Computational study of the transmetalation process in the Suzuki-Miyaura cross-coupling of aryls

The transmetalation step of the Suzuki-Miyaura cross-coupling between aryl groups is analyzed by means of DFT calculations with the Becke3LYP functional. The halide considered is Ph-Br, and the organoboronic acid is Ph-B(OH)₂. The model catalyst is Pd(PH₃)₂, and the base, OH⁻. The transmetalation is considered to start from the Pd(Ph)(PH₃)₂Br complex, the product of the oxidative addition. The results are compared with those of a previous study on the analogous reaction with vinyl groups, and it is shown that the reaction mechanism is very similar.     

Effect of the oxidation state of manganese in the manganese oxides used in the synthesis of Mn-substituted cordierite on the properties of the product

Catalysts based on Mn-substituted cordierite 2MnO · 2Al₂O₃ · 5SiO₂ have been synthesized using different manganese oxides (MnO, Mn₂O₃, and MnO₂) at a calcination temperature of 1100°C. The catalysts differ in their physicochemical properties, namely, phase composition (cordierite content and crystallinity), manganese oxide distribution and dispersion, texture, and activity in high-temperature ammonia oxidation. The synthesis involving MnO yields Mn-substituted cordierite with a defective structure, because greater part of the manganese cations is not incorporated in this structure and is encapsulated and the surface contains a small amount of manganese oxides. This catalyst shows the lowest ammonia oxidation activity. The catalysts prepared using Mn₂O₃ or MnO₂ are well-crystallized Mn-substituted cordierite whose surface contains different amounts of manganese oxides differing in their particle size. They ensure a high nitrogen oxides yield in a wide temperature range. The product yield increases with an increasing surface concentration of Mn³⁺ cations. The highest NO_x yield (about 76% at 800–850°C) is observed for the MnO₂-based catalyst, whose surface contains the largest amount of manganese oxides.     

Steady-shear viscosity of polydimethylsiloxanes over the range from the lower to the upper Newtonian region

The mechanism of non-Newtonian behavior for flow from the lower to the upper Newtonian region is explained by a modification of Graessley's theory. In the theory proposed here, a viscosity η_fric, which is based on friction between polymer segments and is almost shear-independent, is introduced in addition to Graessley's entanglement viscosity η_ent, which decreases with increasing shear rate. The theory is applied to previously obtained data on steady flow of polydimethylsiloxanes of different molecular weights. The agreement between calculated and experimental results is good. In polymers with the molecular weight above the critical molecular weight for entanglement M_c, the major contribution to viscosity near zero shear rate is η_ent. As the shear rate increases, the flow curve has an inflection where η_fric cannot be disregarded in comparison with η_ent. In the upper Newtonian region, η_fric has more influence on the viscosity than η_ent. The theory can also explain the experimental results on flow of polymers with molecular weight below M_c, which were shown to be slightly non-Newtonian in the previous paper.     

Correlation of the position of the bond functions in H-X with electronegativity

S type Gaussian bond functions are optimized for HF, H₂O, NH₃, and CH₄. The optimization is carried out with respect to the exponent and position in the H-X bond. The position is found to correlate well with the electronegativity of Pauling and Allred-Rochow.     

Thermodynamic Approach to the Calculation of the Rate Constants of Monomolecular Reactions in the Strong-Collision Limit

A thermodynamic approach to the calculation of the dissociation (recombination) rate constants of polyatomic molecules in the strong-collision limit is suggested. The approach is based on the density of states obtained by the inverse Laplace transform of the integral expression for the statistical sum of the internal degrees of freedom of the molecule. Although the mathematical apparatus of this model is rather simple, it enables one to take into account the real energy distribution of molecular states. The potential of this method is demonstrated for O₃, H₂O, H₂O₂, CH₄, and C₂H₆. The results obtained are compared with data calculated using other methods. The temperature dependence of the collision efficiency correction β_c is analyzed for water and ethane.     

Effect of the nature of inert solvent on the decay kinetics of the carbocation generated in the photolysis of 1,2-dihydroquinolines in methanol

The dependence of the quantum yield and the decay rate constant for the carbocation generated in the photolysis of 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline (6-EtO-DHQ) in methanol on the solvent composition was studied in the mixtures of methanol with isopropanol, acetonitrile, and pentane by pulse photolysis. The addition of these solvents decreases the yield of the carbocation and differently affects the kinetic parameters of its decay. The carbocation decay in the mixtures MeOH-i-PrOH and MeOH-C₅H₁₂ is described by the pseudo-first order equation (k ₁), with the dependence of k ₁ having a maximum at 50 vol % of MeOH in the MeOH-i-PrOH mixtures, and k ₁ increasing with a decrease in the MeOH concentration in the MeOH-C₅H₁₂ mixtures. In the MeOH-MeCN mixtures, the value of k ₁ decreases with a decrease in the MeOH concentration, and, at the concentration of MeOH lower than 50 vol %, the contribution of the second-order reaction (k ₂) is observed. The activation energies and preexponential factors were determined in the MeOH-C₅H₁₂ mixtures of different compositions, and it was shown that E _act practically did not depend on the solvent composition and were close to E _act for other carbocations obtained in MeOH. The increase in k ₁ with a decrease in the MeOH concentration is caused by an increase in the preexponential factor. The results were discussed on the basis of the reaction mechanism involving the competing reactions of the carbocation combination with two nucleophilic particles, the MeOH molecule and the MeO⁻ anion. The composition of the mixture and the nature of the inert solvent affect strongly the course of these reactions. Published in Russian in Kinetika i Kataliz, 2009, Vol. 50, No. 3, pp. 411–416. The article was translated by the authors.     

Direct Observation of the Binding Mode of the Phosphonate Anchor to Nanosized Polyoxotitanate Clusters

The structures of three newly synthesized phosphonate‐substituted polyoxotitanates are reported. The Ti/O core of [Ti₄O(OEt)₁₂(PhenylPO₃)] ( 1 ) is the building block for two larger phosphonate‐substituted nanoclusters, [Ti₂₅O₂₆(OEt)₃₆(PhenylPO₃)₆] ( 2 ) and [Ti₂₆O₂₆(OEt)₃₉(PhenylPO₃)₆]Br ( 3 ). All compounds exhibit a not previously recognized triply bridging binding mode of the phosphonate anchor with short connecting Ti? O bonds, the average of which is 2.010(7) Å. Comparison with previously reported work suggests that the binding mode of the phosphonate anchor is strongly dependent on the structure of the underlying substrate.     

Investigation of the reaction of quinoline with bromine in nitrobenzene

The reaction of quinoline with bromine in nitrobenzene solvent is investigated using the properties of isomolar series. These properties, viscosity, conductivity, density, depression of the freezing point, are determined using a constant molar concentration of C₉H₇N +Br₂, 0.05 per mole of C₆H₅NO₂. It is shown that the nitrobenzene solution of quinoline forms with bromine a complex with the composition C₉H₇N · Br₂. The molecular conductivity of this complex in nitrobenzene solvent increases with dilution.     

Effect of the mixture t - BuCl/SnCl4 on the cationic polymerization of isobutylene

The polymerizations of isobutylene initiated with the system tert-butyl chloride (t-BuCl)/SnCl₄ and carried out in CH₂Cl₂ at −20°C and −78°C were investigated. The results obtained demonstrate that the presence of t-BuCl in the polymerizing system gives rise to a PIB product with a distinctly bimodal MWD. The higher-molecular weight (HMW) PIB, M̄_n = 20000, I=M̄_w/M̄_n ∼ 2.5, is the result of existence of the protogenic initiation with residual water in the reaction system. The lower-molecular weight (LMW) PIB, M̄_n < 600, M̄_w/M̄_n ≤ 1.4, is the product of polymerization initiated presumably with a complex t-BuCl-SnCl₄-H₂O. To elucidate the reaction mechanism of the polymerization initiated with the complex, a series of similar isobutylene polymerizations using the initiation system 2,5-dichloro-2,5-dimethylhexane (DDH)/SnCl₄ was run and the oily LMW PIB samples were investigated by ¹H-NMR. A new polymerization mechanism describing the role of DDH and t-BuCl is suggested.     

Investigations on the influence of the substrate on the crystal structure of sputtered LiCoO2

The influence of the uppermost substrate layer on the structural properties of sputtered lithium cobalt oxide (LiCoO₂) is discussed in this work. For this purpose, bare, oxidized, and platinum-coated silicon wafers, as well as stainless steel and titanium sheets, were used as substrates. The resulting crystal structure of LiCoO₂ deposited on these substrates was analyzed and discussed. The LiCoO₂ thin films were deposited by RF magnetron sputtering with different film thicknesses. A subsequent annealing step at 700 °C was performed to induce the crystallinity of LiCoO₂. The crystal orientation was determined by X-ray diffraction. The obtained results show a strong dependency of LiCoO₂'s crystal structure on the surface the film is deposited on. However, the strong influence of the film thickness reported in previous publications could not be observed. If LiCoO₂ is deposited on the substrates with a metallic surface, a strong (003) preferential orientation is obtained for a wide range of film thicknesses. In contrast, sputtering of LiCoO₂ on bare and on oxidized silicon wafers results in a (101) dominated crystal structure for the different film thicknesses. These experiments show the importance of the characterization of LiCoO₂'s crystal structure in the intended battery setup.     

On the use of the variational method for interpreting the solubilities of gases in liquids

The Variational theory of mixtures due to Mansoori and Leland is modified to account for translational quantum effects in solution, and this modified form of the theory is applied to the problem of gas solubilities in liquids. The theory is used to derive expressions for the Henry's law constant, the molar heat of solution at infinite dilution, and the partial molar volume at infinite dilution for a solute in a liquid solution. These expressions are applied, over a range of temperatures, to the following systems; H₂ in each of Ar, N₂, and CH₄; He in each of Ar, N₂, and CH₄; and Ne in each of Ar and N₂. Lennard-Jones 6–12 pair potentials are used for these calculations. The Lennard-Jones parameters are taken from gas-phase second virial coefficient data. The results obtained are compared with experimental data and with previous calculations on these systems based on the Leonard-Henderson-Barker theory. The variational results appear to be in better agreement with experiment for the He-containing systems, while the Leonard-Henderson-Barker theory seems better for the other systems. An explanation for this is suggested.     

Impact of the Oxygen Defects and the Hydrogen Concentration on the Surface of Tetragonal and Monoclinic ZrO2 on the Reduction Rates of Stearic Acid on Ni/ZrO2

The role of the specific physicochemical properties of ZrO₂ phases on Ni/ZrO₂ has been explored with respect to the reduction of stearic acid. Conversion on pure m‐ZrO₂ is 1.3 times more active than on t‐ZrO₂, whereas Ni/m‐ZrO₂ is three times more active than Ni/t‐ZrO₂. Although the hydrodeoxygenation of stearic acid can be catalyzed solely by Ni, the synergistic interaction between Ni and the ZrO₂ support causes the variations in the reaction rates. Adsorption of the carboxylic acid group on an oxygen vacancy of ZrO₂ and the abstraction of the α‐hydrogen atom with the elimination of the oxygen atom to produce a ketene is the key to enhance the overall rate. The hydrogenated intermediate 1‐octadecanol is in turn decarbonylated to heptadecane with identical rates on all catalysts. Decarbonylation of 1‐octadecanol is concluded to be limited by the competitive adsorption of reactants and intermediate. The substantially higher adsorption of propionic acid demonstrated by IR spectroscopy and the higher reactivity to O₂ exchange reactions with the more active catalyst indicate that the higher concentration of active oxygen defects on m‐ZrO₂ compared to t‐ZrO₂ causes the higher activity of Ni/m‐ZrO₂.     

An Answer to the Article ‘On the Misleading Interpretation of the Properties of the f-Electron Elements’

The position of the ‘Inclined W theory’, i.e., the linear correlation of the properties (P_i) of the lanthanide and the actinide ions with their ground state total orbital angular quantum numbers (L), P_i = w_iL + k_i, where w_i and k_i are the slopes and intercepts of the least squares lines, is made clear. The position of gadolinium is ambivalent, and it can be a common point for the symmetric ‘Inclined W plots’ or it may either belong to the second or the third tetrad depending on the system investigated. The propionate and the isobutyrate systems are used to elucidate the ‘Inclined W concept’. The ‘Inclined W parameters’ for the solvent extraction of the lanthanides with PMBP ligand have been calculated.     

Enthalpies of formation of radicals and the mass spectra of the products of tetrafluoroethylene polymerization in acetone

The mass spectra of the dissociative electron-impact ionization products of telomers formed upon the radiation-chemical telomerization of tetrafluoroethylene in acetone were measured over the range of m/z from 1 to 204. The most intense bands at m/z = 43, 51, and 57 were attributed to the CH₃CO⁺, CF₂H⁺ and CH₃COCH₂⁺ cations—the main dissociation products of the H(C₂F₄) _n CH₂COCH₃ telomers. The telomer composition was consistent with a radical telomerization mechanism, in which chain growth and chain transfer are due to the formation of the CH₃COCH₂^· radical. Based on published data supplemented with quantum-chemical calculations, the enthalpies of formation of the radicals R(CF₂) _n (n = 2–8; R = H, CH₃, CH₃CO, and CH₃COCH₂) were tabulated. The formation of telomers with the same terminal groups is consistent with thermodynamic data and a polymerization mechanism in which the chain growth reaction is diffusion-limited and the chain transfer reaction is activated hydrogen-atom transfer.     

Relation of the decay of free radicals in irradiated polyethylene to the molecular motion of the polymer and the configurations of the free radicals

Decay reactions of the free radicals produced in irradiated polyethylene (high-density and low-density materials) were examined in connection with the molecular motion of the matrix polymer. Three temperature regions, in which the free radicals decay very rapidly, at around 120, 200, and 250°K, were designated T_A, T_L, and T_B, respectively. The decay of the free radicals at these temperatures had activation energies in high-density polyethylene of 0.4 kcal/mole for T_A, 9.4 kcal/mole for T_L, and 18.4 kcal/mole for T_B. In low-density polyethylene these quantities were 0.7 kcal/mole for T_A, 23.1 kcal/mole for T_L, and 24.8 kcal/mole for T_B. Comparison of time constants for the decay reactions and for molecular motion of the matrix polymer indicate that the decay in T_A and T_B is closely related to molecular motion in the amorphous regions of the polymer. The decay of the free radicals at T_L in high-density polyethylene is due to molecular motion associated with local mode relaxation at lamellar surfaces, while that of low-density polyethylene is due to local mode relaxation in the completely amorphous region. Steric configurations of the free radicals which decay in the respective temperature regions were also investigated.     

Kinetics and mechanism of the oxidation of 2,3,6-trimethylphenol with hydrogen peroxide in the presence of Ti-monosubstituted polyoxometalates

The product composition and reaction kinetics are reported for 2,3,6-trimethylphenol (TMP) oxidation with hydrogen peroxide in acetonitrile catalyzed by a Ti-monosubstituted polyoxometalate (Ti-POM) with a Keggin structure ([Bu₄N]₄[PTi(OMe)W₁₁O₃₉]) and for the stoichiometric reaction between TMP and the peroxo complex [Bu₄N]₄[HPTi(O)₂W₁₁O₃₉] (I). The main products of the stoichiometric reaction are 2,3,5-trimethyl-1,4-benzoquinone (TMBQ) and 2,2′,3,3′,6,6′-hexamethyl-4,4′-biphenol (BP). The TMBQ yield increases as the TMP/I molar ratio is decreased. The catalytic reaction is first-order with respect to H₂O₂ and the catalyst and has a variable order (1-0) with respect to TMP. The rate of the reaction increases as the water concentration in the reaction mixture is raised. The stoichiometric reaction is first-order with respect to peroxo complex I and has a variable order (1-0) with respect to TMP. There is no kinetic isotope effect for this reaction (k _ArOH/k _ArOD = 1). A TMP oxidation mechanism is suggested, which includes the coordination of a TMP molecule and peroxide on a Ti site of the catalyst with the formation of a reactive intermediate. The one-electron oxidation of TMP in this intermediate yields a phenoxyl radical. The subsequent conversions of these ArO^° radicals yield the reaction products.     

Kinetics of the decomposition of calcium carbonate in the presence of Bi2O3

Former studies concerning the formation of the compounds in the pseudobinary systems of Bi₂O₃-MO type (M =Ca, Sr, Ca+Sr) have shown that the reaction which occurs with the highest rate is that between Bi₂O₃ and CaO. In the present work CaCO₃ was used as CaO source. We carried out an investigation of the thermal decomposition of CaCO₃ in the presence of Bi₂O₃ in comparison with the decomposition of pure CaCO₃.The presence of Bi₂O₃ exerts a complex influence on the CaCO₃ decomposition acting on the nucleation as well as on the diffusion of CO₂. The decomposition of the samples with low Bi₂O₃ content follows the mechanism of a contracting sphere. A change from surface nucleation to bulk nucleation is recorded for higher amounts of Bi₂O₃.     

Effect of the carbon atom coordination on the thermoactivated motion of the carbon-bonded trichloromethyl group

The reorientational motion of the trichloromethyl group depending on the environment of the carbon atom is considered based on the chlorine-35 nuclear quadrupole resonance (NQR) data. The study is performed for CCl₃ groups bonded to tri- and tetracoordinated carbon atoms in the crystalline compounds CCl₃CCl=NR (R = CH₂C₆H₅ and CCl₃CCl₃) and CCl₃CXClN=CClC₆H₄NO_2-n (X = H and Cl).₃₅Cl NQR studies of thermoactivated motion in 70 solid trichloromethyl-containing compounds are summarized. The ranges of activation energies of CCl₃ reorientations at tri- and tetracoordinated carbon atoms were determined to be 10-50 and 30-90 kJ/mole, respectively, the activation energy being markedly greater in the latter case. CCl₃ reorientations may be completely frozen by the bulky substituents which may be present along with CCl₃ in the tetracoordinated carbon bonds. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 4, pp. 737-746, July-August, 2000     

Change in the mechanism of the electroreduction of the perfluoro-n-hexyl iodide with varying the nature of the supporting salt Application to electrocarboxylation and sulfoxidation

We have previously reported that reduction of C₆F₁₃I in (DMF, LiClO₄), yields R_FHgR_F, produced through the reduction of R_FHgI itself formed at polarized mercury. The use of LiCl in the place of LiClO₄ affords a possibility to suppress the organometallic route and yields the carbanion C₆F₁₃, formed through a direct bielectronic reduction. This feature is exploited to perform electrocarboxylation and sulfoxidation. In this way, R_FCO₂H and R_FSO₂Cl are easily electrosynthesized in macroscale experiments. The anion effect remarked between Cl^? and ClO₄^? is studied in the case of Br^?, I^?, BF₄^? with various cations.