THE TORSIONAL POTENTIAL OF PERFLUORO N-ALKANES - A DENSITY-FUNCTIONALSTUDY

The structural, vibrational, and conformational properties of small pe rfluoro n-alkanes CnF2n (n less than or equal to 5) have been studied with different density functional models. Our calculations show that t he relative conformational energies are severely underestimated within the local density approximation (LDA). The inclusion of gradient corr ections (GC), on the other hand, leads to results in close agreement w ith experimental values, e.g., the barrier to internal rotation in C2F 6 is calculated to be 2.9 kcal/mol and 3.8 kcal/mol at the LDA and GC level, respectively, whereas corresponding experimental values range f rom 3.7-3.9 kcal/mol. A calculation of the torsional potential about t he central C-C bond in C4F10 gives two degenerate chiral minimum energ y configurations (t(+) and t(-)) shifted away from the usual trans pos ition at zero dihedral angle by similar to +/- 12 degrees. These two m inima are separated by a barrier of similar to 0.3 kcal/mol. At least four local minima were determined on the torsional potential energy su rface. Two enantiomeric gauche minima (g(+) and g(-)) at phi = +/- 125 degrees are similar to 1.0 kcal/mol higher in energy than the t confi guration. Two further minima (g('+) and g('-)) with a relative energy of 1.9 kcal/mol are located at phi = +/- 83 degrees. The role of nonbo nded interactions in determining the conformational energy landscape i s discussed in some detail. (C) 1996 American Institute of Physics.