1,720,998 research outputs found
DFT conformational study of banana-shaped molecules
No full textThe conformational energy of the 1,3-phenylene bis[4-4-(methoxy) benzoyloxy] benzoate and 4-chloro-1,3-phenylene bis[4-4'(methoxy) benzoyloxy] benzoate molecules has been studied using the B3LYP density functional, with a double polarized basis set. These molecules can be seen as a model for the aromatic core of recently discovered banana-shaped mesogens. The relationship between the bent molecular shape and the conformations assumed by the phenyl ester dihedrals is studied together with the effect of substitutions on the central ring. In particular, the inclusion of a chlorine atom in the central ring results in an increase of the bending angle, consistently with recent experimental findings. A discussion of the chirality of some of the resulting energy minima conformations is also given
Atomistic simulation of a nematogen using a force field derived from quantum chemical calculations
No full textBulk phase atomistic computer simulations of 4-n-pentyl-4¢-cyanobiphenyl (5CB) were performed with a
specific force field obtained from ab initio and DFT calculations. The intermolecular potential was previously
derived through the fragmentation reconstruction method (FRM), developed in our group. The description of
some intramolecular interactions, like the torsional potential between the phenyl rings and at the aryl-alkyl
linkage, is achieved through accurate DFT studies. Lengthy (=40 ns) molecular dynamics (MD) simulations
were then carried out at constant atmospheric pressure and different temperatures. The system was stable in
the experimental crystalline structure up to 285 K, where the early stage of the melting process appears with
the loss of positional order. At higher temperatures (between 290 and 305 K) a kinetically stable, orientationally
ordered phase is obtained. This nematic phase was reached starting with three initial configurations, differing
in their orientational order parameter. The calculated values of thermodynamic and structural properties of
each phase were in fairly good agreement with the relevant experimental data
Anomalous Diffusion and Cage Effects in the Isotropic Phase of a Liquid Crystal
No full textThe translational motion of 4-n-hexyl-4'-cyanobiphenyl (6CB) in its isotropic phase has been studied by atomistic molecular dynamics simulation from 280 to 330 K. The mean square displacement shows evidence of a subdiffusive dynamics, with a plateau that becomes very apparent at the lowest temperatures. A three-time self-intermediate scattering function reveals that this plateau is connected with a homogeneous dynamics that, at longer times, becomes heterogeneous and finally exponential. These features are shared by, for example, a high-density system of hard spheres, which supports the universal character of the translational dynamics of liquids in their supercooled condition. As predicted by the idealized version of the mode-coupling theory (MCT), the diffusion coefficient dependence upon temperature is well described by a power law, with a critical temperature very close to that obtained by experimental measurements on orientational relaxation. This agreement might indicate a complete freezing of both rotational and translational intradomain dynamics. The time-temperature superposition principle also holds. The shape of the cage that surrounds a 6CB molecule has been reconstructed, and this analysis suggests a preferential side-by-side arrangement of molecules, which locally tend to align their long axes even in the isotropic phase
Modeling a Liquid Crystal Dynamics by Atomistic Simulation with an Ab Initio Derived Force Field
No full textAtomistic molecular dynamics (MD) simulations of 4-n-pentyl 4'-cyano-biphenyl (5CB) have been performed, adopting a specific ab initio derived force field.(1) Two state points in the nematic phase and three in the isotropic phase, as determined in a previous work,(2) have been considered. At each state point, at least 10 ns have been produced, allowing us to accurately calculate single-molecule properties. In the isotropic phase, the values of the translational diffusion coefficient, and even more so the activation energy for the process, agree well with experimental data. Qualitatively, also the dynamic anisotropy of the nematic phase is correctly accounted for. Rotational diffusion coefficients, which describe spinning and tumbling motions, fall well within the range of experimental values. The reorientational dynamics of our model 5CB covers diverse time regimes. The longest one is strongly temperature dependent and characterized by a relaxation time in accord with experimental dielectric relaxation data. Shear viscosity and Landau-de Gennes relaxation times, typically collective variables, reproduce the experimental results very well in the isotropic phase. In the nematic phase, despite a large statistical uncertainty due to the extremely slow relaxation of the correlation functions involved, our simulation yields the correct relative order of the three experimental Miesowicz viscosities
How the odd-even effects on the inter-molecular potentials propagate to the order parameter in the 4-cyano-4 ' n-alkylbiphenyl series
No full textWe present a study on the dimers of the 4-cyano, 4'n-alkyl biphenyl (nCB) aimed at capturing information connected to the measured odd-even effects on the mesophase properties.
The interaction energy of dimers of the 4-cyano, 4'n-alkyl biphenyl (nCB) is computed by a model atomistic potential. The latter has been derived for the 5CB molecule by ab initio calculations that include correlation energy, with an approach that has already provided accurate results. The analyzed intermolecular energy profiles for selected parallel and antiparallel conformations along the nCB series (n = 5,6,7,8), present some features that might be at the root of the odd-even effect of this class of compounds. Preliminary molecular dynamics results yield a stable nematic phase for each homologue, whose computed order parameter show an evident odd even alternation, in good agreement with the experimental findings
Organic Functionalization and Optimal Coverage of a Silicon(111) Surface in Solvent: A Computational Study
No full textComputer Simulation of Solid and Liquid Benzene with an Atomistic Interaction Potential Derived from ab Initio Calculations
No full textMolecular dynamics atomistic simulations of solid and liquid benzene have been performed, employing a model intermolecular potential derived from quantum mechanical calculations. The ab initio database includes approximately 200 geometries of the benzene dimer with interaction energies computed at the MP2 level of theory. The accuracy of the modeled force field results is satisfactory. The thermodynamic and structural properties, calculated in the condensed phases, are compared with experimental data and previous simulation results. Single particle and collective dynamical properties are also investigated through the calculation of translational and rotational diffusion coefficients, reorientational dynamics, and viscosities. The agreement of these data with experimental measurements confirms the reliability of the proposed force field
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