Electrochemical study of 18-crown-6-Tl+ complexes in binary solvent mixtures

Summary The formation constants,K _S, of the 18-crown-6 complex with thallium(I) ion were studied by polarographic measurements in binary mixtures of acetonitrile, acetone, tetrahydrofuran, and dimethylsulfoxide with water, as a function of the solvent mole fraction. In all the cases, the variation of the stability constant can be described by the empirical relation logK _S=a[(–1)/(2+1)]+b where stands for relative permittivity of a given mixture anda andb mark the regression coefficients. The values ofa calculated for four series of binary mixtures showed correlation with the Gutmann donor numbers of the neat organic solvents which form the mixture.On leave from the Department of Chemistry, Jingzhou Teacher's College, Jingzhou, Hubei, China     

Remote substituent effects on bond dissociation energies of para-substituted aromatic silanes

UB3LYP/6-31G(d) and ROMP2/6-311++G(d,2p) methods were used to calculate the Si-X bond dissociation energies (BDEs) of a number of para-substituted aromatic silanes (4-Y-C(6)H(4)-SiH(2)X, where X = H, F, Cl, or Li). It was found that the substituent effect on the Si-H BDE of 4-Y-C(6)H(4)-SiH(3) was small, as the slope (rho(+)()) of the BDE- regression was only 0.09 kJ/mol. In comparison, the substituent effect on the Si-F BDE of 4-Y-C(6)H(4)-SiH(2)F was much stronger, whose rho(+ )()value was -2.34 kJ/mol. The substituent effect on the Si-Cl BDE of 4-Y-C(6)H(4)-SiH(2)Cl was also found to be strong with a rho(+)() value of -1.70 kJ/mol. However, the substituent effect on the Si-Li BDE of 4-Y-C(6)H(4)-SiH(2)Li was found to have a large and positive slope (+9.12 kJ/mol) against. The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituent to stabilize or destabilize the starting material (4-Y-C(6)H(4)-SiH(2)X) as well as the product (4-Y-C(6)H(4)-SiH(2)* radical) of the homolysis. Therefore, the direction and magnitude of the effects of Y-substituents on the Z-X BDEs in compounds such as 4-YC(6)H(4)Z-X should have some important dependence on the polarity of the Z-X bond undergoing homolysis. This conclusion was in agreement with that from earlier studies (for example, J. Am. Chem. Soc. 1991, 113, 9363). However, it indicated that the proposal from a recent work (J. Am. Chem. Soc. 2001, 123, 5518) was unfortunately not justified.     

Fabrication and characterization of hexagonal mesoporous silica monolith via post-synthesized hydrothermal process

Large-sized, optical transparent mesostructured Brij 56/silica monolith has been fabricated using a lyotropic liquid crystal of Brij 56 (C₁₆EO₁₀) as a template and TMOS as a silica source, combined with a optimizing sol-gel process and a hydrothermal aging process. By programmed temperature drying and calcinations, translucent mesoporous silica monolith with two-dimensional hexagonal structure (P6mm) has bee obtained. The ordered mesoporous silica monoliths have been characterized by small-angle X-ray diffraction, transmission electron microscopy (TEM), and nitrogen adsorption, which shows that the materials have a highly ordered two-dimensional hexagonal mesostructure with the high specific surface area of 837 m² · g⁻¹ and narrow pore distribution with a mean BJH pore diameter of 2.73 nm. Based on calculations and differential scanning calorimetry and thermogravimetric analyses, the action mechanism of the hydrothermal aging process has been proposed: the 100°C hydrothermal conditions and autogenous 2.3 atm pressure promote the condensation and dehydration of silanol groups, with the result that cross-linking degree, the flaws and moisture content in gels are reduced notably. Those processes guarantee the integrity of gels in the following drying process.     

Racemization of chiral amino alcohols III: Effect of Mg or Ca addition on the activity and stability of the Co/γ-Al2O3 catalyst

The racemization of R-(-)-2-amino-1-butanol in a reaction using Co/γ-Al₂O₃ catalysts and catalysts modified by Mg or Ca was investigated in this paper. Complete racemization was achieved with a yield of over 83% at using the Mg modified Co/γ-Al₂O₃ catalyst under optimized reaction conditions of 170°C and 2.5 MPa of H₂. The catalysts were thoroughly characterized by XRD, XPS, TPR, SEM and TEM. The addition of Mg and Ca may be advantageous for dispersing and stabilizing the active species of the Co/γ-Al₂O₃ catalyst, protecting from sintering, significantly improving its catalytic activity and stability.     

Synthesis and structural characterization of a terminal hydroxide containing alumoxane via hydrolysis of aluminum hydrides

A novel terminal hydroxide containing dinuclear alumoxane LAl(OH)OAlL(OCH=N-tBu) (3; L = HC(CMeNAr)2, Ar = 2,6-iPr2C6H3) was prepared by treatment of aluminum dihydride LAlH2 (1) and tert-butyl isocyanate in the presence of trace amounts of water and alternatively from 1 and LAlH(OCH=N-tBu) (2) with water. Compound 2 was obtained from the reaction of 1 and tert-butyl isocyanate.     

Preparation and characterization of spherical V2O3 nanopowder

V₂O₃ nanopowder with spherical particles was prepared by reducing pyrolysis of the precursor, (NH₄)₅[(VO)₆(CO₃)₄(OH)₉]·10H₂O, in H₂ atmosphere. The thermolysis process of the precursor in a H₂ flow was investigated by thermogravimetric analysis and differential thermal analysis. The results indicate that pure V₂O₃ forms at 620°C and crystallizes at 730°C. The effects of various reductive pyrolysis conditions on compositions of V₂O₃ products were studied. Scanning electron micrographs show that the particles of the V₂O₃ powder obtained at 650°C for 1 h are spherical about 30 nm in size with more homogeneous distribution. Experiments show that nanopowder has larger adsorption capacity to gases and is more easily reoxidized by air at room temperature than micropowder. Differential scanning calorimetry experiment indicates that the temperature of phase transition of nano-V₂O₃ powder is −119.5°C on cooling or −99.2°C on heating. The transition heats are −12.55 J g⁻¹ on cooling and 11.42 J g⁻¹ on heating, respectively.     

How is 1,1-dimethyl-1-silacy clopent-2-ene formed in the addition of dimethylsilylene to 1,3-butadiene? evidence for a pathway via a vinylsilirane intermediate

Addition of dimethylsilylene Me₂Si: to 1,4-dideutero-l,3-butadiene in gas-phaee flow pyrolysis experimenss leads to the formation of 2,5-dideutero-1, 1-dimethyl-1-silacyclopent-3-ene and 4,5-dideutero-1,1-dimethyl-1-suacyclopent-2-ene. Pyrolysis of the silacyclopent-3-ene product leads to the same silacyclopent-2-ene. This labelling pattern for the silacyclo-pent-2-ene product is compelling evidence for its formation via carbon-carbon bond-cleavage in a vinylsilirane intermediate, and for the intermediacy of the vinylsilirane in the silylene addition leading to the silacyclopent-3-ene as well.     

Dissociation of sulfur dioxide by ultraviolet multiphoton absorption between 224 and 232 nm

Multiphoton excitation and dissociation of SO(2) have been investigated in the wavelength range from 224 to 232 nm. Strong evidence is found for two-photon excitation to the H Rydberg state, followed by dissociation to SO + O and ionization of the SO product by absorption of a third photon. The two-photon excitation is resonantly enhanced via the C (1)B(2) intermediate state, and the two-photon yield spectrum thus bears a strong resemblance to the spectrum of this intermediate. Imaging of the O((3)P(2)), S((1)D(2)), and SO products suggests that, following dissociation of SO(2) from the H state, SO is produced in the A and B electronic states. S((1)D(2)) is produced both from two-photon dissociation of SO(2) to give S((1)D(2)) + O(2) and by single-photon dissociation of SO(+). In the former process, the O(2) is likely formed in all of its lowest three electronic states.     

Recent advances on deoximation: From stoichiometric reaction to catalytic reaction

Recent advances on the deoximation reactions are reviewed in this review. It was shown that catalytic deoximation with molecular oxygen as the mild oxidant should be the developing trend of the reaction.