Ideal gas thermodynamic properties and isomerization of n-butane and isobutane

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Abstract
Reported Values of Structural parameters, vibrational fundamentals, and potential energy functions for internal rotation of n‐butane and isobutane are reviewed. The selected values were used to calculate the thermodynamic properties (C°p, S°, (H°‐H°0)/T) in the temperature range of O to model. Contributions of internal rotation were evaluated by the direct sum of terms containing energy levels which were calculated with a one‐dimensional potential model. For internal rotation about the central C‐C bond in n‐butane, energy levels were approximated by two procedures. A inique potential function was assumed for each methyl rotor of n‐butane or of isobutane. Top‐top interactions in isobutane were approximated by the potential parameter V‐d6 which was determined empirically by comparison with thermodynamic data. The calculated and observed values of heat capacities and entropies agree well within experimental uncertainties. Standard enthalpies of formation of 298.15 K for the ideal gaseous state were selected from measured values of heats of combustion and third‐law enthalpies for isomerization. Corresponding values of ΔHf°, and log Kf are tabulated over the same temperature range.

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