Journal of Physical Organic Chemistry最新文献

The relative stabilities of unsubstituted bicyclic bridgehead radicals have been previously shown to be dependent primarily upon differences in strain energies between the radicals and the corresponding saturated compounds. Although substituents are known to strongly affect the stability of acyclic carbon-based radicals, the effect of replacing at least one of the one-carbon bridges of bicyclic bridgehead radicals with a substituent was unknown. Because of geometrical constraints imposed by the bicyclic frameworks, the bridging substituents are unable to adopt conformations that optimize their stabilizing effects. Using [2.2.1] substituted derivatives as models, we have shown that substituents exert their effects via two primary modes. First, the presence of substituents influences the extent of hyperconjugative interactions between the SOMO and properly aligned bonds as revealed by NBO calculations. Second, when the substituents are frozen into a geometry reflective of that in the bicyclic radicals, they exert a direct impact upon the radical site that can be very different from that in the corresponding acyclic compounds. Neither of these effects alone rationalizes the relative stabilities of the radicals with various substituents. However, when combined, they offer a reasonable correlation with the observed relative energies. Generally, the effect of substituents on other bicyclic frameworks appears to follow a regular pattern of stabilization versus destabilization. The effect of changing the size of the bicyclic framework while maintaining the same substituent was also investigated. Generally, the order of relative energies mirrored that of the unsubstituted all-carbon analogs, suggesting that strain remains the primary factor in determining stability.

MN15/6-31++G(d,p)/IEFPCM theory level gave a good agreement between the calculated vibronic and experimental absorption spectra of the dianion, in both the maximum position and the shape. Vibronic transitions of the dianion activate only two motions in the S₁ state, namely, torsional vibrations of two aromatic rings (A and B) and the SO₃ group attached to the third ring. Significant photoinduced distortions of the spatial structure of the anion (A and B rings, being almost parallel to each other in the ground state, become mutually perpendicular in the excited state) led to a failure of the Franck-Condon computational procedure for calculating its vibronic spectrum. Photo-induced shifts of the electron density are analyzed. The ring with the SO₃ group attached is not involved in these charge redistributions for both the dianion and the anion. The negative solvatochromism of the dianion and the positive one of the anion observed experimentally were explained both from the point of view of non-specific dipole–dipole interactions with the solvent (changes in the dipole moments of the solutes upon excitation) and specific ones (strengthening/weakening of strong hydrogen bonds of the dianion and anion with water molecules).

Pentazolate compounds have garnered considerable interest as promising building blocks for novel polynitrogen compounds. Leveraging insights from the study of other energetic materials, researchers have enhanced the thermal stability of pentazolate compounds by synthesizing cocrystals, thereby addressing the issue of their poor thermal stability. In this study, the thermal decomposition mechanism of NH₄N₅ and its cocrystals ((N₅)₆(H₃O)₃(NH₄)₄Cl and NH₄N₅·1/6NH₄Cl) was investigated from a theoretical perspective. Laplace bond orders, decomposition pathways, transition states, interaction energies, and aromaticity were employed in the analysis. The computational results indicate that the asymmetric structure R3 of NH₄N₅·1/6NH₄Cl demonstrated incredible thermal stability and aromaticity, with a minimum Laplacian bond order of 1.036, a decomposition barrier of 29.48 kcal mol⁻¹, and an ELF-π value of 0.719. Therefore, co-crystallization with high-energy materials with high decomposition temperatures can enhance the thermal stability of pentazolate compounds.

The topological resonance energy method was employed to analyze [n]circulene systems (n = 3–8) and predict their aromaticity. Calculations revealed that all compounds exhibit some aromatic properties. Local aromaticity of each ring was assessed using bond resonance energy (BRE), circuit resonance energy, and magnetically based superaromatic stabilization energy (m-SSE) indices. Our results indicate that the BRE method overestimates the degree of local aromaticity in the central ring. The local aromaticity, as determined by the m-SSE index, has been compared with other ring indices reported in the literature. Additionally, the Hückel–London ring current model was used to study aromaticity, showing that molecular perimeters sustain diamagnetic bond currents, whereas central rings sustain paramagnetic bond currents. For the individual rings, both the energetic and magnetic criteria of aromaticity predict similar trends in the magnitude of local aromatic and antiaromatic states.

The prooxidant activities of α- and β-lapachones were studied in aqueous media using density functional theory. Although these compounds are well-known prooxidants and have applications as alternatives in the treatment of tumor cells, little is known about the reaction mechanisms involved. The prooxidant activity of α- and β-lapachones considered their reduced forms; they are hydronaphthoquinones $��H_2�Q_{\alpha }�$ and $��H_2�Q_{\beta }�$, whereas the single electron transfer mechanism was considered in the reduction reaction where oxygen, hydrogen peroxide, copper, and iron were substrates. Two reaction routes were identified for the prooxidant activities of H₂Q_α and H₂Q_β: The first considered that the metals were not present in the reduction reactions of oxygen and hydrogen peroxide and obtained reaction rates of 10⁶–10⁷ M⁻¹ s⁻¹; the second considered that the metals copper and iron were present in the reduction reactions and the reaction rates were limited by diffusion. Understanding the reaction mechanism involved in the prooxidant activity of α- and β-lapachones and the physiological importance of these molecules could be employed to comprehend the anticancer properties of compounds in which prooxidant activity is involved.

The [3 + 2] cycloaddition (32CA) reactions of aryl and heteroaryl nitrile oxides with (S)-(−)-β-pinene and (R)-(+)-α-pinene have been theoretically studied from the Molecular Electron Density Theory (MEDT) perspective and a molecular docking evaluation for the antituberculosis activity of the isoxazoline products. The studied 32CA reactions show activation Gibbs free energies between 23.8 and 31.6 kcal·mol⁻¹, consistent with their zwitterionic character and exclusive regioselectivity, in complete agreement with the experiments. Two-stage one-step mechanism with a very low polar character is predicted. The molecular mechanism has been established from the Bonding Evolution Theory (BET) analysis along the reaction path, showing early transition state structures, with weak non-covalent interactions indicated from the quantum theory of atom-in-molecules (QTAIM) analysis. The isoxazoline products for the 32CA reactions of (S)-(−)-β-pinene show appreciable binding affinities towards Mycobacterium tuberculosis transcriptional repressor EthR2, revealing them as promising candidates with antituberculosis potential.

Diphlorethol is a typical phlorotannin with multipharmacological activities. However, its antiradical activity is still ambiguous. The current study aims to evaluate its radical scavenging using thermodynamics and kinetics-based density functional theory (DFT) calculations. The results indicated that the main radical scavenging mechanism in gas and lipid was the formal hydrogen transfer (FHT), and that for the aqueous medium was the sequential proton loss-electron transfer (SPLET). The kinetic reactions with HOO˙ and CH₃OO˙ radicals resulted in the k_overall (overall rate constant) of 1.2 × 10⁸–1.6 × 10⁸ M⁻¹ s⁻¹ in water and 3.0 × 10⁰–2.7 × 10¹ M⁻¹ s⁻¹ in pentyl ethanoate. 4- and 6-OH acted as active centers for radical scavenging. The molecular docking simulation suggested that diphlorethol could serve as a potential inhibitor of the oxidative activity of the Keap1 enzyme, particularly through its interaction with the crucial amino acid residue Arg415. The ADMET (absorption, distribution, metabolism, excretion, and toxicity) analysis demonstrated that diphlorethol exhibited favorable pharmacokinetic properties, including good water solubility, high intestinal absorption, and moderate tissue distribution. Diphlorethol did not induce hepatotoxicity or skin sensitization and showed no inhibitory effects on hERG I or hERG II channels, supporting its potential as a safe antioxidant candidate for further development.

The double-bond isomerization of 1-octene to an equilibrium distribution of all seven linear octene isomers on a dry Amberlyst15 catalyst was followed at 90°C using gas chromatography to resolve five of the seven isomers. The simplest kinetic models that fit observed data over several hours indicated that carbenium ions are likely intermediates in these isomerizations. The C2 octyl carbenium ion appeared to be more stable than the C3 and C4 octyl carbenium ions, leading to a lower equilibrium trans to cis ratio in 2-octene than in 3-octenes and 4-octenes. At equilibrium, [2-octyl carbenium ion] / [3-octyl carbenium ion] = 1.6. The octene adsorption coefficient for double-bond isomerization on this catalyst was 0.4–0.5, indicating a rather weak adsorption. However, the concurrent dimerization of octene to branched hexadecene isomers required an adsorption coefficient of ~40, indicating octene adsorbs onto two different types of sites on this catalyst. The primary alcohol 2-ethyl-1-hexanol was added to reaction mixtures to suppress octene dimerization, and the active isomerization site in this work was a molecule of 2-ethyl-1-hexanol adsorbed onto a supported sulfonic acid group of the initial resin. This new site was less active than the original acid site.

In this article, the structure of the free dicyclopentadiene (DCPD) molecule was studied for the first time by the gas-phase electron diffraction (GED) at 268 K supplemented by quantum chemical calculations (B3LYP and MP2 with the cc-pVTZ basis set). The possibility of transition between endo and exo isomers of DCPD was considered in the gas phase. Three models of the refinement of the experimental (GED) data were applied on the base of two harmonic force fields and molecular dynamic simulations. The theoretical and experimental geometric parameters of the endo isomer are presented. In addition, the geometric parameters of the exo isomer calculated by quantum chemistry are given.

The conductivity of graphene oxide is significantly increases when it is reduced, and oxygen is removed. In this study, the mechanism of deoxygenation of the epoxide sites of small polycyclic aromatic hydrocarbons (PAHs) with triphenylphosphine (PPh₃) was investigated using DFT. When PPh₃ attacks the epoxide oxygen, the carbon–oxygen bond is immediately cleaved by electron transfer, and the oxygen is then abstracted to form triphenylphosphine oxide. The reaction was found to be single-step and different from that of three-membered ring ethers, which are not bound to PAHs. The activation energy for deoxygenation is approximately 20 kcal/mol, and the reaction is exothermic.