Spectroscopic Studies of Rotational Isomerism. III. The Normal Paraffins in the Liquid and Solid States

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Abstract
The Raman spectra of n‐butane, n‐pentane, n‐hexane, and n‐heptane have been investigated in the liquid phase between 310°K and their melting points, and also in the solid phase. The room temperature spectra clearly show the presence of appreciable concentrations of two rotational isomers in butane and pentane, and of three isomers in hexane and probably in heptane. In the solid phase only one of these isomers is present, and in agreement with previous conclusions based on spectroscopic and other evidence, this lowest energy form was assumed to possess the planar, trans‐, zig‐zag configuration. The spectra of the solids yield experimental values for the limiting skeletal stretching frequencies of the zig‐zag form, and these were compared with values calculated by previously established methods. The photoelectric spectrograph was used for quantitative intensity measurements on selected line pairs of each molecule over appropriate temperature intervals, and from the results values were obtained for the energy difference, —ΔH, between rotational isomers. The values obtained were 760±100 cal. mole−1 in n‐butane, 450±60 cal. mole−1 in n‐pentane, 520±70 and 470±60 cal. mole−1 in n‐hexane. No quantitative result could be obtained for n‐heptane. The observed gaseous heat capacities of n‐butane, n‐pentane, and n‐hexane were compared with values calculated by statistical mechanics. It was found that for n‐pentane and n‐hexane the calculated values do not agree accurately with the experimental results, probably because of the simplified partition functions used to obtain the compositions of the isomeric mixtures.