A methodology for consistent development of the quantum chemistry-based force fields with and without many-body polarizable terms is described. Adequate levels of theory and basis sets for determination of the relative conformational energetics, repulsion and dispersion nonbonded parameters, dipole moments, and molecular polarizability are established. Good agreement between the quantum chemistry-based repulsion and dispersion parameters and those previously obtained by fitting crystal structures of poly(oxymethylene) is obtained. Hartree−Fock (HF) calculations with augmented correlation consistent basis sets are adequate for the determination of repulsion parameters, whereas a double extrapolation to improved treatments of electron correlations and larger basis sets is needed to obtain dispersion parameters. Partial charges are obtained by fitting to the electrostatic grid of model compounds. Atomic polarizabilities are fitted to reproduce polarization energy around the model compounds. The density functional B3LYP yields relative conformational energies in better agreement with Møller−Plesset second-order (MP2) perturbation theory than the HF energies; however, the accuracy of the B3LYP density functional was insufficient to provide reliable relative conformational energetics. A molecular mechanics study of the conformational energetics of 1,2-dimethoxyethane indicated that many-body polarizable interactions have little impact on the relative conformational energies.
[Show abstract][Hide abstract] ABSTRACT: Quantum chemistry-based force fields with many-body polarizable interactions and two-body effective polarizability parameters have been developed for the interaction of poly(ethylene oxide) (PEO) with Li+ and BF4-. The Li+/ether repulsion parameters were found to be transferable to another polyether, such as poly(methylene oxide), that is interacting with a Li+ cation. Molecular dynamics (MD) simulations have been performed for PEO (Mw = 2380)/LiBF4 for EO:Li = 15:1 at three temperatures: 363, 393, and 423 K. The Li+ environment was found to be in reasonable agreement with that measured for other lithium salts that have been doped in PEO. MD simulations employing the many-body (MB) polarizable force field predicted ion conductivity, self-diffusion coefficients, and the slowing of the PEO dynamics upon the addition of LiBF4 salt that were in good agreement with experiments. MD simulations employing the two-body (TB) force field yielded polymer and ion dynamics that were slower than those from the simulations employing the MB force field. Analysis of the Li+ cation diffusion mechanism revealed that the Li+ cations with significant motion along PEO chains have a much higher self-diffusion coefficient than do the Li+ cations that do not undergo a noticeable motion along PEO chains, which suggests that the Li+ motion along PEO makes an important contribution to the cation diffusion mechanism.
The Journal of Physical Chemistry B 07/2003; 107(28):6824-6837. DOI:10.1021/jp027539z · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Neutron quasielastic scattering experiments were carried out on aqueous solutions of 500 u molecular weight poly(ethylene glycol) dimethyl ether (PEG-DME) in deuterated water. The intermediate scattering functions extracted from the measured neutron scattering can be fitted by a combination of a fast second-order exponential decay (t < 2 ps) and a slower first-order exponential decay. The analysis of the momentum transfer (Q) dependence of the decay constant for the slower component shows that it has an approximately constant value at Q's less than 13 nm -1 and then increases linearly up to the highest momentum transfer (25 nm -1). Both the slope of the higher-Q linear region and the individual Q-dependent decay constants show a minimum in the PEG-DME weight fraction range of 0.6-0.9. Further analysis of the neutron-scattering data to check the effect of multiple scattering in the sample shows that only the shape of the fast decay (t < 2 ps) is affected by this correction. A direct quantitative comparison is made between experiment and molecular dynamics simulations. Fourier transforming the experimental data from the frequency domain into the time domain to yield the intermediate scattering function allows for a quantitative comparison with the equivalent function calculated from the simulations. Furthermore, a Monte Carlo simulation of the experiment based on simulation results is used to account for the effect of multiple scattering quantitatively, which represents a novel approach to dealing with the complications arising from multiple scattering. This correction is significant, resulting in excellent agreement between experiment and the simulations. Both simulation and experiment give rise to a maximum in the relaxation time for PEG-DME proton motion in the PEG-DME weight fraction range of 0.6-0.9. On the basis of the simulations, this maximum arises from competition between the slowing down of the torsional transitions due to hydrogen bonding between the water and the PEG-DME ether oxygens, and the addition of a sufficient quantity of water results in an increasing fraction of large water clusters and more mobile water (i.e., a low-viscosity solvent). The former dominates at low dilution, and the latter dominates at higher dilution, leading to the enhanced backbone motion of the PEG-DME and the observed maximum in the residence time of the PEG-DME protons as a function of composition.
The Journal of Physical Chemistry B 09/2003; 107(38). DOI:10.1021/jp035234u · 3.30 Impact Factor
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