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Abstract  

The vapour pressures of six para-substituted benzoic acids were measured using the Knudsen effusion method within the pressure range (0.1–1 Pa) in the following temperature intervals: 4-hydroxybenzoic acid (365.09–387.28) K; 4-cyanobenzoic acid (355.14–373.28) K; 4-(methylamino)benzoic acid (359.12–381.29) K; 4-(dimethylamino)benzoic acid (369.29–391.01) K; 4-(acetylamino)benzoic acid (423.10–443.12) K; 4-acetoxybenzoic acid (351.28–373.27) K. From the temperature dependence of the vapour pressure, the standard molar enthalpy, entropy and Gibbs energy of sublimation, at the temperature 298.15 K, were derived for each of the studied compounds using estimated values of the heat capacity differences between the gaseous and the crystalline phases. Equations for estimating the vapour pressure of para substituted benzoic acids at the temperature of 298.15 K are proposed.

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Abstract  

The standard (p° = 0.1 MPa) molar enthalpies of formation in the crystalline state of the 2-, 3- and 4-hydroxymethylphenols,
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $${{\Updelta}}_{\text{f}} H_{\text{m}}^{\text{o}} ( {\text{cr)}} = \, - ( 3 7 7. 7 \pm 1. 4)\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$ \end{document}
,
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $${{\Updelta}}_{\text{f}} H_{\text{m}}^{\text{o}} ( {\text{cr) }} = - (383.0 \pm 1.4) \, \,{\text{kJ}}\,{\text{mol}}^{ - 1}$$ \end{document}
and
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $${{\Updelta}}_{\text{f}} H_{\text{m}}^{\text{o}} ( {\text{cr)}} = - (382.7 \pm 1.4)\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$ \end{document}
, respectively, were derived from the standard molar energies of combustion, in oxygen, to yield CO2(g) and H2O(l), at T = 298.15 K, measured by static bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to measure the dependence of the vapour pressure of the solid isomers of hydroxymethylphenol with the temperature, from which the standard molar enthalpies of sublimation were derived using the Clausius–Clapeyron equation. The results were as follows:
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\Updelta_{\rm cr}^{\rm g} H_{\rm m}^{\rm o} = (99.5 \pm 1.5)\,{\text{kJ}}\,{\text{mol}}^{ - 1}$$ \end{document}
,
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\Updelta_{\rm cr}^{\rm g} H_{\rm m}^{\rm o} = (116.0 \pm 3.7) \,{\text{kJ}}\,{\text{mol}}^{ - 1}$$ \end{document}
and
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\Updelta_{\rm cr}^{\rm g} H_{\rm m}^{\rm o} = (129.3 \pm 4.7)\,{\text{ kJ mol}}^{ - 1}$$ \end{document}
, for 2-, 3- and 4-hydroxymethylphenol, respectively. From these values, the standard molar enthalpies of formation of the title compounds in their gaseous phases, at T = 298.15 K, were derived and interpreted in terms of molecular structure. Moreover, using estimated values for the heat capacity differences between the gas and the crystal phases, the standard (p° = 0.1 MPa) molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, were derived for the three hydroxymethylphenols.
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Enthalpies and entropies of sublimation for N-acetylglycine amide (NAGA), N-acetyl-L-alanine amide (L-NAAA), and N-acetyl-D-leucine amide (D-NALA) were determined from the dependence of their vapour pressures on temperature, as measured by the torsion-effusion method.

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Abstract  

In this paper a review of recent results concerning thermodynamic properties of solid uncharged derivatives of some amino acids and small peptides is reported. The experimental data obtained by different calorimetric methods are: sublimation enthalpies, heat capacities, enthalpies and temperatures of fusion and, in few cases, enthalpies and temperatures of solid-to-solid transitions. The standard molar and specific enthalpies and entropies of sublimation at 298.15 K have been calculated integrating the heat capacities of solids and vapours as function of temperature or directly measuring by calorimetry the heating enthalpies. The first ones have been obtained by interpolation of the values calculated according to the group additivity method of Benson. The sublimation thermodynamic properties have regarded N-acetylamides of glycine (NAGA), L-alanine (L-NAAA), L-valine (L-NAVA), D- and L-leucine (D-NALA and L-NALA, respectively) and L-isoleucine (L-NAIA) as well as the cyclic dipeptides glycyl-glycine (c-Gly-Gly), glycyl-L-alanine (c-Gly-L-Ala), L-alanyl-L-alanine (c-L-Ala-L-Ala) and sarcosyl-sarcosine (c-Sar-Sar). Heat capacities of the solid phases have been included also for N-acetylamide of L-proline (L-NAPA), N-methyl derivatives of the N-acetylamides previously cited and other amino acids, such as phenylalanine (F), isobutyric acid (isoBu), norvaline (norV) and norleucine (norL). In the text these substances are indicated as NAFAMe, etc. The heat capacities of their racemes are also reported. The fusion properties have concerned only two raceme mixtures (D,L-NAAA and D,L-NALA) and N-acetylamides of the cited amino acids, sarcosine (NASarA) and the following di-or tripeptides: glycyl-L-alanine (NAGAA), L-alanyl-L-alanine (NAA2A), glycyl-L-proline (NAGPA), L-prolyl-glycine (NAPGA), L-leucyl-L-proline (NALPA) and L-prolyl-L-leucyl-glycine (NAPLGA). Finally, solid-to-solid transitions have been found and characterized for L-NALA and NAGPA. All thermodynamic properties are discussed in the light of the crystal packing parameters determined during parallel crystallographic studies. It allows a comprehensive rationale of the behaviour of the solid state and its transitions for this interesting family of substances.

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] Table 5 also presents the standard entropies of sublimation {calculated as [Δ cr g H m 0 (298.15 K) − Δ cr g G m 0 (298.15 K)]/298.15} and the selected values of standard enthalpy and Gibbs energy of sublimation of the compounds studied, and of

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. Monte , MJS , Santos , LMNBF , Fonseca , JMS , Sousa , CAD . Vapour pressures, enthalpies and entropies of sublimation of para substituted benzoic acids . J Therm Anal Calorim . 2010 ; 100 : 465 – 474 . 10.1007/s10973

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