An ab initio investigation of the torsional potential function of n-butane

Abstract

The rotational potential function of n-butane has been calculated as a function of the dihedral angle of the skeletal carbon–carbon bonds using a 4–31 Gaussian basis set. Relaxation of the central C—C bond by up to 0.03 Å and of the CCC angles by 2.8° accompanies the rotation. Apart from a vertical displacement in energy, the potential curve, when all coordinates are released, is closely similar to that obtained when the HCC angles are held to tetrahedral values and the C—H bond lengths are fixed at as much as 0.1 Å greater than their equilibrium values. This suggests that the potential curves of more complex systems may be adequately reproduced in this simplifying approximation of not releasing the hydrogen coordinates. The calculated potential curves reproduce the experimental anti to gauche energy differences as well as the optimum gauche dihedral angle of 67°. Significant differences exist between the ab initio curve and the frequently used Scott and Scheraga curves. In particular the energy of the eclipsed configuration over the anti form is reduced from 45 to 27 kJ mol^–1.