Quantum Tunneling Instability in Pericyclic Reactions

Several cycloreversion reactions of the retro-Diels-Alder type were computationally assessed to understand their quantum tunneling (QT) reactivity. N2, CO, and other leaving groups were considered based on their strong exothermicity, as it reduces their thermodynamic and kinetic stabilities. Our results indicate that for many of these reactions, it is essential to take into account their QT decomposition rate, which can massively weaken their molecular stability and shorten their half-lives even at deep cryogenic temperatures. In practical terms, this indicates that many supposedly stable molecules will actually be unsynthesizable or unisolable, and therefore trying to prepare or detect them would be a futile attempt. In addition, we discuss the importance of tunneling to correctly understand the enthalpy of activation and the collective atomic effect on the tunneling kinetic isotope effects to test if third-row atoms can tunnel in a chemical reaction. This project raises the question of the importance of in silico chemistry to guide in vitro chemistry, especially in cases where the latter cannot solve its own uncertainties.

Quantum tunneling instability of the mythical hexazine and pentazine

Through computational analysis we found that pentazine and hexazine, two hypothetical high-energy density materials, exhibit inherent instability due to quantum tunneling effects. This instability remains even near the absolute zero, and therefore they can be deemed as unsynthesizable. We propose substituents that could potentially stabilize pentazine, especially dimethylamine.

Crystal Structures of Xenon(VI) Salts: XeF5Ni(AsF6)3, XeF5AF6 (A = Nb, Ta, Ru, Rh, Ir, Pt, Au), and XeF5A2F11 (A = Nb, Ta)

Experiments on the preparation of the new mixed cations XeF₅M(AF₆)₃ (M = Cu, Ni; A = Cr, Nb, Ta, Ru, Rh, Re, Os, Ir, Pt, Au, As), XeF₅M(SbF₆)₃ (M = Sn, Pb), and XeF₅M(BF₄)_x(SbF₆)_3-x (x = 1, 2, 3; M = Co, Mn, Ni, Zn) salts were successful only in the preparation of XeF₅Ni(AsF₆)₃. In other cases, mixtures of different products, mostly XeF₅AF₆ and XeF₅A₂F₁₁ salts, were obtained. The crystal structures of XeF₅Ni(AsF₆)₃, XeF₅TaF₆, XeF₅RhF₆, XeF₅IrF₆, XeF₅Nb₂F₁₁, XeF₅Ta₂F₁₁, and [Ni(XeF₂)₂](IrF₆)₂ were determined for the first time on single crystals at 150 K by X-ray diffraction. The crystal structures of XeF₅NbF₆, XeF₅PtF₆, XeF₅RuF₆, XeF₅AuF₆, and (Xe₂F₁₁)₂(NiF₆) were redetermined by the same method at 150 K. The crystal structure of XeF₅RhF₆ represents a new structural type in the family of XeF₅AF₆ salts, which crystallize in four different structural types. The XeF₅A₂F₁₁ salts (M = Nb, Ta) are not isotypic and both represent a new structure type. They consist of [XeF₅]⁺ cations and dimeric [A₂F₁₁]^- anions. The crystal structure of [Ni(XeF₂)₂](IrF₆)₂ is a first example of a coordination compound in which XeF₂ is coordinated to the Ni²⁺ cation.

Heavy-Atom Tunneling in the Covalent/Dative Bond Complexation of Cyclo[18]carbon-Piperidine

Recent quantum chemical computations demonstrated the electron-acceptance behavior of this highly reactive cyclo[18]carbon (C₁₈) ring with piperidine (pip). The C₁₈–pip complexation exhibited a double-well potential along the N–C reaction coordinate, forming a van der Waals (vdW) adduct and a more stable, strong covalent/dative bond (DB) complex by overcoming a low activation barrier. By means of direct dynamical computations using canonical variational transition state theory (CVT), including the small-curvature tunneling (SCT), we show the conspicuous role of heavy atom quantum mechanical tunneling (QMT) in the transformation of vdW to DB complex in the solvent phase near absolute zero. Below 50 K, the reaction is entirely driven by QMT, while at 30 K, the QMT rate is too rapid (k_T ∼ 0.02 s^–1), corresponding to a half-life time of 38 s, indicating that the vdW adduct will have a fleeting existence. We also explored the QMT rates of other cyclo[n]carbon–pip systems. This study sheds light on the decisive role of QMT in the covalent/DB formation of the C₁₈–pip complex at cryogenic temperatures.

Stereoelectronic and dynamical effects dictate nitrogen inversion during valence isomerism in benzene imine

Non-classical processes such as heavy-atom tunneling and post transition-state dynamics govern stereoselectivity for benzene imine ⇌ 1H-azepine.

Predicting the structure and NMR coupling constant 1J(129Xe-19F) of XeF6 using quantum mechanics methods

The XeF₆ molecule exists as a monomer in the gas phase and as the (XeF₆)₄ tetramer in solution. Herein we used distinct quantum mechanics methods to study the conformational equilibrium for the XeF₆ monomer, which is represented mainly by O_h and C_3v symmetric geometries, and for the (XeF₆)₄ structure found in condensate phases. The NMR ¹J(¹²⁹Xe-¹⁹F) coupling constant is predicted using our own NMR-DKH basis set, designed for NMR properties. The C_3v conformer of XeF₆ was stable only with HF, CCSD, and hybrid DFT functionals with at least 28% exact HF exchange. Increasing the % of HF exchange improves the description of the geometry and the O_h→C_3v equilibrium. The BMK, BHandHLYP and LC-ωPBE functionals produce results in excellent agreement with experiments and high-level calculations for the XeF₆ molecule. When it comes to the ¹J(¹²⁹Xe-¹⁹F) coupling constant, the (XeF₆)₄ structure must be considered. For that compound, BHandHLYP leads to the best structure, and BMK leads to the best coupling constant; therefore, the generalized protocol BMK/NMR-DKH//BHandHLYP/def2-SVP is recommended to study the XeF₆ molecule in the gas phase and solution.