Rediscovering the Wheel. Thermochemical Analysis of Energetics of the Aromatic Diazines

Thermochemical properties of pyrimidine, pyrazine, and pyridazine have been measured and re-evaluated to provide benchmark quality results. A new internally consistent data set of Δ_f<i>H</i>_m^°(g) has been obtained from combustion calorimetry and vapor pressure measurements. The gas and condensed phase enthalpies of formation of the parent diazines have been re-evaluated, and the results were compared to current theoretical calculations using the highly accurate first-principles methods: G3, G4, CBS-APNO, W1­(RO). Simple “corrected atomization procedures” to derive theoretical Δ_f<i>H</i>_m^°(g) directly from the enthalpies <i>H</i>₂₉₈ have been tested and recommended as an alternative to using the bond separation and isodesmic reaction models for organic cyclic and heterocyclic compounds containing one to three nitrogen atoms

Thermochemistry of Uracils. Experimental and Computational Enthalpies of Formation of 5,6-Dimethyl‑, 1,3,5-Trimethyl‑, and 1,3,5,6-Tetramethyluracils

We describe in the current paper an experimental and computational study of three methylated uracils, in particular, the 5,6-dimethyl-, 1,3,5-trimethyl-, and 1,3,5,6-tetramethyl derivatives. The values of the standard (<i>p</i>⁰ = 0.1 MPa) molar enthalpies of formation in the gas phase at <i>T</i> = 298.15 K have been determined. The energies of combustion were measured by static bomb combustion calorimetry, and from the results obtained, the standard molar enthalpies of formation in the crystalline state at <i>T</i> = 298.15 K were calculated. The enthalpies of sublimation were determined using the transpiration method in a saturated N₂ stream. Values of −(376.2 ± 2.6), −(355.9 ± 3.0), and −(381.7 ± 2.8) kJ·mol^–1 for the gas-phase enthalpies of formation at <i>T</i> = 298.15 K of 5,6-dimethyluracil, 1,3,5-trimethyluracil, and 1,3,5,6-tetramethyluracil, respectively, were obtained from the experimental thermochemical study. An extended theoretical study with the G3 and the G4 quantum-chemical methods has been carried out for all the possible methylated uracils. There is a very good agreement between experimental and calculated enthalpies of formation for the three derivatives studied. A Free–Wilson analysis on G4-calculated enthalpies of formation has been carried out, and the contribution of methylation in the different positions of the uracil ring has been estimated

The Mobility of Water Molecules through Hydrated Pores

To achieve a more exact understanding of the water transport in natural channels, a series of non-natural structures have been developed. They have been studied by far-infrared spectroscopy, solid-state nuclear magnetic resonance, differential scanning calorimetry, thermogravimetric analysis, and variable-temperature powder X-ray diffraction to examine the behavior of water at the molecular level. Water in these predominantly nonpolar pores can be metastable, with filling and emptying occurring upon changes in solvent conditions. The water contained in these pores exhibits a dynamics that might be controlled, since it depends on the structural features of the monomers that form the pore “skeleton”. We have observed changes in the pore diameter depending on the selected isomer. This provokes at a given temperature differences in the arrangement and dynamics of the contained water. The water dynamics increases with both temperature and pore diameter in a process that is reversible over a temperature range specific for each structure. Beyond this particular temperature threshold, the pore water can be irreversibly evacuated, and at this point a decrease of the dynamics is observed. The slower dynamics of the remaining water in partially evacuated pores is probably due to the increased interaction with the inner-pore surface owing to a concomitant narrowing of the pore. These findings not only highlight the need for the presence of freely moving water inside the pore to sustain its permeability by water, but also point to the decrease in the dynamics of the remaining water in partially evacuated pores