Neeraj Rai

Transferable potentials for phase equilibria. 10. Explicit-hydrogen description of substituted benzenes and polycyclic aromatic compounds.

The explicit-hydrogen version of the transferable potentials for phase equilibria (TraPPE-EH) force field is extended to various substituted benzenes through the parametrization of the exocyclic groups -F, -Cl, -Br, -C≡N, and -OH and to polycyclic aromatic hydrocarbons through the parametrization of the aromatic linker carbon atom for multiple rings. The linker carbon together with the TraPPE-EH parameters for aromatic heterocycles constitutes a force field for fused-ring heterocycles. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to compute vapor-liquid coexistence curves for fluorobenzene; chlorobenzene; bromobenzene; di-, tri-, and hexachlorobenzene isomers; 2-chlorofuran; 2-chlorothiophene; benzonitrile; phenol; dihydroxybenzene isomers; 1,4-benzoquinone; naphthalene; naphthalene-2-carbonitrile; naphthalen-2-ol; quinoline; benzo[b]thiophene; benzo[c]thiophene; benzoxazole; benzisoxazole; benzimidazole; benzothiazole; indole; isoindole; indazole; purine; anthracene; and phenanthrene. The agreement with the limited experimental data is very satisfactory, with saturated liquid densities and vapor pressures reproduced to within 1.5% and 15%, respectively. The mean unsigned percentage errors in the normal boiling points, critical temperatures, and critical densities are 0.9%, 1.2%, and 1.4%, respectively. Additional simulations were carried out for binary systems of benzene/benzonitrile, benzene/phenol, and naphthalene/methanol to illustrate the transferability of the developed potentials to binary systems containing compounds of different polarity and hydrogen-bonding ability. A detailed analysis of the liquid-phase structures is provided for selected neat systems and binary mixtures.

Critical behaviour and vapour-liquid coexistence of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids via Monte Carlo simulations

Atomistic Monte Carlo simulations are used to compute vapour-liquid coexistence properties of a homologous series of [C(n)mim][NTf2] ionic liquids, with n = 1, 2, 4, 6. Estimates of the critical temperatures range from 1190 K to 1257 K, with longer cation alkyl chains serving to lower the critical temperature. Other quantities such as critical density, critical pressure, normal boiling point, and accentric factor are determined from the simulations. Vapour pressure curves and the temperature dependence of the enthalpy of vapourisation are computed and found to have a weak dependence on the length of the cation alkyl chain. The ions in the vapour phase are predominately in single ion pairs, although a significant number of ions are found in neutral clusters of larger sizes as temperature is increased. It is found that previous estimates of the critical point obtained from extrapolating experimental surface tension data agree reasonably well with the predictions obtained here, but group contribution methods and primitive models of ionic liquids do not capture many of the trends observed in the present study

Monte Carlo simulations of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB): Pressure and temperature effects for the solid phase and vapor-liquid phase equilibria

The transferable potentials for phase equilibria (TraPPE) force field was extended to nitro and amino substituents for aromatic rings via parametrization to the vapor-liquid coexistence curves of nitrobenzene and aniline, respectively. These groups were then transferred to model 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Without any further parametrization to solid state data, the TraPPE force field is able to predict TATBs unit cell lengths and angles at 295 K with mean unsigned percentage errors of 0.3% and 1.8% and the specific density within 0.5%. These predictions are comparable in accuracy to the GRBF model [Gee et al., J. Chem. Phys. 120, 7059 (2004)] that was parametrized directly to TATBs solid state properties. Both force fields are able to reproduce the pressure dependence of TATBs unit cell volume, but they underestimate its thermal expansion. Due to its energetic nature and unusually large cohesive energy, TATB is not chemically stable at temperature in its liquid range. Gibbs ensemble simulations allow one to determine TATBs vapor-liquid coexistence curve at elevated temperatures and the predicted critical temperature and density for the TraPPE and GRBF model are 937+/-8 and 1034+/-8 K, and 0.52+/-0.02 and 0.50+/-0.02 gcm(3), respectively.

Force Field Development for Actinyl Ions via Quantum Mechanical Calculations: An Approach to Account for Many Body Solvation Effects

Advances in computational algorithms and methodologies make it possible to use highly accurate quantum mechanical calculations to develop force fields (pair-wise additive intermolecular potentials) for condensed phase simulations. Despite these advances, this approach faces numerous hurdles for the case of actinyl ions, AcO2(n+) (high-oxidation-state actinide dioxo cations), mainly due to the complex electronic structure resulting from an interplay of s, p, d, and f valence orbitals. Traditional methods use a pair of molecules (“dimer”) to generate a potential energy surface (PES) for force field parametrization based on the assumption that many body polarization effects are negligible. We show that this is a poor approximation for aqueous phase uranyl ions and present an alternative approach for the development of actinyl ion force fields that includes important many body solvation effects. Force fields are developed for the UO2(2+) ion with the SPC/Fw, TIP3P, TIP4P, and TIP5P water models and are validated by carrying out detailed molecular simulations on the uranyl aqua ion, one of the most characterized actinide systems. It is shown that the force fields faithfully reproduce available experimental structural data and hydration free energies. Failure to account for solvation effects when generating PES leads to overbinding between UO2(2+) and water, resulting in incorrect hydration free energies and coordination numbers. A detailed analysis of arrangement of water molecules in the first and second solvation shell of UO2(2+) is presented. The use of a simple functional form involving the sum of Lennard-Jones + Coulomb potentials makes the new force field compatible with a large number of available molecular simulation engines and common force fields.

Application of the TraPPE Force Field for Predicting the Hildebrand Solubility Parameters of Organic Solvents and Monomer Units.

Configurational-bias Monte Carlo simulations in the isothermal-isobaric and Gibbs ensembles using the transferable potentials for phase equilibria (TraPPE) force field were carried out to compute the liquid densities, the Hildebrand solubility parameters, and the heats of vaporization for a set of 32 organic molecules with different functional groups at a temperature of 298.15 K. In addition, the heats of vaporization were determined at the normal boiling points of these compounds. Comparison to experimental data demonstrates that the TraPPE force field is significantly more accurate than predictions obtained from molecular dynamics simulations with the Dreiding force field [Belmares et al. J. Comput. Chem. 2004, 25, 1814] and an equation of state approach [Stefanis et al. Fluid Phase Equil. 2006, 240, 144]. For the TraPPE force field, the mean unsigned percent errors for liquid density, the Hildebrand solubility parameter, and the heat of vaporization at 298.15 K are 1.3, 3.3, and 4.5%, respectively.

Cassandra: An open source Monte Carlo package for molecular simulation

Cassandra is an open source atomistic Monte Carlo software package that is effective in simulating the thermodynamic properties of fluids and solids. The different features and algorithms used in Cassandra are described, along with implementation details and theoretical underpinnings to various methods used. Benchmark and example calculations are shown, and information on how users can obtain the package and contribute to it are provided.

Microbial cell disruption for improving lipid recovery using pressurized CO2: Role of CO2 solubility in cell suspension, sugar broth, and spent media

The study of in situ gas explosion to lyse the triglyceride‐rich cells involves the solubilization of gas (e.g., carbon dioxide, CO2) in lipid‐rich cells under pressure followed by a rapid decompression, which allows the gas inside the cell to rapidly expand and rupture the cell from inside out. The aim of this study was to perform the cell disruption using pressurized CO2 as well as to determine the solubility of CO2 in Rhodotorula glutinis cell suspension, sugar broth media, and spent media. Cell disruption of R. glutinis was performed at two pressures of 2,000 and 3,500 kPa, respectively, at 295.2 K, and it was found from both scanning electron microscopy (SEM) and plate count that a substantial amount of R. glutinis was disrupted due to the pressurized CO2. We also found a considerable portion of lipid present in the aqueous phase after the disruption at P = 3,500 kPa compared to control (no pressure) and P = 2,000 kPa, which implied that more intracellular lipid was released due to the pressurized CO2. Solubility of CO2 in R. glutinis cell suspension was found to be higher than the solubility of CO2 in both sugar broth media and spent media. Experimental solubility was correlated using the extended Henrys law, which showed a good agreement with the experimental data. Enthalpy and entropy of dissolution of CO2 were found to be −14.22 kJ mol−1 and 48.10 kJ mol−1 K−1, 9.64 kJ mol−1 and 32.52 kJ mol−1 K−1, and 7.50 kJ mol−1 and 25.22 kJ mol−1 K−1 in R. glutinis, spent media, and sugar broth media, respectively.

Composition-directed FeXMo2−XP bimetallic catalysts for hydrodeoxygenation reactions

The development of task-specific bimetallic phosphide catalysts can be accomplished by exploiting the electronic and bi-functional effects of multiple metal combinations, thus providing materials with tunable catalytic properties. Here, we present the modulation of metal compositions (i.e., Fe and Mo) in the synthesis of FeXMo2−XP (0.88 ≤ X ≤ 1.55), leading to a series of iso-structural, orthorhombic FeXMo2−XP catalysts via reduction at 750 °C. Hydrodeoxygenation of phenol was selected as a probe reaction to showcase the effect of metal composition on the catalytic performance. In particular, catalysts with Fe compositions between 0.99 and 1.14 (i.e., Fe0.99Mo1.01P and Fe1.14Mo0.86P) exhibited high selectivities to C–O bond cleavage of phenol with H2 to form benzene. The catalysts with the highest selectivities to C–O scission also exhibited the highest acidity as determined from NH3 temperature programmed desorption experiments. Density functional theory (DFT) calculations indicate the high Lewis acidity for the ∼1 : 1 Fe : Mo compositions resulted from a greater charge separation between metallic species and P species. These compositions led to greater selectivities to benzene due to desired coordination environment of the phenol on catalytic surface, as evidenced by both DFT calculations and a time on stream study using a benzonitrile poison. Enhanced TOFs were also observed with catalysts exhibiting greater Lewis acid character, which reduce the activation energy required to cleave the C–O bond of phenol, as evidenced by DFT calculations. This structure–property study highlights the effects of metal composition in bimetallic phosphides to enhance the activity and selectivity for C–O bond cleavage reactions.

Vapor Liquid Equilibria of Hydrofluorocarbons Using Dispersion-Corrected and Nonlocal Density Functionals

Recent developments in dispersion corrected and nonlocal density functionals are aimed at accurately capturing dispersion interactions, a key shortcoming of local and semilocal approximations of density functional theory. These functionals have shown significant promise for dimers and small clusters of molecules as well as crystalline materials. However, their efficacy for predicting vapor liquid equilibria is largely unexplored. In this work, we examine the accuracy of dispersion-corrected and nonlocal van der Waals functionals by computing the vapor liquid coexistence curves (VLCCs) of hydrofluoromethanes. Our results indicate that the PBE-D3 functional performs significantly better in predicting saturated liquid densities than the rVV10 functional. With the PBE-D3 functional, we also find that as the number of fluorine atoms increase in the molecule, the accuracy of saturated liquid density prediction improves as well. All the functionals significantly underpredict the saturated vapor densities, which also result in an underprediction of saturated vapor pressure of all compounds. Despite the differences in the bulk liquid densities, the local microstructures of the liquid CFH3 and CF2H2 are relatively insensitive to the density functional employed. For CF3H, however, rVV10 predicts slightly more structured liquid than the PBE-D3 functional.

Performance of density functionals for modeling vapor liquid equilibria of CO 2 and SO 2 .

Vapor liquid equilibria (VLE) and condensed phase properties of carbon dioxide and sulfur dioxide are calculated using first principles Monte Carlo (FPMC) simulations to assess the performance of several density functionals, notably PBE‐D3, BLYP‐D3, PBE0‐D3, M062X‐D3, and rVV10. GGA functionals were used to compute complete vapor liquid coexistence curves (VLCCs) to estimate critical properties, while the hybrid and nonlocal van der Waals functionals were used only for computing density at a single state point due to the high computational cost. Our results show that the BLYP‐D3 functional performs well in predicting VLE properties for both molecules when compared with other functionals. In the liquid phase, pair correlation functions reveal that there is not a significant difference in the location of the peak for the first solvation shell while the peak heights are different for different functionals. Overall, the BLYP‐D3 functional is a good choice for modeling VLE of acidic gases with significant environmental implications such as CO2 and SO2.