The enthalpies of formation of pure liquid and gas-phase (Z)-4-hydroxy-3-penten-2-one and 2,4-pentanedione are examined in the light of some more recent NMR studies on the enthalpy differences between gas-phase enthalpies of the two tautomers. Correlation gas chromatography experiments are used to evaluate the vaporization enthalpies of the pure tautomers. Values of (51.2 ( 2.2) and (50.8 ( 0.6) kJ‚mol -1 are measured for pure 2,4-pentanedione and (Z)-4-hydroxy-3-penten-2-one, respectively. The value of (50.8 ( 0.6) kJ‚mol -1 can be contrasted to a value of (43.2 ( 0.2) kJ‚mol -1 calculated for pure (Z)-4-hydroxy-3-penten-2-one when the vaporization enthalpy is measured in a mixture of tautomers. The difference is attributed to an endothermic enthalpy of mixing that destabilizes the mixture relative to the pure components. Calculation of new enthalpies of formation for (Z)-3-hydroxy-3-penten-2-one and 2,4-pentanedione in both the gas, ∆ f H°m(g) ) (-378.2 ( 1.2) and (-358.9 ( 2.5) kJ‚mol -1 , respectively, and liquid phases, ∆ f H°m(l) ) (-429.0 ( 1.0) and (-410.1 ( 1.2) kJ‚mol -1 , respectively, results in enthalpy differences between the two tautomers both in the liquid and gas phases that are identical within experimental error, and in excellent agreement with recent gas-phase NMR studies.
Condensed phase standard (p degrees = 0.1 MPa) molar enthalpies of formation for 1-indanone, 2-indanone, and 1,3-indandione were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation for 1-indanone and 2-indanone, at T = 298.15 K, were measured both by correlation-gas chromatography and by Calvet microcalorimetry leading to a mean value for each compound. For 1,3-indandione, the standard molar enthalpy of sublimation was derived from the vapor pressure dependence on temperature. The following enthalpies of formation in gas phase, at T = 298.15 K, were then derived: 1-indanone, -64.0 +/- 3.8 kJ mol(-1); 2-indanone, -56.6 +/- 4.8 kJ mol(-1); 1,3-indandione, -165.0 +/- 2.6 kJ mol(-1). The vaporization and fusion enthalpies of the indanones studied are also reported. In addition, theoretical calculations using the density functional theory with the B3LYP and MPW1B95 energy functionals and the 6-311G** and cc-pVTZ basis sets have been performed for these molecules and the corresponding one-ring species to obtain the most stable geometries and to access their energetic stabilities.
The vaporization and fusion enthalpies of acetanilide and several of its derivatives have been measured and combined to provide their corresponding sublimation enthalpies. Since all of the materials examined are solid at T = 298.15 K, the vaporization enthalpies measured by correlation gas chromatography at this temperature are for the subcooled liquid state. Fusion enthalpies have also been adjusted to T = 298.15 K. The compounds examined and their vaporization enthalpies measured at T = 298.15 K include the following values (in kJ•mol −1 ): acetanilide (82.1 ± 3.0), 4-methylacetanilide (86.2 ± 3.2), 4-methoxyacetanilide (92.0 ± 3.4), 3-ethoxyacetanilide (94.1 ± 3.5), and 4-ethoxyacetanilide (94.8 ± 3.5). The following fusion enthalpies (kJ•mol −1 ) and fusion temperatures (onset temperatures, K) were measured by differential scanning calorimetry (DSC): acetanilide (22.1 ± 0.1), (386.9 ± 0.1), 4-methylacetanilide (29.4 ± 0.2), (421.3 ± 0.3), 4-methoxyacetanilide (25.4 ± 0.3), (398.2 ± 1.2), 3-ethoxyacetanilide (28.9 ± 0.4), (368.0 ± 0.4), 4-ethoxyacetanilide (32.0 ± 0.1), (407 ± 0.1), and benzanilide (31.2 ± 0.7), (435 ± 0.3). Sublimation enthalpies calculated from vaporization and fusion enthalpies at T = 298.15 K compared favorably with available literature values.
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