TORSIONAL POTENTIAL ABOUT THE CENTRAL C-C-BOND IN PERFLUORO-NORMAL-BUTANE

The potential energy surface for torsion about the central C-C bond in perfluoro-n-butane has been calculated by using ab initio molecular orbital theory. The geometries of the two minima (twist-anti and gauche) and the three transition states separating the minima (anti, 120-degrees, and syn) were optimized at the DZ+D(c) basis set level. The stationary points were confirmed by force constant calculations. Final energies were obtained at the MP-2 level with a full polarized double-zeta basis set. The global minimum is the twist-anti with the gauche 1.48 kcal/mol higher in energy on the electronic energy surface. The anti transition state is 0.38 kcal/mol higher in energy than the twist-anti and the 120-degrees transition state and the syn transition state are 2.41 and 8.02 kcal/mol, respectively, higher in energy than the twist-anti. The calculated vibrational spectra show low frequency torsions that are in agreement with observed transitions. Thermodynamic corrections at 298 K make the anti lower in energy than the twist-anti by 0.22 kcal/mol for DELTA-H but the T-DELTA-S contribution makes the twist-anti lower than the anti by 1.54 kcal/mol for DELTA-G. The implications of this novel temperature dependence are discussed.