Mechanistic studies of the O2-dependent aliphatic carbon-carbon bond cleavage reaction of a nickel enolate complex

The mononuclear nickel(II) enolate complex [(6-Ph(2)TPA)Ni(PhC(O)C(OH)C(O)Ph]ClO(4) (I) was the first reactive model complex for the enzyme/substrate (ES) adduct in nickel(II)-containing acireductone dioxygenases (ARDs) to be reported. In this contribution, the mechanism of its O(2)-dependent aliphatic carbon-carbon bond cleavage reactivity was further investigated. Stopped-flow kinetic studies revealed that the reaction of I with O(2) is second-order overall and is ～80 times slower at 25 °C than the reaction involving the enolate salt [Me(4)N][PhC(O)C(OH)C(O)Ph]. Computational studies of the reaction of the anion [PhC(O)C(OH)C(O)Ph](-) with O(2) support a hydroperoxide mechanism wherein the first step is a redox process that results in the formation of 1,3-diphenylpropanetrione and HOO(-). Independent experiments indicate that the reaction between 1,3-diphenylpropanetrione and HOO(-) results in oxidative aliphatic carbon-carbon bond cleavage and the formation of benzoic acid, benzoate, and CO:CO(2) (～12:1). Experiments in the presence of a nickel(II) complex gave a similar product distribution, albeit benzil [PhC(O)C(O)Ph] is also formed, and the CO:CO(2) ratio is ～1.5:1. The results for the nickel(II)-containing reaction match those found for the reaction of I with O(2) and provide support for a trione/HOO(-) pathway for aliphatic carbon-carbon bond cleavage. Overall, I is a reasonable structural model for the ES adduct formed in the active site of Ni(II)ARD. However, the presence of phenyl appendages at both C(1) and C(3) in the [PhC(O)C(OH)C(O)Ph](-) anion results in a reaction pathway for O(2)-dependent aliphatic carbon-carbon bond cleavage (via a trione intermediate) that differs from that accessible to C(1)-H acireductone species. This study, as the first detailed investigation of the O(2) reactivity of a nickel(II) enolate complex of relevance to Ni(II)ARD, provides insight toward understanding the chemical factors involved in the O(2) reactivity of metal acireductone species.     

A multifaceted approach to hydrogen storage

The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but only if a number of currently intractable problems can be overcome. Not the least of these is the problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN research network, representing an intentionally multi-faceted selection with the goal of advancing the field on a number of fronts simultaneously. For the general reader we provide a concise outline of the main approaches to storing hydrogen before moving on to detailed reviews of recent research in the solid chemical storage of hydrogen, and so provide an entry point for the interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel transition metal catalysts for use with complex hydrides; less common borohydrides; protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides.     

Phase transition induced improvement in H2 desorption kinetics: the case of the high-temperature form of Y(BH4)3

The high-temperature (HT) phase of Y(BH(4))(3) has been prepared by heating of the as mechanochemically synthesised low-temperature (LT) phase of Y(BH(4))(3) to 194-216 °C and subsequent rapid cooling to ambient temperature. Although the differences in the crystal structure and vibrational spectra for these closely-related polymorphs are rather small, yet the NMR MAS (1)H and CP MAS (89)Y spectra reveal clear differences in the chemical shifts for both nuclei. The thermal decomposition process of both forms differs noticeably below 260 °C, decomposition being faster and more facile for the HT phase. The activation energy for thermal decomposition, calculated according to the Kissinger equation, is nearly three times lower for the HT than for the LT polymorph for the first step of the thermal decomposition signalling giant improvement of kinetics of H(2) desorption.     

The role of Mg<Subscript>2</Subscript>FeH<Subscript>6</Subscript> formation on the hydrogenation properties of MgH<Subscript>2</Subscript>-FeF<Subscript>x</Subscript> composites

The hydrogenation properties of magnesium hydride mechanically milled with iron fluorides (FeF₂ and FeF₃), were investigated by Temperature Programmed Desorption (TPD) and volumetric methods using a Sieverts-type apparatus, as prepared upon dehydrogenation and finally upon subsequent hydrogenation. The activation energy of hydrogen desorption (E_a), calculated from the Kissinger formula using TPD measurements obtained with different heating rates, showed significant decreases of Ea in comparison to that of milled MgH₂ without any dopants. Moreover, the influence of these metal fluorides on the thermodynamics of the decomposition process was also examined. In the case of the FeF₂ dopant, rehydrogenation following desorption caused the complete decomposition of the iron fluoride to BCC iron and the formation of a predominant MgH₂ phase. In contrast to FeF₂, the addition of FeF₃ led to the formation of β-MgH₂ as a major phase coexisting with Mg₂FeH₆ and MgF₂ compounds. The presence of pure Fe in the MgH₂+FeF₂ composite, as opposed to MgH₂+FeF₃ containing Mg₂FeH₆ and MgF₂, did not cause any significant influence on the sorption properties of MgH₂. Moreover, the original material doped with FeF₃ predominantly showed iron in the Mg₂FeH₆ compound, while the FeF₂ dopant iron mostly showed the nearly pure BCC metallic phase      

The first Steklov eigenvalue, conformal geometry, and minimal surfaces

We consider the relationship of the geometry of compact Riemannian manifolds with boundary to the first nonzero eigenvalue σ₁ of the Dirichlet-to-Neumann map (Steklov eigenvalue). For surfaces Σ with genus γ and k boundary components we obtain the upper bound σ₁L(∂Σ)?2(γ+k)π. For γ=0 and k=1 this result was obtained by Weinstock in 1954, and is sharp. We attempt to find the best constant in this inequality for annular surfaces (γ=0 and k=2). For rotationally symmetric metrics we show that the best constant is achieved by the induced metric on the portion of the catenoid centered at the origin which meets a sphere orthogonally and hence is a solution of the free boundary problem for the area functional in the ball. For a general class of (not necessarily rotationally symmetric) metrics on the annulus, which we call supercritical, we prove that σ₁(Σ)L(∂Σ) is dominated by that of the critical catenoid with equality if and only if the annulus is conformally equivalent to the critical catenoid by a conformal transformation which is an isometry on the boundary. Motivated by the annulus case, we show that a proper submanifold of the ball is immersed by Steklov eigenfunctions if and only if it is a free boundary solution. We then prove general upper bounds for conformal metrics on manifolds of any dimension which can be properly conformally immersed into the unit ball in terms of certain conformal volume quantities. We show that these bounds are only achieved when the manifold is minimally immersed by first Steklov eigenfunctions. We also use these ideas to show that any free boundary solution in two dimensions has area at least π, and we observe that this implies the sharp isoperimetric inequality for free boundary solutions in the two-dimensional case.     

Rigidity of unary algebras and its application to the $${\mathcal {HS} = \mathcal {SH}}$$ problem

H. P. Gumm and T. Schröder stated a conjecture that the preservation of preimages by a functor T for which |T1| = 1 is equivalent to the satisfaction of the class equality \({{\mathcal {HS}}({\sf K}) = {\mathcal {SH}}({\sf K})}\) for any class K of T-coalgebras. Although T. Brengos and V. Trnková gave a positive answer to this problem for a wide class of Set-endofunctors, they were unable to find the full solution. Using a construction of a rigid unary algebra we prove \({{\mathcal {HS}} \neq {\mathcal {SH}}}\) for a class of Set-endofunctors not preserving non-empty preimages; these functors have not been considered previously.     

Asymmetry and structural information in preferential attachment graphs

Graph symmetries intervene in diverse applications, from enumeration, to graph structure compression, to the discovery of graph dynamics (e.g., node arrival order inference). Whereas Erd?s‐Rényi graphs are typically asymmetric, real networks are highly symmetric. So a natural question is whether preferential attachment graphs, where in each step a new node with m edges is added, exhibit any symmetry. In recent work it was proved that preferential attachment graphs are symmetric for m = 1, and there is some nonnegligible probability of symmetry for m = 2. It was conjectured that these graphs are asymmetric when m ≥ 3. We settle this conjecture in the affirmative, then use it to estimate the structural entropy of the model. To do this, we also give bounds on the number of ways that the given graph structure could have arisen by preferential attachment. These results have further implications for information theoretic problems of interest on preferential attachment graphs.     

The properties of liquid nitrogen

The light-scattering ‘anisotropy’ spectrum of liquid nitrogen has been measured for the liquid along the liquid-vapour coexistence line from 69·4 K (almost the triple point) to near the critical point at 125 K and for the gas under pressure at 128 K. The spectrum is very broad (HWHH～50 cm^-1) due to rapid molecular reorientation. The molecular anisotropy spectrum is approximately gaussian, corresponding to a correlation time for molecular reorientation of order 2 × 10^-13 s at 80 K which is comparable with that obtained from nuclear magnetic resonance. A gaussian rather than a lorentzian form arises because molecular reorientation is not a ‘slow’ variable.

Above the critical temperature the molecular anisotropy spectrum can be roughly described as a collision-broadened rotational line spectrum.

The spectrum and its time Fourier Transform are analysed in terms of the dynamical correlation of orientation of the molecules.

A component of the spectrum due to induced polarization is separated from the anisotropy contribution by a study of the far wing of the spectrum and is observed on the Stokes side out to about 250 cm^-1. This depends exponentially on frequency shift and is interpreted in terms of molecular collision dynamics. The correlation time for this motion is about a factor three shorter than that for molecular reorientation.

The spectra are markedly asymmetric after all experimental corrections for asymmetry have been made. The asymmetry is shown to correspond to the detailed balance factor. It is pointed out that this factor should be allowed for in the case of induced scattering, in particular.

A theoretical analysis is given of the effect of correlation of molecular orientation on the light-scattering spectrum for centrosymmetric linear molecules. In particular it is shown that the normalized second moment of the spectrum is unaffected by correlation of orientation.     

Plasmonic enhancement of blue emission from ZnO nanorods grown on the anodic aluminum oxide (AAO) template

Luminescent properties of ZnO nanorods covered with Ag nanoparticles are examined. Nanorods were synthesized on AAO templates using Atomic Layer Deposition (ALD) technique. Two types of the samples were prepared with different arrangement of ZnO nanorods and doping conditions. Nanorods of the second type were codoped with Al, to stimulate defect-related emissions. The ZnO material fills heterogeneously the interior of the AAO nanopores and has hexagonal, wurtzite structure. Both types of structures exhibit a broad defect-related emission at about 440 nm, most probably related to recombination at zinc interstitial (Zn_i) defects. This emission in samples with a random distribution of ZnO:Al nanorods and finer Ag nanoparticles is enhanced by factor of ～2.5 upon Ag deposition. The so-obtained material is interesting from the point of view of its application in blue range emitting diodes.