Spectroscopic Studies of Rotational Isomerism. I. Liquid n-Butane and the Assignment of the Normal Modes of Vibration

The Raman spectrum of n-butane has been investigated over the temperature range 147°K to 305°K. The results clearly show the presence of two rotational isomers in the liquid. These are the planar, trans-form (symmetry C_2h) and probably the gauche form obtained by a rotation of 120° about the central C-C bond (symmetry C₂). The photoelectric spectrograph was used for quantitative intensity measurements on the line pair at 432 cm^-1 and 325 cm^-1 over the temperature range studied. These lines belong to the two rotational isomers and the results were used to give a value of 770±90 cal. mole^-1 for the energy difference, —ΔH, between the two forms of n-butane in the liquid phase. This result is shown to be in good agreement with a value assumed by Pitzer in his treatment of long chain paraffins. A detailed assignment has been made of the vibrational spectrum of the trans-rotational isomer.