated dynamical signatures in both the diffusivities and the rheological properties of our suspension. Our results suggest that the simulation approach can be used as a one-parameter model to examine quantitatively the rheological properties of colloidal suspensions in complex fluid solvents such as polymeric melts and solutions, as well as allied dynamical phenomena such as phase ordering in mixtures of block copolymers and particles.     

Structural dynamics of supercooled water from quasielastic neutron scattering and molecular simulations

One of the outstanding challenges presented by liquid water is to understand how molecules can move on a picosecond time scale despite being incorporated in a three-dimensional network of relatively strong H-bonds. This challenge is exacerbated in the supercooled state, where the dramatic slowing down of structural dynamics is reminiscent of the, equally poorly understood, generic behavior of liquids near the glass transition temperature. By probing single-molecule dynamics on a wide range of time and length scales, quasielastic neutron scattering (QENS) can potentially reveal the mechanistic details of water's structural dynamics, but because of interpretational ambiguities this potential has not been fully realized. To resolve these issues, we present here an extensive set of high-quality QENS data from water in the range 253-293 K and a corresponding set of molecular dynamics (MD) simulations to facilitate and validate the interpretation. Using a model-free approach, we analyze the QENS data in terms of two motional components. Based on the dynamical clustering observed in MD trajectories, we identify these components with two distinct types of structural dynamics: picosecond local (L) structural fluctuations within dynamical basins and slower interbasin jumps (J). The Q-dependence of the dominant QENS component, associated with J dynamics, can be quantitatively rationalized with a continuous-time random walk (CTRW) model with an apparent jump length that depends on low-order moments of the jump length and waiting time distributions. Using a simple coarse-graining algorithm to quantitatively identify dynamical basins, we map the newtonian MD trajectory on a CTRW trajectory, from which the jump length and waiting time distributions are computed. The jump length distribution is gaussian and the rms jump length increases from 1.5 to 1.9 A? as the temperature increases from 253 to 293 K. The rms basin radius increases from 0.71 to 0.75 A? over the same range. The waiting time distribution is exponential at all investigated temperatures, ruling out significant dynamical heterogeneity. However, a simulation at 238 K reveals a small but significant dynamical heterogeneity. The macroscopic diffusion coefficient deduced from the QENS data agrees quantitatively with NMR and tracer results. We compare our QENS analysis with existing approaches, arguing that the apparent dynamical heterogeneity implied by stretched exponential fitting functions results from the failure to distinguish intrabasin (L) from interbasin (J) structural dynamics. We propose that the apparent dynamical singularity at ～220 K corresponds to freezing out of J dynamics, while the calorimetric glass transition corresponds to freezing out of L dynamics.     

Real-time analysis of secondary organic aerosol particles formed from cyclohexene ozonolysis using a laser-ionization single-particle aerosol mass spectrometer.

A real-time analysis of secondary organic aerosol (SOA) particles formed from cyclohexene ozonolysis in a smog chamber was performed using a laser-ionization single-particle aerosol mass spectrometer (LISPA-MS). The instrument obtains both size and chemical compositions of individual aerosol particles with a high time-resolution (approximately 2 s at the maximum). Both positive and negative-ion mass spectra are obtained. Standard particles generated from dicarboxylic acid solutions using an atomizer were also analyzed. For both standard and SOA particles, the negative-ion mass spectra provided information about the molecular weights of the organic compounds in the particles, since the intense ions in the negative-ion mass spectra are mainly attributable to the molecular-related ions [M-H]-. It was demonstrated that the real-time single-particle analysis of SOA particles by the LISPA-MS technique can reveal the formation and transformation processes of SOA particle in smog chambers.     

Photostability of electronically excited polyacenes: a case study of vibronic coupling in the naphthalene radical cation

Novel issues of electronic nonadiabatic coupling in the excited state dynamics of prototypical naphthalene radical cation of polycyclic aromatic hydrocarbon of the polyacene family are theoretically investigated. A benchmark ab initio quantum dynamical study is performed and its complex vibronic spectra and nonradiative decay are examined. The findings are in very good accord with the experiment, unambiguously establishing the crucial role of intricate electron-nuclear coupling in the photoinduced dynamical processes of this system.     

Supercritical ammonia: a molecular dynamics simulation and vibrational spectroscopic investigation

Combining infrared spectroscopy and molecular dynamics simulations, we have investigated the structural and dynamical properties of ammonia from liquid state (T = 220 and 303 K) up to the supercritical domain along the isotherm T = 423 K. Infrared spectra show that the N-H stretching and bending modes are significantly perturbed which is interpreted as a signature of the change of the local environment. In order to compare the experimental spectra with those obtained using molecular dynamics simulation, we have used a flexible four sites model which allows to take into account the anharmonicity in all the vibration modes particularly that of the inversion mode of the molecule. A good agreement between our experimental and calculated spectra has been obtained hence validating the intermolecular potential used in this study to simulate supercritical ammonia. The detailed analysis of the molecular dynamics simulation results provides a quantitative insight of the relative importance of hydrogen bonding versus nonhydrogen bonded interactions that governs the structure of fluid ammonia.     

Molecular dynamics of liquid acetone determined by depolarized Rayleigh and low-frequency Raman scattering spectroscopy

Slow to ultrafast dynamics of liquid acetone at variable temperature was investigated by depolarized Rayleigh and low-frequency Raman scattering spectroscopy, in the region 0-200 cm(-1). A detailed analysis was performed on the spectra and corresponding time responses, and a consistent view of the molecular dynamics of this dipolar solvent was obtained. The effects of temperature on the spectra were interpreted, and distinct dynamical processes identified. At very low frequencies, or long time scales, acetone dynamics is characterized by a slow diffusive reorientation obeying the Stokes-Einstein-Debye hydrodynamic theory only in the limit of subslip boundary conditions. An alternative model based on the microviscosity concept proved to be able to reproduce this correlation time and its temperature dependence. A comparative analysis of collective and single-molecule reorientational times, these latter estimated from intramolecular Raman spectra, led to an orientational correlation parameter g(2) of unity, which denotes a statistical disorder of molecular polarizability tensors. A fast local restructuring process is putatively responsible for an additional contribution at subpicosecond time scales often referred to as intermediate response in other molecular liquids. The high frequency portion of the dynamical susceptibility showed the signature of librational intermolecular motions, giving rise to an ultrafast decay of the time correlation function of polarizability anisotropy. The overall approach, which provided valuable information on dynamics, structure and molecular interactions of neat acetone, will be applied to acetone electrolytic solutions.     

Dynamics and structure of a flexible columnar liquid crystal based on tetrabenzocyclododecatetraene

The dynamical and conformational behaviour of a flexible tetrabenzocyclododecatetraene derivative exhibiting a columnar mesophase has been studied by a combination of deuteron solid state NMR spectroscopy and molecular dynamics (MD) simulations. As shown by two-dimensional (2D) exchange NMR, the mesophase is characterized by slow axial reorientations (∼10^-3s) of single molecular units where the phenylene rings exhibit a well-defined quasi-fourfold potential, while the 2D spectra of the core methylene sites are sensitive to the molecular conformation and reorientation mechanism. Motional narrowing of one-dimensional (1D) spectra reveals additional fast librations due to the internal flexibility of the mesogenic moiety. The various reorientation pathways comprising interconversions and pseudo-rotations between different energetically stable conformations are elucidated on a microscopic level by molecular dynamics simulations. The mesophase dynamics is ascribed to a complex axial motion involving rotational jumps combined with a pseudo-rotation between two symmetry related sofa forms. This is confirmed quantitatively by comparing the experimental 2D NMR spectra of the core methylene sites and the simulations which are based on the molecular geometries obtained by MD simulations. The lineshapes of one- and two-dimensional spectra of magnetically aligned samples specific to the orientation behaviour of the sofa conformer are discussed.     

A general approach for prediction of motional EPR spectra from Molecular Dynamics (MD) simulations: application to spin labelled protein

A general approach for the prediction of EPR spectra directly and completely from single dynamical trajectories generated from Molecular Dynamics (MD) simulations is described. The approach is applicable to an arbitrary system of electron and nuclear spins described by a general form of the spin-Hamiltonian for the entire motional range. It is shown that for a reliable simulation of motional EPR spectra only a single truncated dynamical trajectory generated until the point when correlation functions of rotational dynamics are completely relaxed is required. The simulation algorithm is based on a combination of the propagation of the spin density matrix in the Liouville space for this initial time interval and the use of well defined parameters calculated entirely from the dynamical trajectory for prediction of the evolution of the spin density matrix at longer times. A new approach is illustrated with the application to a nitroxide spin label MTSL attached to the protein sperm whale myoglobin. It is shown that simulation of the EPR spectrum, which is in excellent agreement with experiment, can be achieved from a single MD trajectory. Calculations reveal the complex nature of the dynamics of a spin label which is a superposition of the fast librational motions within dihedral states, of slow rotameric dynamics among different conformational states of the nitroxide tether and of the slow rotational diffusion of the protein itself. The significance of the slow rotameric dynamics of the nitroxide tether on the overall shape of the EPR spectrum is analysed and discussed.     

Detailed dynamics of three-body recombination of ions in central collisions

The potential energy surface describing quantitatively the dynamics of the collision induced dissociation reactions CsBr + Xe → Cs⁺ + Br⁻ + Xe, CsXe⁺ + Br⁻ is used to explore detailed dynamics of the reverse process of direct three-body recombination of the Cs⁺ and Br⁻ ions undergoing a central collision. For a stepwise analysis of the course of the elementary process, visualization of the trajectory and of the energy state of each pair of the particles is employed. Several different mechanisms of the reaction are found. Their occurrence depends on the impact parameter of the collision of the third body with the recombining pair and on the angles determining the spatial orientation of the ion velocities. The major role in stabilization of the recombination products is played by the repulsion potentials between the ions and the third body, but the amount of energy transferred depends not only on the repulsion interaction strength. Recombination is shown to be able to happen for very low energies of repulsion between the third body and the ions and even in the complete absence of repulsion. In all the cases, the particular recombination mechanism is determined by the dynamical features of the collision. The relation between the kinematic parameters of a collision of the three particles and the recombination mechanism is considered.     

Structure and dynamics of solvated Sn(II) in aqueous solution: an ab initio QM/MM MD approach

Structural and dynamical properties of the hydrated Sn(II) ion have been investigated by ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations at double-zeta HF quantum mechanical level. The results indicate Sn(II)aq to be a rather peculiar, if not unique, case of a hydrated ion: four of its eight first-shell ligands do not take place in the otherwise frequent ligand-exchange processes, forming an approximately tetrahedral cage around the ion. The remaining ligands, however, exchange at a rate that is rather comparable to monovalent than divalent ions. This very surprising behavior of ligand exchange not yet observed in any previous simulation of over 30 hydrated metal ions is consistently confirmed by vibrational spectra, bond lengths, and a detailed analysis of the trajectories of the simulation.     

State-to-state photodissociation dynamics of triatomic molecules: H2O in the B band

State-to-state photodissociation dynamics of H(2)O in its B band has been investigated quantum mechanically on a new set of non-adiabatically coupled potential energy surfaces for the lowest two (1)A' states of H(2)O, which are developed at the internally contracted multi-reference configuration interaction level with the aug-cc-pVQZ basis set. Quantum dynamical calculations carried out using the Chebyshev propagator yield absorption spectra, product state distributions, branching ratios, and differential cross sections, which are in reasonably good agreement with the latest experimental results. Particular focus is placed here on the dependence of various dynamical observables on the photon energy. Detailed analyses of the dynamics have assigned the diffuse structure in absorption spectrum to short-time recurring dynamics near the HOH conical intersection. The non-adiabatic dissociation to the ground state OH product via the HOH conical intersection is facile, direct, fast, and produces rotationally hot OH(X?) products. On the other hand, the adiabatic channel on the excited state leading to the OH(A?) product is dominated by long-lived resonances, which depend sensitively on the potential energy surfaces.     

Molecular dynamics of polystyrene solutions in microwave fields

Equilibrium and nonequilibrium molecular dynamics simulation techniques were used to assess the influence of an applied microwave field on the dynamics of methylamine-methanol and methylamine-dimethylformamide (DMF) solutions bound within atactic polystyrene over a range of polymer densities from 35 to 96 wt % polymer. Atomistically detailed systems were studied, ranging from 3000 to 10 644 particles, using previously established potential models. Structural and dynamical properties were determined in the canonical (NVT) ensemble at 298 K. The simulated DMF self-diffusion coefficients in polystyrene solutions were compared with the zero-field experimental results established with pulsed-gradient spin-echo NMR spectrometry. A simulated external microwave field, with a rms electric field intensity of 0.1 VA, was applied to these systems and the simulated dynamical results over field frequencies up to 10(4) GHz were compared with the zero-field values. Simulated evidence of athermal effects on the diffusive characteristics of these mixtures is reported.     

1,2-邻苯二氧基二乙酸铕及掺杂L(L=La,Y,Yb,Nd)配合物的合成及荧光性质研究

在乙醇体系中,以氯化铕与1,2邻苯二氧基二乙酸和二苯甲酰甲烷反应合成了三元配合物Eu-BDDA-DBM以及弱荧光离子La3+,Y3-,Yb3-和Nd3+掺杂的铕配合物.通过红外、紫外-可见、热重、荧光光谱对配合物进行了表征.红外光谱表明,单一配合物和掺杂配合物具有相似的配位结构.荧光光谱表明,La3+和Nd3+离子掺杂可以大幅度提高的铕配合物的荧光强度,其中La3+掺杂荧光强度增强最明显.     

Molecular dynamics studies of ionically conducting glasses and ionic liquids: wave number dependence of intermediate scattering function

Dynamical heterogeneity is a key feature to characterize both acceleration and slowing down of the dynamics in interacting disordered materials. In the present work, the heterogeneous ion dynamics in both ionically conducting glass and in room temperature ionic liquids are characterized by the combination of the concepts of Lévy distribution and multifractality. Molecular dynamics simulation data of both systems are analyzed to obtain the fractional power law of the k-dependence of the dynamics, which implies the Lévy distribution of length scale. The multifractality of the motion and structures makes the system more complex. Both contributions in the dynamics become separable by using g(k,t) derived from the intermediate scattering function, F(s)(k,t). When the Lévy index obtained from F(s)(k,t) is combined with fractal dimension analysis of random walks and multifractal analysis, all the spatial exponent controlling both fast and slow dynamics are clarified. This analysis is generally applicable to other complex interacting systems and is deemed beneficial for understanding their dynamics.     

过氧化氢氧化硫化钠反应体系的复杂振荡

研究了过氧化氢和硫化钠反应体系在连续流动搅拌反应器中的复杂振荡行为. 观察到11型振荡、12型振荡、非周期振荡以及衰减振荡; 在振荡周期内, 拐点附近区域H+扰动产生的延迟与振荡相位pH有关. 在完全基于硫价态变化的经验速率方程模型的基础上进行数值模拟, 得到的各种复杂振荡行为与实验现象基本一致, 证明了硫价态变化可驱动复杂非线性动力学行为.     

Online aerosol mass spectrometry of single micrometer-sized particles containing poly(ethylene glycol)

The analysis of poly(ethylene glycol) (PEG)-containing particles by online single particle aerosol mass spectrometers equipped with laser desorption/ionization (LDI) is reported. We demonstrate that PEG-containing particles are useful in the development of aerosol mass spectrometers because of their ease of preparation, low cost, and inherently recognizable mass spectra. Solutions containing millimolar quantities of PEGs were nebulized and, after drying, the resultant micrometer-sized PEG-containing particles were sampled. LDI (266 nm) of particles containing NaCl and PEG molecules of average molecular weight<500 Da generated mass spectra reminiscent of mass spectra of PEG collected by other mass spectrometer platforms including the characteristic distribution of positive ions (Na+ adducts) separated by the 44 m/z units of the ethylene oxide units separating each degree of polymerization. PEGs of average molecular weight>500 Da were detected from particles that also contained the tripeptide tyrosine-tyrosine-tyrosine or 2,5-dihydroxybenzoic acid, which were added to nebulized solutions to act as matrices to assist LDI using pulsed 266 nm and 355 nm lasers, respectively. Experiments were performed on two aerosol mass spectrometers, one reflectron and one linear, that each utilize two time-of-flight mass analyzers to detect positive and negative ions created from a single particle. PEG-containing particles are currently being employed in the optimization of our bioaerosol mass spectrometers for the application of measurements of complex biological samples, including human effluents, and we recommend that the same strategies will be of great utility to the development of any online aerosol LDI mass spectrometer platform.     

Theory of relaxation spectra and dielectric relaxation of rigid rodlike particles incorporated in a polymer network

The theory of molecular mobility and relaxation spectra is developed for rodlike particles embedded in a polymer network with allowance for the involvement of the particles in collective network dynamics through topological entanglements with network fragments. A regular cubic coarse-grained network model is used, where the motion of junctions describes the mobility of large fragments (domains) of the initial network with a size equal to the distance between adjacent rodlike particles. The involvement of the rods in collective network dynamics is taken into account by introducing an effective quasi-elastic potential acting between the rods and junctions of the coarse-grained network and preventing long-distance diffusion of the embedded particles. The viscoelastic parameters of the coarse-grained (“renormalized”) network are functions of the viscoelastic characteristics of the initial network. The relaxation time spectra are calculated as well as the frequency dependences of the dielectric loss factor of the embedded particles that possess a permanent dipole moment directed along the major axis of each rod. Depending on the ratio between the viscoelastic characteristics of the rods and the network, the frequency dependence of the dielectric loss factor may have two maxima. The high-frequency maximum corresponds to local orientational movements of particles at fixed junctions of the coarse-grained network, which correspond to the position of the domain centers in the initial network. The low-frequency maximum corresponds to movements of particles involved in large-scale dynamics of network fragments. The dependence of the dielectric loss factor on the ratio between the viscoelastic parameters of the rods and the network is studied.