Chiral discrimination in liquid 1,1,1-trifluoropropan-2-ol: A molecular dynamics study

The structural and thermodynamical properties of the R and S enantiomers of 1,1,1trifluoropropan-2-ol (TFIP) have been investigated by molecular-dynamics simulations. In particular, the chiral discrimination (Ch.D.) between the two enantiomers in a racemic solution has been analyzed in detail. Differences in density and enthalpy of vaporization between the pure enantiomeric liquid and the racemic mixture have been found. The comparison of the radial distribution functions and the distribution of the reciprocal orientations of TFIP molecules have shown the presence of a slightly different packing organization in the aforementioned solutions explaining the difference in density and enthalpy of vaporization. Furthermore, the structural analysis of the racemic mixture has shown a strong dependence of the homo- and heterochiral preference by the nature of the functional groups present in the molecule. At 298 K, in the case of CH3, CF3, and hydroxy groups, the homochiral interaction is followed by a heterochiral one in the order: OH>>CH3>>CF3. The effect of pressure and temperature on the Ch.D. has been also analyzed. As general trend, the Ch.D. is more effective at lower temperatures and decrease as the temperature is raised. At high pressure (1000 bars) the Ch.D. is still present for the OH and CH3 groups while is negligible for the CF3 group. The Ch.D. has been also quantified by calculating the differential free energy of solvation of the R and S enantiomer in a solution of TFIP(R) molecules, at 298 K and 1 bar pressure. A value equal to -1.7+/-0.7 kJ mol(-1) has been found, indicating a slight immiscibility of the two enantiomers. As a general result, the balance between the first neighbor interactions, affecting the geometrical packing and the 2nd-3rd shell organizations, results in different structural and thermodynamical properties (i.e., density and enthalpy of vaporization) between the racemic TFIP mixture and the pure enantiomers. (C) 2003 American Institute of Physics.