Internal Rotation in Propane: Reanalysis of the Microwave Spectrum and Quantum-Mechanical Calculations

Quantum-mechanical calculations are carried out on various conformations of the propane molecule using a Gaussian basis set and the Hartree–Fock–Roothaan formalism. These calculations indicate that internal rotation in propane can be adequately described by a simple equation of the form E(α1, α2) = 12V0 (2 − cos3α1 − cos3α2)−12V1[1 − cos3(α1 + α2)] where α1 and β2 are the two angles describing the rotation of the two methyl groups. The microwave spectrum of propane in the first excited torsional states reported by Hirota, Matsumura, and Morino was analyzed assuming this potential, leading to values for V0 and V1 of 3575 ± 100 cal/mole and 310 ± 40 cal/mole, respectively.