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In this work, comprehensive investigations of l- and dl-methionine have been conducted by the methods of differential scanning calorimetry (DSC), Knudsen effusion mass spectrometry (KEMS), and quantum chemistry. Heat capacities of crystalline l- and dl-methionine were measured by DSC in the temperature range of 209–473 K and their thermodynamic functions (TFs) have been determined. The structures and molecular parameters of 29 gaseous conformers were computed in the framework of DFT theory (B3LYP/cc-pVTZ) and TFs of methionine in the ideal gas state have been obtained. Sublimation of l- and dl-methionine was investigated by KEMS in the temperature range 369–431 K. Both compounds were found to sublime congruently in the form of monomers. The saturated vapor pressure in the studied temperature range has been obtained as ln (p/Pa) = −(16905 ± 166)/(T/K) + (38.145 ± 0.411) (l-methionine) and ln­(p/Pa) = −(16969 ± 140)/(T/K) + (38.473 ± 0.350) (dl-methionine). Sublimation enthalpies Δsub H°(298.15 K) equal to 147.9 ± 1.2 kJ·mol–1 (l-methionine) and 148.7 ± 2.6 kJ·mol–1 (dl-methionine) have been determined by the second and third law techniques.

The processes of the sublimation and thermal decomposition of the 1-ethyl-3-methylimidazolium hexafluorophosphate ionic liquid (EMImPF6) were studied by a complex approach including Knudsen effusion mass spectrometry, IR and NMR spectroscopy, and quantum chemical calculations. It was established that the vapor over the liquid phase primarily consists of decomposition products under equilibrium conditions. Otherwise, the neutral ion pairs are the only vapor components under Langmuir conditions. To identify the nature of the decomposition products, an experiment on the distillation of the ionic liquid was performed and the collected distillate was analyzed. It was revealed by the IR and NMR spectroscopy that EMImPF6 decomposes to substituted imidazole-2-ylidene (C6N2H10PF5) and HF. The measured vapor pressure of C6N2H10PF5 reveals a very low activity of the decomposition products (<10−4) in the liquid phase. The absence of a significant accumulation of decomposition products in the condensed phase makes it possible to determine the enthalpy of sublimation of the ionic liquid assuming its unchanged activity. The thermodynamics of the EMImPF6 sublimation was studied by Knudsen effusion mass spectrometry. The formation enthalpy of EMImPF6 in the ideal gas state was found from a combination of the sublimation enthalpy and formation enthalpy of the ionic liquid in the condensed state. The obtained value is in good agreement with those calculated by quantum chemical methods.