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 CF4 were performed first to determine the optimal van der Waals radius R* and well depth epsilon for fluorine by adjusting these parameters to reproduce the experimental molar volume and enthalpy of vaporization of CF4. The best values of R* and epsilon were determined to be 1.75 angstrom and 0.061 kcal/Mol. Using these fluorine parameters, the simulations of CHF3 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 epsilon for carbon and hydrogen to reproduce the experimental molar volume and enthalpy of vaporization. It was found that an R* of 1.21 angstrom was optimal, significantly smaller than the R* = 1.49 angstrom found by Spellmeyer for "normal" hydrocarbon hydrogens. This value of R* is in good agreement with the R* for the hydrogen in CHF3 derived independently using ab initio calculations and molecular mechanics on F3C-H ... OH2 by Veenstra et al.