Boon-Keng Teo

The Discotic Phase of Uro-Porphyrin I Octa-N-Dodecyl Estei

Abstract A number of materials which exhibit the relatively new discotic phase have recently been discovered. All of the materials known which show this type of phase have molecular structures that are flat and hexagonal in shape. In this current work, we have shown that a porphyrin compound, uro-porphyrin I octa-n-dodecyl ester, which has a flat, octagonally shaped molecular structure can also exhibit a discotic phase. This is the first report of the discotic liquid crystal phase being exhibited by porphyrin compounds.

Nonparameterized MO calculations of ligand-bridged M2(CO)8-(U2-X)2-type dimers containing metalmetal interactions: Evidence for dictation of stereochemistry by one-electron and two-electron metalmetal σ-type bonds☆

Nonparameterized MO calculations performed on the (edge-bridged)-bioctahedral metal dimers of the Dessy-characterized [Cr2(CO)8(μ2-PR2)2](n-2) series and of the [Mn2(CO)8(μ2-PR2)2]n series (n = 0, +1, +2) have revealed that the corresponding dimeric pairs with n = 0, +1, and +2 have two, one, and no electrons, respectively, in the antibonding 2b3u MO corresponding to a “net” no-electron metalmetal bond, a “net” one-electron metalmetal bond, and a two electron metalmetal bond. Of prime significance is that this 2b3u MO, which is the LUMO in both electron-pair (metalmetal)-bonded dimers (n = +2) and the HOMO in the corresponding dimers to which one or two electrons have been added, is found to be largely composed of in-plane antibonding σ★-type dimetal orbital character rather than either out-of-plane π★-type dimetal antibonding orbital character or bridging-ligand orbital character. These MO results are also shown to be completely compatible with the available spectral and X-ray data.

Local structure of amorphous MO50Ni50 determined by anomalous x-ray scattering using synchroton radiation

Abstract Anomalous (resonance) x-ray scattering technique using synchrotron radiation was applied to determine the compositionally resolved local structure of sputter deposited amorphous Mo50Ni50. The local environments of Mo atoms and Ni atoms were found to be significantly different from each other, but similar to the corresponding local environments in crystalline MoNi. The results compare favorably with those of the EXAFS measurement.

EXAFS of glassy metallic alloys: Amorphous and crystalline MoNi

Abstract The Ni and Mo K-edge extended X-ray absorption fine structure (EXAFS) spectra of MoNi alloy in the amorphous and crystalline δ-phase have been measured in the transmission mode. The EXAFS spectrum of a Mo foil has also been measured for comparison. The EXAFS data were analyzed using both symmetrical (Gaussian) and asymmetrical (liquid-metal type) pair distribution functions. A newly developed fine adjustment technique based on model (FABM) was applied to the best fit results based on theoretical backscattering amplitude and phase functions. The following results were obtained. (1) There exist differences in distances between the crystalline (rc) and the amorphous (ra) alloys, which follow the general trend: Ni  Ni (r c − r a ≈ 0 A ⩽ Mo  Ni (0.03 A ) Ni  Mo (0.10 A ) ⪡ Mo  Mo (0.2 A ) . (2) The distances between unlike atoms obtained from the Ni K-edge spectra (NiMo) and from the Mo K-edge spectra (MoNi) are different for both amorphous and crystalline states. The differences, in terms of (NiMo)−(MoNi), amount to 0.06 A in the amorphous and 0.13 A in the crystalline states. (3) The coordination numbers obtained by EXAFS for the metallic alloys are abnormally low (N = 1–4) in sharp contrast to those (N = 5–8) expected from crystallography. These features are interpreted in terms of large local atomic disorder, asymmetry in the pair distribution functions, and phase cancellation between the nickel and the molybdenum waves (the backscattering phases are off by π radians). Fits with the asymmetric pair distribution function are, however, of similar or somewhat poorer quality than those with symmetric ones and follow the same trends. Parameter correlation characteristics reveal distinct differences among metal foils, crystalline metal alloys, and amorphous metal alloys. These differences can be attributed to the different pair distribution functions, indicating that neither the crystalline metal foils nor the crystalline metal alloy can be used as model for the amorphous metal alloy. Our results indicate that in the MoNi alloy the nearest neighbor interactions are to some extent preserved while the higher shell coordinations are increasingly diminished in going from the crystalline phase to amorphous phase.

Extended X-Ray Absorption Fine Structure (EXAFS) Spectroscopy: Techniques and Applications

Extended X-ray absorption fine structure (EXAFS) refers to the oscillatory variation of the X-ray absorption as a function of photon energy beyond an absorption edge. The absorption, normally expressed in terms of absorption coefficient (µ), can be determined from a measurement of the attenuation of X-rays upon their passage through a material. When the X-ray photon energy (E) is tuned to the binding energy of some core level of an atom in the material, an abrupt increase in the absorption coefficient, known as the absorption edge, occurs. For isolated atoms, the absorption coefficient decreases monotonically as a function of energy beyond the edge. For atoms either in a molecule or embedded in a condensed phase, the variation of absorption coefficient at energies above the absorption edge displays a complex fine structure called EXAFS.

A new cluster model for the FeMo-cofactor of nitrogenase

Abstract A new structural model for the FeMo-cofactor of nitrogenase, consisting of two Fe 4 S 4 clusters bridged by an S 2 Mo 2 unit, is proposed. Available chemical, spectroscopic, and EXAFS data are shown to be consistent with the proposed structure. In particular, EXAFS data are in agreement with m(Mo-Fe):n(Mo-S) of either 2:4 or 3:4; comparison with known Mo-Fe-S and Fe-S systems leads us to favor the former. Based on the proposed structural model, a possible mechanism of reduction of N 2 is suggested.

Local atomic structure in amorphous Mo50Ni50 by resonance x-ray diffraction using synchrotron radiation

Anomalous (resonance) x-ray diffraction technique using synchrotron radiation was applied to determine the compositionally resolved local structure of sputter deposited amorphous Mo50Ni50. The local environments are similar to the ones in the crystalline compound.

Use of a semiconductor detector in anomalous (resonance) X-ray scattering measurement of local structure of an amorphous alloy

Abstract Anomalous (resonance) X-ray scattering is a powerful technique to determine the compositionally resolved local structure of disordered materials, such as liquids or amorphous solids. However, a very high accuracy of measurements is required for the succesful application of this technique and the removal of the fluorescent radiation from the sample is crucial in achieving the necessary accuracy. We show that this can be done satisfactorily using an energy sensitive semiconductor detecor.

Bond Angle Determination by EXAFS: A New Dimension

While single scattering EXAFS formalism can provide structural information about the local environment of the absorbing atoms in terms of radial distribution functions (distances), angular information is generally not available; except, perhaps, for polarization dependent measurements on single crystals. Furthermore, the very same advantageous characteristics of EXAFS (short-range, single-scattering) are also its serious limitations: distance determinations out to only ca 4A. The situation, however, changes dramatically when atoms (including the X-ray absorbing atom and its neighbors) are arranged in a linear or nearly colinear fashion. In such cases, EXAFS contributions from neighboring atoms as far as 8A can be observed. For these systems, both the amplitude and the phase of the EXAFS of a more distant neighbor are significantly affected by the intervening atom(s). In particular, the amplitude is greatly enhanced and is therefore commonly called “focusing” effect [1]. The short-range single-scattering theory of EXAFS fails in these situations and one must take into account multiple scattering processes involving the intervening atoms.