Calculation of the Isobaric Heat Capacities of the Liquid and Solid Phase of Organic Compounds at and around 298.15 K Based on Their "True" Molecular Volume.

Rudolf Naef

doi:10.3390/molecules24081626

Abstract

A universally applicable method for the prediction of the isobaric heat capacities of the liquid and solid phase of molecules at 298.15 K is presented, derived from their “true” volume. The molecules’ “true” volume in A3 is calculated on the basis of their geometry-optimized structure and the Van-der-Waals radii of their constituting atoms by means of a fast numerical algorithm. Good linear correlations of the “true” volume of a large number of compounds encompassing all classes and sizes with their experimental liquid and solid heat capacities over a large range have been found, although noticeably distorted by intermolecular hydrogen-bond effects. To account for these effects, the total amount of 1303 compounds with known experimental liquid heat capacities has been subdivided into three subsets consisting of 1102 hydroxy-group-free compounds, 164 monoalcohols/monoacids, and 36 polyalcohols/polyacids. The standard deviations for Cp(liq,298) were 20.7 J/mol/K for the OH-free compunds, 22.91 J/mol/K for the monoalcohols/monoacids and 16.03 J/mol/K for the polyols/polyacids. Analogously, 797 compounds with known solid heat capacities have been separated into a subset of 555 OH-free compounds, 123 monoalcohols/monoacids and 119 polyols/polyacids. The standard deviations for Cp(sol,298) were calculated to 23.14 J/mol/K for the first, 21.62 J/mol/K for the second, and 19.75 J/mol/K for the last subset. A discussion of structural and intermolecular effects influencing the heat capacities as well as of some special classes, in particular hydrocarbons, ionic liquids, siloxanes and metallocenes, has been given. In addition, the present method has successfully been extended to enable the prediction of the temperature dependence of the solid and liquid heat capacities in the range between 250 and 350 K.

Highlights

The heat capacity is a fundamental extensive property characterizing a molecule’s thermophysical response towards the addition of heat energy, knowledge of which is important e.g., in connection with the adjustment calculation of further thermodynamic properties such as the heats and entropies of sublimation, vaporization or solvation to a defined temperature
Both the lists of molecules used in the correlation calculations of the heat capacities of the solid and liquid phase are collected in standard SDF files and are stored in the Supplementary Material, ready to be imported by external chemistry software
This paper has presented a universally applicable method for the calculation of the heat capacities of the solid and liquid phase of all molecules, based on a property that is common to every molecule, its molecular volume

Summary

Introduction

The heat capacity is a fundamental extensive property characterizing a molecule’s thermophysical response towards the addition of heat energy, knowledge of which is important e.g., in connection with the adjustment calculation of further thermodynamic properties such as the heats and entropies of sublimation, vaporization or solvation to a defined temperature. The theoretical approach for the evaluation of the heat capacity Cp at constant pressure and at a given physical phase state and temperature is feasible for single atoms and basically only for very small molecules due to the rapid increase of the degrees of freedom of translational, rotational, vibrational and electronic motion with the number of atoms in a molecule and the question as to which of these degrees of freedom are excited at all at a certain temperature. Based on these findings, it is clear that the heat capacity of a molecule is temperature-dependent, being zero at 0K and increasing with temperature. These methods have successfully been applied to all kinds of molecules besides ordinary organic compounds: literature references of experimental data for inorganic and organic salts, liquid crystals, ionic liquids (ILs) as well as metal-organics have been found and will be cited later on

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Conclusion