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

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Abstract
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 C2h) 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.