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A number of hexaarylbenzene compounds were studied theoretically, in order to compare energy changes as a result of the toroidal delocalization effect that is characteristic of all these species. The energy was studied taking advantage of locally designed isodesmic reactions. Results indicate that the amount of aromaticity manifested by each substituent is a factor that should be considered when assessing the quantity of energy dissipated from each aromatic center. The influence of different substituents on electronic delocalization is also analyzed, as well as the role played by their frontier molecular orbitals. [ABSTRACT FROM AUTHOR]

Recently, an unusual elongation of the C-S bond was observed experimentally for some sulfur-containing heterocycles. Using a superior ab initio (SCS-MP2/cc-pVTZ) level of theory, we showed that the phenomenon can be explained by a contribution of a donor–acceptor adduct of a carbene with an unsaturated ligand. One may achieve further elongation of the C-S bond, eventually turning it to a coordinate one, by increasing the stability of each part of the system as, e.g., in the utmost case of spiro adducts with Arduengo carbenes. The effect of carbene stability was quantified by employing the isodesmic reactions of carbene exchange. [ABSTRACT FROM AUTHOR]

Macromolecular associates, such as membraneless organelles or lipid-protein assemblies, provide a hydrophobic environment, i.e., a liquid protein phase (LP), where folding preferences can be drastically altered. LP as well as the associated phase change from water (W) is an intriguing phenomenon related to numerous biological processes and also possesses potential in nanotechnological applications. However, the energetic effects of a hydrophobic yet water-containing environment on protein folding are poorly understood. Here, we focus on small β-sheets, the key motifs of proteins, undergoing structural changes in liquid–liquid phase separation (LLPS) and also model the mechanism of energy-coupled unfolding, e.g., in proteases, during W → LP transition. Due to the importance of the accurate description for hydrogen bonding patterns, the employed models were studied by using quantum mechanical calculations. The results demonstrate that unfolding is energetically less favored in LP by ~0.3–0.5 kcal·mol−1 per residue in which the difference further increased by the presence of explicit structural water molecules, where the folded state was preferred by ~1.2–2.3 kcal·mol−1 per residue relative to that in W. Energetics at the LP/W interfaces was also addressed by theoretical isodesmic reactions. While the models predict folded state preference in LP, the unfolding from LP to W renders the process highly favorable since the unfolded end state has >1 kcal·mol−1 per residue excess stabilization. [ABSTRACT FROM AUTHOR]

An analysis of the effects induced by F, Cl, and Br-substituents at the α-position of both, the hydroxyl or the amino group for a series of amino-alcohols, HOCH2(CH2)nCH2NH2 (n = 0–5) on the strength and characteristics of their OH···N or NH···O intramolecular hydrogen bonds (IMHBs) was carried out through the use of high-level G4 ab initio calculations. For the parent unsubstituted amino-alcohols, it is found that the strength of the OH···N IMHB goes through a maximum for n = 2, as revealed by the use of appropriate isodesmic reactions, natural bond orbital (NBO) analysis and atoms in molecules (AIM), and non-covalent interaction (NCI) procedures. The corresponding infrared (IR) spectra also reflect the same trends. When the α-position to the hydroxyl group is substituted by halogen atoms, the OH···N IMHB significantly reinforces following the trend H < F < Cl < Br. Conversely, when the substitution takes place at the α-position with respect to the amino group, the result is a weakening of the OH···N IMHB. A totally different scenario is found when the amino-alcohols HOCH2(CH2)nCH2NH2 (n = 0–3) interact with BeF2. Although the presence of the beryllium derivative dramatically increases the strength of the IMHBs, the possibility for the beryllium atom to interact simultaneously with the O and the N atoms of the amino-alcohol leads to the global minimum of the potential energy surface, with the result that the IMHBs are replaced by two beryllium bonds. [ABSTRACT FROM AUTHOR]

Intramolecular interactions are shown to be key for favoring a given structure in systems with a variety of conformers. In ortho-substituted benzene derivatives including a beryllium moiety, beryllium bonds provide very large stabilizations with respect to non-bound conformers and enthalpy differences above one hundred kJ·mol−1 are found in the most favorable cases, especially if the newly formed rings are five or six-membered heterocycles. These values are in general significantly larger than hydrogen bonds in 1,2-dihidroxybenzene. Conformers stabilized by a beryllium bond exhibit the typical features of this non-covalent interaction, such as the presence of a bond critical point according to the topology of the electron density, positive Laplacian values, significant geometrical distortions and strong interaction energies between the donor and acceptor quantified by using the Natural Bond Orbital approach. An isodesmic reaction scheme is used as a tool to measure the strength of the beryllium bond in these systems in terms of isodesmic energies (analogous to binding energies), interaction energies and deformation energies. This approach shows that a huge amount of energy is spent on deforming the donor–acceptor pairs to form the new rings. [ABSTRACT FROM AUTHOR]

In the present work, we studied in detail the thermochemistry, thermal stability, mechanical sensitivity, and detonation performance for 20 nitro-, cyano-, and methyl derivatives of 1,2,5-oxadiazole-2-oxide (furoxan), along with their bis-derivatives. For all species studied, we also determined the reliable values of the gas-phase formation enthalpies using highly accurate multilevel procedures W2-F12 and/or W1-F12 in conjunction with the atomization energy approach and isodesmic reactions with the domain-based local pair natural orbital (DLPNO) modifications of the coupled-cluster techniques. Apart from this, we proposed reliable benchmark values of the formation enthalpies of furoxan and a number of its (azo)bis-derivatives. Additionally, we reported the previously unknown crystal structure of 3-cyano-4-nitrofuroxan. Among the monocyclic compounds, 3-nitro-4-cyclopropyl and dicyano derivatives of furoxan outperformed trinitrotoluene, a benchmark melt-cast explosive, exhibited decent thermal stability (decomposition temperature >200 °C) and insensitivity to mechanical stimuli while having notable volatility and low melting points. In turn, 4,4′-azobis-dicarbamoyl furoxan is proposed as a substitute of pentaerythritol tetranitrate, a benchmark brisant high explosive. Finally, the application prospects of 3,3′-azobis-dinitro furoxan, one of the most powerful energetic materials synthesized up to date, are limited due to the tremendously high mechanical sensitivity of this compound. Overall, the investigated derivatives of furoxan comprise multipurpose green energetic materials, including primary, secondary, melt-cast, low-sensitive explosives, and an energetic liquid. [ABSTRACT FROM AUTHOR]