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

Abstract

The structural, vibrational, and conformational properties of small perfluoro n-alkanes CnF2n (n≤5) have been studied with different density functional models. Our calculations show that the relative conformational energies are severely underestimated within the local density approximation (LDA). The inclusion of gradient corrections (GC), on the other hand, leads to results in close agreement with experimental values, e.g., the barrier to internal rotation in C2F6 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 from 3.7-3.9 kcal/mol. A calculation of the torsional potential about the central C-C bond in G4F10 gives two degenerate chiral minimum energy configurations (t+ and t-) shifted away from the usual trans position at zero dihedral angle by ∼±12°. These two minima are separated by a barrier of ∼0.3 kcal/mol. At least four local minima were determined on the torsional potential energy surface. Two enantiomeric gauche minima (g+ and g-) at φ=±125° are ∼1.0 kcal/mol higher in energy than the t configuration. Two further minima (g′+ and g′-) with a relative energy of 1.9 kcal/mol are located at φ=±83°. The role of nonbonded interactions in determining the conformational energy landscape is discussed in some detail. © 1996 American Institute of Physics.