Benchmark thermodynamic properties of alkanediamines: Experimental and theoretical study

Vapor pressures of (dl)-1,2-propanediamine and 2-methyl-1,2-propanediamine were measured using the transpiration method. Molar enthalpies of vaporization were derived from the vapor pressure temperature dependence. Thermodynamic data on alkanediamines available in the literature were collected and treated uniformly. Consistency of the experimental data set for alkanediamines was evaluated with group-contribution and quantum-chemical methods.The standard molar entropy of formation and the standard molar Gibbs function of formation have been calculated. Vaporization and formation enthalpies of alkanediamines of benchmark quality are recommended for practical thermochemical calculations and validation of empirical and theoretical methods.     

Experimental and theoretical study of thermodynamic properties of levoglucosan

The heat capacity of levoglucosan was measured over the temperature range (5 to 370) K by adiabatic calorimetry. The temperatures and enthalpies of a solid-phase transition and fusion for the compound were found by DSC. The obtained results allowed us to calculate thermodynamic properties of crystalline levoglucosan in the temperature range (0 to 384) K. The enthalpy of sublimation for the low-temperature crystal phase was found from the temperature-dependent saturated vapor pressures determined by the Knudsen effusion method. The thermodynamic properties of gaseous levoglucosan were calculated by methods of statistical thermodynamics using the molecular parameters from quantum chemical calculations. The enthalpy of formation of the crystalline compound was found from the experiments in a combustion calorimeter. The gas-phase enthalpy of formation was also obtained at the G4 level of theory. The thermodynamic analysis of equilibria of levoglucosan formation from cellulose, starch, and glucose was conducted.     

Thermodynamic properties of bromine fluorene derivatives: An experimental and computational study

This report presents a comprehensive experimental and computational study of the thermodynamic properties of two bromine fluorene derivatives: 2-bromofluorene and 2,7-dibromofluorene. The standard (p° = 0.1 MPa) molar enthalpies of formation in the crystalline phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The vapour pressures of the crystalline phase of the two compounds were measured using the Knudsen effusion method and a static method that has also been used to measure the liquid vapour pressures of 2-bromofluorene. From these results the standard molar enthalpies, entropies and Gibbs energies of sublimation of the two compounds studied and of vapourisation of 2-bromofluorene were derived. The enthalpies and temperatures of fusion were determined from DSC experiments. Derived results of standard enthalpies and Gibbs energies of formation, in both gaseous and crystalline phases, were compared with the ones reported in the literature for fluorene.The experimental values of the gas-phase enthalpies of formation of each compound were compared with estimates based on density functional theory calculations using the B3LYP hybrid exchange–correlation energy functional with the 6-311++G(d,p) basis set.     

Low-temperature heat capacities and thermodynamic properties of 2,2-dimethyl-1,3-propanediol

The molar heat capacities C _p,m of 2,2-dimethyl-1,3-propanediol were measured in the temperature range from 78 to 410 K by means of a small sample automated adiabatic calorimeter. A solid-solid and a solid-liquid phase transitions were found at T-314.304 and 402.402 K, respectively, from the experimental C _p-T curve. The molar enthalpies and entropies of these transitions were determined to be 14.78 kJ mol⁻¹, 47.01 J K⁻¹ mol⁻ for the solid-solid transition and 7.518 kJ mol⁻¹, 18.68 J K⁻¹ mol⁻¹ for the solid-liquid transition, respectively. The dependence of heat capacity on the temperature was fitted to the following polynomial equations with least square method. In the temperature range of 80 to 310 K, C _p,m/(J K⁻¹ mol⁻¹)=117.72+58.8022x+3.0964x ²+6.87363x ³−13.922x ⁴+9.8889x ⁵+16.195x ⁶; x=[(T/K)−195]/115. In the temperature range of 325 to 395 K, C _p,m/(J K⁻¹ mol⁻¹)=290.74+22.767x−0.6247x ²−0.8716x ³−4.0159x ⁴−0.2878x ⁵+1.7244x ⁶; x=[(T/K)−360]/35. The thermodynamic functions H _T−H _298.15 and S _T−S _298.15, were derived from the heat capacity data in the temperature range of 80 to 410 K with an interval of 5 K. The thermostability of the compound was further tested by DSC and TG measurements. The results were in agreement with those obtained by adiabatic calorimetry.     

Experimental and computational study of the thermodynamic properties of 2-nitrofluorene and 2-aminofluorene

This report presents a comprehensive experimental and computational study of the thermodynamic properties of two fluorene derivatives: 2-aminofluorene and 2-nitrofluorene. The standard (p° = 0.1 MPa) molar enthalpies of formation in the crystalline phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. A Knudsen effusion method was used to perform the vapour pressure study of the referred compounds, yielding an accurate determination of the standard molar enthalpies and entropies of sublimation. The enthalpies of sublimation were also determined using Calvet microcalorimetry and the enthalpy and temperature of fusion were derived from DSC experiments. Derived results of standard enthalpy and Gibbs energy of formation in both gaseous and crystalline phases were compared with the ones reported in literature for fluorene. A theoretical study at the G3 and G4 levels has been carried out, and the calculated enthalpies of formation have been compared to the experimental values.     

Structural studies of cyclic ureas: 2. Enthalpy of formation of parabanic acid

Thermophysical and thermochemical studies have been carried out for crystalline parabanic acid. The thermophysical study was made by differential scanning calorimetry, DSC, over the temperature interval between T = (263 and 473) K. Two phase transitions were found: at T = (392.3 ± 1.6) K with the enthalpy of transition of (2.1 ± 0.4) kJ · mol⁻¹ and at T = (509.8 ± 1.5) K, when the compound was scanned to its fusion temperature. The standard (p = 0.1 MPa) molar enthalpy of formation, at T = 298.15 K, for crystalline parabanic acid was determined using static-bomb combustion calorimetry as −(590.2 ± 1.0) kJ · mol⁻¹. The standard molar enthalpy of sublimation, at T = 298.15 K, was derived from the variation of their vapour pressures, measured by the Knudsen-effusion method, with the temperature. These two thermochemical parameters yielded the standard molar enthalpy of formation in the gaseous phase, at T = 298.15 K, as −(470.8 ± 1.2) kJ · mol⁻¹.     

Thermochemical study of 1-, 3- and 4-piperidinecarboxamide derivatives

The standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase, of three piperidinecarboxamide derivatives, namely 1-, 3- and 4-piperidinecarboxamide, were determined from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry in oxygen and by Calvet microcalorimenty, respectively.The final results are analysed and discussed in terms of molecular structure.     

S6 Isomer of C60(CF3)12: Synthesis,properties and thermodynamic functions

A crystalline form of S₆-С₆₀(CF₃)₁₂ was synthesized in an amount sufficient for reliable experimental investigation. We determined the enthalpy of combustion of S₆-С₆₀(CF₃)₁₂ in oxygen and its heat capacity, which made possible to derive the thermodynamic functions of S₆-С₆₀(CF₃)₁₂, namely the enthalpy of formation, the entropy and the Gibbs energy at T = 298.15 K. These experimental thermochemical data enabled estimation of the formation energy for a broad range of other trifluoromethylated compounds C₆₀(CF₃)_n, with n = (2–18; 24), on the basis of their DFT calculated relative energies.     

Cyclic alkylene carbonates. Experiment and first principle calculations for prediction of thermochemical properties

The standard molar enthalpies of formation of ethylene carbonate, propylene carbonate, and butylene carbonate were measured using combustion calorimetry. Ab initio calculations of molar enthalpies of formation of alkylene carbonates were performed using the G3MP2 method. The calculated values are in excellent agreement with available experimental data. Ring strain corrections were quantified for the refinement of the group-contribution method for prediction of enthalpies of formation and vaporization of alkylene carbonates.     

Thermochemical properties of 4-N,N-dialkylamino-7-nitrobenzofurazan derivatives (alkyl = methyl,ethyl)

The standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase, for two nitrobenzofurazan derivatives, 4-N,N-dimethylamino-7-nitrobenzofurazan (DMANBF) and 4-N,N-diethylamino-7-nitrobenzofurazan (DEANBF), were derived from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry and by the Knudsen effusion technique, respectively. The results are compared with the corresponding data calculated by the G3(MP2)//B3LYP approach. Computationally, the molecular structures of both compounds were established and the geometrical parameters were determined at the B3LYP/6-31G(d) level of theory.     

Thermodynamic properties of caffeine: Reconciliation of available experimental data

Molar enthalpies of sublimation of two crystal forms of caffeine were obtained from the temperature dependence of the vapour pressure measured by the transpiration method. A large number of primary experimental results on the temperature dependences of vapour pressure and phase transitions have been collected from the literature and have been treated in a uniform manner in order to derive sublimation enthalpies of caffeine at T = 298.15 K. This collection together with the new experimental results reported here has helped to resolve contradictions in the available sublimation enthalpies data and to recommend a consistent and reliable set of sublimation and formation enthalpies for both crystal forms under study. Ab initio calculations of the gaseous molar enthalpy of formation of caffeine have been performed using the G3MP2 method and the results are in excellent agreement with the selected experimental data.     

Heat-capacity measurements and thermodynamic functions of crystalline adenine; revised thermodynamic properties of aqueous adenine

The standard molar heat capacity C^°_p,m of adenine(cr) has been measured using adiabatic calorimetry over the range 6<(T/K)<310 and the results used to derive thermodynamic functions for adenine(cr) at smoothed temperatures. At T=298.15 K, C^°_p,m=(142.67±0.29) J · K⁻¹ · mol⁻¹ and the third law entropy S^°_m=(145.62±0.29) J · K⁻¹ · mol⁻¹. The standard molar Gibbs free energy of formation Δ_fG^°_m at T=298.15 K for crystalline adenine was calculated, using the standard molar enthalpy of formation for the compound and entropies of the elements from the literature, and found to be Δ_fG^°_m=(301.4±1.0) kJ · mol⁻¹. The results were combined with solution calorimetry and solubility measurements from the literature to yield revised values for the standard molar thermodynamic properties of aqueous adenine at T=298.15 K: Δ_fG^°_m=(313.4±1.0) kJ · mol⁻¹, Δ_fH^°_m=(129.5±1.4) kJ · mol⁻¹, and S_m^°=(217.68±0.44) J · K⁻¹ · mol⁻¹.     

Thermochemical study of 5-methyluracil, 6-methyluracil, and 5-nitrouracil

The standard (p° = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, at T = 298.15 K, for 5-methyluracil, 6-methyluracil, and 5-nitrouracil were derived from the values of the standard massic energies of combustion measured by static bomb combustion calorimetry. The results obtained together with literature values of the enthalpies of sublimation yielded the standard molar enthalpies of formation, in gaseous phase, at T = 298.15 K. These values are discussed in the terms of structural enthalpic increments.     

Calculation of thermodynamic properties and transport coefficients for SF6-N2 mixtures in the temperature range 1,000–30,000 K

We have computed the equilibrium composition, the transport coefficients (viscosity, electrical and thermal conductivities), the thermodynamic properties (Gibbs and Helmholtz potentials, entropy, enthalpy, specific heats), and the derived quantities (mass density, sound velocity) for SF₆-N₂ mixtures in conditions relevant to circuit-breaker arcs: temperatures between 1000 and 30,000 K, pressures in the range 1–10 atm. The validity of our computation has been checked by a detailed comparison of our results with those available in the literature concerning pure SF₆ and pure N₂. In SF₆-N₂ mixtures the chemical reactions (dissociation, ionization) have a strong influence on thermal conduction and heat capacities. The effect of SF₆ on the properties of such mixtures is elucidated: in a mixture containing 40% SF₆, the amplitude of the thermal conduction peak appearing around 7500 K is reduced by a factor of 4 relative to that of pure N₂. The influence of pressure on the properties of the plasma between 1 and 10 atm is relatively low.     

Thermochemistry of uracil and thymine revisited

Thermochemical properties of uracil and thymine have been evaluated using additional experiments. Standard (p⁰ = 0.1 MPa) molar enthalpies of formation in the gas phase at T = 298.15 K for uracil −(298.1 ± 0.6) and for thymine −(337.6 ± 0.9) kJ · mol⁻¹ have been derived from energies of combustion measured by static bomb combustion calorimetry and molar enthalpies of sublimation determined using the transpiration method. The G3 and G4 quantum-chemical methods were used for calculations of theoretical gaseous enthalpies of formation being in very good agreement with the re-measured experimental values.     

Electrochromic properties of poly(alkoxy-terthiophenes): an experimental and theoretical investigation

Much effort has been made to synthesize novel compounds and enhance the optical properties of poly(terthiophenes). The electrochromic properties of poly(4,4′′-dimetoxy-3′-methyl-2′-5′,2′′-terthiophene) (PDMMT) and poly(4,4′′-dipentoxy-3′-methyl-2′-5′,2′′-terthiophene) (PDPMT) have been studied focusing on the differences in the alkoxy-group length. Theoretical calculations were employed to elucidate the structural and thermodynamic stability of the monomers and dimmers. The results showed that in this type of thiophenes large alkoxy chains assist positive charge dispersion through hyperconjugative effect. Thus, PDPMT is thermodynamically more stable than PDMMT in the oxidized state, leading to better electrochromic stability and optical memory.     

2-phenoxathiinyl-5-phenyloxazole and 5-phenoxathiinyl-2-phenyloxazole derivatives: experimental and theoretical study of emission properties

The absorption and emission spectra in cyclohexane and methanol of the title phenoxathiinyl-phenyloxazole derivatives are presented and discussed. Comparing to the unsubstituted diphenyloxazole (PPO), the experimental results show a bathochromic shift of the emission band, a significant dependence of the maximum on the solvent polarity and a drastic decrease of the fluorescence quantum yield. Semiempirical MO calculations (AM1) in both the ground and excited states support the experimental findings. A charge transfer from the phenoxathiin fragment to the oxazole ring is predicted in the excited state explaining the solvatochromism of the compounds. The values for the singlet-triplet gap, 3500-5000 cm-1 point to an enhanced probability of intersystem crossing (ISC) non-radiative deactivation processes, in agreement with the low fluorescence quantum yields.