Root-Mean-Square Dipole Moment and Neutron Scattering Function of 18-Crown-6 in Cyclohexane: Comparison of Three Potential Models

Molecular dynamics simulations have been performed in order to study the effect of a new charge density distribution for 18-crown-6, recently described in the literature [J. Mol. Struct. (THEOCHEM) 1994, 305, 2491, on different properties of the crown ether. Comparisons are made with results discussed in a previous paper where other potential models were employed. Remarkable shifts in the spectrum of conformations are observed, and for the first time an average dipole moment is calculated that is in good agreement with experiment. We have calculated neutron scattering cross sections and X-ray intensities for two potential models. Of these, only the neutron scattering cross section turned out to be sensitive to the differences in the structures resulting from these potentials

Perfluoropolyethers: Development of an All-Atom Force Field for Molecular Simulations and Validation with New Experimental Vapor Pressures and Liquid Densities

A force field for perfluoropolyethers (PFPEs) based on the general optimized potentials for liquid simulations all-atom (OPLS-AA) force field has been derived in conjunction with experiments and ab initio quantum mechanical calculations. Vapor pressures and densities of two liquid PFPEs, perfluorodiglyme (CF3−O−(CF2−CF2−O)2−CF3) and perfluorotriglyme (CF3−O−(CF2−CF2−O)3−CF3), have been measured experimentally to validate the force field and increase our understanding of the physical properties of PFPEs. Force field parameters build upon those for related molecules (e.g., ethers and perfluoroalkanes) in the OPLS-AA force field, with new parameters introduced for interactions specific to PFPEs. Molecular dynamics simulations using the new force field demonstrate excellent agreement with ab initio calculations at the RHF/6-31G* level for gas-phase torsional energies (<0.5 kcal mol−1 error) and molecular structures for several PFPEs, and also accurately reproduce experimentally determined densities (<0.02 g cm−3 error) and enthalpies of vaporization derived from experimental vapor pressures (<0.3 kcal mol−1). Additional comparisons between experiment and simulation show that polyethers demonstrate a significant decrease in enthalpy of vaporization upon fluorination unlike related molecules (e.g., alkanes and alcohols). Simulation suggests this phenomenon is a result of reduced cohesion in liquid PFPEs due to a reduction in localized associations between backbone oxygen atoms and neighboring molecules

Doctor of Philosophy

dissertationA new quantum chemistry-based, atomic point polarizable dipole potential was developed for molecular dynamic (MD) simulations of poly (ethylene oxide) (PEO) and poly (propylene oxide) (PPO) aqueous solutions employing a modified version of a single water molecule with four interaction sites and Drude polarizability (SWM4-DP). A twoextended charge ether model has been chosen as best describing electrostatic potential of DME. Ether/water interactions were parameterized to reproduce the binding energy of water with 1,2-dimethoxyethane (DME) that was determined from high-level quantum chemistry calculations. The DME/water nonbonded parameters were found to be transferrable to 1,2-dimethoxypropane (DMP). An accuracy of the developed force field was justified by comparing thermodynamics properties obtained from molecular dynamics simulations with experimental data including free energy, enthalpy, and entropy of DME solvation. Free energy of DME solvation in water was obtained employing a new interface transit method (ITM) followed by calculations using perturbation theory. Simulations of DME/water solutions at room temperature using the new polarizable force field yielded enthalpy of solvation in a good agreement with experiment. Simulations of PEO/water and PPO/water solutions improved ability of the new force field to capture, at least qualitatively, low critical solution temperature (LCST) behavior in these solutions. The predicted miscibility of PEO and water as a function of temperature was found to be strongly correlated with the predicted free energy of solvation of DME in water for the various force fields investigated. Intermolecular pair correlations are employed to analyze phase behavior of nonionic polymers in aqueous solution

Dielectric Constant and Structure of Liquid 18-Crown-6 Calculated from Molecular Dynamics Simulations

The results are presented for molecular dynamics simulations of liquid 18-crown-6 using different potential models. The results offer the possibility of investigating the influence of the flexibility of the dihedral angles and the effects of the united atom approach. The radial distribution functions and the correlation between the molecular separations and relative orientations are found to be rather insensitive to the specific potential model used. The relation between orientation correlations and dipole−dipole correlations on the other hand is found to be very sensitive to the flexibility of the molecule. The contributions of the dipole−dipole correlations to the dielectric constant are found to be small compared to those of the molecular dipoles. The calculated dielectric constants are very much in disagreement with the experimental one. It is believed that adding electronic polarization terms to the potential models will very much enhance the contributions of the dipole−dipole correlations to the dielectric constant without necessarily changing the molecular and structural properties