Mathematical modelling of the oxidation of nickel titanium alloys

A one-dimensional bulk reaction model for the oxidation of nickeltitanium is formulated, with preferential oxidation of titaniumbeing included. The modelling is directed at the better understandingof the dominant mechanisms involved in the oxidation processand their significance for the biocompatibility of the alloy.Two different regimes for the relative diffusivities of oxygenand the metals are investigated. By assuming fast bulk reactions,different asymptotic structures emerge in different parameterregimes and the resulting models take the form of moving boundaryproblems. Different profiles of nickel concentration are obtained:in particular a nickel-rich layer (observed in practice) ispresent below the oxide/metal interface for the case when oxygenand the metals diffuse at comparable rates.     

Transient-field strength measurements for52Cr traversing Fe hosts at high velocity and polarization transfer mechanisms

Transient-field strengths were measured for⁵²Cr ions traversing polarized Fe hosts at velocities up to 12υ₀ (12υ₀ =c/137 = Bohr velocity). The results are compared with predictions of various transient-field parametrizations and discussed in terms of possible mechanisms by which polarization might be transferred from the Fe host to inner vacancies of the moving Cr ions. Theg-factor of the first 2⁺ state of⁵²Cr was also measured by the transient-field technique and found to be in accord with shell model calculations.     

The use of temporary tethers in the meta photocycloaddition reaction

The use of temporary tethers in facilitating meta photocycloaddition reactions between phenol and allyl alcohol derivatives has been investigated. The merits of silicon, carbonate and methylene acetal tethers were assessed, whilst considering strategies for the preparation of the natural products gymnomitrol and gelsemine. The photoadducts were epoxidised, and then subjected to acid catalysed fragmentation with concomitant cleavage of the tether. Depending on whether water or methanol was used during the fragmentation stage of the methylene tethers, the methylene group was either removed altogether or transformed into a MOM group.     

Complex II from a structural perspective

The super-macromolecular complex, succinate:quinone oxidoreductase (SQR, Complex II, succinate dehydrogenase) couples the oxidation of succinate in the matrix / cytoplasm to the reduction of quinone in the membrane. This function directly connects the Krebs cycle and the aerobic respiratory chain. Until the recent first report of the structure of SQR from Escherichia coli (E. coli) the structure-function relationships in SQR have been inferred from the structures of the homologous QFR, which catalyses the same reaction in the opposite direction. The structure of SQR from E. coli, analogous to the mitochondrial respiratory Complex II, has provided new insight into SQR's molecular design and mechanism, revealing the electron transport pathway through the enzyme. Comparison of the structures of SQR, QFR and other related flavoproteins shows how common amino acid residues at the interface of two domains facilitate the inter-conversion of succinate and fumarate. Additionally, the structure has provided a possible explanation as to why certain organisms utilise both SQR and QFR despite the fact that both can catalyse the inter-conversion of succinate and fumarate, in vitro and in vivo. Here we review how this structure has advanced our knowledge of this important enzyme and compare the structural information to other members of the Complex II superfamily and related flavoproteins.     

Enhanced chromatographic resolution of amine enantiomers as carbobenzyloxy derivatives in high-performance liquid chromatography and supercritical fluid chromatography

The carbobenzyloxy (cbz) protecting group is evaluated for it's potential to enhance the resolution of chiral amine enantiomers using high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC). A series of cbz derivatives of commercially available racemates was prepared and analyzed by enantioselective chromatography using a variety of mobile phases and polysaccharide and Pirkle-type chiral stationary phases (CSPs). The cbz-derivatized product consistently demonstrated enhanced chiral resolution under HPLC and SFC conditions. Improved selectivity and resolution combined with an automated preparative HPLC or SFC system can lead to the rapid generation of highly purified enantiomers of desirable starting materials, intermediates or final products.     

Some trace elements in the waters,marine organisms and sediments of the Adriatic by neutron activation analysis

A number of investigations of trace elements in the waters, organisms and sediments of the Adriatic, using neutron activation analysis with radiochemical, separations are reported. These include studies of Hg in mussels from the Northern Adriatic, of Hg and Se in edible animals from the Rijeka region, and of seven elements (As, Cd, Cu, Hg, Mn, Se and Zn) in marine invertebrates from the Slovene coast. Additionally, plankton, sediment cores and water samples were taken from a grid of stations covering the whole Adriatic and analyzed for 6 to 11 of the trace elements As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Sb and Zn (Hg only in water). Generally, levels found were not indicative of pollution as compared with oceanic samples, but some evidence of locally increased levels was found, especially for Hg. The levels of eleven trace elements in three marine Intercomparison samples prepared by the IAEA Monaco Laboratory are also presented.     

Simultaneous radiochemical neutron activation analysis of iodine, uranium and mercury in biological and environmental samples

The determination of medium and long-lived nuclides can be combined with short-lived ones if a medium or long irradiation is made prior to the short irradiation and radiochemical processing. Thus, an RNAA method previously developed for determination of iodine based on the reaction¹²⁷I(n,)¹²⁸I (T _1/2=25 m) using oxygen flask ignition of the irradiated sample, followed by solvent extraction with an iodine-iodide redox cycle, was combined with an overnight preirradiation to induce the²³⁵U fission product¹³³I (T _1/2=20.8 h). By reactivating the sample, cooled 1–2 days after the first irradiation, for few minutes both¹²⁸I and¹³³I could be quantified in the separated iodine fraction. Non-combustible inorganic materials (e.g., sediment, soil, etc.) can be successfully ignited after mixing with excess cellulose powder. Chemical yields for iodine were determined spectrophotometrically in the organic phase, while homogeneously spiked Whatman cellulose powder was used as uranium standard. Mercury is also released on ignition and collected in the absorbing solution, from where it was separated by toluene extraction. Its chemical yield was determined for each aliquot using²⁰³Hg tracer and counting on an LEPD. Results for some suitable SRMs are presented, and the general features of the double irradiation technique discussed.     

Competition between N and O: use of diazine N-oxides as a test case for the Marcus theory rationale for ambident reactivity

The preferred site of alkylation of diazine N-oxides by representative hard and soft alkylating agents was established conclusively using the ¹H–¹⁵N HMBC NMR technique in combination with other NMR spectroscopic methods. Alkylation of pyrazine N-oxides (1 and 2) occurs preferentially on nitrogen regardless of the alkylating agent employed, while O-methylation of pyrimidine N-oxide (3) is favoured in its reaction with MeOTf. As these outcomes cannot be explained in the context of the hard/soft acid/base (HSAB) principle, we have instead turned to Marcus theory to rationalise these results. Marcus intrinsic barriers (ΔG^‡₀) and Δ_rG° values were calculated at the DLPNO-CCSD(T)/def2-TZVPPD/SMD//M06-2X-D3/6-311+G(d,p)/SMD level of theory for methylation reactions of 1 and 3 by MeI and MeOTf, and used to derive Gibbs energies of activation (ΔG^‡) for the processes of N- and O-methylation, respectively. These values, as well as those derived directly from the DFT calculations, closely reproduce the observed experimental N- vs. O-alkylation selectivities for methylation reactions of 1 and 3, indicating that Marcus theory can be used in a semi-quantitative manner to understand how the activation barriers for these reactions are constructed. It was found that N-alkylation of 1 is favoured due to the dominant contribution of Δ_rG° to the activation barrier in this case, while O-alkylation of 3 is favoured due to the dominant contribution of the intrinsic barrier (ΔG^‡₀) for this process. These results are of profound significance in understanding the outcomes of reactions of ambident reactants in general.

Marcus theory enables rationalisation and quantification of selectivities in reactions of ambident nucleophiles for which the HSAB principle cannot operate.