Synthesis and characterization of two polymorphic crystalline phases and an amorphous powder of nickel(II) bisimidazolate

Nickel(II) bisimidazolate is polymorphic. Depending on the synthetic strategy adopted, two crystalline phases (alpha- and beta-Ni(im)(2)) or an amorphous material of the same composition can be prepared. The thermodynamically stable alpha-Ni(im)(2) phase, which can be prepared in water at elevated temperatures, contains a two-dimensional polymer (of nearly square meshes) with square-planar NiN(4) chromophores and exo-bidentate imidazolate ligands bridging nickel atoms that are ca. 5.73 A apart. The beta-Ni(im)(2) phase can be kinetically stabilized at lower temperatures, but the structural complexity and the lack of single crystals prevented its full structural characterization, even in the presence of an indexed powder diffraction pattern. The spectroscopic features of these crystalline phases are compared with those of the amorphous material.     

Study on particle removal efficiency of an impinging jet by an image-processing method

 An image-processing method is proposed to obtain the distribution of the removal efficiency of particles on a plate by an air jet. This method can be used to measure particle removal from a flat surface by processing the image of the reflected light from the surface. Factors affecting the particle removal efficiency such as air pressure, distance between the nozzle and the impinging surface and the impinging angle are discussed. Optimal conditions are determined to obtain the most effective particle removal by the air jet. Received: 10 April 2001 / Accepted: 2 August 2001     

Reversible microwave-assisted cycloaddition of aziridines to carbon nanotubes

We report a new approach that uses microwaves to rapidly functionalize carbon nanotubes by using 1,3-dipolar cycloaddition of aziridines, well-known precursors to azomethine ylides, in solvent-free conditions. The efficiency of our microwave-mediated protocol is confirmed by comparison to a similar protocol in classical conditions for the azomethine ylides in DMF. Under these latter conditions, the reaction proceeds in 5 days (against 1 h under microwave irradiation), and the functionalization degree is much lower, as confirmed by thermogravimetric analysis and Raman spectroscopy. With our procedure, we easily scale-up the reaction up to 250 mg of functionalized MWNT in 1 h. We also provide an indirect proof of the covalent sidewall functionalization of the tubes.     

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

Stable isotopic studies of earthworm feeding ecology in tropical ecosystems of Puerto Rico

Feeding strategies of earthworms and their influence on soil processes are often inferred from morphological, behavioral and physiological traits. We used (13)C and (15)N natural abundance in earthworms, soils and plants to explore patterns of resource utilization by different species of earthworms in three tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus and Estherella sp. showed less (15)N enrichment ((15)N = 3-6 per thousand) than exotic Pontoscolex corethrurus ((15)N =7-9 per thousand) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in (15)N enrichment ((15)N = 6-9 per thousand), suggesting the potential for interspecific competition for N resources. A tabonuco forest converted to pasture contained only P. corethrurus which were less enriched in (15)N than those in the forest sites, but more highly enriched in (13)C suggesting assimilation of C from the predominant C(4) grass. These results support the utility of stable isotopes to delineate resource partitioning and potential competitive interactions among earthworm species. Copyright 1999 John Wiley & Sons, Ltd.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.     

Synthesis of Hexahydro‐2‐pyrindine (=Hexahydrocyclopenta[c]pyridine) Derivatives as Conformationally Restricted Analogs of the Nicotinic Ligands Arecolone and Isoarecolone

Two hexahydropyrindine derivatives, 1,2,3,4,6,7‐hexahydro‐2‐methyl‐5H‐cyclopenta[c]pyridin‐5‐one ( 1 ) and 1,2,3,4,5,6‐hexahydro‐2‐methyl‐7H‐cyclopenta[c]pyridin‐7‐one ( 2 ), and their methiodides 14 and 26 , respectively, were synthesized. They can be considered rigid analogues of the known nicotinic agonists arecolone (=1‐(1,2,5,6‐tetrahydro‐1‐methylpyridin‐3‐yl)ethanone) and isoarecolone (=1‐(1,2,3,6‐tetrahydro‐1‐methylpyridin‐4‐yl)ethanone). The affinity for the central nicotinic receptor were measured on rat cerebral cortex. Although only the methiodide 14 , among the four conformationally restricted compounds, shows an appreciable affinity, the results obtained provide useful information on the molecular requirements at the interaction site of the central nicotinic receptors.