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At least 1,009 records · Page 56Linked to original sources
PubMed2003

Towards graphite: magnetic properties of large polybenzenoid hydrocarbons.

The geometries of four different series of D(6h)-symmetric polybenzenoid hydrocarbons (PBH) up to and including C(222)H(42) have been optimized at the B3LYP/6-31G(d) level of theory. Excluding C(48)H(24) and C(138)H(42), which have D(3d) minima due to 1,5 H...H repulsions between adjacent perimeter rings, optimized geometries are planar D(6h) minima. Nucleus Independent Chemical Shifts (NICS), at the same level, indicate the presence of individual aromatic rings, which correspond to Clar's qualitative sextets rule (Clar, E. TheAromatic Sextet; Wiley: London, 1972). NICS and the Clar valence electron topologies agree perfectly in the molecule plane; however, the NICS values computed in parallel planes further away from the molecular surface converge, indicating the presence of a uniform magnetic shielding field. For each series, PBH total NICS values (i.e., the sum of NICS values for all rings in the PBH) correlate linearly with the number of carbon atoms, indicating constant magnetic field development within a series. The C-C lengths depend on their proximity to the more olefinic rich molecular perimeters. However, the large PBH (> or =C(48)H(24)) internal C-C distances converge to approximately 1.426 A. In agreement with Clar's rule, HF/6-31G(d)//B3LYP/6-31G(d) vertical ionization potentials and B3LYP/6-31G(d) HOMO-LUMO gaps are largest within the "fully benzenoid" series, where all carbon atoms are members of a single sextet. The largest members of the four series studied are predicted to exhibit semiconducting properties.

Journal Article↗
PubMed2005

Thiophene dendron jacketed poly(amidoamine) dendrimers: nanoparticle synthesis and adsorption on graphite.

The peripheral functionalization of amine-terminated fourth-generation poly(amidoamine) (PAMAM) with thiophene dendrons and the preparation of dendrimer-encapsulated metal nanoparticles are described. Interesting nanoparticle stabilization and energy-transfer properties were observed with these hybrid materials. The synthesis involved imine coupling of the dendron derivatives to the peripheral amine groups of PAMAM followed by reduction. The formation of these metal-organic nanoparticle hybrid materials was monitored by UV-vis spectroscopy. The complexation of metal ions and the stabilization effect of PAMAM on metal nanoparticles were investigated by FT-IR. Energy transfer was observed between the metal surface plasmon absorption and fluorescence of the terthiophene dendrons. Noncontact magnetic-AC mode AFM imaging revealed the formation of monodispersed and very stable nanoparticles adsorbed on an HOPG flat substrate.

Journal Article↗
PubMed1999

Enzymatic digestion-high-pressure homogenization prior to slurry introduction graphite furnace atomic absorption spectrometry for selenium determination in plant and animal tissues.

Homogenization using a new flat valve homogenizer in combination with enzymatic digestion with a crude protease was investigated as a means of releasing Se compounds from zoological and botanical matrixes prior to slurry introduction GFAAS. Timed trials with four zoological certified reference materials (CRMs), three botanical reference materials (RMs), and a food crop indicated that Se release into 5% (v/v) ethanol-0.03 M TRIS containing 20 mg of protease was quantitative after homogenization or became quantitative within 1 h of digestion at 60 degrees C. For each of the zoological RMs (whole egg powder, dogfish muscle, and dogfish liver), three passes through the homogenizer in the presence of protease provided a quantitative release of selenium, and incubation with the enzyme was not necessary. No separation of the Se between the liquid phase and the particulate phase was evident even after several days of subsequent storage at 4 degrees C. The botanical matrixes (three milled wheat RMs and a rapeseed sample) were more resistant to selenium release and required up to 1 h of digestion with protease at 60 degrees C. Alternatively, 10 passes through the homogenizing valve (in the presence of the enzyme) resulted in the quantitative release of analyte.

Animals↗
PubMed2005

Covalent bonding of alkene and alkyne reagents to graphitic carbon surfaces.

Various aromatic and aliphatic alkynes and one alkene were covalently bonded to sp(2)-hybridized carbon surfaces by heat treatment in an argon atmosphere. X-ray photoelectron spectroscopy, Raman, and FTIR spectra of the modified surfaces showed that the molecules were intact after the 400 degrees C heat treatment but that the alkyne group had reacted with the surface to form a covalent bond. Alkynes with ferrocene and porphyrin centers exhibited chemically reversible voltammetric waves that could be cycled many times. Atomic force microscopy of the modified surfaces indicated a thickness of the molecular layer consistent with monolayer coverage, and surface coverage determined by voltammetry was also in the monolayer range. Raman spectroscopy of the porphyrin monolayers formed from a porphyrin alkyne showed no evidence for dimer formation, although multilayer formation may occur at undetected levels. FTIR spectra of the porphyrin-modified carbon surfaces were well-defined, similar to the parent molecule, and indicative of an average tilt angle between the porphyrin plane and the surface normal of 37 degrees . The bond between the molecular monolayer and the carbon surface was quite stable, withstanding sonication in tetrahydrofuran, mild aqueous acid and base, and repeated voltammetric cycling in propylene carbonate electrolyte. Heat treatment of alkynes and alkenes appears to be a generally useful method for modifying carbon surfaces, which can be applied to both aromatic and aliphatic molecules.

Journal Article↗