SPECTROSCOPICALLY DETERMINED FORCE-FIELDS FOR MACROMOLECULES .1. N-ALKANE CHAINS

Detailed spectroscopic and dynamics studies of macromolecules require force fields that not only predict structures and thermodynamic quantities correctly but also reliably reproduce, i.e., to spectroscopic standards, vibrational frequencies. We have developed a methodology for transforming ab initio force fields, suitably scaled to experimentally assigned bands, into a molecular mechanics potential energy function that retains the original frequency as well as structure and energy predictions. With such inherent spectroscopic constraints, we obtain what we call a spectroscopically determined force field (SDFF). We have applied this approach to a set of 14 conformers of n-pentane and n-hexane, with the condition that the intrinsic geometry and force constant parameters be common to the set. We find that, with a systematically reduced parameter set, this SDFF gives very good agreement with experimental data. By virtue of the explicit dependence of its second derivatives on conformation, this potential should be transferable to conformers of other large molecules of this class and should provide more reliable molecular dynamics simulations than are obtained with spectroscopically less accurate potentials.