Computation of entropy values for non-electrolyte solute molecules in solution based on semi-empirical corrections to a polarized continuum model

A simple heuristic model was developed for estimating the entropy of a solute molecule in an ideal solution based on quantum mechanical calculations with polarizable continuum models (QM/PCMs). A translational term was incorporated that included free-volume compensation for the Sackur-Tetrode equation and a rotational term was modeled based on the restricted rotation of a dipole in an electrostatic field. The configuration term for the solute at a given concentration was calculated using a simple lattice model that considered the number of configurations of the solute within the lattice. The configurational entropy was ascertained from this number based on Boltzmann's principle. Standard entropy values were determined for 41 combinations of solutes and solvents at a set concentration of 1 mol dm^-3 using the proposed model, and the computational values were compared with experimental data. QM/PCM calculations were conducted at the ωB97X-D/6-311++G(d,p)/IEF-PCM level using universal force field van der Waals radii scaled by 1.2. The proposed model accurately reproduced the entropy values reported for solutes in non-aqueous solvents within a mean absolute deviation of 9.2 J mol^-1 K^-1 for 33 solutions. This performance represents a considerable improvement relative to that obtained using the method based on the ideal gas treatment that is widely utilized in commercially available computation packages. In contrast, computations for aqueous molecules overestimated the entropies because hydrophobic effects that decrease the entropy of aqueous solutions were not included in the present model.

Detailed Kinetic Model for the Thermal Decomposition of Hydrazine Nitrate in Nitric Acid Solution Based on Quantum Chemistry Calculations Combined with the Polarizable Continuum Model

The decomposition mechanism of hydrazine nitrate in nitric acid solutions was investigated using quantum chemistry calculations combined with the polarizable continuum model at the CBS-QB3//ωB97X-D/SMD level of theory. These calculations provided a detailed kinetic model incorporating rate coefficients and thermodynamic data. Rate coefficients were determined using traditional transition state theory, while diffusion-limited reactions were modeled based on the Einstein-Stokes equations. The resulting model comprised the kinetics for 108 reactions and thermodynamic data for 58 species. This model was validated by comparing simulations of the variations in chemical species during the decomposition process to experimental data acquired under isothermal conditions at 100 °C. The model was found to accurately reproduce the concentration changes of N₂H₄, HN₃, and NH₃ and also explained the reaction mechanism. The thermal decomposition was found to proceed via two parallel paths: N₂H₄ + HNO₃ → H₂O + HONO + N₂H₂ and N₂H₄ + HONO → HN₃ + 2H₂O. Following these reactions, a portion of the HN₃ decomposes to produce NH₃ through a multistep process. A sensitivity analysis showed that the rate of decomposition is greatly affected by the pH of the solution.

Self-assembly of tripeptides into γ-turn nanostructures

Self-assembling phenylalanine-based peptides have garnered interest owing to their potential for creating new functional materials. Here, we designed four diastereomers, l-Phe-l-Phe-l-Phe (FFF), d-Phe-l-Phe-l-Phe (fFF), l-Phe-d-Phe-l-Phe (FfF) and l-Phe-l-Phe-d-Phe (FFf), to analyze the effect of the d-isomer on the self-assembly. Using SEM, TG, VCD, and solid-state NMR measurements, we found that only FFf forms a γ-turn conformation and self-assembles into a nanoplate with higher thermal stability. The supramolecular structure of FFf consists of intra- and intermolecular hydrogen bonds and π-π stackings. From our results, we have discovered that FFf forms a new type of self-assembling γ-turn conformation, clarifying the structural role of a d-amino acid residue in supramolecular formation.

Direct Self-Sustained Fragmentation Cascade of Reactive Droplets

A traditional hand-held firework generates light streaks similar to branched pine needles, with ever smaller ramifications. These streaks are the trajectories of incandescent reactive liquid droplets bursting from a melted powder. We have uncovered the detailed sequence of events, which involve a chemical reaction with the oxygen of air, thermal decomposition of metastable compounds in the melt, gas bubble nucleation and bursting, liquid ligaments and droplets formation, all occurring in a sequential fashion. We have also evidenced a rare instance in nature of a spontaneous fragmentation process involving a direct cascade from big to smaller droplets. Here, the self-sustained direct cascade is shown to proceed over up to eight generations, with well-defined time and length scales, thus answering a century old question, and enriching, with a new example, the phenomenology of comminution.