Vladimir N. Emel’yanenko

Making Sense of Enthalpy of Vaporization Trends for Ionic Liquids: New Experimental and Simulation Data Show a Simple Linear Relationship and Help Reconcile Previous Data

Vaporization enthalpy of an ionic liquid (IL) is a key physical property for applications of ILs as thermofluids and also is useful in developing liquid state theories and validating intermolecular potential functions used in molecular modeling of these liquids. Compilation of the data for a homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide ([C(n)mim][NTf2]) ILs has revealed an embarrassing disarray of literature results. New experimental data, based on the concurring results from quartz crystal microbalance, thermogravimetric analyses, and molecular dynamics simulation have revealed a clear linear dependence of IL vaporization enthalpies on the chain length of the alkyl group on the cation. Ambiguity of the procedure for extrapolation of vaporization enthalpies to the reference temperature 298 K was found to be a major source of the discrepancies among previous data sets. Two simple methods for temperature adjustment of vaporization enthalpies have been suggested. Resulting vaporization enthalpies obey group additivity, although the values of the additivity parameters for ILs are different from those for molecular compounds.

Thermodynamics of ionic liquids precursors: 1-methylimidazole.

The standard molar enthalpy of formation in the liquid state for 1-methylimidazole (MeIm) was obtained from combustion calorimetry. The enthalpy of vaporization of the compound was derived from the temperature dependence of the vapor pressure measured by the transpiration method. Additionally, the enthalpy of vaporization for MeIm was measured directly using Calvet-type calorimetry. In order to verify the experimental data, first-principles calculations of MeIm were performed. The enthalpy of formation evaluated at the G3MP2 level of theory is in excellent agreement with the experimental value. The heat capacity and parameters of fusion of MeIm were measured in the temperature range (5 to 370) K using adiabatic calorimetry. The thermodynamic functions for the compound in the crystal and liquid states were calculated from these data. Based on the experimental spectroscopic data and the results of quantum-chemical calculations, the ideal-gas properties for MeIm were calculated by methods of statistical thermodynamics.

Structure, conformations, vibrations, and ideal-gas properties of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic pairs and constituent ions.

Energies, geometries, and frequencies of normal vibrations have been calculated by quantum-chemical methods for different conformers of a bis(trifluoromethylsulfonyl)imide anion (NTf2-), 1-alkyl-3-methylimidazolium cations ([C(n)mim]+, n = 2, 4, 6, 8), and [C(n)mim]NTf2 ionic pairs. The assignment of frequencies for NTf2-, [C2mim]+, and [C4mim]+ in the vibrational spectra of ionic liquids have been performed. Thermodynamic properties of [C(n)mim]NTf2, [C(n)mim]+, and NTf2- in the gas state have been calculated by the statistical thermodynamic methods. The resulting entropies are in satisfactory agreement with the values obtained from the experimental data previously reported in literature.

Vaporization and Formation Enthalpies of 1-Alkyl-3-methylimidazolium Tricyanomethanides

Thermochemical studies of the ionic liquids 1-ethyl-3-methylimidazolium tricyanomethanide [C(2)MIM][C(CN)(3)] and 1-butyl-3-methylimidazolium tricyanomethanide [C(4)MIM][C(CN)(3)] have been performed in this work. Vaporization enthalpies have been obtained using a recently developed quartz crystal microbalance (QCM) technique. The molar enthalpies of formation of these ionic liquids in the liquid state were measured by means of combustion calorimetry. A combination of the results obtained from QCM and combustion calorimetry lead to values of gaseous molar enthalpies of formation of [C(n)MIM][C(CN)(3)]. First-principles calculations of the enthalpies of formation in the gaseous phase for the ionic liquids [C(n)MIM][C(CN)(3)] have been performed using the CBS-QB3 and G3MP2 theory and have been compared with the experimental data. Furthermore, experimental results of enthalpies of formation of imidazolium-based ionic liquids with the cation [C(n)MIM] (where n = 2 and 4) and anions [N(CN)(2)], [NO(3)], and [C(CN)(3)] available in the literature have been collected and checked for consistency using a group additivity procedure. It has been found that the enthalpies of formation of these ionic liquids roughly obey group additivity rules.

Structure-Energy Relationship in Barbituric Acid: A Calorimetric, Computational, and Crystallographic Study

This paper reports the value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K for barbituric acid. The enthalpies of combustion and sublimation were measured by static bomb combustion calorimetry and transference (transpiration) method in a saturated N2 stream and a gas-phase enthalpy of formation value of -(534.3 +/- 1.7) kJ x mol(-1) was determined at T = 298.15 K. G3-calculated enthalpies of formation are in very good agreement with the experimental value. The behavior of the sample as a function of the temperature was studied by differential scanning calorimetry, and a new polymorph of barbituric acid at high temperature was found. In the solid state, two anhydrous forms are known displaying two out of the six hydrogen-bonding patterns observed in the alkyl/alkenyl derivatives retrieved from the Cambridge Crystallographic Database. The stability of these motifs has been analyzed by theoretical calculations. X-ray powder diffraction technique was used to establish to which polymorphic form corresponds to the commercial sample used in this study and to characterize the new form at high temperature.

Ionic Liquids: Differential Scanning Calorimetry as a New Indirect Method for Determination of Vaporization Enthalpies

Differential scanning calorimetry (DSC) has been used to measure enthalpies of synthesis reactions of the 1-alkyl-3-methylimidazolium bromide [C(n)mim][Br] ionic liquids from 1-methylimidazole and n-alkyl bromides (with n = 4, 5, 6, 7, and 8). The optimal experimental conditions have been elaborated. Enthalpies of formation of these ionic liquids in the liquid state have been determined using the DSC results according to the Hess Law. The ideal-gas enthalpies of formation of [C(n)mim][Br] were calculated using the methods of quantum chemistry. They were used together with the DSC results to derive indirectly the enthalpies of vaporization of the ionic liquids under study. In order to validate the indirect determination, the experimental vaporization enthalpy of [C(4)mim][Br] was measured by using a quartz crystal microbalance (QCM). The combination of reaction enthalpy measurements by DSC with modern high-level first-principles calculations opens valuable indirect thermochemical options to obtain values of vaporization enthalpies of ionic liquids.

Thermochemistry of Dihalogen-Substituted Benzenes: Data Evaluation Using Experimental and Quantum Chemical Methods

Temperature dependence of vapor pressures for 12 dihalogen-substituted benzenes (halogen = F, Cl, Br, I) was studied by the transpiration method, and molar vaporization or sublimation enthalpies were derived. These data together with results available in the literature were collected and checked for internal consistency using structure-property correlations. Gas-phase enthalpies of formation of dihalogen-substituted benzenes were calculated by using quantum-chemical methods. Evaluated vaporization enthalpies in combination with gas-phase enthalpies of formation were used for estimation liquid-phase enthalpies of formation of dihalogen-substituted benzenes. Pairwise interactions of halogens on the benzene ring were derived and used for development of simple group additivity procedures for estimation of vaporization enthalpies, gas-phase, and liquid-phase enthalpies of formation of dihalogen-substituted benzenes.

Thermodynamic Analysis of Strain in Heteroatom Derivatives of Indene

Thermochemical properties of indene, 2,3-benzofuran, indole, and N-methylindole have been studied to obtain a better quantitative understanding of the energetics associated with these compounds containing five-membered ring units. We used combustion calorimetry, transpiration, and high-level first-principles calculations to derive gaseous enthalpies of formation of the five-membered heterocyclic compounds. Our new values together with selected values for the parent heterocyclic compounds, available from the literature were used for calculation of the strain enthalpies H(S) of five-membered C, N, and O-containing cycles. The quantitative analysis of the resulting stabilization or destabilization of a molecule due to interaction of the benzene ring with the heteroatom has been performed.

Experimental and Computational Thermochemical Study and Solid-Phase Structure of 5,5-Dimethylbarbituric Acid

This paper reports an experimental and computational thermochemical study on 5,5-dimethylbarbituric acid and the solid-phase structure of the compound. The value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -(706.4 +/- 2.2) kJ x mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated NB(2) stream, and a value of the enthalpy of sublimation at T = 298.15 K was derived as (115.8 +/- 0.5) kJ x mol(-1). From these results a value of -(590.6 +/- 2.3) kJ x mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 level were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in reasonable agreement with the experimental value. 5,5-Dimethylbarbituric acid was characterized by single crystal X-ray diffraction analysis. In the crystal structure, N-H...O=C hydrogen bonds lead to the formation of ribbons connected further by weak C-H...O=C hydrogen bonds into a three-dimensional network. The molecular and supramolecular structures observed in the solid state were also investigated in the gas phase by DFT calculations.

Thermodynamics of Imidazolium-Based Ionic Liquids Containing PF6 Anions

Imidazolium-based ionic liquids (ILs) with PF6(-) anions are considered as low-cost solvents for separation processes, but they exhibit restricted thermal stabilities. Reliable measurements of vaporization thermodynamics by conventional methods have failed. In this work, we applied a quartz-crystal microbalance method to determine for the first time the absolute vapor pressures for the [Cnmim][PF6] family, with n = 2, 4, 6, 8, and 10, in the temperature range 403-461 K. An absence of decomposition of ILs in experimental conditions was determined by the attenuated total reflection-infrared spectroscopy. The consistency of the experimental results within the homologous series was established through enthalpy and entropy analyses of the liquid and gas phases as well as by molecular dynamics simulations.