Improved evaluation of liquid densities using van der Waals molecular models

Pencil and Paper Estimation of Hansen Solubility Parameters

Simple procedures to estimate Hansen solubility parameter (HSP) components from structural formulas are investigated. The best results are obtained using a simple relationship with molar volume and refractivity for the dispersion component, and using additivity models based on tailored fragments specifically designed for the polar and hydrogen bonding components. Despite large errors for some classes of chemicals, including small inorganic molecules, ionic liquids, and high halogen compounds, these models yield average absolute deviations from reference on par with state-of-the-art models and lower than reported using molecular dynamics simulations or nonlinear quantitative structure-property relationship models based on a limited set of quantum chemical descriptors. In contrast to group contribution methods that are either more restricted in scope or heavily parameterized, they are thoroughly validated and very easy to apply. Furthermore, the errors observed are easy to rationalize and may usually be anticipated. This work sheds light on some limitations inherent to pure additivity approaches for HSP prediction and provides a first step toward better models. A Python script implementing the procedure and the fully detailed results are provided as the Supporting Information.

Using elemental ratios to predict the density of organic material composed of carbon, hydrogen, and oxygen

A governing equation was developed to predict the density ρ(org) of organic material composed of carbon, oxygen, and hydrogen using the elemental ratios O:C and H:C as input parameters: ρ(org) = 1000 [(12 + 1(H:C) + 16(O:C)]/[7.0 + 5.0(H:C) + 4.15(O:C)] valid for 750 < ρ(org) < 1900 kg m(-3). Comparison of the actual to predicted ρ(org) values shows that the developed equation has an accuracy of 12% for more than 90% of the 31 atmospherically relevant compounds used in the training set. The equation was further validated for secondary organic material (SOM) produced by isoprene photo-oxidation and by α-pinene ozonolysis. Depending on the conditions of SOM production, ρ(org/SOM) ranged from 1230 to 1460 kg m(-3), O:C ranged from 0.38 to 0.72, and H:C ranged from 1.40 to 1.86. Atmospheric chemistry models that simulate particle production and growth can employ the developed equation to simulate particle physical properties. The equation can also extend atmospheric measurements presented as van Krevelen diagrams to include estimates of the material density of particles and their components. Use of the equation, however, is restricted to particle components having negligible quantities of additional elements, most notably nitrogen.

Inductive modeling of physico-chemical properties: flash point of alkanes

The problem of predicting flash points (T(*)) of alkanes from their molecular formula is revisited. Starting from an examination of the dependence of T(*) on the length of the carbon chain for n-alkanes, a new model is proposed. Despite its extreme simplicity, it performs better than published alternatives based on advanced regression techniques. This illustrates the interest of an inductive approach to quantitative structure-property relationships, whereby a model is first developed for restricted series of simple compounds before being generalized.

Exhaustive QSPR Studies of a Large Diverse Set of Ionic Liquids: How Accurately Can We Predict Melting Points?

Several popular machine learning methods--Associative Neural Networks (ANN), Support Vector Machines (SVM), k Nearest Neighbors (kNN), modified version of the partial least-squares analysis (PLSM), backpropagation neural network (BPNN), and Multiple Linear Regression Analysis (MLR)--implemented in ISIDA, NASAWIN, and VCCLAB software have been used to perform QSPR modeling of melting point of structurally diverse data set of 717 bromides of nitrogen-containing organic cations (FULL) including 126 pyridinium bromides (PYR), 384 imidazolium and benzoimidazolium bromides (IMZ), and 207 quaternary ammonium bromides (QUAT). Several types of descriptors were tested: E-state indices, counts of atoms determined for E-state atom types, molecular descriptors generated by the DRAGON program, and different types of substructural molecular fragments. Predictive ability of the models was analyzed using a 5-fold external cross-validation procedure in which every compound in the parent set was included in one of five test sets. Among the 16 types of developed structure--melting point models, nonlinear SVM, ASNN, and BPNN techniques demonstrate slightly better performance over other methods. For the full set, the accuracy of predictions does not significantly change as a function of the type of descriptors. For other sets, the performance of descriptors varies as a function of method and data set used. The root-mean squared error (RMSE) of prediction calculated on independent test sets is in the range of 37.5-46.4 degrees C (FULL), 26.2-34.8 degrees C (PYR), 38.8-45.9 degrees C (IMZ), and 34.2-49.3 degrees C (QUAT). The moderate accuracy of predictions can be related to the quality of the experimental data used for obtaining the models as well as to difficulties to take into account the structural features of ionic liquids in the solid state (polymorphic effects, eutectics, glass formation).

Hydration of Aprotic Donor Solvents Studied by Means of FTIR Spectroscopy

The paper attempts to explain the mutual influence of nonpolar and electron-donor groups on solute hydration, the problem of big importance for biological aqueous systems. Aprotic organic solvents have been used as model solutes, differing in electron-donating power. Hydration of acetonitrile, acetone, 2-butanone, and triethylamine has been studied by HDO and (partially) H2O spectra. The quantitative version of difference spectra method has been applied to determine solute-affected water spectra. Analysis of the data suggests that solvent-water interaction via the donor center of the solute is averaged between water-water interactions around the solute. Such behavior can be simply explained by the model of solute rotating in a cavity of water structure, which is formed by clathratelike hydrogen-bonded water network. On the basis of the band shape of solute-affected HDO spectra and the corresponding distribution of intermolecular distances, the criterion for hydrophobic type hydration has been proposed. From that point of view, all the studied solutes could be treated as hydrophobic ones. The limiting band position and the corresponding intermolecular distance of affected water, gained with increasing electron-donating power of solutes, has been inferred from the data obtained. These observations are important for interpretation of vibrational spectra of water as well as for volumetric measurements of solutions. The simple model of hydration, proposed to better justify the results, connects the values obtained from the methods providing microscopic and macroscopic characteristics of the system studied.