Carbonyl allenylation/free radical cyclization sequence as a new regio- and stereocontrolled access to bi- and tricyclic beta-lactams

A novel approach to racemic and enantiopure nonconventional fused bi- and tricyclic beta-lactams has been developed by using regio- and stereocontrolled intramolecular free radical reactions in monocyclic 2-azetidinone-tethered allenynes and haloallenes. The access to allene cyclization precursors was achieved by metal-mediated carbonyl allenylation of appropriately substituted 4-oxoazetidine-2-carbaldehydes in an aqueous environment. The tin-promoted radical cyclizations of allene-beta-lactams are totally regioselective for the central allenic carbon, providing bi- and tricyclic beta-lactams containing a seven-membered ring.     

Macroturbulence measurements with electromagnetic and ultrasonic sensors: a comparison under high-turbulent flows

 Experimental results of turbulent flows for both, field and laboratory tests are presented. The spatial and temporal resolution is limited by the measuring instruments, which result in “filtering” out the very small scales, thus observing only macroturbulence. The experimental field-results have been obtained during the large-scale surf zone experiments carried out in the Ebro Delta, (Spain), under spilling/plunging breaking waves. The field-measurements include several tests across the surf zone with high vertical resolution. The measured turbulent properties are compared with state-of-art macroturbulence characteristics and length parameterisations. Furthermore, a detailed comparison of two-dimensional electromagnetic and three-dimensional ultrasonic velocity measurements has been carried out for hydraulic jumps in a laboratory flume to validate the field measurements and to study the frequency response of electromagnetic sensors. Received: 22 December 1996/Accepted: 8 September 1998     

Structure and Bonding in First‐Row Transition‐Metal Dicarbides: Are They Related to the Stability of Met‐cars?

First-row transition-metal dicarbides MC(2) (M=Sc-Zn) have been investigated by using quantum-mechanical techniques. The competition between cyclic and linear isomers in these systems has been studied and the bonding scheme for these compounds is discussed through topological analysis of electron density. All of the systems have been found to prefer a C(2v)-symmetric arrangement, although for ZnC(2) the energy difference between this and the linear isomer is rather small. In most cases the C(2v)-symmetric structure corresponds to a T-shaped structure, with the exceptions of TiC(2), CoC(2), and NiC(2) which have been shown to be true rings. A detailed analysis of the variation of the energy of the system with geometry has been carried out. An analysis of the bonding, taking into account the main interactions between the valence orbitals of both fragments, the M atom and the C(2) molecule, has allowed the main features of these compounds to be interpreted. A clear correlation between the dissociation energies of the first-row transition-metal dicarbides and the bonding energies of the corresponding met-cars was observed.     

Theoretical study of AlC3+

A theoretical study of the AlC₃⁺ species has been carried out. Predictions have been made for some of the molecular properties (geometries, dipole moments, and harmonic vibrational frequencies) which could help in their possible experimental detection. In addition, a topological analysis of the electron density and its associated Laplacian has also been carried out. The global ground state is predicted to be a linear species with ¹Σ electronic state, but a rhombic four‐membered ring (¹A₁) lies close in energy. It seems that both isomers could be accessible to experimental detection. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Reaction of 4-acylaminomethylpyridine N-oxides with phenylbutazone in the presence of acetic anhydride

Reaction of 4-acylaminomethylpyridine N-oxides with phenylbutazone in the presence of acetic anhydride is described. In the pharmacological screening one compound shows an interesting anticonvulsant activity.     

Report on Turbulence and Mixing in Geophysical Flows II

We describe the presentations at the international meeting/workshop on Mixing in Geophysical Flows that took place at Vilanova i la Geltru, near Barcelona during the 20, 21 and 22nd of March 1997. There were more than 100 participants from 20 countries with 66 oral and poster presentations covering experimental and theoretical aspects of rotating and stratified fluids as well as field observations. The main topics discussed at the workshop were stratified flows, rotating stratified flows, gravity waves, instabilities and mixing, convection, experiments and numerical simulations of Geophysical flows and turbulent mixing. The papers are summarised in this report giving a state-of-the-art overview of present research in geophysical turbulent mixing.     

连续波锁模Pr3+:YLF激光器

本文报导了采用氩离子激光器来泵浦Pr³⁺:YLF晶体,应用声光调制器实现了主动锁模;同时应用振动─高反射平面镜也实现了被动锁模,两种锁模均得到了ps光脉冲.据作者了解这是这种晶体材料的第一次锁模运转.     

Solution of sparse quasi-square rectangular systems by Gaussian elimination

We present a general method for the linear least-squares solutionof overdetermined and underdetermined systems. The method isparticularly efficient when the coefficient matrix is quasi-square,that is when the number of rows and number of columns is almostthe same. The numerical methods for linear least-squares problemsand minimum-norm solutions do not generally take account ofthis special characteristic. The proposed method is based onLU factorization of the original quasi-square matrix A, assumingthat A has full rank. In the overdetermined case, the LU factorsare used to compute a basis for the null space of A^T. The right-handside vector b is then projected onto this subspace and the least-squaressolution is obtained from the solution of this reduced problem.In the case of underdetermined systems, the desired solutionis again obtained through the solution of a reduced system.The use of this method may lead to important savings in computationaltime for both dense and sparse matrices. It is also shown inthe paper that, even in cases where the matrices are quite small,sparse solvers perform better than dense solvers. Some practicalexamples that illustrate the use of the method are included.     

Structure Functions in Complex Flows

The turbulence in the ocean and atmosphere is most of the time non-homogeneous in nature. These spatial changes could affect the structure of the turbulence. In this work a classification is proposed to determine the intermittency and mixing ability. The variation of the structure functions and the scaling exponent in decaying non-homogeneous turbulence produced by a grid and by a jet is measured with a sonic velocimeter SONTEK3-D. We use Extended Self Similarity (ESS) to obtain better estimates of the scaling exponents of the structure functions of order up to the 6th. We study the variation of the absolute scaling exponents _p and relative scaling exponents ¯_p as a function of distance from the source of turbulence. In most cases, the absolute scaling exponent ₃ is shown to vary as function of the separation distance l. On the other hand the relative scaling exponents ¯_p depend on the location of the flow and in most cases the deviations from the Kolmogorov 1941 scaling are related to the intermittency.