Vincenzo Mollica

Group contributions to the thermodynamic properties of non-ionic organic solutes in dilute aqueous solution

The thermodynamic properties ΔGho,ΔHho, and ΔCp,hoassociated with the transfer of non-ionic organic compounds from gas to dilute aqueous solution and the limiting partial molar properties Cpo,2 and V22 of these compounds in water are described through a simple scheme of group contributions. A distinction is made between groups made only of carbon and hydrogen, and functional groups i.e. groups containing at least one atom different from carbon and hydrogen. Each group is assigned a contribution, for each property, through a least squares procedure which utilizes only molecules containing at most one functional group. Finally, for compounds containing more than one functional group, correction parameters are evaluated as the differences between the experimental values and those calculated by means of the group contributions. The different behavior of hydrophilic compared with hydrophobic groups is discussed for the various properties. A rationale for the correction parameters, i.e. for the effects of the interactions among hydrophilic groups on the thermodynamic properties, is attempted.

Predicting Physical−Chemical Properties of Compounds from Molecular Structures by Recursive Neural Networks

In this paper, we report on the potential of a recently developed neural network for structures applied to the prediction of physical chemical properties of compounds. The proposed recursive neural network (RecNN) model is able to directly take as input a structured representation of the molecule and to model a direct and adaptive relationship between the molecular structure and target property. Therefore, it combines in a learning system the flexibility and general advantages of a neural network model with the representational power of a structured domain. As a result, a completely new approach to quantitative structure-activity relationship/quantitative structure-property relationship (QSPR/QSAR) analysis is obtained. An original representation of the molecular structures has been developed accounting for both the occurrence of specific atoms/groups and the topological relationships among them. Gibbs free energy of solvation in water, Delta(solv)G degrees , has been chosen as a benchmark for the model. The different approaches proposed in the literature for the prediction of this property have been reconsidered from a general perspective. The advantages of RecNN as a suitable tool for the automatization of fundamental parts of the QSPR/QSAR analysis have been highlighted. The RecNN model has been applied to the analysis of the Delta(solv)G degrees in water of 138 monofunctional acyclic organic compounds and tested on an external data set of 33 compounds. As a result of the statistical analysis, we obtained, for the predictive accuracy estimated on the test set, correlation coefficient R = 0.9985, standard deviation S = 0.68 kJ mol(-1), and mean absolute error MAE = 0.46 kJ mol(-1). The inherent ability of RecNN to abstract chemical knowledge through the adaptive learning process has been investigated by principal components analysis of the internal representations computed by the network. It has been found that the model recognizes the chemical compounds on the basis of a nontrivial combination of their chemical structure and target property.

Thermodynamic study of the partitioning of organic compounds between water and octan-1-ol. Effects of water as cosolvent in the organic phase

The effect of water, as cosolvent, in changing the affinity of octan-1-ol towards organic solutes according to their chemical nature, has been examined by determining the thermodynamic functions of solvation, ΔsolvX°(j)(X=G, H, S), in anhydrous (j= oct) and in water-saturated octan-1-ol (j= oct*) of organic compounds of the RY type (R = hydrocarbon frame, Y = H, O, CO, NH2, NH, N). From these data and the analogous thermodynamic functions of hydration, ΔsolvX°(W), the thermodynamic functions, ΔtrX°(j→j*), for the three ideal or practical processes of transfer (namely, w→oct, w→oct*, oct→oct*) have been calculated.The ΔtrX°(j→j*) functions have been correlated with the structure of the solutes by using their intrinsic volume as a unique structural parameter. The coefficients of the linear equations obtained give information about the contributions to the envisaged thermodynamic function, connected with the formation of a cavity containing the solute and to the solute–solvent interactions. In the case of the ΔtrG°(j→j*) functions, the LSERs (linear solvation energy relationships) have also been used for correlating the thermodynamic functions to the structural features of the solutes.

Isothermal vapour/liquid equilibria of binary mixtures with dibutyl ether at 298.15 K

A head-space gas chromatographic technique has been used to determine vapour/liquid equilibria at 298.15 K of binary mixtures of di-n-butyl ether plus an organic compound (hexane, octane, diethyl ether, tetrahydrofuran, propylamine, butylamine, propanone, cyclopentanone, methanol, butan-1-ol, acetonitrile). Excess molar Gibbs energies, GE, for the systems investigated have been obtained by a least-squares treatment of equilibrium data. Mixtures with hexane, octane and diethyl ether revealed an almost ideal behaviour. Other systems showed positive deviations, which increase with the polarity of the second component. Dibutyl ether + butanol and + cyclopentanone revealed the formation of azeotropic compositions. The Kirkwood–Buff integrals over the entire composition range have been calculated from the composition dependence of GE, and briefly discussed.

Free energy and enthalpy changes for the process of transfer from gas and from dilute aqueous solutions of some alkanes and monofunctional saturated organic compounds

Standard free energies, ΔsolvG°(oct), and enthalpies, ΔsolvH°(oct), of solvation in octan-1-ol of some alkanes (heptane, octane), ketones (propan-2-one, butan-2-one, hexan-2-one, heptan-4-one), ethers (dipropyl ether, dibutyl ether, tetrahydropyran), alkanols (butan-1-ol, butan-2-ol, 2-methylpropan-2-ol) and amines (pyrrolidine, N-methylpyrrolidine, and piperidine) with open-chain and cyclic structure have been determined at 298.15 K. The ΔsolvG°(oct) values were obtained from measurements of partial vapour pressures of dilute solutions and the ΔsolvH°(oct) values, by adding the heats of solution, determined by calorimetry, to known values of enthalpy of vaporization.These data are compared with the standard free energies and enthalpies of solvation in water, and the standard thermodynamic functions for the ideal transfer process of the solutes from pure water to pure octan-1-ol, δtrX°(w–oct)(X=G, H) are calculated.For the examined solutes, hydrocarbons and monofunctional saturated organic compounds, the thermodynamic functions of solvation in octanol and in water are closely correlated to the position of the functional group in the molecular skeleton. The values of the enthalpy of transfer from water to octan-1-ol are also related more to the topologic characteristics of the solute molecules than to their size or to the nature of their functional group. In contrast, the size of the molecules, as well as the presence of a functional group, are important with regard to ΔtrG°(w–oct). An increase of 1 cm3 mol–1 in the intrinsic volumes corresponds to an increase in the value of the partition coefficients of ca. 15%. The substitution of a part of the hydrocarbon surface with a polar surface produces a very large increase in the values of the partition coefficients.The values of the enthalpy of transfer from water to octan-1-ol are always positive, in contrast to the standard free energies of transfer. The entropic term operates by always favouring the transfer towards the alcoholic phase.The potential ability of anhydrous octanol to extract organic solutes of various molecular structure from water-saturated octanol and from hexadecane is also evaluated and discussed.

Thermodynamic study of organic compounds in octan-1-ol. Processes of transfer from gas and from dilute aqueous solution

Free energies and enthalpies of solvation of water and some hydrocarbons (hexane, cyclohexane), ethers (diethyl ether, tetrahydrofuran) and ketones (propanone, pentan-3-one, cyclopentanone) in octan-1-ol have been determined at 298.15 K from vapour-pressure measurements of dilute solutions and from limiting heats of solution. These solvation functions in octanol have been used together with the corresponding hydration functions in order to obtain water–octan-1-ol partition coefficients and their dependence on temperature. A comparison is made with the practical partition coefficients relative to mutually saturated solvents.

Thermodynamic study of organic compounds in di-n-butyl ether. Enthalpy and Gibbs energy of solvation

The Gibbs energies and enthalpies of solvation of some hydrocarbons (n-hexane, n-octane, cyclohexane), alcohols (methanol, propan-1-ol, butan-1-ol, butan-2-ol), ethers (diethyl ether, tetrahydrofuran), ketones (propanone, pentan-3-one, cyclopentanone), amines (n-propylamine, n-butylamine), and acetonitrile in di-n-butyl ether have been determined at 298.15 K from vapour–liquid equilibrium measurements and from limiting enthalpies of solution. The data obtained have been compared with the corresponding values of the solvation functions in octan-1-ol and hexadecane. The phenomenology has been discussed in terms of a simple group additivity scheme. The interaction effects of polar and non-polar groups with the solvents have been deduced from the above group contributions combined with the cavity terms estimated through the scaled particle theory. The linear solvation energy relationships (LSER) have also been used for correlating the thermodynamic solvation function to the structural features of the solutes. All the approaches consistently highlight that the hydrophobic groups exhibit interactions with the solvent of nearly the same strength in the three media, while clearly different interactions are shown by polar groups.

Thermodynamic study of dilute aqueous solutions of organic compounds. Part 4.—Cyclic and straight chain secondary alcohols

Values of the enthalpy, entropy and free energy of solution and hydration are reported for cyclopentanol, cyclohexanol, cycloheptanol, butan-2-ol, pentan-3-ol, hexan-3-ol and heptan-4-ol. Aqueous solutions of cyclic alcohols are thermodynamically more stable, mainly because of entropy effects, than those of the corresponding open chain secondary alcohols. ΔH°h varies linearly with ΔS°h, but the values for cyclic and secondary aliphatic alcohols lie on different straight lines.Parameters in the equation ΔH°h=α+β(ΔS°h+ 45) were calculated for all classes of monofunctional compounds so far investigated. The dependence of α on the type of functional group and on the type of hydrocarbon chain, open or cyclic, in which the latter is inserted is discussed.

Limiting partial molar volumes at 298.15 K of some open chain and cyclic organic compounds in 1—octanol

Abstract Partial molar volumes at 298.15 K in 1—octanol have been determined for some hydrocarbons, ethers, ketones, and water from density measurements carried out with a vibrating-tube density meter. In the transfer process from the pure liquid state to the infinitely dilute solution in 1—octanol, a slight shrinkage is generally observed for solutes showing density values lower than that of the solvent. On the contrary, for solutes with higher density values, a weak expansion is produced. Comparisons are made among the partial molar volumes of organic solutes in 1—octanol, in water, and in other organic solvents. The case of water as a solute in 1—octanol and in many other organic liquids is carefully considered. In non-polar solvents the value of the limiting partial molar volume of water is always larger in respect to the value of the molar volume of pure water, but in polar solvents the contrary occurs. An explanation of this phenomenon is provided and a rationale is given to the value of the limiting partial molar volume of water in 1—octanol and to the trend exhibited by the partial molar volume of water in the 1—octanol/water mixture as water concentration is increased.

Thermodynamic study of dilute aqueous solutions of organic compounds. Part 5.—Open-chain saturated bifunctional compounds

Values have been determined at 25°C of the changes in the free energy, enthalpy and entropy corresponding to the process of transfer from the ideal gas state to dilute aqueous solution for ethylene-diamine, 2-methoxyethylamine, 3-methoxypropylamine, 1,2-dimethoxymethane and four 2-alkoxy-ethanols (methoxy to n-butoxy).These data have been used to calculate the variations in the thermodynamic functions of hydration for the hypothetical process of introducing a Y group (Y = O, NH) into a monofunctional RX compound (X = O, NH, NH2, OH), either by breaking a C—C or a C—H bond. Evidence of strong interactions between hydrophilic centres in bifunctional compounds emerges. The strength and nature (entropic or enthalpic) of these interactions depend on both the type of functional groups and their relative distance.