Infrared spectroscopy of physisorbed and chemisorbed N2 in the Pt(111)(3x3)N2 structure

Using infrared spectroscopy and low electron energy diffraction, we have investigated the adsorption of N(2), at 30 K, on the Pt(111) and the Pt(111)(1x1)H surfaces. At monolayer coverage, N(2) orders in commensurate (3x3) structures on both surfaces, and we propose that the unit cells contain four molecules in each case. The infrared spectra reveal that N(2) exclusively physisorbs on the Pt(111)(1x1)H surface, while both physisorbed and chemisorbed N(2) is detected on the Pt(111) surface. Physisorbed N(2) is the majority species in the latter case, and the two adsorption states show an almost identical uptake behavior, which indicates that they are intrinsic constituents of the growing (3x3) N(2) islands. An analysis of the infrared absorbance data, based on a simple scaling concept suggested by density functional theory calculations, supports a model in which the (3x3) unit cell contains one chemisorbed molecule in end-on atop configuration and three physisorbed molecules. We note that a classic "pinwheel" structure on a hexagonal lattice, with the end-on chemisorbed N(2) molecules acting as "pins," is compatible with this composition.     

Capillary electrophoresis with end-capillary potentiometric detection using a copper electrode.

Potentiometric end-capillary detection in capillary electrophoresis has the advantage of relatively easy miniaturisation without having to compromise the concentration sensitivity. Potentiometric end-capillary detection using a copper electrode is also attractive because of the sensitive detection of many inorganic and organic UV-transparent ions and the ability to work in both direct and indirect mode. In this work, detection of a number of common anions in a tartrate electrolyte at pH 3 was studied. The influence of the end-capillary detection geometry on the detection performance was investigated. An end-capillary detection cell allowing the separation capillary to be changed without the need to realign the detection electrode was constructed and fitted into a commercial CE apparatus. Under the optimal configuration, which was a 25 microm diameter copper electrode aligned coaxially with a 25 microm capillary and positioned at a distance of about 25 microm from the capillary end, excellent peak shapes were achieved and comparison with simultaneous on-capillary photometric detection showed no additional peak broadening. Good sensitivity was obtained, resulting in concentration limits of detection (LODs) in the low microM range and mass LODs in the low amol range. Examples of separations of inorganic and organic anions are presented and the analytical potential of the detection method is assessed.     

Silicon nanoclusters formed through self-assembly on CaF2 substrates: morphology and optical properties

We studied the influence of the size and shape of silicon nanoclusters on their optical response. For this purpose, clusters were prepared by deposition, and subsequent diffusion and nucleation, of Si atoms on CaF₂ substrate surfaces. By varying the growth parameters, oblate aggregates with sizes of between 5 and 30 nm and axial ratios between almost unity and 0.1 were generated. We found that the substrate temperature during growth predominantly influences the diameter and number density of the particles, whereas the coverage and deposition rate determine the axial ratio. Optical extinction was measured in the photon energy range between 1.0-6.0 eV and compared to model calculations. The mean size of the Si clusters mainly determines the absolute magnitude of the optical extinction. In contrast, the axial ratio drastically affects the overall structure of the spectra and the relative importance of three identified maxima. The most essential and interesting reason for the pronounced influence of the particle shape on the optical properties is a shift of the valence band plasmon in the silicon nanoparticles from an energy of above 9.0 eV for spherical clusters into the ultraviolet spectral range to about 5 eV for axial ratios below 0.3.     

Volume dependence of thermal conductivity and bulk modulus for poly(propylene glycol)

The thermal conductivity λ and heat capacity per unit volume of poly(propylene glycol) PPG (0.4 and 4.0 kg·mol⁻¹ in number-average molecular weight) have been measured in the temperature range 150–295 K at pressures up to 2 GPa using the transient hot-wire method. At 295 K and atmospheric pressure, λ = 0.147 W m⁻¹K⁻¹ for PPG (0.4 kg·mol⁻¹) and λ = 0.151 W m⁻¹K⁻¹ for PPG (4.0 kg·mol⁻¹). The temperature dependence of λ is less than 4 × 10⁻⁴ W m⁻¹K⁻² for both molecular weights. The bulk modulus has been measured in the temperature range 215–295 K up to 1.1 GPa. At atmospheric pressure, the room temperature bulk moduli are 1.97 GPa for PPG (0.4 kg·mol⁻¹) and 1.75 GPa for PPG (4.0 kg·mol⁻¹). These data were used to calculate the volume dependence of $ \lambda ,g\, = - \left( {\frac{{\partial \lambda /\lambda }}{{\partial V/V}}} \right)_T $. At room temperature and atmospheric pressure (liquid phase) we find g = 2.79 for PPG (0.4 kg·mol⁻¹) and g = 2.15 for PPG (4.0 kg·mol⁻¹). The volume dependence of g, (∂g/∂ log V)_T varies between −19 to −10 for both molecular weights. Under isochoric conditions, g is nearly independent of temperature. The difference in g between the glassy state and liquid phase is small and just outside the inaccuracy of g of about 8%. The theoretical model for λ by Horrocks and McLaughlin yields an overestimate of g by up to 120%. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 345–355, 1998     

Novel Vinylpyridine Based Cationic MIP Monoliths for Enantiomer Separation in CEC

Cationic vinylpyridine based molecularly imprinted polymer monoliths were, for the first time, applied to enantiomer separation using capillary electrochromatography. In order to map the synthesis conditions under which superporous monoliths are obtained, capillary columns were prepared by an in situ polymerization technique using varying monomer and porogen compositions. Both electrolyte pH and percentage of the organic modifier acetonitrile were found to affect the electrochromatographic behaviour on such columns. An interesting observation is that the electroosmotic flow changed direction from cathodic at high pH to anodic at low pH of the electrolyte, opening up the opportunity to manipulate the flow. This is attributed to the vinylpyridine based MIP becomes protonated and positively charged at low pH, whereas it is uncharged at high pH where instead negatively charged silanol groups drive the electroosmotic flow. Improved enantiomer resolution, as well as increased retention, was observed when the concentration of acetonitrile in the electrolyte decreased, indicating a significant element of hydrophobic effects in the molecular recognition of the imprinted enantiomer. Enantiomer separation of the non-steroidal anti-inflammatory drug ibuprofen was studied and efficiencies as high as 30,000 plates per meter with an asymmetry factor below 4 were obtained for the last eluting imprinted enantiomer. These values are better than that obtained generally using MIP based chromatography.     

Selective adsorption, bound states, and potential parameters for He, Ne, and Ar interacting with a Cu(110) surface

Using nozzle beams of He, Ne, and Ar, we have measured diffractive selective adsorption resonances from a Cu(110) surface kept at 20 K. Bound state energies of the atom-surface potentials have been determined from plots of the measured resonance energies versus incident angle and their fits to calculated kinematical dispersion relations. For 3He and 4He we have found a unique level assignment that is compatible with a single gas-surface potential curve with a well depth of 6.05 meV of the He-Cu(110) potential. This value is about 10% larger than the prediction of 5.55 meV from the current physisorption theory. The Ne and Ar data reveal a large number of closely spaced levels with level separations and estimated van der Waals coefficients that are compatible with available theoretical data.     

Dissociative recombination of ammonia clusters studied by storage ring experiments

Dissociative recombination of ammonia cluster ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for dissociative recombination of H+(NH3)2, H+(NH3)3, D+(ND3)2, and D+(ND3)3 in the collision energy range of 0.001-27 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 1000 K are calculated from the experimental data and compared with earlier results. The fragmentation patterns for the two ions H+(NH3)2 and D+(ND3)2 show no clear isotope effect. Dissociative recombination of X+(NX3)2 (X=H or D) is dominated by the product channels 2NX3+X [0.95+/-0.02 for H+(NH3)2 and 1.00+/-0.02 for D+(ND3)2]. Dissociative recombination of D+(ND3)3 is dominated by the channels yielding three N-containing fragments (0.95+/-0.05).     

Synthesis and characteristics of a nonaggregating tris(tetrathiafulvaleno)dodecadehydro[18]annulene

A new tris(tetrathiafulvaleno)dodecadehydro[18]annulene with six peripheral n-hexyl substituents was prepared by oxidative Glaser-Hay cyclization of a corresponding diethynylated tetrathiafulvalene (TTF) precursor. The electronic properties of the neutral and oxidized species were studied by both UV/Vis absorption spectroscopy and electrochemistry. From these studies, it transpires that the strongly violet-colored macrocycle does not aggregate in solution to any significant degree, which was confirmed by (1)H NMR spectroscopy. This reluctance towards aggregation contrasts that observed for related TTF-annulenes containing other peripheral substitutents. Oxidation of the TTF-annulene occurs in two three-electron steps as inferred from both the peak amplitudes and the spectroelectrochemical study. We find that the tris(TTF)-fused dehydro[18]annulene is more difficult to oxidize (by +0.20 V) than the silyl-protected diethynylated mono-TTF precursor. In contrast, the first vertical ionization energy calculated at the B3 LYP/6-311+G(2d,p) level for the parent tris(TTF)-fused dehydro[18]annulene devoid of peripheral hexyl substituents is in fact lower (by 0.44 eV). Moreover, the surface morphology of 1 d drop-cast on a mica substrate was investigated by atomic force microscopy (AFM). Crystalline domains with slightly different orientations were observed. The thickness of individual layers seen in the crystalline domains and the thickness of a monolayer obtained from a very dilute solution were determined to 1.8-1.9 nm. This thickness corresponds to the diameter of the macrocycle and the layers seen in the film are apparently formed when the molecules stack in the horizontal direction relative to the substrate.     

Chlorine isotope fractionation of a semi-volatile organochlorine compound during preparative megabore-column capillary gas chromatography

Chlorine isotope fractionation during preparative capillary gas chromatography (pcGC) was investigated using 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) as a model compound for semi-volatile organochlorine (OCl) molecules. Chlorine isotope analysis by thermal ionization mass spectrometry revealed no significant alteration of the chlorine isotope composition when the whole peaks were collected in pcGC (delta37Cl -3.2 per thousand versus -3.6 per thousand for the unprocessed DDT, +/-0.5 per thousand SD). However, distinct isotope fractionations were measured for the front (delta37Cl -5.1 per thousand) and tail (delta37Cl -1.8 per thousand) segments of partially collected samples. Isolation of individual OCls by pcGC enables accurate off-line chlorine isotope analysis, and thus facilitates the investigation of naturally occurring OCls.