The molecular dynamics computer simulation method has been used to study sulfuric and methanesulfonic acids. Calculations have been carried out between 200 K and 400 K using reliable force fields. Thermodynamic properties, such as the density, the heat of vaporization and the melting temperature, have been computed. Moreover, structural and dynamical quantities, such as the radial distribution functions, the shear viscosity and the diffusion coefficients, have also been calculated. The results display a noticeable good agreement with the available experimental data. A hydrogen bond analysis has also been performed, which shows, on one hand, that sulfuric acid has a hydrogen bond network which resembles the one of water; and, on the other hand, that methanesulfonic acid has a hydrogen bond structure which, in some details, recalls the one of methanol, but with a more important presence of single bonds and, to a lesser extent, of branching. Finally, the dynamics of the formation and rupture of hydrogen bonds has also been analyzed. To this end, the interrupted or slow hydrogen bonding lifetimes have been calculated using two different procedures. Our findings suggest that the sulfuric acid hydrogen bond network is more labile than the methanesulfonic acid one.
We present a detailed study on the liquid structure of hexachloroethane (C2Cl6) within the first shell using the proper Euler-angles convention. Molecular dynamics is used and the van der Waals picture of a liquid is taken as the main phenomenology. Particular attention is paid to the first neighbor structure studying the distance of the center of mass (CM) and relative orientation arrangements. The distance dependence of the orientation and location of closer neighbors is studied up to the fourth neighbor (the maximum of first peak of the CM radial distribution function). An unusual arrangement in the first neighbor is seen.
The role of the torsional potential on the glass transition of a linear polymer has been investigated by means of Molecular Dynamics simulations. The study has been performed on a polyethylene model polymer, which has been simulated at decreasing temperatures towards its glass transition. The united atom description has been adopted, with every methyl and methylene group along the chains being regarded as an individual interacting site. Three rotational isomers are allowed by the reference torsional potential, which has been parameterized so that the two torsional barriers can be tuned separately. Then, two sets of torsional parameters have been considered. The first set is characterized by increasing values for the trans–gauche (t–g) barrier and by no gauche–gauche (g–g) barrier. In the second set, the t–g barrier is kept constant, and the g–g barrier takes increasing values. The zero potential situation has also been analyzed. The glass transition temperature (Tg) has been obtained by following the specific volume of the system as a function of temperature. It has been found that Tg increases with increasing values of the t–g barrier. Nevertheless, Tg becomes independent of the t–g barrier for very high barriers. As for the role of the g–g barrier, Tg is almost independent of it, provided that the t–g barrier already exists.
A series of molecular dynamics simulations have been performed to study the solvation shell of Na+ and Cl- ions in acetonitrile. Both structural and dynamic properties including reorientational and residence times, velocity autocorrelation functions, self-diffusion coefficients and spectral densities have been analysed. The present results are compared with those obtained in earlier studies of single ions in water and methanol.
Molecular dynamics simulations of one HCl molecule in liquid Ar at three different thermodynamic states have been carried out. The dynamic properties of both the solute molecule and solvent atoms are discussed. Results for Ar in the first solvation shell of HCl are compared with those for atoms in the bulk. The study includes radial distribution functions, residence times, velocity autocorrelation functions, spectral densities, self - diffusion coefficients, reorientational time correlation functions and infrared spectra.