The torsional conformations of butane: Definitive energetics from ab initio methods

The torsional potential function for butane was probed theoretically using increasingly complete basis sets (up to 840 functions) and treatments of electron correlation [up to the CCSD(T) method] until it was shown that the sequence of relative energies approached convergence. The Schrödinger limit in the Born–Oppenheimer approximation was thus estimated. The equilibrium energies relative to the anti conformation (ΔEe) as obtained by this focal-point extrapolation were 0.62, 3.31, and 5.51 kcal mol−1 for the geometrically optimized stationary points having carbon backbone torsion angles of 64.8°, 119.6°, and 0°, respectively. The final prediction of the anti–syn difference is 5.40±0.15 kcal mol−1. Some consequences of this result are discussed.