Perfluoroalkanes:  Conformational Analysis and Liquid-State Properties from ab Initio and Monte Carlo Calculations

Classical OPLS-AA force-field parameters are developed for perfluoroalkanes primarily by fitting to conformational profiles from gas-phase ab initio calculations (LMP2/cc-pVTZ(-f)//HF/6-31G*) and to experimental data for pure liquids. The ab initio C−C−C−C profile of n-C4F10 (perfluorobutane) is similar to those from prior high-level calculations and indicates the presence of gauche (g) and ortho (o) minima and of anti (a) minima slightly offset from 180°. Ab initio torsional profiles for n-C5F12 (perfluoropentane) and (CF3)2CFCF2CF3 (perfluoro-2-methylbutane) also show three sets of energy minima. Special OPLS-AA torsional parameters for these three molecules closely match ab initio and experimental geometries, conformational energies (ΔEmin), and conformational energy barriers. These specialized force fields were merged to provide a generalized force field for linear, branched, and cyclic perfluoroalkanes. The resultant parameters yield key ΔEmin values within 0.6 kcal/mol of the ab initio results for the three test compounds but more poorly represent the energy barriers. The parametrization also included reproduction of experimental liquid properties of these compounds, CF4 (perfluoromethane) and c-C5F10 (perfluorocyclopentane) via Monte Carlo (MC) simulations. MC simulations of six additional molecules were also performed in order to test the transferability of the force field.