Internal Rotation in Butane

Abstract

Quantum-mechanical calculations within the Hartree–Fock framework and using two Gaussian basis sets are carried out on several conformations of the butane molecule. The smaller basis set predicts a transgauche barrier of 3.536 kcal/mole, an energy difference of 0.822 kcal/mole between the gauche and trans forms, and a barrier of 6.821 kcal/mole for interconversion of the two gauche conformations. The values of these quantities as given by the larger basis set are 3.619, 0.761, and 6.834 kcal/mole. Taking the angle of rotation about the central bond to be 0 for the trans conformation, the gauche form is predicted to occur at 110.9° for the smaller and at 111.3° for the larger basis set. The barrier to internal rotation of a methyl group is predicted to be 2.92 kcal/mole and 2.94 kcal/mole for the smaller and larger basis sets, respectively.