Theory of the Rotational Barriers in Ethyl Fluoride and Ethane

Ab initio LCAO-MO-SCF wave functions have been computed with an atomic orbital basis set of double-ξ accuracy for the staggered and eclipsed conformations of ethyl fluoride and ethane. Total energy differences, ΔE_T, yield barriers of 2.59 and 2.58 kcal/mol for ethyl fluoride and ethane, respectively. These calculations are in reasonable agreement with the experimental result that barriers in these two molecules differ by about 10%. Our wave functions also substantiate and extend a previous conclusion that the origin of barriers is contained within the framework of the Hartree-Fock approximation. To obtain a more detailed understanding of the barrier mechanism, the total energy has been separated into its two competing and out-of-phase components: V_attractive ≡ V_ne and V_repulaive = V_nn + T + V_ee. Both ethyl fluoride and ethane are repulsive dominant. One result of the repulsive dominance is a small difference in dipole moment between staggered and eclipsed ethyl fluoride. A physical rationalization of the rough equality of these two barriers also follows from their commnn repulsive dominance. The sum of one-electron molecular orbital energies (Walsh criterion) is investigated as a means of predicting barrier magnitudes. This is shown to be reasonable for ethyl fluoride and ethane but not for some other molecules.