Derivation of fluorine and hydrogen atom parameters using liquid simulations

Simulations of periodic boxes of tetrafluoromethane and trifluoromethane were run to determine van der Waals parameters for fluorine and for hydrogen attached to a fluorine‐bearing carbon. The simulations of CF 4 were performed first to determine the optimal van der Waals radius R * and well depth ε for fluorine by adjusting these parameters to reproduce the experimental molar volume and enthalpy of vaporization of CF 4 . The best values of R * and ε were determined to be 1.75 Å and 0.061 kcal/mol. Using these fluorine parameters, the simulations of CHF 3 were then performed to determine if the hydrogen of this molecule required a smaller R * than that used for the “normal” hydrocarbon hydrogen determined by Spellmeyer and Kollman (results in preparation). That R * was determined by running Monte Carlo simulations on methane, ethane, propane, and butane and adjusting R * and ε for carbon and hydrogen to reproduce the experimental molar volume and enthalpy of vaporization. It was found that an R ε of 1.21 Å was optimal, significantly smaller than the R * = 1.49 Å found by Spellmeyer for “normal” hydrocarbon hydrogens. This value of R * is in good agreement with the R * for the hydrogen in CHF 3 derived independently using ab initio calculations and molecular mechanics on F 3 C H … OH 2 by Veenstra et al. © 1992 by John Wiley & Sons, Inc.