Temperature-Dependent Physicochemical Properties and Solvation Thermodynamics of Nitrotoluenes from Solvation Free Energies

Abstract

Expanded ensemble molecular dynamics simulations are used to calculate the free energies of hydration and self-solvation of low polarity nitrotoluenes over the temperature range of 273 K to 330 K. From this information the liquid, subcooled, and solid-phase vapor pressures, solubilities, Henry’s law constants, hydration and self-solvation entropies, enthalpies, isobaric heat capacities, and enthalpies of vaporization or sublimation are then computed. The values obtained are compared to the limited experimental data available. At a reference temperature of 300 K, the hydration enthalpies are found to be larger in magnitude than hydration entropies for the nitrotoluenes, and vary with the number of nitro groups, while the hydration entropies are almost unchanged as functions of either the number of nitro groups or the solvent accessible surface area. Consequently the variation in the hydration free energies among the nitrotoluenes is due to the variation in their hydration enthalpies. In contrast, both enthalpies and entropies change with the number of nitro groups in self-solvation, and both contribute to the variation of the self-solvation free energy. Also, the isobaric hydration heat capacities are found to change only slightly with temperature.

Molecular Theory of Hydration at Different Temperatures

“…Figure compares the theoretical predictions of the hydration free energies for the five nitrotulenes with MD simulation and experimental values at four different temperatures­(273, 290, 310, and 330 K). Using the experimental data as a benchmark, we find that the average unsigned error (AUE) of the theoretical predictions is 0.74 kcal/mol, which is comparable to the AUE of 0.68 kcal/mol for MD simulation . The magnitude of the solvation free energy increases with the number of nitro groups due to more extensive hydrophilic hydration.…”

“…Nitrotoluenes are important industrial agents used as pigments, photographic chemicals, pesticides, and explosives. We test our theoretical procedure against the hydration free energies of nitrotoluenes not only because of the practical relevance of such properties for environmental regulations but also for the readily availability of such results from previous experiment and simulation . Specifically, we consider solvation of five nitrotoluenes in water from 273 to 330 K. Figure illustrates schematically the chemical structure of these nitrotoluenes: 2-nitrotoluene (2-NT), 4-nitrotoluene (4-NT), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), and 2,4,6-trinitrotoluene (2,4,6-TNT).…”

“…Linear fitting of the reduced solvation free energy Δ G sol / RT versus 1/ T . The points are from (A) DFT predictions, (B) experimental data, and (C) MD data …”

“…Except for 2,4,6-TNT, both theory and simulation underestimate the enthalpy of solvation in absolute values. The discrepancy may be attributed to strong conjugation between the nitro group with the π-electrons of the benzene ring that is not adequately captured by conventional force fields …”

Solubility of Polar and Non-Polar Aromatic Molecules in Subcritical Water: The Role of the Dielectric Constant

“…(2) has been shown to hold for various liquid and solid solutes. 54,55,66,67 From eq. (2), solubility can be calculated from the hydration free energy and the pure solute A vapor pressure, which in turn can be obtained from the solute self-solvation free energy 54,68 .…”