Polysiloxanes: Ab initio force field and structural, conformational and thermophysical properties

Various levels of ab initio calculation have been performed to determine the optimum strategy for parameterization of the valence parameters of a CFF-type force field for siloxanes and polysiloxanes. Electrostatic nonbond parameters have been determined using scaled electrostatic potential (ESP) charges adjusted for known systematic differences between ab initio and experimental data, while van der Waals nonbond parameters have been determined using a classical approach involving fitting to experimental liquid density and cohesive energy density data measured at atmospheric pressure and a single temperature for a set of four small molecules. Simulations have been performed on molecular crystals, liquids and isolated molecules, yielding results which agree favorably with available experimental data. Properties calculated include unit cell parameters and crystal densities, liquid densities from 303-473 K and 0-1800 bar, dependence of oligomer density and solubility parameters on chain length and temperature, gas-phase geometries and vibrational frequencies, and gas and liquid-phase conformational behavior. (C) 1997 Elsevier Science B.V.