High yield incorporation and washing properties of halides incorporated into single walled carbon nanotubes

We describe here the high yield filling (i.e. >50%) of single walled nanotubes (SWNTs) with a variety of halides, achieved according to various modified filling procedures. Both bundles and discrete SWNTs can be filled continuously up to lengths of several hundred nm, often with filling yields approaching 60–70% or better. In addition some high yield filled SWNTs were subjected to long-term washing in either boiling or room temperature. aqueous media, which does not remove the filling from the tubules, but enables effective removal of water-soluble extraneous materials . Received: 10 May 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +44-1865/272-690, E-mail: jeremy.sloan@chem.ox.ac.uk     

TEM and HREM of diamond crystals grown on Si tips: structure and results of ion-beam-treatment

Diamond single crystals were grown on the silicon whiskers by a hot filament chemical vapor deposition technique at the filament temperature about 2100 degrees C and the temperature of support 800 degrees C. Specimens were examined by SEM, TEM, HRTEM and SAED. When the filament temperature was about 1900 degrees C globular polycrystalline diamond particles were grown. At a support temperature more then 800 degrees C SiC nanoparticles were formed. To investigate the ion etching process of the silicon tip/diamond system, tips were treated with an Ar(+) beam with energy up to 30 kV. The results depend on fluence: at 4 x 10(18)ion/cm(2) diamonds and partially Si tips were destroyed, amorphous layer was formed (sometimes with nanometric size fragments of diamond); at 1 x 10(18)ion/cm(2) sharpened diamonds (radius of curvature about 20 nm) covered with amorphous layer (radius about 80 nm) probably with nanoclusters of diamond were observed; at 4.4 x 10(17) ion/cm(2) there was no visible tip sharpening but formation of amorphous thick layer occurred. The emission characteristics of Si tips covered with diamond were improved due to ion treatment. Since such tips in our case were covered with amorphous layer containing nanometric size fragments of diamond, we suppose this layer is responsible for electron emission improvement.     

Chemical models for aspects of the photosynthetic reaction centre: synthesis and photophysical properties of tris- and tetrakis-porphyrins that resemble the arrangement of chromophores in the natural system

Tris-porphyrin and tetrakis-porphyrin arrays 1 and 2 are proposed as models for the arrangement of the chromophores that constitute photosynthetic reaction centres (PRC's). Their porphyrinic chromophores are similar in distance apart to the key chromophores of PRC's and the C2 symmetric arrangement of the macrocycles that constitute the 'special pair' where charge separation occurs is also incorporated. The use of zinc(II) and gold(III) chelation establishes an energy gradient for photoinduced electron transfer across each compound. Synthesis was achieved in good yields through a strategy that used the construction of biquinoxalinyl and Tr?ger's base linkages between the porphyrinoid components. Compounds which are bis-porphyrin molecular components of the arrays were also synthesised. Photophysical analyses indicate that long-range photoinduced energy and electron transfer processes occur in the extended arrays in addition to those occurring in the component bis-porphyrins. Evidence for step-wise electron transfer between terminal zinc(II)-chelated and gold(III)-chelated porphyrins has been detected in both porphyrins 1 and 2 in polar solvents, representing charge transfer across 35 A and 50 A, respectively. At 298 K, in deaerated benzonitrile, the lifetime of the charge transfer state of the tris-porphyrin 1 is 150 ns and the lifetime of the charge transfer state of tetrakis-porphyrin 2 is 59.4 micros; very long when compared to simpler chemical model systems, but still much shorter than the 1 s lifetime of the charge separated state of natural PRC's in cell membranes.     

Mapon and single passage NMRON spectroscopy on125SbFe and125SbNi single crystals

The signs, mode magnitudes and distributions of the electric quadrupole hyperfine interactions (EQI) in single crystal¹²⁵SbFe and¹²⁵SbNi have been examined along a principal hard axis for each system and are compared with earlier easy axis results. The surface preparation was essentially the same for both hosts but the results of the distribution width and anisotropy in the mode values of the measured EQI’s are remarkably different between the two hosts. For¹²⁵SbFe the mode value of the EQI is a factor of two larger along the hard axis <111> and the same sign, negative, as for the easy axis <100>. For¹²⁵SbNi the mode values of the EQI along the same two principal directions are comparable in magnitude but the efg distributions are much broader than in the¹²⁵SbFe case. Single passage results on¹²⁵SbNi provide a weak sweep asymmetry with indications of an apparent change in sign in EQI from negative along the <111> easy axis to positive along the hard <100> axis.     

Stereospecific synthesis and antibiotic activity of some cephalosporin analogs

The strategy used for the stereospecific synthesis of benzofused octams as cepham analogs was the conversion of a 3,4-dihydrothioisocarbostyril to a thioimidate, cycloaddition reaction to form a fused β-lactam and subsequent removal of the alkylthio group by Raney nickel hydrogenolysis. Using azidoacetyl chloride for forming the β-lactam and following known procedures, an amido side chain was generated. Two of the carba analogs of cephalosporin so produced showed low levels of antibacterial activity against a number of gram positive and gram negative organisms.     

Foliacenes: ab initio modeling of metallocomplexes exhibiting a unique form of 16-electron, metal-induced aromaticity

Ab initio calculations predict that the cyclic trefoilenes 2 can be stabilized by formation of a complex 4 with early transition metals. The metal atom within the complex is nested within the carbon ring and is considerably closer to the ring centroid than in traditional metallocene complexes. Stabilization is explained by a unique form of 16-electron delocalization involving the metal atom, for which we suggest the name "foliate aromaticity". The aromaticity of various polyfoliate systems such as 9 suggests this 16-electron motif is more robust than Clar-like aromatic 6pi-sextets. The open hemisphere of the metal in such foliacene complexes is predicted to coordinate a variety of ligands.     

Addition of lithiated 9-deazapurine derivatives to a carbohydrate cyclic imine: convergent synthesis of the aza-C-nucleoside immucillins

Means have been developed for the synthesis and addition of 9-deaza-9-lithiopurine derivatives to the carbohydrate-derived cyclic imine 6 in facile convergent syntheses of biologically active aza-C-nucleosides.     

MAPON EFG Result for 54Mn in Cubic Cobalt

Modulated adiabatic passage on oriented nuclei (MAPON) spectroscopy has been used to measure the electric quadrupole interaction at dilute ⁵⁴Mn impurity probes in crystallographically cubic (fcc) cobalt. The measured value is P/h=+5.2(5) kHz and using Q=+0.33(3) b, leads to an electric field gradient of V _zz =+1.3(2)×10¹⁹ V m⁻². This result is consistent with established trends for Mn and Co probes in the 3d ferromagnetic hosts. This revised version was published online in September 2006 with corrections to the Cover Date.     

Continuous-Wave Magnetic Resonance Imaging of Short T2 Materials

There is growing interest in the use of magnetic resonance imaging (MRI) to examine solid materials where the restricted motion of the probed spins leads to broad lines and short T₂ values, rendering many interesting systems invisible to conventional 2DFT pulsed imaging methods. In EPR T₂ seldom exceeds 0.1 μs and continuous-wave methods are adopted for spectroscopy and imaging. In this paper we demonstrate the use of continuous-wave MRI to obtain 2-dimensional images of short T₂ samples. The prototype system can image samples up to 50 mm in diameter by 60 mm long and has been used to image polymers and water penetration in porous media. Typical acquisition times range between 10 and 40 min. Resolution of 1 to 2 mm has been achieved for samples with T₂ values ranging from 38 to 750 μs. There is the possibility of producing image contrast that is determined by the material properties of the sample.