Saturation vapor pressures and vaporization enthalpies of esters of ethylene glycol and lower carboxylic acids

Saturation vapor pressures and vaporization enthalpies of ethylene glycol and C₁–C₅ carboxylic acid disubstituted esters of normal and branched structures are determined by the transfer method in the temperature range of 295 to 327 K. Dependences of vaporization enthalpies versus the number of carbon atoms in a molecule and the retention indices are determined. An analysis of existing calculation schemes is given to help predict the vaporization enthalpy of the compounds under study.     

Temperature dependences of saturated vapor pressure and the enthalpy of vaporization of <Emphasis Type="Italic">n</Emphasis>-pentyl esters of dicarboxylic acids

The saturated vapor pressures and enthalpies of vaporization of n-pentyl esters of linear С₂–С₆ dicarboxylic acids are determined by the transpiration method in the temperature range of 309.2–361.2 K. The dependences of enthalpies of vaporization on the number of carbon atoms in the molecule and on the retention indices have been determined. The predictive capabilities of the existing calculation schemes for estimation of enthalpy of vaporization of the studied compounds have been analyzed.     

The enthalpies of vaporization and intermolecular interaction energies of 1-chloroalkanes

The enthalpies of vaporization of 1-chloroalkanes containing n = 3–20 carbon atoms were calculated from saturated vapor pressure data. It was shown that the enthalpies of vaporization of 1-chloroalkanes could be calculated on the basis of model concepts of the structure of liquids from experimental liquid density and sound velocity data.     

Predicting the enthalpies of melting and vaporization for pure components

A mathematical model of the melting and vaporization enthalpies of organic components based on the theory of thermodynamic similarity is proposed. In this empirical model, the phase transition enthalpy for the homological series of n-alkanes, carboxylic acids, n-alcohols, glycols, and glycol ethers is presented as a function of the molecular mass, the number of carbon atoms in a molecule, and the normal transition temperature. The model also uses a critical or triple point temperature. It is shown that the results from predicting the melting and vaporization enthalpies enable the calculation of binary phase diagrams.     

Prediction of enthalpies of vaporization based on modified Randič indices. Esters

In the work a prediction method based on modified Randi? indices to estimate the enthalpies of vaporization under standard conditions Δ_vapH⁰(298.2) is proposed for esters with different structures and numbers of ester groups. It is shown that the proposed method enables the prediction of enthalpies of vaporization of esters with an accuracy as good as experimental.     

Intentional selection of coordination compounds with the required thermochemical properties on the basis of the cambridge bank of structural data

A structural-thermochemical analysis of the Cambridge bank of structural data was performed according to specified number and types of intermolecular contacts. The TOPOS package and the SFERA and KONTAKT programs were used to analyze the structure of mononuclear barium molecular complexes containing completely screened barium atoms. Contacts between their molecules are formed by ligand hydrogen, carbon, and fluorine atoms only, whereas contacts of the type barium-barium or barium-any ligand atom of a neighboring molecule are absent. The enthalpies of vaporization of the selected complexes were estimated and a series of their volatility was established. The results were in agreement with the currently available experimental data.     

Enthalpies of solvation in cyclohexane and in water for homologous aliphatic ketones and esters

The standard enthalpies of solution at infinite dilution were determined for homologous aliphatic ketones and esters in water and in cyclohexane, using a rotating Calvet calorimeter, and solution concentrations about 5×10^–4 mole fraction. Vaporization enthalpies, obtained for each compound with an effusion calorimetric cell, were added to calculate the solvation enthalpies. Their dependence on the number of carbon atoms in the chain is discussed in terms of the Friedman and Krishnan treatment. The effect of polarization of the functional groups is evaluated, and separation from the influence of chain length and the hydrophobic interactions of the methylenes is attempted. For the aqueous solutions, the rearrangement in the structure of the solvent around solute molecules is also considered in relation to deviations from linearity. Comparisons are made with solvation enthalpies obtained for ketones and esters with branched or cyclic substitutes.     

Enthalpies of solvation for N-alkylamides in water and in carbon tetrachloride at 25°C

Molar enthalpies of solution at infinite dilution have been determined at 25°C for several N-alkyl and N,N-dialkylamides in water and in carbon tetrachloride, using a Calvet-type rotating calorimeter, and solution concentrations below 5×10^–2 molal. Relevant enthalpies of transfer between the two solvents also have been derived. Molar enthalpies of solvation have been obtained by adding enthalpies of vaporization to solution values. Results are compared with those of other laboratories on other substituted amides, and their dependence on the number of carbon atoms in the chain is discussed. A possible computation of solvation enthalpies of functional groups is suggested and results for hydration of peptide or similar groups present in the compounds examined are discussed in terms of current models of their hydration and hydrogen bond formation.Presented at the sixth Italian meeting on Calorimetry and Thermal Analysis (AICAT) held in Naples, December 4–7, 1984.     

Saturated vapor pressures and enthalpies of evaporation of esters of glycerol and lower carboxylic acids

Saturated vapor pressures and enthalpies of evaporation of trisubstituted esters of glycerol and carboxylic acids C₁–C₅ with normal and branched structure were determined by the transpiration method in the temperature range of 300–371 K. Dependences of enthalpies of evaporation on the number of carbon atoms in a molecule and on the retention indices are established. Prognostic possibilities of existing calculation schemes are analyzed for compounds with three carboxyl groups in a molecule.     

Standard molar enthalpies of solvation of aliphatic esters in 1,2-dichloroethane andn-hexane

The molar enthalpies of solution at infinite dilution of methyl-, ethyl-, n-butyl-and n-pentylacetate, and ethylpropionate, ethylbutanoate and ethylhexanoate were obtained by means of calorimetric determinations in 1,2-dichloroethane and n-hexane. From these values and the standard molar enthalpy of vaporization, the standard molar enthalpy of solvation of these esters in both solvents were calculated. These calculated values are shown to correlate with the number of methylene groups in the esters.     

Energetics of intra- and intermolecular bonds in ω-alkanediols

The enthalpies of combustion and vaporization at 298.15 K of the-alkanediols were determined with a CRMT rocking calorimeter equipped with a micro-bomb and a Tian-Calvet calorimeter equipped with a Knudsen effusion cell, respectively. The enthalpies of formation in the condensed and gaseous phases and the enthalpies of atomization are calculated. They depend linearly on the number of carbon atoms. Furthermore, the results permit derivation of the enthalpy of an intermolecular hydrogen bond in liquid-alkanediols and calculation of mean enthalpies of the C-C and C-OH bonds. All these thermochemical quantities are discussed in relation to the structural particulars of the molecules.     

Three-dimensional quantitative structure-property relationship (3D-QSPR) models for prediction of thermodynamic properties of polychlorinated biphenyls (PCBs): enthalpy of vaporization

Three-dimensional Quantitative Structure-Property Relationship (QSPR) models have been derived using Comparative Molecular Field Analysis (CoMFA) to correlate the vaporization enthalpies of a representative set of polychlorinated biphenyls (PCBs) at 298.15 K with their CoMFA-calculated physicochemical properties. Various alignment schemes, such as inertial, as is, and atom fit, were employed in this study. The CoMFA models were also developed using different partial charge formalisms, namely, electrostatic potential (ESP) charges and Gasteiger-Marsili (GM) charges. The most predictive model for vaporization enthalpy (Delta(vap)H(m)(298.15 K)), with atom fit alignment and Gasteiger-Marsili charges, yielded r2 values 0.852 (cross-validated) and 0.996 (conventional). The vaporization enthalpies of PCBs increased with the number of chlorine atoms and were found to be larger for the meta- and para-substituted isomers. This model was used to predict Delta(vap)H(m)(298.15 K) of the entire set of 209 PCB congeners.     

Vaporization of Molecular Strontium and Barium β-Diketonates [Sr(15C5)(C5O2F6H)2] and [Ba(18C6)(C5O2F6H)2]. Structure-Thermochemical Approach

Vaporization of the barium molecular complex [Ba(18C6)(C₅O₂F₆H)₂] and the newly prepared strontium complex [Sr(15C5)(C₅O₂F₆H)₂] was studied using a semiempirical structure-thermochemical approach. The studies of intermolecular steric shielding of individual atoms and analysis of the possible intermolecular contacts in these complexes made it possible to identify the atoms and atom groups with significant contributions to the vaporization enthalpy. The hypothetical vaporization enthalpies were calculated by summing the contributions of groups. The melting and sublimation enthalpies were determined experimentally.     

Experimental and computational thermochemical study of 2- and 3-thiopheneacetic acid methyl esters

Thiophene-based compounds have widespread use in modern drug design, biodiagnostics, electronic and optoelectronic devices, and conductive polymers. The present study reports an experimental and computational thermochemical study on the relative stabilities of 2- and 3-thiopheneacetic acid methyl esters. The enthalpies of combustion and vaporization were measured by a rotating-bomb combustion calorimeter, Calvet microcalorimetry, and correlation gas chromatography, and the gas-phase enthalpies of formation at T=298.15 K were determined. Standard ab initio molecular orbital calculations at the G3 level were performed, and a theoretical study of the molecular and electronic structure of the compounds studied was carried out. Calculated enthalpies of formation, using atomization and isodesmic reactions are in very good agreement with the experimental results.     

Estimation des enthalpies de vaporisation des composes organiques liquides. Partie 1. Applications aux alcanes,cycloalcanes, alcenes,hydrocarbures benzeniques,alcools, alcanes thiols,chloro et bromoalcanes,nitriles, esters,acides et aldehydes

Development of the method used by Benson's group for estimating enthalpies of vaporization permits calculation of the enthalpies of formation of the following classes of organic compound in the liquid phase: alkanes, cycloalkanes, alkenes, chloro- and bromoalkanes, alcohols, acids, aldehydes, esters, nitriles, aromatic hydrocarbons, and mercaptans. For high polarity molecules, some correction terms must be introduced into the calculation.     

Interatomic Lennard-Jones potentials of linear and branched alkanes calibrated by Gibbs ensemble simulations for vapor-liquid equilibria

We propose Lennard-Jones potential parameters for interatomic interactions of linear and branched alkanes based on matching the results of Gibbs ensemble simulations of vapor-liquid equilibria to experimental data. The alkane model is similar as in the OPLS-AA, but multiple atom types for carbon based on the number of covalently bonded hydrogen atoms are necessary to accurately reproduce liquid densities and enthalpies of vaporization with the errors of 2.1% and 3.3%, respectively, for hydrocarbons of various chain lengths and structures. We find that the attraction energies of the carbon atoms are almost proportional to the number of covalent hydrogen atoms with each increasing the carbon energy parameter by approximately 0.033 kcal/mol. Though the present force field outperforms the OPLS-AA force field for alkanes we studied, systematic deviations for vapor pressures are still observed with errors of 15%-30%, and critical temperatures are slightly underestimated. We think that these shortcomings are probably due to the inadequacy of the two-parameter Lennard-Jones potential, and especially its behavior at short distances.     

Enthalpies and entropies of vaporization of propan-2-ol-2-methylpropan-1-ol solutions

P-T-x dependences are measured for the solutions of a propan-2-ol-2-methylpropan-1-ol binary system and the enthalpies and entropies of vaporization are determined. Dimerization in propan-2-ol and 2-methylpropan-1-ol is rationalized and the contribution from energy introduced by isostructural methyl groups to the enthalpy of vaporization is determined. Structural and energy analyses of solutions with networks of specific interactions are performed. The formation of heterodimers in solutions and vapors with reduced hydrogen bond energies and specific interactions with the 2s ²(C) unshared electron pairs of the carbon atoms of terminal methyl groups in ethyl and propyl fragments of propan-2-ol and 2-methylpropan-1-ol, respectively, is substantiated. The hydrogen bond energy of heterodimers is estimated.     

Thermochemistry of heteroatomic compounds

The enthalpies of vaporization of different classes three-coordinated arsenic compounds have been determined according to their enthalpies of solution in hexane and molar refraction. The enthalpies of solvation of cyclic and acyclic As(III)-derivatives in hexane, carbon tetrachloride,p-xylene and pyridine are obtained and discussed. Part 6, see Ref. [1].     

Thermochemistry of Heteroatomic Compounds X. The thermochemistry of solution and solvation of substituted alkylphosphonic derivatives

The enthalpies of vaporization of different classes of phosphorylated alcohols and amines were determined from their enthalpies of solution in hexane and carbon tetrachloride. The enthalpies of specific (hydrogen-bond) interaction with the solvents (chloroform and pyridine) of derivatives containing X-H groups (X=O or N) in the α-position to the P=O group were determined. The results were explained in terms of the spatial structure of such compounds. This revised version was published online in August 2006 with corrections to the Cover Date.     

Calculation of Fundamental Thermochemical Parameters of Phosphoranes by the Additivity Scheme

Using the additivity scheme, experimental enthalpies of vaporization and formation of phosphoranes were analyzed, group contributions to the enthalpies of vaporization and formation were determined, and thermochemical parameters of five-coordinate phosphorus derivatives were calculated.