Grant D. Smith

A quantum chemistry study of benzene dimer

We have performed a detailed quantum chemistry study of the gas‐phase benzene dimer. Large atomic orbital basis sets with multiple polarization functions were used. The effects of basis set size, electron correlation, and basis set superposition error were investigated for the low‐energy planar sandwich (D6h and C6v), parallel displaced (C2h), and T‐shaped (C2v) dimer structures. Our studies indicate that the C2h‐symmetry parallel displaced geometry is the lowest‐energy structure for the benzene dimer. The T‐shaped structure was found to be a low‐energy saddle point for interconversion between parallel displaced structures, while the planar sandwich structure was found to be a saddle point on a higher‐energy interconversion path between parallel displaced structures. Detailed analysis of the low‐energy (T‐shaped saddle point) path revealed the presence of a shallow minimum corresponding to a tilt angle between phenyl ring planes of about 45°. Much of the behavior of the benzene dimer observed through mole...

Dynamics of poly(oxyethylene) melts: Comparison of 13C nuclear magnetic resonance spin-lattice relaxation and dielectric relaxation as determined from simulations and experiments

Molecular dynamics simulations of poly(oxyethylene) (POE) melts have been performed using a previously derived quantum-chemistry-based force field. Local chain dynamics for POE melts in the form of 13C nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) and NOE values and dielectric relaxation behavior have been determined from molecular dynamics simulations and compared with experimental measurements. 13C NMR T1 and NOE values for the methyl, α, and interior carbons from molecular dynamics simulations are in good agreement with experimental values over a wide range of temperatures. Similar agreement is seen for the dielectric relaxation strength and maximum dielectric loss frequency as a function of temperature. Non-Arrhenius behavior is seen for the correlation times of the P2CH orientational autocorrelation function (OACF), the molecular dipole moment OACF, and the torsional autocorrelation function (ACF). Very close correspondence between the torsional ACF and the molecular dipole mome...

Polymer force fields from ab initio studies of small model molecules: can we achieve chemical accuracy?

Abstract We endeavor to demonstrate, using poly(ethylene oxide) (PEO) as a model system, that atomistic force fields based upon high-level quantum chemistry calculations on small model molecules can accurately reproduce static and dynamic properties of polymer melts and solutions. The validity of the force field is demonstrated through (1) comparison of predicted properties of the small molecules obtained from quantum chemistry with experiment, (2) comparison of properties predicted by the force field for larger molecules with those determined from quantum chemistry and (3) comparison of static and dynamic properties obtained from molecular dynamics simulations of PEO melts and PEO/LiI solutions using the quantum chemistry based force field with experimental neutron scattering and NMR measurements on these systems.