The torsional potential of perfluoro n-alkanes: a density functional study

The structural, vibrational, and conformational properties of small perfluoro n-alkanes CnF2n (n ? 5) have been studied with different d. functional models. The relative conformational energies are severely underestimated within the local d. approxn. (LDA). The inclusion of gradient corrections (GC), on the other hand, leads to results in close agreement with exptl. values, e.g., the barrier to internal rotation in C2F6 is calcd. to be 2.9 kcal/mol and 3.8 kcal/mol at the LDA and GC level, resp., whereas corresponding exptl. values range from 3.7-3.9 kcal/mol. A calcn. of the torsional potential about the central C-C bond in C4F10 gives two degenerate chiral min. energy configurations (t+ and t-) shifted away from the usual trans position at zero dihedral angle by .apprx.+-12 Deg. These two min. are sepd. by a barrier of .apprx.0.3 kcal/mol. At least four local min. were detd. on the torsional potential energy surface. Two enantiomeric gauche min. (g+ and g-) at f=+-125 Deg are .apprx.1.0 kcal/mol higher in energy than the t configuration. Two further min. (g'+ and g'-) with a relative energy of 1.9 kcal/mol are located at f=+-83 Deg. The role of nonbonded interactions in detg. the conformational energy landscape is discussed in some detail. [on SciFinder (R)]