Electron Paramagnetic Resonance Study of the Interaction of Surface Titanium Species with AlR₃ Cocatalyst in Supported Ziegler–Natta Catalysts with a Low Titanium Content

The electron paramagnetic resonance (EPR) method was used to investigate the formation of alkylated Ti­(III) species in superactive titanium–magnesium catalysts with a low titanium content during their interaction with an organoaluminum activator (AlMe₃), as well as the interaction of alkylated Ti­(III) surface species with carbon monoxide. EPR data on the content of alkylated Ti­(III) species in these catalysts agree well with the number of Ti–R bonds that are determined after the interaction of radioactive carbon monoxide (¹⁴CO) with catalyst activated by triethylaluminum in the absence of monomer. Parameters of EPR spectra of the Ti­(III) species having different structure and composition on the surface of titanium–magnesium catalysts were calculated by quantum-chemical simulations. The calculated <i>g</i>-values are consistent with the <i>g</i>-values observed in EPR spectra of the catalysts. Analysis of the literature data and results of our study made it possible to propose the parameters of EPR spectra characterizing the alkylated Ti­(III) species that can serve as precursors of the active sites in supported Ziegler–Natta catalysts

Photochemistry of Dithiocarbamate Cu(S₂CNEt₂)₂ Complex in CHCl₃. Transient Species and TD-DFT Calculations

Nanosecond laser flash photolysis was used to study the mechanism of photochemical transformations of the diethyldithiocarbamate Cu­(II) complex (Cu­(dtc)₂, where dtc^– ≡ ^–S₂CNEt₂ anion) in chloroform solutions. The electron transfer from the excited Cu­(dtc)₂ complex to a solvent molecule leads to the appearance of the primary intermediate, the [ClCu­(dtc)­(dtcCHCl₂)] complex, where a dtcCHCl₂ molecule is coordinated with a copper ion via one sulfur atom. In the fast reaction (<i>k</i> = 2.1 × 10⁹ M^–1 s^–1) with Cu­(dtc)₂, this complex forms a long-lived dimer [ClCu­(dtc)­(dtcCHCl₂)­Cu­(dtc)₂]. This intermediate decays during several seconds (<i>k</i> = 5.6 × 10^–2 s^–1) into the final product, i.e., a diamagnetic dimer [ClCu­(dtc)­Cu­(dtc)₂]. To determine the structure of intermediate complexes the quantum chemical calculations were carried out using DFT, TD-DFT, and PCM (Polarizable Continuum Model) methods

Effect of Impregnation on the Structure of Niobium Oxide/Alumina Catalysts Studied by Multinuclear Solid-State NMR, FTIR, and Quantum Chemical Calculations

Multinuclear solid-state ¹H, ²⁷Al, and ⁹³Nb NMR experiments and DFT calculations were carried out for structural characterization of alumina-supported niobium oxide catalysts with high niobium content following an every stage in the catalyst preparation. It was found that the first stage of the impregnation procedure plays a key role in determining the catalyst structure and acidity. In order to monitor the presence in catalysts of aluminum niobate phase, AlNbO₄, a series of ²⁷Al and ⁹³Nb NMR experiments was performed for several different individual AlNbO₄ samples. Aluminum and niobium NMR parameters were determined for AlNbO₄, which crystal structure contains two different crystallographic sites for each element. The compound was investigated through a combination of experimental ⁹³Nb and ²⁷Al NMR spectroscopy methods at several magnetic field strengths (9.4, 11.7, 19.4, and 21.1 T) and complemented by ab initio quantum chemical calculations of NMR parameters for these nuclei. The chemical shielding and the quadrupole coupling tensor parameters were determined for both ⁹³Nb and ²⁷Al