The standard molar enthalpy of formation,molar heat capacities,and thermal stability of anhydrous caffeine

The constant-volume energy of combustion of crystalline anhydrous caffeine (C₈H₁₀N₄O₂) in α (lower temperature steady) crystal form was measured by a bomb combustion calorimeter, the standard molar enthalpy of combustion of caffeine at T = 298.15 K was determined to be −(4255.08 ± 4.30) kJ · mol⁻¹, and the standard molar enthalpy of formation was derived as −(322.15 ± 4.80) kJ · mol⁻¹. The heat capacity of caffeine in the same crystal form was measured in the temperature range from (80 to 387) K by an adiabatic calorimeter. No phase transition or thermal anomaly was observed in the above temperature range. The thermal behavior of the compound was further examined by thermogravimetry (TG), differential thermal analysis (DTA) over the range from (300 to 700) K and by differential scanning calorimetry (DSC) over the range from (300 to 540) K, respectively. From the above thermal analysis a (solid–solid) and a (solid–liquid) phase transition of the compound were found at T = (413.39 and 509.00) K, respectively; and the corresponding molar enthalpies of these transitions were determined to be (3.43 ± 0.02) kJ · mol⁻¹for the (solid–solid) transition, and (19.86 ± 0.03) kJ · mol⁻¹ for the (solid–liquid) transition, respectively.     

Low-temperature heat capacity and standard molar enthalpy of formation of 9-fluorenemethanol (C14H12O)

Low-temperature heat capacities of the 9-fluorenemethanol (C₁₄H₁₂O) have been precisely measured with a small sample automatic adiabatic calorimeter over the temperature range between T=78 K and T=390 K. The solid–liquid phase transition of the compound has been observed to be T_fus=(376.567±0.012) K from the heat-capacity measurements. The molar enthalpy and entropy of the melting of the substance were determined to be Δ_fusH_m=(26.273±0.013) kJ · mol⁻¹ and Δ_fusS_m=(69.770±0.035) J · K⁻¹ · mol⁻¹. The experimental values of molar heat capacities in solid and liquid regions have been fitted to two polynomial equations by the least squares method. The constant-volume energy and standard molar enthalpy of combustion of the compound have been determined, Δ_cU(C₁₄H₁₂O, s)=−(7125.56 ± 4.62) kJ · mol⁻¹ and Δ_cH_m^∘(C₁₄H₁₂O, s)=−(7131.76 ± 4.62) kJ · mol⁻¹, by means of a homemade precision oxygen-bomb combustion calorimeter at T=(298.15±0.001) K. The standard molar enthalpy of formation of the compound has been derived, Δ_fH_m^∘(C₁₄H₁₂O,s)=−(92.36 ± 0.97) kJ · mol⁻¹, from the standard molar enthalpy of combustion of the compound in combination with other auxiliary thermodynamic quantities through a Hess thermochemical cycle.     

Determination of the energies of combustion and enthalpies of formation of nitrobenzenesulfonamides by rotating-bomb combustion calorimetry

The energies of combustion for 2-nitrobenzenesulfonamide (cr), 3-nitrobenzenesulfonamide (cr), and 4-nitrobenzenesulfonamide (cr) were determined using a recently described rotating-bomb combustion calorimeter. The condensed phase molar energies of combustion obtained were ?(3479.2 ± 1.0) kJ · mol^?1 for 2-nitrobenzenesulfonamide (cr), ?(3454.2 ± 1.1) kJ · mol^-1 for 3-nitrobenzenesulfonamide (cr), and ?(3450.1 ± 1.9) kJ · mol^-1 for 4-nitrobenzenesulfonamide (cr). From these combustion energy values, the standard molar enthalpies of formation in the condensed phase were obtained as: ?(341.3 ± 1.3) kJ · mol^?1, ?(366.3 ± 1.3) kJ · mol^?1, and ?(370.4 ± 2.1) kJ · mol^?1, respectively. Polyethene bags were used as an auxiliary material in the combustion experiments. The heat capacities and purities of the compounds were determined using a differential scanning calorimeter.     

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.     

(Solid + liquid) equilibria for the ternary system (CsBr + LnBr3 + H2O) (Ln = Pr,Nd, Sm) at T = 298.2 K and atmospheric pressure,thermal and fluorescent properties of Cs2LnBr5·10H2O

The (solid + liquid) phase equilibria of the ternary systems (CsBr + LnBr₃ + H₂O) (Ln = Pr, Nd, Sm) at T = 298.2 K were studied by the isothermal solubility method. The solid phases formed in the systems were determined by the Schreinemakers wet residues technique, and the corresponding phase diagrams were constructed based on the measured data. Each of the phase diagrams, with two invariant points, three univariant curves, and three crystallization regions corresponding to CsBr, Cs₂LnBr₅·10H₂O and LnBr₃·nH₂O (n = 6, 7), respectively, belongs to the same category. The new solid phase compounds Cs₂LnBr₅·10H₂O are incongruently soluble in water, and they were characterized by chemical analysis, XRD and TG-DTG techniques. The standard molar enthalpies of solution of Cs₂PrBr₅·10H₂O, Cs₂NdBr₅·10H₂O and Cs₂SmBr₅·10H₂O in water were measured to be (52.49 ± 0.48) kJ · mol⁻¹, (49.64 ± 0.49) kJ · mol⁻¹ and (50.17 ± 0.48) kJ · mol⁻¹ by microcalorimetry under the condition of infinite dilution, respectively, and their standard molar enthalpies of formation were determined as being −(4739.7 ± 1.4) kJ · mol⁻¹, −(4728.4 ± 1.4) kJ · mol⁻¹ and −(4724.4 ± 1.4) kJ · mol⁻¹, respectively. The fluorescence excitation and emission spectra of Cs₂PrBr₅·10H₂O, Cs₂NdBr₅·10H₂O and Cs₂SmBr₅·10H₂O were measured. The results show that the upconversion spectra of the three new solid phase compounds all exhibit a peak at 524 nm when excited at 785 nm.     

Energetic and structural properties of 4-nitro-2,1,3-benzothiadiazole

The energetic study of 4-nitro-2,1,3-benzothiadiazole has been developed using experimental techniques together with computational approaches. The standard (p^° = 0.1 MPa) molar enthalpy of formation of crystalline 4-nitro-2,1,3-benzothiadiazole (181.9 ± 2.3 kJ · mol⁻¹) was determined from the experimental standard molar energy of combustion −(3574.3 ± 1.3) kJ · mol⁻¹, in oxygen, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The standard (p^° = 0.1 MPa) molar enthalpy of sublimation, at T = 298.15 K, (101.8 ± 4.3) kJ · mol⁻¹, was determined by a direct method, using the vacuum drop microcalorimetric technique. From the latter value and from the enthalpy of formation of the solid, it was calculated the standard (p^° = 0.1 MPa) enthalpy of formation of gaseous 4-nitro-2,1,3-benzothiadiazole as (283.7 ± 4.9) kJ · mol⁻¹. Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several working reactions in order to derive the standard molar enthalpy of formation 4-nitro-2,1,3-benzothiadiazole. The ab initio results are in good agreement with the experimental data.     

Synthesis,characterization and thermochemistry of Cs(ASP) · nH2O (n = 0, 1)

New compounds of aspartic acid Cs(ASP) · nH₂O (n = 0, 1) have been synthesized and characterized by XRD, IR and Raman spectroscopy as well as TG. The structural formula of this new compound was Cs(ASP) · nH₂O (n = 0, 1). The enthalpy of solution of Cs(ASP) · nH₂O (n = 0, 1) in water were determined. With the incorporation of the standard molar enthalpies of formation of CsOH(aq) and ASP(s), the standard molar enthalpy of formation of −(1202.9 ± 0.2) kJ · mol⁻¹ of Cs(ASP) and −(1490.7 ± 0.2) kJ · mol⁻¹ of Cs(ASP) · H₂O were obtained.     

Thermochemistry of some barium compounds

The molar enthalpies of reaction of metallic barium with 0.047 mol·dm⁻³ HClO₄ as well as the molar enthalpies of dissolution of BaCl₂ in 1.01 mol·dm⁻³ HCl and in water have been measured at T=298.15 K in a sealed swinging calorimeter with an isothermal jacket. From these results the standard molar enthalpy of formation of the barium ion in an aqueous solution at infinite dilution, as well as the enthalpies of formation of barium chloride and barium perchlorate, are calculated to be: Δ_fH⁰_m(Ba²⁺,aq)=−(535.83±1.25) kJ · mol⁻¹; Δ_fH⁰_m(BaCl₂,cr)=−(855.66±1.28) kJ · mol⁻¹; and Δ_fH⁰_m(BaClO₄,cr)=−(796.26±1.35) kJ · mol⁻¹. The results obtained are discussed and compared with previous experimental values.     

Standard enthalpies of formation of 2-aminobenzothiazoles in the crystalline phase by rotating-bomb combustion calorimetry

The standard molar enthalpies of combustion of 2-aminobenzothiazole (2AB), 2-amino-4-methyl-benzothiazole (2A4MB), and 2-amino-6-methyl-benzothiazole (2A6MB) were determined in the crystalline phase at T = 298.15 K using a rotating-bomb combustion calorimeter. The molar energies of combustion of these compounds were found to be: (−4273.6 ± 0.9), (−4896.9 ± 1.1), and (−4906.9 ± 1.2) kJ · mol⁻¹, respectively. From these values, the corresponding standard molar enthalpies of formation in the solid phase were obtained as: (59.55 ± 1.28), (2.71 ± 1.50), and (13.53 ± 1.53) kJ · mol⁻¹, respectively. The enthalpies of formation in the gas phase were determined using the experimental enthalpies of formation in the solid phase and predicted values of the enthalpies of sublimation. Additionally, the enthalpies of formation in the gas phase were calculated by means of the Gausian-4 theory, using several gas-phase working reactions, and were compared with those found using the predicted enthalpies of sublimation.     

Molar heat capacity and thermodynamic properties of 1-cyclohexene-1,2-dicarboxylic anhydride [C8H8O3]

The molar heat capacity C_p,m of 1-cyclohexene-1,2-dicarboxylic anhydride was measured in the temperature range from T=(80 to 360) K with a small sample automated adiabatic calorimeter. The melting point T_m, the molar enthalpy Δ_fusH_m and the entropy Δ_fusS_m of fusion for the compound were determined to be (343.46 ± 0.24) K, (11.88 ± 0.02) kJ · mol⁻¹ and (34.60 ± 0.06) J · K⁻¹ · mol⁻¹, respectively. The thermodynamic functions [H_(T)−H_(298.15)] and [S_(T)−S_(298.15)] were derived in the temperature range from T=(80 to 360) K with temperature interval of 5 K. The mass fraction purity of the sample used in the adiabatic calorimetric study was determined to be 0.9928 by using the fractional melting technique. The thermal stability of the compound was investigated by differential scanning calorimeter (DSC) and thermogravimetric (TG) technique, and the process of the mass-loss of the sample was due to the evaporation, instead of its thermal decomposition.     

The molar enthalpies of solution and vapour pressures of saturated aqueous solutions of some cesium salts

Vapour pressures of water over saturated solutions of cesium chloride, cesium bromide, cesium nitrate, cesium sulfate, cesium formate, and cesium oxalate were determined as a function of temperature. These vapour pressures were used to evaluate the water activities, osmotic coefficients and molar enthalpies of vapourization. Molar enthalpies of solution of cesium chloride, Δ_solH_m(T = 295.73 K; m = 0.0622 mol · kg⁻¹) = (17.83 ± 0.50) kJ · mol⁻¹; cesium bromide, Δ_solH_m(T = 293.99 K; m = 0.0238 mol · kg⁻¹) = (26.91 ± 0.59) kJ · mol⁻¹; cesium nitrate, Δ_solH_m(T = 294.68 K; m = 0.0258 mol · kg⁻¹) = (37.1 ± 2.3) kJ · mol⁻¹; cesium sulfate, Δ_solH_m(T = 296.43 K; m = 0.0284 mol · kg⁻¹) = (16.94 ± 0.43) kJ · mol⁻¹; cesium formate, Δ_solH_m(T = 295.64 K; m = 0.0283 mol · kg⁻¹) = (11.10 ± 0.26) kJ · mol⁻¹ and Δ_solH_m(T = 292.64 K; m = 0.0577 mol · kg⁻¹) = (11.56 ± 0.56) kJ · mol⁻¹; and cesium oxalate, Δ_solH_m(T = 291.34 K; m = 0.0143 mol · kg⁻¹) = (22.07 ± 0.16) kJ · mol⁻¹ were determined calorimetrically. The purity of the chemicals was generally greater than 0.99 mass fraction, except for HCOOCs and (COOCs)₂ where purities were approximately 0.95 and 0.97 mass fraction, respectively. The uncertainties are one standard deviations.     

Destabilization in the isomeric nitrobenzonitriles: an experimental thermochemical study

The enthalpies of combustion and of sublimation, respectively, of the three isomeric nitrobenzonitriles have been measured: o-, {(−3456.3±2.9), (88.1±1.4)} kJ·mol⁻¹; m-, {(−3442.8±3.3), (92.8±0.3)} kJ·mol⁻¹; p-, {(−3448.2±3.6), (91.1±1.3)} kJ·mol⁻¹. In turn, from these values, the standard molar enthalpies of formation for the condensed and gaseous state, respectively, have been derived: o-, {(130.1±3.1), (218.2±3.4)} kJ·mol⁻¹; m-, {(116.5±3.5), (209.3±3.5)} kJ·mol⁻¹; p-, {(122.0±3.8), (213.1±4.0)} kJ·mol⁻¹. Destabilization energies associated with the presence of the two electron-withdrawing groups have been determined, for o-, m-, and p-nitrobenzonitrile, {(17.6±4.1), (8.7±4.2), and (12.5±4.6)} kJ·mol⁻¹, respectively, and are consistent with those obtained for the corresponding sets of isomeric methyl benzenedicarboxylates, dicyanobenzenes, dinitrobenzenes, and (neutral and ionized) nitrobenzoic acids.     

Energetics of aminomethylpyrimidines: An examination of the aromaticity of nitrogen heteromonocyclic derivatives

The standard (p^o = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at the reference temperature of 298.15 K, of 2-amino-4-methylpyrimidine ((98.1 ± 1.6) kJ · mol⁻¹), 2-amino-4,6-dimethylpyrimidine ((55.9 ± 1.8) kJ · mol⁻¹) and 4-amino-2,6-dimethylpyrimidine ((60.1 ± 1.8) kJ · mol⁻¹) were calculated from the enthalpies of formation, in the crystalline phase, and enthalpies of sublimation, derived, respectively, from static bomb combustion calorimetry and Knudsen effusion technique results. In order to quantify the resonance effects arising from the substitution on the pyrimidine ring, hypothetical isodesmic reactions were used to analyze the experimental gaseous-phase enthalpies of formation. The aromaticity of benzene, pyridine, pyrimidine and the substituted pyrimidines was investigated in terms of magnetic (NICS), geometric (HOMA), electronic (Shannon aromaticity, QTAIMs ring critical point properties and HOMO–LUMO gap), reactive (hardness), vibrational (Kekulé mode) and spectroscopic (UV–Vis) properties.     

Micro-combustion calorimetry employing a Calvet heat flux calorimeter

Two micro-combustion bombs developed from a high pressure stainless steel vessel have been adapted to a Setaram C80 Calvet calorimeter. The constant of each micro-bomb was determined by combustions with benzoic acid NIST 39j, giving for the micro-combustion bomb in the measurement sensor k_m=(1.01112±0.00054) and for the micro-combustion bomb in the reference sensor k_r=(1.00646±0.00059) which means an uncertainty of less than 0.06 per cent for calibration. The experimental methodology to get results of combustion energy of organic compounds with a precision also better than 0.06 per cent is described by applying this micro-combustion device to the measurement of the enthalpy of combustion of the succinic acid, giving Δ_cH^∘_m(cr, T=298.15 K)=−(1492.89 ± 0.77) kJ · mol⁻¹.     

Thermodynamic properties of two zinc borates: 3ZnO·3B2O3·3.5H2O and 6ZnO·5B2O3·3H2O

Two pure zinc borates with microporous structure 3ZnO·3B₂O₃·3.5H₂O and 6ZnO·5B₂O₃·3H₂O have been synthesized and characterized by XRD, FT-IR, TG techniques and chemical analysis. The molar enthalpies of solution of 3ZnO·3B₂O₃·3.5H₂O(s) and 6ZnO·5B₂O₃·3H₂O(s) in 1 mol · dm⁻³ HCl(aq) were measured by microcalorimeter at T = 298.15 K, respectively. The molar enthalpies of solution of ZnO(s) in the mixture solvent of 2.00 cm³ of 1 mol · dm⁻³ HCl(aq) in which 5.30 mg of H₃BO₃ were added were also measured. With the incorporation of the previously determined enthalpy of solution of H₃BO₃(s) in 1 mol · dm⁻³ HCl(aq), together with the use of the standard molar enthalpies of formation for ZnO(s), H₃BO₃(s), and H₂O(l), the standard molar enthalpies of formation of −(6115.3 ± 5.0) kJ · mol⁻¹ for 3ZnO·3B₂O₃·3.5H₂O and −(9606.6 ± 8.5) kJ · mol⁻¹ for 6ZnO·5B₂O₃·3H₂O at T = 298.15 K were obtained on the basis of the appropriate thermochemical cycles.     

The role of water in the thermodynamics of drug binding to cyclodextrin

The thermodynamic parameters, Δ_BG^∘, Δ_BH^∘, Δ_BS^∘, and Δ_BC_p, of the drugs flurbiprofen (FLP), nabumetone (NAB), and naproxen (NPX) binding to β-cyclodextrin (βCD) and to γ-cyclodextrin (γCD) in 0.10 M sodium phosphate buffer were determined from isothermal titration calorimetry (ITC) measurements over the temperature range from 293.15 K to 313.15 K. The heat capacity changes for the binding reactions ranged from −(362 ± 48) J · mol⁻¹ · K⁻¹ for FLP and −(238 ± 90) J · mol⁻¹ · K⁻¹ for NAB binding in the βCD cavity to 0 for FLP and −(25.1 ± 9.2) J · mol⁻¹ · K⁻¹ for NPX binding in the larger γCD cavity, implying that the structure of water is reorganized in the βCD binding reactions but not reorganized in the γCD binding reactions. Comparison of the fluorescence enhancements of FLP and NAB upon transferring from the aqueous buffer to isopropanol with the maximum fluorescence enhancements observed for their βCD binding reactions indicated that some localized water was retained in the FLP–βCD complex and almost none in the NAB–βCD complex. No fluorescence change occurs with drug binding in the larger γCD cavity, indicating the retention of the bulk water environment in the drug–γCD complex. Since the specific drug binding interactions are essentially the same for βCD and γCD, these differences in the retention of bulk water may account for the enthalpically driven nature of the βCD binding reactions and the entropically driven nature of the γCD binding reactions.     

Construction,calibration and testing of a micro-combustion calorimeter

An isoperibolic micro-combustion calorimeter was designed, built and set up in our laboratory, taking as base a 1107 Parr combustion bomb of 22 cm³ of volume. Taken into account the geometrical form of the bomb, it was designed and constructed a vessel and a submarine chamber in brass. All of the pieces of the calorimeter were chromium-plated to reduce heat loss by radiation. The calorimeter was calibrated by using pellets of standard benzoic acid (mass approximate of 40 mg) leading to the energy equivalent of ε(calor) = (1283.8 ± 0.6) J · K⁻¹. In order to test the calorimeter, combustion experiments of salicylic acid were performed leading to a value of combustion energy of Δ_cu^∘ = −(21,888.8 ± 10.9) J · g⁻¹, which agrees with the reported literature values. The combustion of piperonylic acid was carried out as a further test leading to a value of combustion energy of Δ_cu^∘ = −(20,215.9 ± 10.4) J · g⁻¹ in accordance with the reported literature value. The uncertainty of the calibration and the combustion of salicylic acid and piperonylic acid was 0.05%.     

1,2,4-Triazole as a reference material for combustion calorimetry of N-containing compounds

A micro-bomb combustion calorimeter recently designed for samples of mass  ≈ 80 mg has been improved and tested with m -methoxybenzoic acid in order to verify the chemistry of the combustion process and the accuracy of the energy corrections involved in the analysis of results. From measurements in this calorimeter, the standard massic energy of combustion of 1,2,4-triazole was determined to beΔ_cu^o =  − (19200.3  ±  3.4)J · g ^− 1. Some new measurements with our macro combustion calorimeter confirm an earlier result from this laboratory of  − (19203.1  ±  1.2)J · g ^− 1. Determination of the purity by d.s.c. of 1,2,4-triazole purified some 10 years ago reveals that samples of this compound remained unchanged and suggest that 1,2,4-triazole be used as a possible reference material for organic compounds with a high content of nitrogen. From the experimental results with the micro-bomb combustion calorimeter, the actual and earlier results from macro-bomb combustion calorimetry, and those obtained in other laboratories, the standard massic energy of combustion of 1,2,4-triazole was deduced to beΔ_cu^o =  − (19202.5  ±  1.7)J · g ^− 1.     

High-precision micro-combustion calorimetry of anthracene

A high-precision micro-combustion calorimeter was developed. Details of the calorimeter are described. The energy equivalent of the calorimeter and the standard deviation of the mean were determined to be (67.8330  ±  0.0024)J · K ^− 1after calibration with thermochemical standard benzoic acid (five experiments). The standard enthalpy of combustion of anthracene was determined to be  − (7065.0  ±  1.1)kJmol ^− 1.     

Vapour pressure of C60 by a transpiration method using a horizontal thermobalance

A horizontal thermal analysis system was adopted for the measurement of vapour pressure of C₆₀ using the vapour transport technique. The experimental precautions taken in order to ensure measurement of equilibrium vapour pressure by the transpiration method are described. The equilibrium nature of these measurements was ensured by the existence of plateau regions in the isothermal plots of apparent vapour pressure as a function of flow rate of the carrier gas. To verify the applicability of this TG based transpiration method, vapour pressure of CsI was measured to be log(p/Pa)=11.667±0.013−(9390±0.078)/T (K) over the range 737–874 K yielding a value of 195.6 kJ mol⁻¹ for the third-law enthalpy of sublimation, ΔH⁰_sub,298 of CsI, the value which compares well with the literature data. The vapour pressure measurements on C₆₀ over the range 789–907 K could be represented by log(p/Pa)=9.018±0.061−(7955±0.280)/T(K). Third-law treatment of the data yielded a value of 183.5±1.0 kJ mol⁻¹ for ΔH⁰_sub,298 of C₆₀ which is in good agreement with some of the other vapour pressure measurements in the literature, if subjected to third-law processing using the same set of free energy functions reliably reported in the literature.