Silicon-29 cross-polarization/magic-angle-spinning NMR study of inorganic-organic hybrids: homogeneity of sol-gel derived hybrid gels

The structures of sol-gel derived hybrid gels prepared by co-hydrolysis of tetraethoxysilane (TEOS)-organotrialkoxysilane (RTES, RSi(OC₂H₅)₃, R = CH₃, C₅H₁₁, C₈H₁₇ and C₆H₅) mixtures (TEOS : RTES : CH₃CH₂OH : H₂O : HNO₃ = 0.5 : 0.5 : 10 : x : 0.3; x = 2, 5, 10 and 20) were characterized based on signal intensities of ²⁹Si CP/MAS NMR as a function of the contact time. The contact time dependencies of the signals for Q⁴ (Si(OSi)₄) units were successfully simulated by assuming distributed cross relaxation times (T_SiH) and an intrinsic ¹H spin-lattice relaxation time in the rotating frame (); the latter was different from the apparent value estimated by assuming single T_SiH. The distribution ranges of T_SiH for the TEOS-RTES gels broadened as the water content increased, suggesting that the Q⁴ units tended to be separated from the T units and that the local concentration of ¹H spins around the Q⁴ unit tended to decrease.     

Thermal Activation of Molecular Tungsten Halide Clusters with the Retention of an Octahedral Metal Framework and the Catalytic Dehydration of Alcohols to Olefins as a Solid Acid Catalyst

When the molecular tungsten halide cluster (H₃O)₂[(W₆Cl₈)Cl₆]·6H₂O, with an octahedral metal framework, is heated to 50 and 150 °C in flowing helium gas, it changes into (H₃O)₂[(W₆Cl₈)Cl₆] and [(W₆Cl₈)Cl₄(H₂O)₂], respectively. Activation at 250 °C yields a poorly crystallized solid state cluster, [W₆Cl₈]Cl₂Cl_4/2, which exhibits catalytic activity for the dehydration of ethanol to yield ethylene and a small amount of ethyl ether and acetal. The activity is attributed to the Br?nsted acidity of the hydroxo ligand that is produced by elimination of hydrogen chloride from the chloro and aqua ligands. The catalytic activity increases with increasing temperature, and reaches a maximum at 300 °C. The catalytic activity then disappears above 350 °C, at which temperature the crystallinity of the cluster improves and the active sites are included in the crystal. In the case of primary alcohols, the reactivity decreases with increasing length of the carbon chain, and secondary alcohols are more reactive than the corresponding primary alcohols. Halide clusters of niobium, molybdenum, and tantalum having the same metal framework are also active catalysts for these reactions.     

Ir-Catalyzed substitution of propargylic-type esters with enoxysilanes

Propargylic-type acetates react readily with enoxysilanes in the presence of 1 mol % of [Ir(cod){P(OPh)3}2]OTf activated preliminarily with molecular H2 to give beta-alkynyl ketones in high to excellent yields. Substitution at the propargyl carbon proceeds exclusively or selectively in most types of propargylic esters. Alternatively, the formation of the allenyl products is predominant in the reaction of esters, which have two phenyl groups on the propargyl carbon.     

Weak second-order stochastic Runge–Kutta methods for non-commutative stochastic differential equations

A new explicit stochastic Runge–Kutta scheme of weak order 2 is proposed for non-commutative stochastic differential equations (SDEs), which is derivative-free and which attains order 4 for ordinary differential equations. The scheme is directly applicable to Stratonovich SDEs and uses 2m-1$2m-1$ random variables for one step in the m-dimensional Wiener process case. It is compared with other derivative-free and weak second-order schemes in numerical experiments.     

Study of the Complexation of Protactinium(V) with EDTA

Journal of Solution Chemistry - The complex formation of protactinium(V) with ethylenediaminetetraacetic acid (EDTA) was studied at tracer concentration...     

Symmetric and Non-Symmetric Bases of Quantum Integrable Particle Systems with Long-Range Interactions

We study in an algebraic manner the symmetric basis of the Calogero model and the non-symmetric basis of the corresponding Calogero model with distinguishable particles. The Rodrigues formulas are presented for the polynomial parts of both bases. The square norm of the non-symmetric basis is evaluated. Symmetrization of the non-symmetric basis reproduces the symmetric basis and enables us to calculate its square norm.     

On the improvement of the SIMPLE-like method for flows with complex geometry

For strongly nonorthogonal grids, the convergence difficulty is often encountered in the SIMPLE method unless very low relaxation factors are adopted. This is because that the cross derivatives of pressure corrections are usually neglected in deriving the pressure-correction equation. In this study, an approximate treatment is proposed to deal with the cross derivatives in the pressure-correction equation. Computations are performed for various laminar flows in a nonorthogonal cavity and the turbulent flows through the compressor cascades. It is found that the convergence difficulty encountered in the SIMPLE method for strongly nonorthogonal grids can be overcome when the SIMPLEC method is employed. In addition, when the present approximate treatment for the cross derivatives is adopted in the SIMPLEC method, the convergence rate can be speeded up in comparison with the original SIMPLEC method. In some cases, the CPU time can be decreased by more than 60%. The computational results are also compared with the numerical results of others and the available experimental data. Received on 30 November 1998