Rotational potential surface for alkanes: Basis set and electron correlation effects on the conformations of n-butane

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Abstract
The rotational potential surface of n‐butane is explored by means of accurate ab initio molecular orbital methods. Complete geometry optimizations are performed to determine the structures of the two minima (trans and gauche) and the two transition states (C2 and C2v) at several theoretical levels. Basis set effects are investigated in a systematic manner by performing single point computations with several progressively larger basis sets (up to triple zeta plus polarization functions on all atoms). Electron correlation effects are included by means of Mo/ller–Plesset perturbation calculations to third order. Vibrational frequencies and zero point energies are evaluated for all the stationary points. The best computed 0 K relative energies for the two minima and the two transition states are 0.0, 0.7, 3.3, and 6.0 kcal/mol, respectively. The results suggest that some of the previously accepted values need some revision.