Russian Chemical Bulletin最新文献

The DFT CAM-B3LYP/6-311++G(d,p) method showed that two migration mechanisms are possible for the 4-methoxybenzyl group in the pentamethoxycarbonylcyclopentadiene ring, namely, 1,5-sigmatropic shifts with an energy barrier ΔE^≠_ZPE = 32.5 kcal mol⁻¹ in the gas phase and a dissociative pathway through the formation of a tight ion pair with a lower activation barrier ΔE^≠_ZPE = 24.2 kcal mol⁻¹ in methanol (PCM). In contrast, the 4-nitrobenzyl group migrates in this system only via 1,5-sigmatropic shifts both in the gas phase and in methanol with higher activation barriers ΔE^≠_ZPE = 33.6 and 33.3 kcal mol⁻¹, respectively, whereas migrations of the benzhydryl group in the pentamethoxycarbonylcyclopentadiene ring occur via the dissociation—recombination mechanism both in the gas phase and in methanol with fairly low activation barriers ΔE^≠_ZPE = 24.0 and 21.5 kcal mol⁻¹, respectively.

The review is concerned with specific features of the molecular and electronic structure as well as spectral characteristics of palladium carboxylate complexes Pd₄(μ-L)₄(μ-RCO₂)₄ (R is an organic or organoelement substituent; L = CO, SR′, OOR′, CNR′, NO) having a planar rectangular metal core. The results of density functional quantum chemical calculations (including energy and vibrational frequency calculations) are analyzed. It was demonstrated that the most favorable arrangement of ligands around the metal core depends primarily on the nature of the ligand L, whereas the properties of the substituents R and R′ have little effect. The results of theoretical studies are consistent with experimental data.

Quantum chemical calculations performed reproduce well the experimental data on the temperature dependence of the magnetic susceptibility of mononuclear complexes [Ln(PyrCOO)(acac)₂(H₂O)₂] (Ln = Dy, Er, Yb, Eu, Gd, Tb, Ho, Tm; PyrCOOH is pyrazine-2-carboxylic acid) in a DC magnetic field. The calculated signs of zero-field splitting energy, which are negative for all the complexes studied, make it possible to consider the complexes as single-molecule magnets.

The formation of binuclear complexes of bis(18-crown-6)stilbene with Ho^III ions in anhydrous MeCN was studied by spectrophotometry and NMR spectroscopy. The quantum yields of the reversible E—Z photoisomerization of the complex under study were measured. The changes in the molar magnetic susceptibility of the binuclear complex caused by E—Z photoisomerization in solution at room temperature were demonstrated by the Evans method. The distances between the paramagnetic Ho^III ions in the complexes of E and Z isomers of bis(18-crown-6)stilbene were evaluated by density functional theory calculations.

This review analyzes the experimental and theoretical works in the field of spin chemistry, which is the basis for describing physical processes that occur when influencing the strength and plasticity of solids using constant magnetic fields, magnetic resonance excited in structure defects, as well as hyperfine interaction of magnetic nuclei. The conditions for observing these analogs of spin-dependent chemical processes in crystals are determined, and the various methods for identifying magnetic field-stimulated elementary events are considered.

According to the NMR studies, N-(2-bromocyclohexyl)triflamide and N-(2-bromocyclohexyl)-N′-(triflyl)amidine exist as trans-isomers with equatorially oriented substituents. The quantum chemical calculations of all conformers of cis- and trans-isomers of these compounds and their fluorine-substituted analogs in the gas phase and in the solution confirm a higher stability of the experimentally proved eq-eq-conformers. No direct correlation between the H⋯X hydrogen bond length (X is halogen) and stability of the conformers (because of a lower ability of the NH group than OH to be the hydrogen bond donor) for the compounds under study and the presence of intramolecular interactions of the oxygen or fluorine atoms of the triflamide group with the hydrogen atoms of the cycle were proved by a comparison of the calculation results with those for 2-halocyclohexanols.

Halogenation of 3-nitro-6-phenyl- and 3-formyl-6-phenyl-2H-thiopyrans with bromine or iodine nitrate led to 5-halo-2H-thiopyrans. Chlorination of 3-nitro-6-phenyl-2H-thiopyrans and imides of 6-phenyl-2H-thiopyran-2,3-dicarboxylic acids resulted in (2R*,3S*,4R*)-configured 2,3,4-trichloro-3,4-dihydro-2H-thiopyrans. Bromination of 3-nitro-2H-thiopyrans was shown to proceed under conditions of both electrophilic and radical halogenation.

The selective monochlorination of 2,3,4-trisubstituted pyrroles was performed by a two-step one-pot method. It involves the oxidative chlorination of pyrrole substrates with trichloroisocyanuric acid giving dichlorinated non-aromatic intermediates followed by their reduction with tin(ii) chloride or sodium dithionite to afford the target products in yields of up to 100%.

The cohydrogenation of carbon oxides in the presence of complex oxides with the perovskite structure GdFe_1−xCo_xO₃ (x = 0, 0.5, 1) was studied. The catalyst samples were synthesized via the sol—gel technology and characterized by XRD, XPS, FT-IR spectroscopy, N₂ adsorption, TG, and DSC. The iron-containing samples demonstrated a high catalytic efficiency in the hydrogenation of simulated biosynthesis gas to light hydrocarbons. An increase in the carbon dioxide content in the reaction mixture to the ratio CO₂/[CO + CO₂] = 0.5 suppressed methane formation and favors the synthesis of light olefins. The hydrogenation of CO₂ proceeds in two steps: the reverse water gas shift reaction to form CO followed by CO hydrogenation to yield olefins and paraffins. The tandem effect of Fe and Co enhances the selectivity for light olefins. These findings provide valuable information useful for the rational design of efficient bimetallic catalysts for biosyngas hydrogenation.