ANALYSIS OF TORSIONAL SPECTRA OF MOLECULES WITH 2 INTERNAL C3V ROTORS .12. LOW-FREQUENCY VIBRATIONAL-SPECTRA, METHYL TORSIONAL POTENTIAL FUNCTION, AND INTERNAL-ROTATION OF NORMAL-BUTANE

The vibrational spectrum of gaseous n-butane has been investigated below 500 cm-1. A vibrational assignment for the observed bands in this region has been given for both s-trans and the high-energy gauche conformers. The asymmetric potential function has been calculated from three observed transitions, leading to a value of the enthalpy difference between the conformers of 0.89 kcal/mol and a gauche dihedral angle of 62°. The values of the potential coefficients were V1 = 418 ± 6 cm-1 (1.19 kcal/mol); V3 = 639 ± 67 cm-1 (1.83 kcal/mol); V6 = 136 ± 23 cm-1 (0.40 kcal/mol). Bands have been assigned to the methyl torsions of both conformers and from these assignments the barriers to internal rotation of the methyl groups for both the s-trans and gauche conformers have been calculated to be 3.21 ± 0.01 and 4.30 ± 0.03 kcal/mol, respectively. © 1979 American Chemical Society.