Mechanistic Aspects of Gas‐Phase Hydrogen‐Atom Transfer from Methane to [CO].+ and [SiO].+: Why Do They Differ?

The reactivity of the two diatomic congeneric systems [CO]^.+ and [SiO]^.+ towards methane has been investigated by means of mass spectrometry and quantum‐chemical calculations. While [CO]^.+ gives rise to three different reaction channels, [SiO]^.+ reacts only by hydrogen‐atom transfer (HAT) from methane under thermal conditions. A theoretical analysis of the respective HAT processes reveals two distinctly different mechanistic pathways for [CO]^.+ and [SiO]^.+, and a comparison to the higher metal oxides of Group 14 emphasizes the particular role of carbon as a second‐row p element.     

Formation of DNA‐Copolymer Fibrils Through an Amyloid‐Like Nucleation Polymerization Mechanism

Conjugation of a hydrophobic poly(2‐oxazoline) bearing tertiary amide groups along its backbone with a short single stranded nucleotide sequence results in an amphiphilic comb/graft copolymer, which organizes in fibrils upon direct dissolution in water. Supported by circular dichroism, atomic force microscopy, transmission electron microscopy, and scattering data, fibrils are formed through inter‐ and intramolecular hydrogen bonding between hydrogen accepting amide groups along the polymer backbone and hydrogen donating nucleic acid grafts leading to the formation of hollow tubes.

     

Excitation dependent quenching of luminescence in LED phosphors

Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu²⁺ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm². The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu²⁺ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Matrix Isolation Spectroscopic and Relativistic Quantum Chemical Study of Molecular Platinum Fluorides PtFn (n=1–6) Reveals Magnetic Bistability of PtF4

Molecular platinum fluorides PtF_n, n=1–6, are prepared by two different routes, photo-initiated fluorine elimination from PtF₆ embedded in solid noble-gas matrices, and the reaction of elemental fluorine with laser-ablated platinum atoms. IR spectra of the reaction products isolated in rare-gas matrices under cryogenic conditions provide, for the first time, experimental vibrational frequencies of molecular PtF₃, PtF₄ and PtF₅. Photolysis of PtF₆ enabled a highly efficient and almost quantitative formation of molecular PtF₄, whereas both PtF₅ and PtF₃ were formed simultaneously by subsequent UV irradiation of PtF₄. The vibrational spectra of these molecular platinum fluorides were assigned with the help of one- and two-component quasirelativistic DFT computation to account for scalar relativistic and spin–orbit coupling effects. Competing Jahn-Teller and spin–orbit coupling effects result in a magnetic bistability of PtF₄, for which a spin-triplet (³B_2g, D_2h) coexists with an electronic singlet state (¹A_1g, D_4h) in solid neon matrices.     

Thermal Ethane Activation by Bare [V2O5]+ and [Nb2O5]+ Cluster Cations: on the Origin of Their Different Reactivities

The gas‐phase reactivity of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane has been investigated by means of mass spectrometry and density functional theory (DFT) calculations. The two metal oxides give rise to the formation of quite different reaction products; for example, the direct room‐temperature conversions C₂H₆→C₂H₅OH or C₂H₆→CH₃CHO are brought about solely by [V₂O₅]⁺. In distinct contrast, for the couple [Nb₂O₅]⁺/C₂H₆, one observes only single and double hydrogen‐atom abstraction from the hydrocarbon. DFT calculations reveal that different modes of attack in the initial phase of C?H bond activation together with quite different bond‐dissociation energies of the M?O bonds cause the rather varying reactivities of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane. The gas‐phase generation of acetaldehyde from ethane by bare [V₂O₅]⁺ may provide mechanistic insight in the related vanadium‐catalyzed large‐scale process.     

Revisiting the Intriguing Electronic Features of the BeOBeC Carbyne and Some Isomers: A Quantum-Chemical Assessment

Extensive high-level quantum-chemical calculations reveal that the rod-shaped molecule BeOBeC, which was recently generated in matrix experiments, exists in two nearly isoenergetic states, the ⁵Σ quintet (⁵ 6 ) and the ³Σ triplet (³ 6 ). Their IR features are hardly distinguishable at finite temperature. The major difference concerns the mode of spin coupling between the terminal beryllium and carbon atoms. Further, the ground-state potential-energy surface of the [2Be,C,O] system at 4 K is presented and differences between the photochemical and thermal behaviors are highlighted. Finally, a previously not considered, so far unknown C_2v-symmetric rhombus-like four-membered ring ³[Be(O)(C)Be] (³ 5 ) is predicted to represent the global minimum on the potential-energy surface.     

Online Monitoring of Electrochemical Carbon Corrosion in Alkaline Electrolytes by Differential Electrochemical Mass Spectrometry

Carbon corrosion at high anodic potentials is a major source of instability, especially in acidic electrolytes and impairs the long‐term functionality of electrodes. In‐depth investigation of carbon corrosion in alkaline environment by means of differential electrochemical mass spectrometry (DEMS) is prevented by the conversion of CO₂ into CO₃^2?. We report the adaptation of a DEMS system for online CO₂ detection as the product of carbon corrosion in alkaline electrolytes. A new cell design allows for in situ acidification of the electrolyte to release initially dissolved CO₃^2? as CO₂ in front of the DEMS membrane and its subsequent detection by mass spectrometry. DEMS studies of a carbon‐supported nickel boride (Ni_xB/C) catalyst and Vulcan XC 72 at high anodic potentials suggest protection of carbon in the presence of highly active oxygen evolution electrocatalysts. Most importantly, carbon corrosion is decreased in alkaline solution.     

Counter‐Intuitive Gas‐Phase Reactivities of [V2]+ and [V2O]+ towards CO2 Reduction: Insight from Electronic Structure Calculations

[V₂O]⁺ remains “invisible” in the thermal gas‐phase reaction of bare [V₂]⁺ with CO₂ giving rise to [V₂O₂]⁺; this is because the [V₂O]⁺ intermediate is being consumed more than 230 times faster than it is generated. However, the fleeting existence of [V₂O]⁺ and its involvement in the [V₂]⁺ → [V₂O₂]⁺ chemistry are demonstrated by a cross‐over labeling experiment with a 1:1 mixture of C¹⁶O₂/C¹⁸O₂, generating the product ions [V₂¹⁶O₂]⁺, [V₂¹⁶O¹⁸O]⁺, and [V₂¹⁸O₂]⁺ in a 1:2:1 ratio. Density functional theory (DFT) calculations help to understand the remarkable and unexpected reactivity differences of [V₂]⁺ versus [V₂O]⁺ towards CO₂.     

SLIM: A Short‐Linked,Highly Redox‐Stable Trityl Label for High‐Sensitivity In‐Cell EPR Distance Measurements

The understanding of biomolecular function is coupled to knowledge about the structure and dynamics of these biomolecules, preferably acquired under native conditions. In this regard, pulsed dipolar EPR spectroscopy (PDS) in conjunction with site‐directed spin labeling (SDSL) is an important method in the toolbox of biophysical chemistry. However, the currently available spin labels have diverse deficiencies for in‐cell applications, for example, low radical stability or long bioconjugation linkers. In this work, a synthesis strategy is introduced for the derivatization of trityl radicals with a maleimide‐functionalized methylene group. The resulting trityl spin label, called SLIM, yields narrow distance distributions, enables highly sensitive distance measurements down to concentrations of 90 nm , and shows high stability against reduction. Using this label, the guanine‐nucleotide dissociation inhibitor (GDI) domain of Yersinia outer protein O (YopO) is shown to change its conformation within eukaryotic cells.