Gemma Sesé

Molecular dynamics study of Na+ and Cl− in methanol

Molecular dynamics simulations of solutions made up by one ion in methanol have been performed. The ions under study have been Na+ and Cl−. Structural and dynamical data as well as the dynamics of solvation have been analyzed. Both translational and reorientational motions of solvent molecules have been studied. An analysis of the solvent response to instantaneous changes of the electrical distribution of the solute in the linear response approximation has been undertaken. Special attention has been paid to differences between solvent molecules in the first shell and in the bulk, which happen to be more important in the Na+ shell. The influence of the ionic mass on the solvent properties has also been studied.

On the hydrogen bonding effects in liquid methanol: a molecular dynamics simulation study

Abstract The influence of hydrogen bonding on the microscopic properties of liquid methanol is investigated by molecular dynamics simulation. Both structural and dynamical properties of methanol are compared with those of an ideal methanol-like system whose molecules have the same dipole moment as methanol but lack sites for hydrogen bonding. The influence of the molecular dipole moment is also analysed by considering molecules without any electric charge. It is observed that the absence of hydrogen bonding produces dramatic changes in the properties of methanol whereas the effects of the molecular polarity are smaller.

Molecular dynamics studies of supercooled ethanol

Molecular dynamics simulations have been performed to get some insight into the dynamical properties of supercooled ethanol. The temperature dependence of translational and reorientational dynamics of the system has been analyzed. For all the correlators under study, a two-step relaxation behavior has been encountered. For short times, there is an initial Gaussian decay. For very long times, the relaxation shows a nonexponential dependence. For intermediate times and for temperatures close to a critical temperature, a nondiffusive regime appears (β relaxation). The predictions of the ideal version of the mode-coupling theory have been tested and a reasonable agreement has been found for the dynamics of the molecular centers-of-mass.

Rotational dynamics of a dipolar supercooled liquid

We study the rotational dynamics of a supercooled molecular liquid by means of molecular dynamics simulations. The system under investigation is composed of rigid diatomic molecules with an associate dipole moment. At room temperature, orientational correlations decrease rapidly with increasing distances. Upon cooling, angles between dipole moments of molecules within the first coordination shell decrease. As for the dynamical properties, rotational diffusion coefficients decrease with temperature at a smaller rate than translational diffusion coefficients do, and the critical temperature associated with the former is lower than the one corresponding to their translational counterparts. Translation and rotation about an inertial axis are uncorrelated, whereas some coupling between translation and dipole reorientation is obtained.

Study of spatial correlations in a supercooled molecular system

Spatial heterogeneities have been investigated in a supercooled system composed of diatomic molecules with an associated dipole moment by using the molecular dynamics simulation technique. Pair distribution functions of molecules with different mobilities have been evaluated, and it has been found that molecules belonging to the same dynamic domain are spatially correlated. Molecules with extremely large mobilities form larger clusters than those resulting from random statistics. These clusters are stringlike shaped. The mean cluster size displays a maximum at times between the ballistic and the diffusive regime, approximately at the end of the beta-relaxation zone. The value of this maximum increases upon cooling the system. An analogous profile has been observed for the characteristic cluster length when plotted against time. Agreement with Adam-Gibbs predictions has been encountered when considering these clusters as the basic dynamic units of the theory. For the extremely slow molecules, a cluster distribution has also been encountered. These clusters are smaller than the ones composed by fast molecules; they do not have a quasilinear geometry and no maximum is observed for their mean cluster size.

On the analysis of conformational dynamics in polymers with several rotational isomers

The ability of different correlation functions to shed some light onto the conformational dynamics of an amorphous polymer has been analyzed. The study has been performed on a polyethylene model polymer, which has been simulated at decreasing temperatures towards its glass transition, via the molecular dynamics technique. Three rotational isomers are allowed by the considered torsional potential. The correlation times associated with the evaluated transition rates have shown to be Arrhenius in nature, with activation energies resulting basically from internal rotation barriers. Overall torsional autocorrelation functions have been calculated. We have observed that they are dominated by slow events. Alternatively, a set of torsional autocorrelation functions associated with every isomeric state has been evaluated. Stretched exponential fits lead to correlation times that display Vogel-Fulcher temperature dependence.

Microscopic dynamics of supercooled low weight alcohols

Dynamical properties of low weight alcohols have been analyzed both in the liquid and the supercooled states. Realistic interatomic potential models for methanol and ethanol have been used. The influence of temperature on the hydrogen-bonded structure has been undertaken. Remarkable similarities have been obtained in both systems. Velocity autocorrelation functions have been evaluated for molecules participating in zero, one, and two hydrogen bonds at a wide range of temperatures. A backscattering area preceded by a shoulder has been identified as a signature of this function when evaluated for the subset of molecules that participate in two hydrogen bonds. Memory functions have also been evaluated. Their initial decay depends only slightly upon temperature. A more marked temperature dependence is observed for the nonassociated molecules. For them, reasonable agreement with the mode-coupling approach predictions has been encountered.

Influence of microscopic interactions on the spectra of polyacetylene

The influence of interatomic forces on the spectra of polyacetylene has been analyzed in detail by means of molecular dynamics simulations. Several simulations have been undertaken in which different terms have been added to the force field. Stretching, bending, torsional, and nonbonded forces have been subsequently incorporated in the simulations, in order to point out the specific spectral effects of every interaction. It has been obtained that the influence of all interatomic forces is qualitatively the same in both the trans and cis systems except for the torsions involving hydrogen atoms, which are relevant for the spectra parallel to the chain axis in the cis system, but not in the trans. Stretching and bending forces have a strong influence on the spectra parallel to the chain axis. The interchain forces do not significantly affect any of the spectra. Moreover, the results obtained in our simulations have been used to assign the experimental frequencies of polyacetylene to particular microscopic in...

Free energy and entropy of a dipolar liquid by computer simulations

Thermodynamic properties for a system composed of dipolar molecules are computed. Free energy is evaluated by means of the thermodynamic integration technique, and it is also estimated by using a perturbation theory approach, in which every molecule is modeled as a hard sphere within a square well, with an electric dipole at its center. The hard sphere diameter, the range and depth of the well, and the dipole moment have been calculated from properties easily obtained in molecular dynamics simulations. Connection between entropy and dynamical properties is explored in the liquid and supercooled states by using instantaneous normal mode calculations. A model is proposed in order to analyze translation and rotation contributions to entropy separately. Both contributions decrease upon cooling, and a logarithmic correlation between excess entropy associated with translation and the corresponding proportion of imaginary frequency modes is encountered. Rosenfeld scaling law between reduced diffusion and excess entropy is tested, and the origin of its failure at low temperatures is investigated.

On the Relationship between Intramolecular Dynamics and the Density of States

The relationship between the intramolecular dynamics and the spectra has been analyzed by means of the molecular dynamics technique. Time autocorrelation functions of bond lengths, bending angles and torsional angles have been evaluated in a crystalline trans-polyacetylene system. The Fourier transforms of such functions have been compared with the densities of states obtained both for carbon and hydrogen atoms. This comparison is aimed at investigating the microscopic origin of the peaks which appear in the densities of states. This approach can be used in the analysis of the spectroscopic data of any molecular or polymeric system.