The adsorption and desorption of ethanol ices from a model grain surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98 K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98 K. Annealing multilayer ethanol ices (exposures >50 L) beyond 120 K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3+/-1.7 kJ mol(-1). At exposures exceeding 50 L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160 K.     

Simultaneous multielement‐specific detection of a novel glutathione–arsenic–selenium ion [(GS)2AsSe]− by ICP AES after micellar size‐ exclusion chromatography

In order to confirm the solution structure of [(GS)₂AsSe]⁻ (GS = glutathione), we have investigated the retention behaviour of a [(GS)₂AsSe]⁻/oxidized glutathione (GSSG) mixture on a Sephadex G‐25 (SF) column with Tris buffers (0.1 mol dm⁻³, pH 8.0) containing ­various surfactants at concentrations above the critical micellar concentration (CMC): hexadecyltrimethlammonium bromide (HDTAB; 30, 40 and 50 mmol dm⁻³); dodecyltrimethylammonium bromide (DDTAB; 50 mmol dm⁻³); and sodium lauryl sulfate (SLS; 50 mmol dm⁻³). ­An inductively coupled plasma atomic emission spectrometer (ICP AES) provided simultaneous on‐line detection of arsenic, selenium and ­sulfur in the column effluent. The chromatographic retention behaviour was used to investigate the association of both compounds with the positively charged micelles (HDTAB and DDTAB mobile phases). The relative strength of association with the micelles provided insight into the effective negative charge on [(GS)₂AsSe]⁻ and GSSG. The chromatograms obtained with 50 mmol dm⁻³ HDTAB indicated that two glutathione molecules are associated with the elution of an arsenic–selenium compound. Combined, these chromatographic data strongly support the spectroscopically derived solution structure of [(GS)₂AsSe]⁻. Copyright ­© 2000 John Wiley & Sons, Ltd.     

Experimental confirmation of the importance of orientation in the anomalous chiral sensitivity of second harmonic generation

Macromolecular interactions were demonstrated to yield large chiroptical effects in second harmonic generation measurements of ultrathin surface films. Second harmonic generation (SHG) has recently shown to be several orders of magnitude more sensitive to chirality in oriented systems than common linear methods, including absorbance circular dichroism (CD) and optical rotary dispersion (ORD). Numerous mechanisms have been developed to explain this anomalous sensitivity, with a general emphasis on understanding the molecular origins of the chromophore chirality. In this work, orientational effects alone are shown to be the dominant factor for generating large SHG chiral dichroic ratios in many surface systems. Three distinct uniaxial surface films of SHG-active achiral chromophores oriented at chiral templated surfaces were observed to yield chiral dichroic ratios as great as 40% in magnitude.     

Synthesis of sialic acids via desymmetrization by ring-closing metathesis

Formal total syntheses of the naturally occurring deaminated sialic acids KDN (2), a potential oncofetal antigen, and N-acetylneuraminic acid (Neu5Ac, 1), the most naturally abundant sialic acid, have been accomplished in 46% and 9.3% overall yield, respectively, via a novel ketalization/ring-closing metathesis sequence. The rapid introduction of all oxygen and nitrogen functionality in a completely stereocontrolled manner exploited a rigid 6,8-dioxabicyclo[3.2.1]oct-2-ene template. The 2,7-anhydro-KDN derivative 40 served as an advanced intermediate in each of the two syntheses.     

Characterization of a new external neutron beam facility at the Ohio State University

A thermal neutron beam facility has been designed and implemented at the Ohio State University Research Reactor. A project is underway to construct a large vacuum chamber such that the facility could have neutron depth profiling and neutron radiography capabilities as intended. The neutron beam is extracted from the reactor through a neutron collimator emplaced in Beam Port #2. The neutron spectrum entering the neutron collimator was unfolded from foil activation analysis results and also simulated with a full reactor core model in the MCNP Monte Carlo code. The neutron collimator uses polycrystalline bismuth as a gamma ray filter and single-crystal sapphire as a fast neutron filter. The beam is defined by multiple 3.0 cm diameter apertures made of borated aluminum. Characterization of the beam was performed using foil activation to find the flux and a low-budget neutron imaging apparatus to see the beam profile. The modulation transfer function was calculated to offer insight into the resolution of the imaging system and the collimation of the beam. The neutron collimator delivers the filtered thermal neutron beam with a ~4 cm diameter and a thermal equivalent flux of (1.27 ± 0.03) × 10⁷ n/(cm²s) at 450 kW power at the end of the collimator.     

Double excitations within time-dependent density functional theory linear response

Within the adiabatic approximation, time-dependent density functional theory yields only single excitations. Near states of double excitation character, the exact exchange-correlation kernel has a strong dependence on frequency. We derive the exact frequency-dependent kernel when a double excitation mixes with a single excitation, well separated from the other excitations, in the limit that the electron--electron interaction is weak. Building on this, we construct a nonempirical approximation for the general case, and illustrate our results on a simple model.     

Oxidative Allene Amination for the Synthesis of Azetidin‐3‐ones

Regioselectivity in the aziridination of silyl‐substituted homoallenic sulfamates is readily diverted to the distal double bond of the allene to yield endocyclic bicyclic methyleneaziridines with excellent stereocontrol. Subsequent reaction with electrophilic oxygen sources initiates facile rearrangement to densely functionalized, fused azetidin‐3‐ones in excellent d.r., effectively transferring the axial chirality of the allene to central chirality in the products. The steric nature of the silyl group dictates which of the two rings of the fused azetidin‐3‐one will undergo further functionalization, providing an additional element of diversity for the preparation of enantioenriched azetidine scaffolds with potential biological activity.     

Effects of Metal Electrode Support on the Catalytic Activity of Fe(oxy)hydroxide for the Oxygen Evolution Reaction in Alkaline Media

FeO_xH_y and Fe-containing Ni/Co oxyhydroxides are the most-active catalysts for the oxygen evolution reaction (OER) in alkaline media. However, the activity of Fe sites appears strongly dependent on the electrode-substrate material and/or the elemental composition of the matrix in which it is embedded. A fundamental understanding of these interactions that modulate the OER activity of FeO_xH_y is lacking. We report the use of cyclic voltammetry and chronopotentiometry to assess the substrate-dependent activity of FeO_xH_y on a number of commonly used electrode substrates, including Au, Pt, Pd, Cu, and C. We also evaluate the OER activity and Tafel behavior of these metallic substrates in 1 M KOH aqueous solution with Fe³⁺ and other electrolyte impurities. We find that the OER activity of FeO_xH_y varies by substrate in the order Au>Pd≈Pt≈Cu>C. The trend may be caused by differences in the adsorption strength of the Fe oxo ion on the substrate, where a stronger adhesion results in more adsorbed Fe at the interface during steady-state OER and possibly a decreased charge-transfer resistance at the FeO_xH_y-substrate interface. These results suggest that the local atomic and electronic structure of [FeO₆] units play an important role in catalysis of the OER as the activity can be tuned substantially by substrate interactions.     

Structure-optical property relationships in organometallic sydnones

As part of an effort to develop a spectroscopic structure-property relationship in platinum acetylide oligomers, we have prepared a series of mesoionic bidentate Pt(PBu3)2L2 compounds containing sydnone groups. The ligand is the series o-Syd-(C6H4-C[triple bond]C)n-H, where n = 1-3, designated as Syd-PEn-H. The terminal oligomer unit consists of a sydnone group ortho to the acetylene carbon. We synthesized the platinum complex (Syd-PEn-Pt), the unmodified ligands (PEn-H), and the unmodified platinum complexes (PEn-Pt). The compounds were characterized by various methods, including X-ray diffraction, 13C NMR, ground-state absorption, fluorescence, phosphorescence, and laser flash photolysis. From solving the structure of Syd-PE1-Pt, we find the angle between the sydnone group and the phenyl group is 45 degrees . By comparison of the 13C NMR spectra of the sydnone-containing ligands, the sydnone complexes with the corresponding unmodified ligands and complexes not containing the sydnone group, the sydnone group is shown to polarize the nearest acetylenes and have a charge-transfer interaction with the platinum center. Ground-state absorption spectra of the complexes in various solvents give evidence that the Syd-PE1-Pt complex has an excited state less polar than the ground state, while the PE1-Pt complex has an excited state more polar than the ground state. In all the higher complexes the excited state is more polar than the ground state. The phosphorescence spectrum of the Syd-PE1-Pt complex has an intense vibronic progression distinctly different from the PE1-Pt complex. The sydnone effect is small in Syd-PE2-Pt and negligible in Syd-PE3-Pt. From absorption and emission spectra, we measured the singlet-state energy E(S), the triplet-state energy E(T), and the singlet-triplet splitting Delta E(ST). By comparison with energies obtained from the unmodified complexes, attachment of the sydnone lowers E(S) by approximately 0.1 eV and raises E(T) by approximately 0.1 eV. As a result, the sydnone group lowers Delta E(ST) by approximately 0.2 eV. The trends suggest one of the triplet-state singly occupied molecular orbitals (SOMOs) is localized on the sydnone group, while the other SOMO resides on the rest of the ligand.