Spectroscopic Studies of Rotational Isomerism. IV. 2-Methyl Butane and 2,3-Dimethyl Butane

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Abstract
The Raman spectra of 2‐methyl butane and 2,3‐dimethyl butane were investigated between room temperature and 90°K. In marked contrast to the behavior of the normal paraffins all the main lines persist in the solid phase. The slight change with temperture in the relative intensity of one line pair in liquid 2‐methyl butane was investigated quantitatively with the photoelectric spectrograph. No measurable intensity changes were observed in the 2,3‐dimethyl butane spectrum. The results were interpreted in terms of both a very small (<100 cal. mole−1) and a very large (1600 cal. mole−1) energy difference between the rotational isomers. Although both interpretations appear consistent with the spectroscopic results, neither affords a completely straightforward explanation of all the facts. It is shown for 2‐methyl butane that accurate gaseous heat capacities would distinguish between these alternatives. The spectroscopic data on 2,3‐dimethyl butane are inconsistent with a single molecular configuration possessing a center of symmetry.