Torsion Energy Profiles and Force Fields Derived from Ab Initio Calculations for Simulations of Hydrocarbon−Fluorocarbon Diblocks and Perfluoroalkylbromides

Force fields with explicit atoms are proposed for the molecular simulation of two families of substituted perfluorocarbons: hydrocarbon−perfluorocarbon diblocks, or partially fluorinated alkanes, and 1-bromoperfluoroalkanes, or perfluoroalkylbromides. Both types of molecules are of interest for formulations of synthetic in vivo gas carriers, or blood substitutes, and for drug-delivery systems. Some semifluorinated alkanes have amphiphile character between hydrogenated and fluorinated liquid phases. The present molecular models are based on the OPLS all-atom force field, published in 2001 for perfluoroalkanes, completed with potential energy functions for the torsion around chemical bonds near the junction between the hydrocarbon and fluorocarbon blocks and close to the bromine atom, respectively. The dihedral terms have been derived from ab initio calculations on series of molecules of both families containing between two and five carbon atoms. Whenever possible, comparisons were made with experimental barriers to internal rotation. An analysis of the atomic charges was also performed, but no modification was introduced in the specification of the original force fields for hydrocarbons and fluorocarbons. Force-field parameters for the bromine atom were developed from quantum chemical and molecular dynamics studies on bromotrifluoromethane. As a test of the proposed models, a molecular dynamics simulation was used to calculate the properties of perfluorooctylbromide (PFOB) and perfluorooctylethane (PFOE) and to predict the solubility of oxygen and carbon dioxide in these liquids. The results of the predictions agree well with experiment.