Thermodynamic Properties of Thionyl Fluoride from 13°K to Its Boiling Point. Entropy from Molecular and Spectroscopic Data

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Abstract
The heat capacity of thionyl fluoride has been measured from 12° to 230°K for a sample of 99.976 mole% purity as determined by the melting-point method. The heat of fusion was 1505.9 cal/mole at the solid—liquid—vapor equilibrium temperature of 143.25°K. The heat of vaporization at the normal boiling point of 228.84°K was 5091 cal/mole. The vapor pressure of the liquid to the normal boiling point has been measured, and the data are represented by log10Pmm=24.41879–1719.074/T−5.94452 log10Twhich was obtained by minimizing the square of the residuals of the pressure. The density of the liquid is represented by d(g/cc)=2.389–0.003398T. The entropy of thionyl fluoride gas in the standard state at the normal boiling point calculated from the experimental data is 63.56 cal/(mole·°K). A spectroscopic entropy of 63.28 cal/(mole·°K) is calculated from the product of the moments of inertia, IAIBIC=1.6562×10−114 g3·cm6 from microwave data and the following frequency assignment: v1(a′)=1336 cm−1; v2(a′)=808 cm−1; v3(a′)=530 cm−1; v4(a′)=378 cm−1; v5(a″)=748 cm−1; and v6(a″)=396 cm−1. The discrepancy between the two entropies is accountable in terms of the experimental uncertainty and therefore, the molecular parameters and frequency assignment are confirmed.