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.

A simple heuristic approach to estimate the thermochemistry of condensed-phase molecules based on the polarizable continuum model.

A simple model based on a quantum chemical approach with polarizable continuum models (PCMs) to provide reasonable translational and rotational entropies for liquid phase molecules was developed. A translational term was evaluated with free-volume compensation for the Sackur-Tetrode equation. We assumed that the free-volume corresponds to the cavity volume in the PCM. A rotational term was modeled as restricted rotation of a dipole in the electrostatic field. Entropies were assessed for twenty species in the liquid-phase using the proposed model, and the computed values were compared with experimental values. Quantum chemistry calculations were conducted at the ωB97X-D/6-311++G(d,p) level with the conductor-like PCM method. Predicted entropies were in good agreement with the experimental entropies, and the root mean square deviation was 17.2 J mol-1 K-1. The standard enthalpy change of formation was then investigated for eleven specific species. The CBS-QB3//ωB97X-D method provides a reasonable standard enthalpy of formation for gasified species; however, improvement of the accuracy is required for liquid species. Finally, the dependence of the Gibbs energy on temperature was investigated for the eleven specific species. When the ideal gas treatment is used, the Gibbs energy trends for the gaseous and liquid phases are quasi-parallel for all of the species, although the Gibbs energy trends for liquids based on the proposed model intersected the gaseous trend (i.e. the intersection is the boiling point). However, the model significantly under or overestimated the experimental boiling points. The error of the boiling points was predominantly due to the inaccuracy of the enthalpy.

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.

Initial Decomposition Pathways of Aqueous Hydroxylamine Solutions.

This work examined the reaction pathways involved in the initial decomposition of aqueous hydroxylamine solutions via the overall reaction, 2NH2OH → NH3 + HNO + H2O, using quantum chemistry calculations incorporating solvent effects. Several possible decomposition mechanisms were identified and investigated: three neutral-neutral bimolecular, two water-catalyzed, one neutral trimolecular, two ion-neutral bimolecular, and one cation-catalyzed. Optimized structures for the reactants, products, and transition states were obtained at the ωB97XD/6-311++G(d,p)/SCRF = (solvent = water) level of theory, and the total electron energies of such structures were calculated at the CBS-QB3 level of theory. The cation-catalyzed reaction 2NH2OH + NH3OH+ → NH4+ + HNO + H2O + NH2OH (maximum energy barrier (ΔE0‡) = 53.6 kJ/mol) and the anion-neutral bimolecular reaction NH2OH + NH2O- → NH3 + 1NO- + H2O (ΔE0‡ = 79.0 kJ/mol) were both found to be plausible candidates for the dominant step in the initial decomposition. The results of this study indicate that both acidic and basic conditions can affect the thermal stability of hydroxylamine in water.