Quantenchemische untersuchungen zum mechanismus der elektrophilen substitution—III : Zur stabilität und struktur von [H,CH3]-σ-komplexen (ipso-Addukten) protonierter methylbenzole

The [H,CH₃]-σ-complexes of toluene and the three isomeric xylenes formed by electrophilic H⁺-attack on substituted positions or by H-shifts from the isomeric σ-complexes are studied by the CNDO/2-FK MO method. These complexes (ipso-adducts) occur as minima on the energy surfaces in agreement with the results for [H,H]-σ-complexes. Their geometrical structure and tendencies of formation are presented and a comparison with experimental results in solution is given. Suggestions for different stabilization of the isomeric σ-complexes of substituted benzene by solvent molecules are found.     

Si chemical shift tensors of silyl silicate cages

Static and ²⁹Si CP MAS spectra (spinning rates of 2000 and 500 Hz) of six silyl silicate cages are analyzed by deconvolution and simulation. The principal values of the various ²⁹Si chemical shift tensors provide information about the point symmetry of the Si sites. The intensities of the signals correspond sufficiently well with the stoichiometries.     

The landscape of the proximal point method for nonconvex–nonconcave minimax optimization

Mathematical Programming - Minimax optimization has become a central tool in machine learning with applications in robust optimization, reinforcement learning, GANs, etc. These applications are...     

Kinetics and mechanism of hydroboration reactions of HBBr2 · SMe2 and HBCl2 · SMe2 - Application of B NMR spectroscopy

The kinetics and mechanism of the hydroboration reactions of 1-octene with HBBr₂ · SMe₂ and HBCl₂ · SMe₂, in CH₂Cl₂ as a solvent, were studied. Rates of hydroboration were monitored using ¹¹B NMR spectroscopy. The reactions exhibited simple second-order kinetics of the form . The HBCl₂ · SMe₂ was found to be 20 times more reactive than the HBBr₂ · SMe₂. The overall activation parameters (ΔH^≠, ΔS^≠) for the reaction of HBBr₂ · SMe₂ with 1-octene were found to be 82 ± 1 kJ mol⁻¹, −18 ± 4 J K⁻¹ mol⁻¹ and with 1-hexyne were 78 ± 4 kJ mol⁻¹ −34 ± 12 J K⁻¹ mol⁻¹. For the reaction of HBCl₂ · SMe₂ with 1-octene, ΔH^≠ and ΔS^≠ were 104 ± 5 kJ mol⁻¹ and 43 ± 16 J K⁻¹ mol⁻¹, respectively. The activation parameters (ΔH^≠, ΔS^≠) for the dissociation of Me₂S from HBBr₂ · SMe₂ were found to be 104 ± 2 kJ mol⁻¹, +33 ± 8 J K⁻¹ mol⁻¹, respectively. Based on the activation parameters, it was concluded that the detaching of Me₂S from the boron centre follows a dissociative mechanism, while the hydroboration process follows an associative pathway. It was also concluded that the dissociation of Me₂S from the boron centre is the rate determining step.     

Hochauflösende Festkörper-NMR-Untersuchungen an Kieselsäuren. II. Hochauflösende 1H-Festkörper-NMR-Untersuchungen an polykristallinen Phyllokieselsäuren (H2Si2O5)x mit Hilfe von Vielimpulsverfahren bei tiefem und hohem Magnetfeld

Solid-State High Resolution N.M.R. Studies of Silicic Acids. II. Solid-State High Resolution ¹H N.M.R. Studies of Polycrystalline Phyllosilicic Acids (H₂Si₂O₅)_x by Using Combined Multiple Pulse Magic Angle Spinning Techniques at Low and High Magnetic Fields The α- and δ-phases of the phyllosilicic acid (H₂Si₂O₅)_x are studied by means of multiple pulse techniques (MREV8, BR24) and combined multiple pulse-magic angle spinning experiments at low (1.42 T) and high (6.35 T) magnetic fields. In addition to the main signal due to SiOH groups the NMR spectra of both samples show a small peak which can be assigned to water molecules. For the SiOH groups of both phases the anisotropy of the proton shielding tensors Δσ and the isotropic chemical shift values δ are obtained. According to the N.M.R. results a model for the hydrogen bonds is derived for the phyllosilicic acids. Compared with the α-phase, the SiOH groups in the δ-phase form weaker hydrogen bonds.