Standard Enthalpy Of Fusion Research Articles

QSPR Models for the Prediction of Some Thermodynamic Properties of Cycloalkanes Using GA-MLR Method.

Cycloalkanes have been largely used in the field of medicine, components of food, pharmaceutical drugs, and they are mainly used to produce fuel. In present study the relationship between molecular descriptors and thermodynamic properties such as the standard enthalpies of formation (∆H°f), the standard enthalpies of fusion (∆H°fus), and the standard Gibbs free energy of formation (∆G°f)of the cycloalkanes is represented. The Genetic Algorithm (GA) and multiple linear regressions (MLR) were successfully used to predict the thermodynamic properties of cycloalkanes. A large number of molecular descriptors were obtained with the Dragon program. The Genetic algorithm and backward method were used to reduce and select suitable descriptors. QSPR models were used to delineate the important descriptors responsible for the properties of the studied cycloalkanes. The multicollinearity and autocorrelation properties of the descriptors contributed in the models were tested by calculating the Variance Inflation Factor (VIF), Pearson Correlation Coefficient (PCC) and the Durbin-Watson (DW) statistics. The predictive powers of the MLR models were discussed using Leave-One-Out Cross-Validation (LOOCV) and test set validation methods. The statistical parameters of the training, and test sets for GA-MLR models were calculated. The results of the present study indicate that the predictive ability of the models was satisfactory and molecular descriptors such as: the Functional group counts, Topological indices, GETAWAY descriptors, Constitutional indices, and molecular properties provide a promising route for developing highly correlated QSPR models for prediction the studied properties.

Calculation of Five Thermodynamic Molecular Descriptors by Means of a General Computer Algorithm Based on the Group-Additivity Method: Standard Enthalpies of Vaporization, Sublimation and Solvation, and Entropy of Fusion of Ordinary Organic Molecules and Total Phase-Change Entropy of Liquid Crystals.

The calculation of the standard enthalpies of vaporization, sublimation and solvation of organic molecules is presented using a common computer algorithm on the basis of a group-additivity method. The same algorithm is also shown to enable the calculation of their entropy of fusion as well as the total phase-change entropy of liquid crystals. The present method is based on the complete breakdown of the molecules into their constituting atoms and their immediate neighbourhood; the respective calculations of the contribution of the atomic groups by means of the Gauss-Seidel fitting method is based on experimental data collected from literature. The feasibility of the calculations for each of the mentioned descriptors was verified by means of a 10-fold cross-validation procedure proving the good to high quality of the predicted values for the three mentioned enthalpies and for the entropy of fusion, whereas the predictive quality for the total phase-change entropy of liquid crystals was poor. The goodness of fit (Q2) and the standard deviation (σ) of the cross-validation calculations for the five descriptors was as follows: 0.9641 and 4.56 kJ/mol (N = 3386 test molecules) for the enthalpy of vaporization, 0.8657 and 11.39 kJ/mol (N = 1791) for the enthalpy of sublimation, 0.9546 and 4.34 kJ/mol (N = 373) for the enthalpy of solvation, 0.8727 and 17.93 J/mol/K (N = 2637) for the entropy of fusion and 0.5804 and 32.79 J/mol/K (N = 2643) for the total phase-change entropy of liquid crystals. The large discrepancy between the results of the two closely related entropies is discussed in detail. Molecules for which both the standard enthalpies of vaporization and sublimation were calculable, enabled the estimation of their standard enthalpy of fusion by simple subtraction of the former from the latter enthalpy. For 990 of them the experimental enthalpy-of-fusion values are also known, allowing their comparison with predictions, yielding a correlation coefficient R2 of 0.6066.

Enthalpies of fusion, vaporisation and sublimation of crown ethers determined by thermogravimetry and differential scanning calorimetry

The phase change enthalpies of 18-crown-6 ether (18C6), dibenzo-18-crown-6 ether (DB18C6), dibenzo-24-crown-8 ether (DB24C8) and dibenzo-30-crown-10 ether (DB30C10), which are chemical structures that are of interest in supramolecular chemistry, were determined by thermogravimetry (TGA) and differential scanning calorimetry (DSC). The vapour pressure data as a function of the temperature for the compounds under investigation were estimated by using thermogravimetry and applying the Langmuir equation; thus, the enthalpies of vaporisation were derived. The standard enthalpies of fusion were measured by differential scanning calorimetry then added to those of vaporisation to obtain reliable results for the enthalpy of sublimation at the fusion temperatures. For each crown ether, the heat capacities of the solid and liquid phase were also measured by DSC and that of the gas phase was theoretically estimated by DFT calculations, then all of them were applied to derive the enthalpies of sublimation at 298.15K, which were determined to be (139.7±3.6) kJmol−1 for 18-crown-6 ether, (190.6±3.6) kJmol−1 for dibenzo-18-crown-6 ether, (232.9±3.6) kJmol−1 for dibenzo-24-crown-8 ether and (255.7±5.3) kJmol−1 for dibenzo-30-crown-10 ether. The meaning of these results with regard to the energy of the cohesion in the solid of each crown ether is discussed herein.

Gas phase enthalpies of formation of nitrobenzamides using combustion calorimetry and thermal analysis

The standard molar energies of combustion of 2-nitrobenzamide, 3-nitrobenzamide and 4-nitrobenzamide were determined with an isoperibolic, static-bomb, combustion calorimeter. From the combustion results, the standard molar enthalpies of combustion and formation for these compounds in the condensed phase at T=298.15K were derived. Subsequently, to determine the enthalpies of sublimation, the vapour pressure data as a function of the temperature for the compounds under investigation were estimated using thermogravimetry by applying Langmuir’s equation, and the enthalpies of vaporisation were derived. Standard enthalpies of fusion were measured by differential scanning calorimetry then added to those of vaporisation to obtain reliable results for the enthalpy of sublimation. From the combustion and sublimation data, the gas phase enthalpies of formation were determined to be (−138.9±3.5)kJ·mol−1, (−122.9±2.9)kJ·mol−1 and (−108.5±3.7)kJ·mol−1 for the ortho, meta and para isomers of nitrobenzamide, respectively. The meaning of these results with regard to the enthalpic stability of these molecular structures is discussed herein.

Role of vapor pressure of 1,4-bis(trimethylsilyl)benzene in developing silicon carbide thin film using a plasma-assisted liquid injection chemical vapor deposition process

The temperature dependence equilibrium vapor pressure (pe)T data yielded a straight line when ln(pe) was plotted against the reciprocal temperature in the range of 312.82–367.12K, leading to a standard enthalpy of sublimation (ΔsubH°) value of 68.2±0.8kJmol−1 for 1,4-bis(trimethylsilyl)benzene (TMSB). From the depression of the melting point in the DTA-mode, the standard enthalpy of fusion (ΔfusH°) was found to be 26.9±2.5kJmol−1. A thin film of silicon carbide was grown on graphite substrate at 573K using TMSB or bis(trimethylsilyl)acetylene as precursors. The deposited films were characterized by scanning electron microscopy and energy dispersive X-ray analysis for their composition and morphology.

Calorimetric Studies on the Phenolic Glycoside D(–)-Salicin

A pure orthorhombic phase sample of D(–)-salicin was purified and characterized for calorimetric measurements. From differential scanning calorimetry (DSC) measurements it was found that the onset and maximum temperatures of the fusion peak were Ton = (473.30 ± 0.05) K and Tmax = (474.74 ± 0.05) K, respectively, and that the corresponding standard enthalpy of fusion was ΔcrlHm° = (55.5 ± 0.4) kJ mol−1. From the last two values the standard entropy of fusion is calculated as ΔcrlSm° = (116.9 ± 0.4) J mol−1 K−1. The standard molar enthalpy of formation of orthorhombic D(–)-salicin at T = 298.15 K, was determined as ΔfHm°(C13H18O7, cr, orthorhombic) = −(1366.9 ± 3.2) kJ mol−1, by combustion calorimetry. From the results of solution calorimetry obtained in this work and some auxiliary values taken from the literature the enthalpy of reaction of hydrolysis of D(–)-salicin to produce β-glucose and o-hydroxybenzyl alcohol was found marginally thermoneutral, if the uncertainty interval was considered. Additionally, specific heat capacity measurements on the orthorhombic phase, glass and liquid-quenched glass obtained by DSC was reported. © 2008 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:5354–5362, 2008

Temperature dependence vapour pressure measurements of Mg(tmhd) 2(tmeda) [(tmhd = 2,2,6,6-tetramethyl-3,5-heptanedione, tmeda = N, N, N′, N′-tetramethylethylenediamine

The reaction between the magnesium β-diketonate complex Mg(tmhd) 2(H 2O) 2 and 1 equiv. of N, N, N′, N′-tetramethylethylenediamine (tmeda = Me 2NCH 2CH 2NMe 2) in hexane at room temperature yielded Mg(tmhd) 2(tmeda). The standard enthalpy of sublimation (83.2 ± 2.3 kJ mol −1) and entropy of sublimation (263 ± 6.3 J mol −1 K −1) of Mg(tmhd) 2(tmeda) were obtained from the temperature dependence vapour pressure, determined by adopting a horizontal dual arm single furnace thermogravimetric analyser as a transpiration apparatus. From the observed melting point depression DTA, the standard enthalpy of fusion (58.3 ± 5.2 kJ mol −1) was evaluated, using the ideal eutectic behaviour of Mg(tmhd) 2(tmeda) as a solvent with bis(2,4-pentanedionato)magnesium(II), Mg(acac) 2 as a non-volatile solute.

Group-contribution based estimation of pure component properties

A new method for the estimation of properties of pure organic compounds is presented. Estimation is performed at three levels. The primary level uses contributions from simple groups that allow describing a wide variety of organic compounds, while the higher levels involve polyfunctional and structural groups that provide more information about molecular fragments whose description through first-order groups is not possible. The presented method allows estimations of the following properties: normal boiling point, critical temperature, critical pressure, critical volume, standard enthalpy of formation, standard enthalpy of vaporization, standard Gibbs energy, normal melting point and standard enthalpy of fusion. The group-contribution tables have been developed from regression using a data set of more than 2000 compounds ranging from C=3–60, including large and complex polycyclic compounds. Compared to the currently used group-contribution methods, the new method makes significant improvements both in accuracy and applicability.

Thermodynamic stability of solid intermolecular compounds

Standard enthalpies of formation for 25 solid intermolecular compounds have been calculated from solid–liquid phase equilibria information, and standard enthalpies of fusion have been calculated for 11 of their pure components. The enthalpies of formation of the solid intermolecular compounds range from high negative to moderate positive values. Their stabilities are discussed in terms of enthalpic and entropic effects.

Vaporization behaviour of uranium tetrafluoride

The vapour pressure of solid and liquid UF 4 has been measured in the temperature range 1169 to 1307 K and 1312 to 1427 K respectively, by both transpiration and evaporation-temperature methods. The vapour pressures obtained by the two methods agree well and were, therefore, combined to give the equations: log 10{p( UF 4, s)/ kPA} = (12.03±0.14)−(15994±176)( K T ) log 10{p( UF 4, l)/ kPA} = (8.99±0.24)−(12014±335)( K T ) The melting temperature, normal boiling temperature, and the standard enthalpy of fusion at the melting temperature were calculated to be 1309 K, 1720 K, and (70.3 ± 7.3) kJ mol −1 respectively. The standard enthalpy of vaporization ΔH o(vap, 298.15 K), as calculated from the vapour pressures of UF 4(s) by the second law, is (326.1±3.3) kJ mol −1 and the standard entropy of vaporization ΔS o(vap, 298.15 K) is (217.6±15.7) J K −1 mol −1. The vapour pressures and other thermodynamic quantities are compared with data from the literature.