Mechanistic aspects of the formation of aldehydes and nitriles in photosensitized reactions of aldoxime ethers

The photooxidation of a series of aldoxime ethers was studied by laser flash photolysis and steady-state (product studies) methods. Nanosecond laser flash photolysis studies have shown that chloranil (CA)-sensitized reactions of the O-methyl (1), O-ethyl (2), O-benzyl (3), and O-tert-butyl (4) benzaldehyde oximes result in the formation of the corresponding radical cations. In polar non-nucleophilic solvents such as acetonitrile, there are several follow-up pathways available depending on the structure of the aldoxime ether and the energetics of the reaction pathway. When the free energy of electron transfer (DeltaGET) becomes endothermic, syn-anti isomerization is the dominant pathway. This isomerization pathway is a result of triplet energy transfer from CA to the aldoxime ether. For substrates with alpha-protons (aldoxime ethers 1-3), the follow-up reactions involve deprotonation at the alpha-position followed by beta-scission to form the benziminyl radical (and an aldehyde). The benziminyl radical reacts to give benzaldehyde, the major product under these conditions. A small amount of benzonitrile is also observed. In the absence of alpha-hydrogens (aldoxime ether 4), the major product is benzonitrile, which is thought to occur via reaction of the excited (triplet) sensitizer with the aldoxime ether. Abstraction of the iminyl hydrogen yields an imidoyl radical, which undergoes a beta-scission to yield benzonitrile. An alternative pathway involving electron transfer followed by removal of the iminyl proton was not deemed viable based on charge densities obtained from DFT (B3LYP/6-31G*) calculations. Similarly, a rearrangement pathway involving an intramolecular hydrogen atom transfer process was ruled out through experiments with a deuterium-labeled benzaldehyde oxime ether. Studies involving nucleophilic solvents have shown that all aldoxime ethers reacted with MeOH by clean second-order kinetics with rate constants of 0.7 to 1.2 x 10(7) M(-1) s(-1), which suggests that there is only a small steric effect in these reactions. The steady-state experiments demonstrated that under these conditions no nitrile is formed. This is explained by a mechanistic scheme involving nucleophilic attack on the nitrogen of the aldoxime ether radical cation, followed by solvent-assisted [1,3]-proton transfer and elimination of an alcohol, similar to the results obtained for a series of acetophenone oxime ethers.     

A scanning tunneling microscopy study of distyrylbenzene on Ag/Ge(111)-(sqr rt of 3 x sqr rt of 3)R30 degrees

The adsorption and self-organized monolayers of trans,trans-distyrylbenzene (tt-DSB) and cis,cis-distyrylbenzene (cc-DSB) on Ag/Ge(111)-(sqr rt of 3 x sqr rt of 3)R30 degrees (Ag/Ge(111)-sqr rt of 3) were studied by low-temperature scanning tunneling microscopy (STM) in ultrahigh vacuum. tt-DSB and cc-DSB overlayers were prepared by vapor deposition at a substrate temperature of 200 K and imaged after the samples were cooled to 100 K. High-resolution images allow identification of the internal structure of individual tt-DSB molecules with three phenyl rings and their molecular arrangements on the Ag/Ge(111)-sqr rt of 3 surface. It is found that the intermolecular distance between two terminal phenyl rings in tt-DSB is about twice the lattice constant of Ag/Ge(111)-sqr rt of 3. Such a lattice match makes Ag/Ge(111)-sqr rt of 3 an ideal substrate for tt-DSB self-organization and the formation of a (3 x 1) overlayer unit cell. The structural model and the molecule registry corresponding to STM images for the adlayers of tt-DSB on Ag/Ge(111)-sqr rt of 3 are proposed and discussed. For cc-DSB adsorption on Ag/Ge(111)-sqr rt of 3, uniform molecular overlayers with two discernible molecular images corresponding to two major types of cc-DSB conformers were observed. The coexistence of multiple conformers and the mismatch of molecular dimension of cc-DSB with the substrate unit cell length limit the growth of large cc-DSB domains.     

Non-abelian local invariant cycles

Let be a degeneration of Kähler manifolds. The local invariant cycle theorem states that for a smooth fiber of the degeneration, any cohomology class, invariant under the monodromy action, comes from a global cohomology class. Instead of the classical cohomology, one may consider the non-abelian cohomology. This note demonstrates that the analogous non-abelian version of the local invariant cycle theorem does not hold if the first non-abelian cohomology is the moduli space (universal categorical quotient) of the representations of the fundamental group.

     

Adaptive Algorithms of Nonlinear Approximation with Finite Terms

This paper deals with realizable adaptive algorithms of the nonlinear approximation with finite terms based on wavelets. We present a concrete algorithm by which we may find the required index set Am for the greedy algorithm Gm^P（., Ψ）. This makes the greedy algorithm realize the near best approximation in practice. Moreover, we study the efficiency of the finite-term approximation of another Mgorithm introduced by Birge and Massart.     

Interior Regularity Criteria for Suitable Weak Solutions of the Navier-Stokes Equations

We present new interior regularity criteria for suitable weak solutions of the 3-D Navier-Stokes equations: a suitable weak solution is regular near an interior point z if either the scaled -norm of the velocity with 3/p + 2/q ≤ 2, 1 ≤ q ≤ ∞, or the -norm of the vorticity with 3/p + 2/q ≤ 3, 1 ≤ q < ∞, or the -norm of the gradient of the vorticity with 3/p + 2/q ≤ 4, 1 ≤ q, 1 ≤ p, is sufficiently small near z.     

Novel Synthesis and Properties of Smart Core-Shell Microgels

A new method has been developed to prepare smart microgels that consist of well-defined temperature-sensitive cores with pH-sensitive shells. The microgels were obtained directly from aqueous graft copolymerization of N-isopropylacrylamide and N,N′-methylenebisacrylamide from water-soluble polymers containing amino groups such as poly(ethyleneimine) and chitosan. The gel diameters ranged from 300 to 400 nm with narrow size distribution. The unique core-shell nanostructures exhibited tuneable responses to pH and temperature.     

Synthesis and Properties of Poly(N-isopropylacrylamide-co-acrylamide) Hydrogels

Poly (N-isopropylacrylamide-co-acrylamide) [Poly(NIPAAm-co-AAm)] with different feed ratios were obtained by radiation polymerization using Co⁶⁰γ-rays. Swelling equilibrium data in various media: deionized water, aqueous NaCl solutions and different pH buffer solutions, were determined. It appeared that the lower critical transition temperature (LCST) of the hydrogels increased with an increasing acrylamide content and decreased with increasing ionic strength. Moreover, LCST was affected by pH.     

Chaotic behavior of a nonlinear oscillator

Behavior of bifurcation and chaos in a forced oscillator(?)containing a square nonlinear term is investigated by using Mel’nikov method and digital computer simulations.     

Investigation on High Efficient Catalysts for the Oxidative Dehydrogenation of Ethane

Since the pioneer work of Thorsteinson fot me ox(?) ethane, a series of V-Mo based catalysts mainly for the oxidative dehydrogenation of ethane have been patented. On the surfaces of these catalysts, a C_2H_4 selectivity of 70％ was achieved, but the space velocity was only about 340 h~(-1). Lunsford, et al. reported a C_2H_6 conversion of 75％ and a C_2H_4 selectivity of 76％ over the     

二维振荡流动中污染云团的收缩*

当垂向扩散时间尺度与流动的周期相当时,在转流过程中,污染云团将会出现收缩.这时水平剪切分散导数将会出现负值奇性.本文根据作者两维延迟扩散方程[7]: 其中u(t),v (t)为深度平均水平速度.导出X(t,τ),Y(t,τ)坐标位移,D_ij(t,τ)为剪切扩散导数的方程.一般情况下,?D_ij(t,τ)/?τ是正的.不存在奇异性.但在转流的初期.记忆函数D_ij(t,τ)就有可能是负的.本文给出了D_ij和X、Y的解析表示式.