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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