Thermodynamic properties of an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) in the ideal gas state were calculated from molecular and spectral data. Because quantum chemical calculations demonstrated that K dis for [C4mim][PF6] did not exceed 10-11 at temperatures below 1000 K, the gas was assumed to consist of ion pairs. The product of the principal moments of inertia was found to be 16.49 × 10-132 kg3·m6. The frequencies of normal vibrations were obtained from the experimental and calculated spectra. Rotation of CH3N was assumed to be free. The parameters for all alkyl tops were taken to be close to those in alkanes. The parameters for Bu- and PF6 were calculated ab initio. The calculated thermodynamic functions of the ideal gas (S°, C p, and −(G° − H°(0))/T) were (657.4, 297.0, and 480.3) J·K-1·mol-1, respectively, at 298 K and were (843.1, 424.4, and 252.8) J·K-1·mol-1, respectively, at 500 K. Experiments were performed to better characterize the thermal stability and vapor pressure P sat of this substance. DSC experiments were carried out in a temperature range from (303 to 523) K and suggest that the substance starts to decompose at temperatures greater than 473 K. Knudsen effusion experiments were attempted to measure P sat for [C4mim][PF6] in the temperature range (433 to 522) K, but no reproducible values of P sat were obtained. By combining a published value of the cohesive energy density, measured heat capacities, and thermodynamic properties in the ideal gas state, thermodynamic properties of vaporization ( C p, Δvap S, Δvap H) and vapor pressure (P sat) were calculated. At room temperature, the calculated P sat was found to be 10-10 Pa, a value that is much smaller than the lower detection limit for effusion measurements.

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