New spiro derivatives of penicillin

The first example of carbenoid ring expansion of thiophene to a 2H-thiopyran has been observed in its Rh-catalysed reaction with benzhydryl 6-diazopenicillanate. Buchner ring expansion of anisole with the same carbenoid gives two isomeric methoxy-cycloheptatrienes by attack on the upper face of the β-lactam ring. In one of the isomers, the methoxyl group shows a strong conformational preference in solution, revealed by n.O.e. difference speotroscopy.     

Dynamics of viscous fingers in Hele-Shaw cells of liquid crystals Theory and experiment

The theoretical and experimental developments in the interfacial dynamics and the formation of viscous fingering patterns in Hele-Shaw cells of liquid crystal-air systems are summarized and discussed. These include radial and linear cells with or without grooves engraved on the cell plates. Instabilities of fingers, the role of intrinsic and extrinsic anisotropies, etc., are emphasized. In a linear cell, when the injected air is kept at constant pressure, a whole sequence of successive instabilities of fingers (hump, tip-splitting, sidewrinkling, sidebranching and DLA-like structure) is observed in a single run of the experiment. In our theory, the equations of motion of nematic flows in Hele-Shaw cells are derived from the Ericksen-Leslie equations. In the linear approximation, the equations resemble those of isotropic liquids with the presence of effective viscosities and anisotropic surface tension. Experimental observations are interpreted with the introduction of an effective control parameter which may be time dependent. Special features of viscous fingers in liquid crystals in contrast to those in isotropic liquids, such as asymmetric dendritics, displacement of the finger from the central axis of the linear cell, and reentrant sequence of patterns, are pointed out. Plausible explanations of these phenomena are given. In this newly developed field, a large number of interesting problems remain to be solved.     

Parametric study of Xe2 * dimer in high-pressure electrical discharges

Optical and electrical properties of a high-pressure discharge of pure xenon and xenon-helium and xenon-neon mixtures are studied experimentally. Uniform discharge at total gas pressure as high as 10 atm is achieved in xenon-lean mixtures. Vacuum ultraviolet emissions due to the first and second continua of Xe₂ ^* are examined spectroscopically. The vibrational relaxation rate constant k_He of Xe₂ ^* by helium is determined to be of the order of 10^-11 cm³ s^-1 from spectroscopic data. Laser oscillation for the 172-nm band was attempted without success. Discharge instability at high gas pressures is considered to the cause of the unsuccessful laser experiments. Some issues related to discharge instability in a high-pressure rare-gas discharge are discussed. Received: 1 October 2001 / Revised version: 11 June 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +852-2603/5204, E-mail: dlo@phy.mhk.edu.hk     

Measurement of carbonaceous aerosols: validation and comparison of a solvent extraction-gas chromatographic method and a thermal optical transmittance method

A solvent extraction-gas chromatographic method was developed for the measurement of four classes of solvent-extractable organic compounds (SEOC), viz., aliphatic hydrocarbons, polynuclear aromatic hydrocarbons, fatty acids and alkanols, in ambient carbonaceous aerosols. The selectivity and recovery of the method using different solvents commonly used in the analysis of SEOC was evaluated. The composition of SEOC in aerosol and urban dust samples was found to vary with the choice of solvent. Such variation was not observed in filters spiked with standards of SEOC. The developed method was validated by the analysis of reference urban dust SRM 1649a from the National Institute of Standards and Technology. The concentrations of selected polynuclear aromatic hydrocarbons in the reference dust were found to fall within the certified and reference concentrations. The thermal characteristics of aerosol samples collected in an urban area of Hong Kong and the four classes of identified SEOC were also studied using a thermal optical transmittance (TOT) method as specified in NIOSH Method 5040. Correlations were made to compare the method-dependent SEOC content obtained by the solvent extraction method and the results for total carbon, organic carbon and elemental carbon obtained by the TOT method.     

Stabilization of D5h and C2v valence tautomers of the croconate dianion

The non-benzenoid aromatic D(5h) and enediolate C(2v) form of C5O5(2-) have been stabilized by hydrogen bonding with urea and 1,3-dimethylurea, respectively, in the host lattices of two novel crystalline inclusion compounds.     

Synthesis and reactions of 7-fluoro-4-methyl-6-nitro-2-oxo-2H-1-benzopyran-3-carboxylic acid: a novel scaffold for combinatorial synthesis of coumarins

7-Fluoro-4-methyl-6-nitro-2-oxo-2H-1-benzopyran-3-carboxylic acid has been prepared as a novel scaffold for combinatorial synthesis of coumarins. The scaffold has three points of diversity. The optimal conditions for its reactions with different nucleophiles in solid phase were obtained. Sixteen coumarin derivatives with different structures were designed and synthesized in solid phase to demonstrate its application as a scaffold for combinatorial synthesis of coumarins. Thirteen of these derivatives were obtained in high yields. Many of these model compounds fluoresce. Combinatorial libraries constructed with this novel scaffold may have interesting biological or physical properties.