Mechanistic Studies on the Gas‐Phase Dehydrogenation of Alkanes at Cyclometalated Platinum Complexes

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L? H)]⁺ (L=2,2′‐bipyridine (bipy), 2‐phenylpyridine (phpy), and 7,8‐benzoquinoline (bq)) with linear and branched alkanes C_nH_2n+2 (n=2–4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L? H)]⁺/C₂H₆, loss of C₂H₄ dominates clearly over H₂ elimination; however, the mechanisms significantly differs for the reactions of the “rollover”‐cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen‐atom transfer from C₂H₆ to [Pt(bipy? H)]⁺, followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)]⁺, for the phpy and bq complexes [Pt(L? H)]⁺, the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L? H)(H₂)]⁺ as the ionic product accounts for C₂H₄ liberation. In the latter process, [Pt(L? H)(H₂)(C₂H₄)]⁺ (that carries H₂ trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C₂H₄ rather than H₂ is ejected. For both product‐ion types, [Pt(H)(bipy)]⁺ and [Pt(L? H)(H₂)]⁺ (L=phpy, bq), H₂ loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy? H)]⁺ with the higher alkanes C_nH_2n+2 (n=3, 4), H₂ elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C₃H₅)(bipy)]⁺. In the reactions of [Pt(L? H)]⁺ (L=phpy, bq) with propane and n‐butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of “rollover”‐cyclometalated [Pt(bipy? H)]⁺ with C_nH_2n+2 (n=2–4) less than 15 % of the generated product ions are formed by C? C bond‐cleavage processes, this value is about 60 % for the reaction with neo‐pentane. The result that C? C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2‐elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1‐ and 1,3‐elimination pathways are only of minor importance.     

On a Kohn-Vogelius like formulation of free boundary problems

The present paper is concerned with the solution of a Bernoulli type free boundary problem by means of shape optimization. Two state functions are introduced, namely one which satisfies the mixed boundary value problem, whereas the second one satisfies the pure Dirichlet problem. The shape problem under consideration is the minimization of the L ²-distance of the gradients of the state functions. We compute the corresponding shape gradient and Hessian. By the investigation of sufficient second order conditions we prove algebraic ill-posedness of the present formulation. Our theoretical findings are supported by numerical experiments.     

One-dimensional confinement in heterogeneous catalysis: Trapped oxygen on RuO2(110) model catalysts

With temperature programmed reaction (TPR) experiments and kinetic Monte Carlo (kMC) simulations of coadsorbed oxygen and HCl on the RuO₂(110) surface we studied the thermal stabilization of dissociatively adsorbed oxygen. Due to one-dimensional confinement single surface O atoms can be trapped by surface chlorine atoms so that surface oxygen is not able to desorb from the RuO₂(110) surface at the expected temperature of 420 K. Trapped oxygen needs desorption temperatures as high as 700 K where it recombines with bridging O from RuO₂(110) to form O₂. Kinetic modeling of catalytic reactions with dimensional confinement of their reaction intermediates on the catalyst's surface requires the application of kinetic Monte Carlo simulations which are beyond the mean field approach.     

Periodic nanostructures on Si(100) surfaces generated by high-repetition rate sub-15 fs pulsed near-infrared laser light

Nanoscale rifts and ripples at a periodicity of 130 nm were generated on Si(100) surfaces immersed in water using tightly focused 800 nm 12 fs pulsed 85 MHz laser light at subnanojoule pulse energies. At radiant exposure close to the ablation threshold rifts were typically 20-50 nm in width and 70 nm in depth running perpendicular to the laser polarization. On increase of the irradiance, the rifts broadened and formed periodic ripples, whereas at highest exposure, a random nanoporous surface topology emerged. Rift and ripple formation is explained by laser-induced standing surface plasma waves, which result in periodic variation of dissipation and ablation.     

Thermal Ammonia Activation by Cationic Transition‐Metal Hydrides of the First Row – Small but Mighty

The thermal reactions of cationic 3d transition‐metal hydrides MH⁺ (M=Sc–Zn, except V and Cu) with ammonia have been studied by gas‐phase experiments and computational methods. There are three primary reaction channels: 1) H₂ elimination by N? H bond activation, 2) ligand exchange under the formation of M(NH₃)⁺, and 3) proton transfer to yield NH₄⁺. Computational studies of these three reaction channels have been performed for the couples MH⁺/NH₃ (M=Sc–Zn) to elucidate mechanistic aspects and characteristic reaction patterns of the first row. For N? H activation, σ‐bond metathesis was found to be operative.     

Disordered carbon nanofibers/LiCoPO<Subscript>4</Subscript> composites as cathode materials for lithium ion batteries

LiCoPO₄-coated disordered carbon nanofibers (CNFs/LiCoPO₄) were obtained by a sol–gel method, using triethyl phosphite or triethyl phosphate as the phosphorous source. The crystal structure of the products was analyzed by X-ray powder diffraction, while morphology was studied using scanning electron microscopy, transmission electron microscopy, Auger electron spectroscopy and X-ray photoelectron spectroscopy. Optimal synthesis conditions for the CNFs/LiCoPO₄ in light of the best electrochemical performance are discussed. The best discharge capacity 105 mAh/g (or ca. 63% of the theoretical capacity) shows the material with 40% CNFs/LiCoPO₄ and addition coating by carbon black. This composition has a best purity of active materials and point coverage of CNFs. The X-ray photoelectron C1s spectra of the CNFs surface without and with sputter erosion show enhancement of C–O bonds at the fiber surface, which does not influence significantly electrochemical behavior of the composite materials.     

Concerted, highly asynchronous, enzyme-catalyzed [4 + 2] cycloaddition in the biosynthesis of spinosyn A; computational evidence

A theoretical study has been carried out on model systems to study a recently reported, (Nature, 2011, 473, 109) biosynthetic, [4 + 2] cycloaddition catalyzed by a stand-alone enzyme (the cyclase SpnF). It was suggested in this paper that SpnF is the first known example of a Diels-Alderase (DA). In the present study, for a model system of the substrate a transition structure was found with density functional calculations (DFT). In addition, the intrinsic reaction coordinate calculations indicated that the transition structure is that of a concerted, but highly asynchronous, DA reaction. Based on the DFT and M?ller-Plesset second order calculations the activation energy was estimated to be about 15 kcal mol(-1). The results of a natural population analysis indicated that there is significant charge transfer in the transition state, and it is proposed that possibly the enzyme plays a dual role of not only folding the substrate into the proper conformation for the DA reaction to occur, but also lowering its activation energy by stabilization of the highly polarized transition structure.     

Neutron Matter Wave Quantum Optics

Neutron matter-wave optics provides the basis for new quantum experiments and a step towards applications of quantum phenomena. Most experiments have been performed with a perfect crystal neutron interferometer where widely separated coherent beams can be manipulated individually. Various geometric phases have been measured and their robustness against fluctuation effects has been proven, which may become a useful property for advanced quantum communication. Quantum contextuality for single particle systems shows that quantum correlations are to some extent more demanding than classical ones. In this case entanglement between external and internal degrees of freedom offers new insights into basic laws of quantum physics. Non-contextuality hidden variable theories can be rejected by arguments based on the Kochen-Specker theorem.     

Chlorodifluoroacetyl Azide, ClF(2)CC(O)N(3): Preparation, Properties, and Decomposition

Chlorodifluoroacetyl azide, ClF(2)CC(O)N(3), was prepared and characterized by IR (gas, Ar matrix), Raman (liquid), UV-vis (gas), and (19)F, (13)C NMR spectroscopy. The vibrational spectra were analyzed in terms of a single conformer, gauche-syn, where the Cl-C and the N(α)═N(β) bonds are gauche and syn to the C═O bond, respectively. The photo and thermal decomposition reactions of the azide were studied with the aid of matrix isolation. In both cases, a new isocyanate species ClF(2)CNCO was produced and characterized by matrix IR spectroscopy. The conformational properties and the Curtius rearrangement pathways of this new carbonyl azide were theoretically explored, which suggest the preference of a concerted over stepwise decomposition for the global minimum gauche-syn conformer.