Phase equilibrium in a quadrupolar hard sphere interaction site model of benzene

Coarse-grained modeling of the nucleation of polycyclic aromatic hydrocarbons into soot precursors

The aggregation and physical growth of polycyclic aromatic hydrocarbon (PAH) molecules was simulated using a coarse-grained (CG) approach based on the Paramonov-Yaliraki (PY) potential and a stochastic Monte Carlo framework, following earlier efforts in which the structure [Phys. Chem. Chem. Phys., 2016, 18, 13736] and equilibrium thermodynamics [Phys. Chem. Chem. Phys., 2017, 19, 1884] were investigated and critically compared to the predictions of all-atom models. Homomolecular and heteromolecular assemblies of pyrene, coronene, and circumcoronene were considered at various temperatures and compositions, and the distributions of aggregation products were characterized. Under the simulated conditions, and in agreement with earlier studies, the clusters are rather small and, in the case of pyrene-rich systems, only formed below 1000 K. The clusters obtained by spontaneous aggregation of isolated molecules are statistically analysed. For the selected sizes of tetramers and octamers, broad distributions of isomers are obtained with a clear entropic stabilization. In heteronuclear assemblies, our results suggest a minor spontaneous segregation towards pure and equi concentrations at variance with purely statistical expectations.

Dynamics and thermodynamics of the coronene octamer described by coarse-grained potentials

Coarse-grained models developed for polycyclic aromatic hydrocarbons based on the Paramonov–Yaliraki potential have been employed to investigate the finite temperature thermodynamics, out-of-equilibrium dynamics, energy landscapes, and rearrangement pathways of the coronene octamer.

Coarse-graining the structure of polycyclic aromatic hydrocarbons clusters

Clusters of polycyclic aromatic hydrocarbons are essential components of soot and may concentrate a significant fraction of carbon matter in the interstellar medium.

Fluid-solid equilibrium of carbon dioxide as obtained from computer simulations of several popular potential models: the role of the quadrupole

In this work the solid-fluid equilibrium for carbon dioxide (CO2) has been evaluated using Monte Carlo simulations. In particular the melting curve of the solid phase denoted as I, or dry ice, was computed for pressures up to 1000 MPa. Four different models, widely used in computer simulations of CO2 were considered in the calculations. All of them are rigid non-polarizable models consisting of three Lennard-Jones interaction sites located on the positions of the atoms of the molecule, plus three partial charges. It will be shown that although these models predict similar vapor-liquid equilibria their predictions for the fluid-solid equilibria are quite different. Thus the prediction of the entire phase diagram is a severe test for any potential model. It has been found that the Transferable Potentials for Phase Equilibria (TraPPE) model yields the best description of the triple point properties and melting curve of carbon dioxide. It is shown that the ability of a certain model to predict the melting curve of carbon dioxide is related to the value of the quadrupole moment of the model. Models with low quadrupole moment tend to yield melting temperatures too low, whereas the model with the highest quadrupole moment yields the best predictions. That reinforces the idea that not only is the quadrupole needed to provide a reasonable description of the properties in the fluid phase, but also it is absolutely necessary to describe the properties of the solid phase.

Computing the free energy of molecular solids by the Einstein molecule approach: ices XIII and XIV, hard-dumbbells and a patchy model of proteins

The recently proposed Einstein molecule approach is extended to compute the free energy of molecular solids. This method is a variant of the Einstein crystal method of Frenkel and Ladd [J. Chem. Phys. 81, 3188 (1984)]. In order to show its applicability, we have computed the free energy of a hard-dumbbell solid, of two recently discovered solid phases of water, namely, ice XIII and ice XIV, where the interactions between water molecules are described by the rigid nonpolarizable TIP4P/2005 model potential, and of several solid phases that are thermodynamically stable for an anisotropic patchy model with octahedral symmetry which mimics proteins. Our calculations show that both the Einstein crystal method and the Einstein molecule approach yield the same results within statistical uncertainty. In addition, we have studied in detail some subtle issues concerning the calculation of the free energy of molecular solids. First, for solids with noncubic symmetry, we have studied the effect of the shape of the simulation box on the free energy. Our results show that the equilibrium shape of the simulation box must be used to compute the free energy in order to avoid the appearance of artificial stress in the system that will result in an increase in the free energy. In complex solids, such as the solid phases of water, another difficulty is related to the choice of the reference structure. As in some cases there is no obvious orientation of the molecules; it is not clear how to generate the reference structure. Our results will show that, as long as the structure is not too far from the equilibrium structure, the calculated free energy is invariant to the reference structure used in the free energy calculations. Finally, the strong size dependence of the free energy of solids is also studied.

Transferable Potentials for Phase Equilibria. 9. Explicit Hydrogen Description of Benzene and Five-Membered and Six-Membered Heterocyclic Aromatic Compounds

The explicit hydrogen version of the transferable potentials for phase equilibria (TraPPE-EH) force field is extended to benzene, pyridine, pyrimidine, pyrazine, pyridazine, thiophene, furan, pyrrole, thiazole, oxazole, isoxazole, imidazole, and pyrazole. While the Lennard-Jones parameters for carbon, hydrogen (two types), nitrogen (two types), oxygen, and sulfur are transferable for all 13 compounds, the partial charges are specific for each compound. The benzene dimer energies for sandwich, T-shape, and parallel-displaced configurations obtained for the TraPPE-EH force field compare favorably with high-level electronic structure calculations. Gibbs ensemble Monte Carlo simulations were carried out to compute the single-component vapor-liquid equilibria for benzene, pyridine, three diazenes, and eight five-membered heterocycles. The agreement with experimental data is excellent with the liquid densities and vapor pressures reproduced within 1 and 5%, respectively. The critical temperatures and normal boiling points are predicted with mean deviations of 0.8 and 1.6%, respectively.

Study of the Thermal Diffusion Behavior of Alkane/Benzene Mixtures by Thermal Diffusion Forced Rayleigh Scattering Experiments and Lattice Model Calculations

In this work the thermal diffusion behavior of binary mixtures of linear alkanes (heptane, nonane, undecane, tridecane, pentadecane, heptadecane) in benzene has been investigated by thermal diffusion forced Rayleigh scattering (TDFRS) for a range of concentrations and temperatures. The Soret coefficient ST of the alkane was found to be negative for these n-alkane/benzene mixtures indicating that the alkanes are enriched in the warmer regions of the liquid mixtures. For the compositions investigated in this work, the magnitude of the Soret coefficient decreases with increasing chain length and increasing alkane content of the mixtures. The temperature dependence of the Soret coefficient depends on mixture composition and alkane chain length; the slope of ST versus temperature changes from positive to negative with increasing chain length at intermediate compositions. To study the influence of molecular architecture on the Soret effect, mixtures of branched alkanes (2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane, and 2,2,4-trimethylpentane) in benzene were also investigated. Our results for the Soret coefficients show that the tendency for the alkanes to move to the warmer regions of the fluid decreases with increasing degree of branching. The branching effect is so strong that for 2,2,4-trimethylpentane/benzene mixtures the Soret coefficient changes sign at high alkane content and that equimolar 2,2,3-trimethylbutane/benzene mixtures have positive Soret coefficients in the investigated temperature range. In order to investigate the effect of molecular interactions on thermal diffusion, we adapted a recently developed two-chamber lattice model to n-alkane/benzene mixtures. The model includes the effects of chain-length, compressibility, and orientation dependence of benzene-benzene interactions and yields good qualitative predictions for the Soret effect in n-alkane/benzene mixtures. For the branched isomers, we find some correlations between the moments of inertia of the molecules and the Soret coefficients. PACS numbers: 66.10.Cb, 61.25.Hq.

Vapor−Liquid and Vapor−Solid Phase Equilibria for United-Atom Benzene Models near Their Triple Points: The Importance of Quadrupolar Interactions

Gibbs ensemble Monte Carlo simulations were used to calculate the vapor-liquid and vapor-solid coexistence curves for benzene using two simple united-atom models. An extension of the Gibbs ensemble method that makes use of an elongated box containing a slab of the condensed phase with a vapor phase along one axis was employed for the simulations of the vapor-solid equilibria and the vapor-liquid equilibria at very low reduced temperatures. Configurational-bias and aggregation-volume-bias Monte Carlo techniques were applied to improve the sampling of particle transfers between the two simulation boxes and between the vapor and condensed-phase regions of the elongated box. An isotropic united-atom representation with six Lennard-Jones sites at the positions of the carbon atoms was used for both force fields, but one model contained three additional out-of-plane partial charge sites to explicitly represent benzene's quadrupolar interactions. Both models were fitted to reproduce the critical temperature and density of benzene and yield a fair representation of the vapor-liquid coexistence curve. In contrast, differences between the models are very large for the vapor-solid coexistence curve. In particular, the lack of explicit quadrupolar interactions for the 6-site model greatly reduces the energetic differences between liquid and solid phases, and this model yields a triple point temperature that is about a factor of 2 too low. In contrast, the 9-site model predicts a triple point of benzene at T = 253 +/- 6 K and p = 2.3 +/- 0.8 kPa in satisfactory agreement with the experimental data (T = 278.7 K and p = 4.785 kPa).

The impact of the multipolar distribution on chiral discrimination in racemates

This article explores the impact of the multipolar distribution on chiral discrimination in a series of racemic fluids. Discrimination is measured via the difference between the like-like (LL) and the like-unlike (LU) radial distributions in the liquid. We have found previously that the magnitude and orientation of the molecular dipole have a decisive impact on the short-ranged enantiomeric imbalance in racemates. Although quadrupolar and octupolar interactions decrease more rapidly with intermolecular separation, they can be significant at small separations, where enantiomeric imbalances occur. We have carefully selected a number of models in which we isolate the effects of the molecular quadrupole and octupole. We find that discrimination can be greatly enhanced by changes in the quadrupole moments. However, for octupole moments, changes in discrimination are small and some octupoles inhibit discrimination. We identify the quadrupole moment closest to the plane perpendicular to the direction of the molecular dipole as the moment that has the greatest favorable effect on chiral discrimination in racemates. In racemates where this moment is large, we have found differences of up to 40% between the LL and the LU radial distributions.

Modeling benzene with single-site potentials fromab initiocalculations: A step toward hybrid models of complex molecules

Extensive ab initio calculations at the MP2/6-31G* level have been carried out to sample the energy surface for the interactions of the benzene dimers. This database has been used to parameterize two anisotropic single-site models, meant to be used as building blocks in hybrid models of complex, liquid crystal forming molecules. A quadrupolar Gay-Berne (GBQIII) and an S-function (SF) Corner potentials have been obtained in this way. Their ability to reproduce, qualitatively at least, the phase diagram as well as energetic and structural properties of benzene has been tested with Monte Carlo simulations and compared with previous literature potentials, GBQI [S. Gupta et al., Mol. Phys. 65, 961 (1988)] and GBQII [T. R. Walsh, Mol. Phys. 100, 2867 (2002)]. It turned out that GBQI showed no melting transition in the temperature range explored (100-400 K), while GBQII underwent a phase transition from solid to gas, with no liquid phase. Conversely, both models parameterized on our database of ab initio interaction energies (GBQIII and SF) gave rise to a stable liquid phase. Melting has been observed between 100 and 150 K (GBQIII) and in the range 300-350 K (SF), i.e., substantially below and slightly above the experimental value at ambient pressure, 278 K. The description of the crystal structure of benzene at atmospheric pressure is also in better agreement with experimental data if the SF model is used, while positional correlations in the liquid are better described by the GBQIII potential. The S-function potential is also computationally more convenient. These results could be useful in the semirealistic modeling of more complex molecules.