1,720,988 research outputs found
Distribution and Dynamic Properties of Xenon Dissolved in the Ionic Smectic Phase of [C16mim][NO3]: MD Simulation and Theoretical Model
No full textWe have investigated the structural and dynamic properties of Xe dissolved in the
ionic liquid crystal (ILC) phase of 1-hexadecyl-3-methylimidazolium nitrate using classical
molecular dynamics (MD) simulations. Xe is found to be preferentially dissolved within the
hydrophobic environment of the alkyl chains rather than in the ionic layers of the smectic phase.
The structural parameters and the estimated local diffusion coefficients concerning the short-time
motion of Xe are used to parametrize a theoretical model based on the Smoluchowski equation for
the macroscopic dynamics across the smectic layers, a feature which cannot be directly obtained
from the relatively short MD simulations. This protocol represents an efficient combination of
computational and theoretical tools to obtain information on slow processes concerning the
permeability and diffusivity of the xenon in smectic ILCs
Aggregation Behavior of Octyl Viologen Di[bis(trifluoromethanesulfonyl)amide] in Nonpolar Solvents
No full textThe aggregation behavior in nonpolar solvents of the octyl viologen (OV) salt with the hydrophobic anion bis(trifluoromethanesulfonyl)amide (Tf2N−) has been investigated. 1H and 19F NMR, ESI-MS and DFT calculations suggest that large aggregates are formed in toluene, benzene and chloroform, where the salt is highly soluble. The lifetime of the aggregates is long enough to be detected as independent species by 1H and 19F NMR spectroscopy, together with the smaller neutral OV(Tf2N)2 cluster. This behavior is quite at variance with usual NMR detected equilibria where only average signals are generally observed. Large aggregates are also observed in ESI-MS spectra of toluene and chloroform solutions despite the well-known low-coordinating ability of Tf2N−. It is suggested that the structure of the large aggregates mimics the thermotropic smectic phase that this system exhibits near room temperature
A diffusive model for interpreting solvation dynamics in isotropic and ordered liquid phases
No full textSolutions are discussed for a two-body diffusive model in which a rotating probe is coupled to a solvent polarisation field, in the case of anisotropic diffusion. The model describes the diffusional rotational behaviour of a rigid molecule, carrying a permanent electric dipole and coupled, via a dipole-field term, to a polarisation vector or reaction field coordinate, which is also relaxing diffusively. When the solvent relaxation coordinate relaxes faster or slower than the probe rotation, a semi-analytical treatment of the system is possible, based on the separation of timescales. This treatment can be applied in the presence of a mean field potential acting on the probe orientation, thus allowing to consider liquid crystalline phases. We specialize our treatment to the case of solvation dynamics: a theoretical transient Stokes shift (TSS) correlation function is defined and its behaviour is discussed
Simulated time resolved fluorescence in ordered phases
No full textAn interpretation of time resolved fluorescence emission experiments, performed on probes in isotropic and ordered polar phases, is developed. The system under scrutiny is provided by a dipolar molecule undergoing rotational diffusion, whose dipole moment is coupled to a stochastic reaction field generated by the ordered polar medium. Under quite general conditions, the fluorescence emission is defined as an integral over all possible orientations of the ground and excited states. Since the model allows to reproduce experimental results at a semi-quantitative level and a negligible computational cost, a user-friendly package with graphics utilities has been created, which can be of help to investigate interactively the dependence of the simulated spectra upon several macroscopical parameters such as the dielectric constants and the viscosity of the medium
The structuring effect of the alkyl domains on the polar network of ionic liquid mixtures: a molecular dynamics study
Full text linkBy using molecular dynamics simulations, we investigate the structural and dynamic properties of mixtures of 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, [C(1)C(1)im][Tf2N] and 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C(12)C(1)im][Tf2N] (also C-1 and C-12 in short). Such mixtures feature an imidazolium bistriflimide salt with a very short alkyl chain, not giving rise to any nano-segregation as a pure component, with another one with a longer alkyl chain that exhibits a substantial nano-segregation as a pure liquid. As the mole fraction of the long-chain component C-12 is increased, the so-called pre-peak of the structure factor S(q), occurring in the region 1-3 nm(-1), shows a shift to higher values of the wavevector q, mirroring a decrease of the corresponding correlation length. Moreover, the intensity of the pre-peak strongly increases with the C-12 concentration. These results are in very good agreement with experimental X-ray scattering data in the literature. On the other hand, the diffusion of the ions is found to exhibit a simple behaviour consistent with the increased viscosity of the mixture, and these results are also in good agreement with NMR experimental data from the literature. The simulation results are rationalized as caused by a structuring effect, similar to the hydrophobic effect, of the alkyl domains of the C-12 component dissolved in the "solvent" represented by the C-1 cation, the Tf2N- anion and the C-12 cation head. In short, the exclusion of the alkyl chains from the polar network, a process mostly governed by electrostatic interactions, favours the formation of hydrophobic domains, which in turn exert a structuring effect on the ions of the polar domains, favouring a stronger ionic interaction. This is finally reflected in a shorter correlation length and a higher intensity of the pre-peak of the structure factor S(q) as the C-12 mole fraction is increased. At variance with the microscopic structure, the diffusion of all three types of ions is not strongly influenced by the nano-segregation and is essentially dependent on the viscosity of the mixture
A pairwise additive potential for the elastic interaction energy of a chiral nematic
No full textWe have developed a pairwise additive potential model to describe the macroscopic elastic interactions of a chiral nematic liquid crystal. The potential is obtained from the expression for the elastic free energy density discretized onto a cubic lattice and mapped onto a suitable expansion in scalar invariants. ne value of the pair potential is explored by means of a Monte Carlo lattice computer simulation. It allows, in a simple and efficient way, the simulation of liquid crystal devices and samples in confined geometries, taking into account the effect of the temperature of the sample and of the thermal fluctuations in the director distribution, as well as the elastic interactions
An experimental and theoretical study of the mechanism of cleavage of an RNA-model phosphate diester by mononuclear Zn(II) complexes
No full textKinetic investigations and density functional theory calculations were used to explore the mechanism of the cleavage of the RNA analogue 2-hydroxypropyl-p-nitrophenyl phosphate promoted by mononuclear Zn(II) complexes of polyamine ligands, focusing the attention on the attack of the 2-hydroxyl to the phosphorous atom. The results obtained indicate that mononuclear complexes of tridentate polyamine ligands are more reactive than the tetradentate ones notwithstanding similar Lewis acidity. Three reaction pathways were found corresponding to the three reaction mechanisms so far proposed: general base catalysis by a metal coordinated species, specific base catalysis and specific base catalysis involving coordination of the 2-hydroxyl to the metal ion. The three mechanisms have similar reaction barriers with a slightly lower one for the general base catalysis one. Among the two specific base catalysis mechanisms, which are in agreement with previous experimental observation of low isotopic solvent kinetic effects for the reaction, the one involving metal coordination of the substrate alcoholic group appears the more likely and is also capable to explain the higher activity of mononuclear complexes of triamine ligands compared with tetramine ones
Theoretical model of photoinduced intramolecular charge transfer processes
No full textMolecular systems which undergo upon photoexcitation intramolecular charge transfer processes accompanied by conformational changes are known to exhibit complex spectroscopic and dynamical behaviour. Typical spectroscopic effects are dual fluorescence and pronounced solvatochromism. Dynamical features are revealed by the temperature dependence of fluorescence emission intensities and shifts under stationary conditions; the precursor-successor behaviour for the observed decay of the high-frequency band and the rise of the low-frequency band, after pulse excitation in correspondence of the high-frequency absorption; the fluorescence depolarization effects; the dynamic Stokes shifts measured in time-dependent experiments. All these effects can be rationalized by a stochastic model in which coupling of the fluorescent probe with the polar environment is explicitly taken in account. Ingredients of the model are the potential energy surface for the probe internal coordinates, and hydrodynamic and dielectric properties of the solvent. The model reduces to the simple kinetic scheme based on two-state interconversion process only in the limiting case of a relatively high energy barrier separating the two emitting states, and rapid equilibration of the polar solvent with respect to the instantaneous probe dipole moment
LiquidLiquid Phase Separation of Viologen Bistriflimide/Benzene Mixtures: Role of the Dual Ionic and Organic Nature of Ionic Liquids
No full textLiquid-liquid phase separation occurs at room temperature when mixing an excess of benzene with solid viologen bistriflimide salts with various alkyl side-chain lengths. A liquid phase composed of (almost) pure benzene is above the other sponge-like liquid phase with salt absorbed in benzene. Nuclear magnetic resonance experiments indicate that the mole ratio of benzene/salt in the sponge-like phase remains unchanged upon varying the amounts of (nonexcessive) salt or benzene. Moreover, the benzene/viologen salt mole ratio in the sponge-like phase increases linearly with respect to the side-chain length of the cation. Similarly, when an excess of viologen salt is added in benzene, a sponge-like liquid phase composed of salt absorbed by benzene is observed in equilibrium with some solid viologen salt neither dissolved nor absorbed by the solvent. The mole ratio of the sponge-like liquid phase again increases linearly with side-chain length, while it remains independent of the relative amount of benzene and viologen salt as long as the latter is in excess. Finally, when appropriate amounts of benzene and viologen salt are mixed, a single sponge-like liquid phase is observed at an intermediate composition between the lower and upper limits. Molecular dynamics simulations reveal that because of their dual ionic and organic nature, when absorbed in benzene, the studied salts form nanoscale segregated liquid structures, akin ionic liquids, with a continuous polar network composed of anions and cationic charged groups, along with nonpolar domains composed of alkyl cationic side chains. Benzene molecules are preferentially absorbed inside the nonpolar region, which effectively expands the nonpolar region to be sponge-like and consequently liquidizes the viologen salt. The linearity of the benzene/salt ratio in the upper and lower phase boundaries comes from the fact that the effective volume of the nonpolar region for accommodating benzene molecules grows linearly with cationic alkyl side-chain length. The occurrence of the above phenomena is attributed to the nonpolar feature of benzene molecules, and there is no evidence of pi-pi or ion-pi interaction between the ions and benzene molecules. Moreover, the diffusion of benzene in the sponge-like phase is found to be close to that in n-alkanes, supporting the idea of nanoscale segregation of polar and nonpolar regions in the sponge-like phase. The revealed mechanism is anticipated to be general for understanding liquid-liquid phase separation observed in mixtures of organic salts (ionic liquids) having relatively long alkyl chains with small organic molecules
Solvation dynamics of Coumarin 503 in the liquid-crystal mixture ZLI 1167
No full textThe solvation dynamics of the dye Coumarin 503 in the nematic mixture ZLI 1167 has been investigated by means of time-resolved fluorescence spectroscopy, both in the nematic and isotropic phases of the sample. No alignment was imposed on the micro-domains of the nematic phase, so that a complete depolarization of the emitted light was obtained. For the isotropic phase the usual set-up with vertically polarized excitation light and magic-angle detection was used. A time-dependent frequency shift of the maximum of the fluorescence band, caused by solvent reorganization after pulse excitation of the fluorescent probe, was observed. The analysis of the transient Stokes shift correlation function clearly shows biexponential behavior in the nematic phase, the slowest time constant varying from 1670 ps at 311.5 K to 230 ps at 373 K. The decay of the correlation function appears to be largely unaffected by the nematic–isotropic transition, suggesting that the local environment, rather than long-range ordering, determines Stokes shift dynamics. A theoretical model, which takes into account probe reorientations in the presence of a nematic field, and fluctuation of the local solvent polarization, has been developed to interpret the experimental findings
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