An evaluation of various computational methods for the treatment of organoselenium compounds

A reliable computational method for the prediction of organoselenium geometries and bond dissociation energies (BDEs) has been determined on the basis of the performance of density functional theory (DFT: B3LYP and B3PW91) and ab initio molecular orbital procedures (Hartree-Fock (HF)) in conjunction with various Pople basis sets including (but not limited to) the 6-31G(d), 6-31G(d,p), 6-311G(d), 6-311G(d,p), 6-311G(2df,p), and 6-311G(3df,3pd) sets. Predicted geometries and BDEs are compared with available experimental data and quadratic configuration interaction including single and double substitutions (QCISD) results. The B3PW91/6-311G(2df,p) level of theory is recommended for the prediction of the geometries and energetics of organoselenium compounds.     

Heterogeneous Solution NMR Signal Amplification by Reversible Exchange

A novel variant of an iridium‐based organometallic catalyst was synthesized and used to enhance the NMR signals of pyridine in a heterogeneous phase by immobilization on polymer microbead solid supports. Upon administration of parahydrogen (pH₂) gas to a methanol mixture containing the HET‐SABRE catalyst particles and the pyridine, up to fivefold enhancements were observed in the ¹H NMR spectra after sample transfer to high field (9.4 T). Importantly, enhancements were not due to any residual catalyst molecules in solution, thus supporting the true heterogeneity of the SABRE process. Further significant improvements may be expected by systematic optimization of experimental parameters. Moreover, the heterogeneous catalyst is easy to separate and recycle, thus opening a door to future potential applications varying from spectroscopic studies of catalysis, to imaging metabolites in the body without concern of contamination from expensive and potentially toxic metal catalysts or accompanying organic molecules.     

Experimental investigation of the molecular ions of the xylenes

A mass spectrometric method of distinguishing between molecular ions of the three isomeric xylenes (dimethylbenzenes) was sought, in light of recent findigs that photoexcited ions could be distinguished via measurements of kinetic energy release accompanying expulsion of a methyl radical. Provided the molecular ions are formed with low internal energies, reproducible differences were found between relative intensities of collision induced reactions of higher critical energies, than for methyl expulsion. These differences exist both for collision energies in the kilovolt range (double focusing mass spectrometers) and in the range of a few tens of volts (triple quadrupole instrument). Though statistically significant, these differences were small. The mechanism of isomerization and fragmentation was investigated via isotopic labelling studies and measurements of kinetic energy release. Most of the present findings can be rationalized in terms of the most recent version of established mechanisms for reactions of ionized methylbenzenes.     

Cross section measurements of the Pd(p, γ)Ag, Sn(p, γ)Sb, and Sn(α, γ)Te reactions relevant to the astrophysical rp- and γ-processes

Total cross section measurements for the ¹⁰²Pd(p, γ)¹⁰³Ag and ¹¹⁶Sn(p, γ)¹¹⁷Sb reactions have been performed in the proton energy range 2.6 to 4.25 MeV, and for the ¹¹²Sn(α, γ)¹¹⁶Te reaction over the alpha beam energy range 7.0 to 10.5 MeV. An activation technique was used in which gamma rays from decays of the reaction products were detected off-line by two hyper-pure germanium detectors in a low background environment. Where possible, reaction rates are derived and the results compared to those of calculations generated by the NON-SMOKER and the MOST statistical model codes so as to judge their applicability for describing the cross sections needed for network calculations of nucleosynthesis in explosive astrophysical environments via the γ- and rp-processes.     

Large-area In2O3 ordered pore arrays and their photoluminescence properties

Large-area In₂O₃ ordered pore arrays were prepared on glass and silicon substrates by the sol–gel technique based on colloidal monolayer spheres. The morphologies of such arrays are determined by precursor concentration used and colloidal sphere size, and are thus controllable. It has been shown that the formed ordered pore arrays consist of In₂O₃ polycrystallites. The photoluminescence measurement of the In₂O₃ ordered pore arrays shows that there is a strong photoluminescence band in the blue-green region centered around 465 nm, which does not exist in the bulk materials. Further experiments reveal that this peak originates from the oxygen deficiencies in In₂O₃ skeletons. This polydomain ordered pore-structured array could be of great potential for Si-based integrated nanophotonics and optoelectronic devices of the next generation, in addition to new gas sensors. PACS  01.30.-y; 78.20.-e; 78.55.-m; 78.55.Et     

Detection of impurity segregation in zone-confined, polycrystalline tin-doped indium oxide thin films by STM and AFM

This paper discusses the possibility of using STM and AFM to image the dopant material in a segregated state. Samples of tin-doped indium oxide were prepared using a zone-confining process. The resultant material has dopant species segregated over certain grain boundaries at desired positions while the others remain dopant free. Samples were then imaged using STM, AFM and STEM. Enhanced contrast from the dopant rich grain boundaries and a larger grain size are observed at the surface making an interface with the substrate as compared to the free surface of the sample, while secondary electron STEM images show these grains to be smaller in size and the boundaries to be almost physically flat. This is interpreted to be a consequence of the zone-confining effect.     

Delayed predissociation and collision-induced processes of the ammonia di-cation NH32+

Reactions of NH₃²⁺ di-cations, observed a few microseconds after formation by electron ionisation of ammonia, have been studied in a double-focusing mass spectrometer using techniques of ion translational energy spectroscopy. Fragmentation reactions occuring under unimolecular conditions correspond to predissociation processes, but lack of knowledge about states of NH₃²⁺ precludes any definitive interpretation. A partial tentative interpretation, using arguments by analogy with isoelectronic neutral species, is proposed. Almost all collision-induced fragmentations required electron transfer forming NH₃⁺ as an intermediate step, so the non-dissociative electron transfer processes were also studied. The most unexpected collision-induced fragmentation reaction was that which formed H₃⁺ fragment ions.     

Vibrations of stiffened cylinders with cutouts

An approximate method of determining the free vibration characteristics of ring and/or stringer-stiffened cylindrical shells with cutouts is presented in this paper. The method is based on the Rayleigh-Ritz technique in which beam characteristic functions (axially) and trigonometric functions (circumferentially) are used in the displacement series for the shell reference surface. It was found that the cutouts generally tend to decrease the frequencies. This effect is the largest on the fundamental frequency. Physically this means that a cutout reduces the effective shell stiffness to a greater extent than it does the effective mass. The mode shapes display strong coupling of the distinct wave forms of an otherwise uniform shell. They also reveal the possibility of peak amplitudes in the normal displacements both near and away from the edges of the cutouts. The reductions in the lower frequencies (caused by cutouts) for the stiffened shell were found to be less than those for the unstiffened shell.     

Cutting planes for mixed-integer knapsack polyhedra

An algorithm for generating cutting planes for mixed-integer knapsack polyhedra is described. The algorithm represents an exact separation procedure and is based on a general methodology proposed by one of the authors in an earlier paper. Computational results are presented. © 1998 The Mathematical Programming Society, Inc. Published by Elsevier Science B.V.