A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water

Using light energy and O₂ for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O₂ by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)₃]²⁺, generating the highly oxidized chromophore and the powerful reductant methyl‐viologen radical MV^+.. MV^+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O₂ to O₂^.? in an aqueous medium to generate an active iron(III)‐(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.     

Sensitivity to Initial Noise in Measurement-Induced Nonlinear Quantum Dynamics

We consider a special iterated quantum protocol with measurement-induced nonlinearity for qubits, where all pure initial states on the Bloch sphere can be considered chaotic. The dynamics is ergodic with no attractive fixed cycles. We show that initial noise radically changes this behavior. The completely mixed state is an attractive fixed point of the dynamics induced by the protocol. Our numerical simulations strongly indicate that initially mixed states all converge to the completely mixed state. The presented protocol is an example, where gaining information from measurements and employing it to control an ensemble of quantum systems enables us to create ergodicity which, in turn, is destroyed by any initial noise.     

On strong affine representations of the polycyclic monoids

Jones and Lawson have discovered in (Period Math Hungar 64(1):53–87 2012) that certain representations of the so-called polycyclic monoids are closely related to some permutative representations of the Cuntz algebras \({\mathcal {O}}_{n}\) studied by Bratteli and Jorgensen in (Mem Am Math Soc 139(663):x+89 1999). We investigate these representations of the polycyclic monoids, and we generalize some results from Jones and Lawson (2012). We give a (sharp) upper bound on the number of atoms in case one of the parameters tends to infinity and present an infinite family of representations having only one atom. Furthermore, by making use of a C++ program we present some observations regarding the number of atoms in the case \(n=3\).     

Perceptual recalibration in human sound localization: learning to remediate front-back reversals

The efficacy of a sound localization training procedure that provided listeners with auditory, visual, and proprioceptive/vestibular feedback as to the correct sound-source position was evaluated using a virtual auditory display that used nonindividualized head-related transfer functions (HRTFs). Under these degraded stimulus conditions, in which the monaural spectral cues to sound-source direction were inappropriate, localization accuracy was initially poor with frequent front-back reversals (source localized to the incorrect front-back hemifield) for five of six listeners. Short periods of training (two 30-min sessions) were found to significantly reduce the rate of front-back reversal responses for four of five listeners that showed high initial reversal rates. Reversal rates remained unchanged for all listeners in a control group that did not participate in the training procedure. Because analyses of the HRTFs used in the display demonstrated a simple and robust front-back cue related to energy in the 3-7-kHz bandwidth, it is suggested that the reductions observed in reversal rates following the training procedure resulted from improved processing of this front-back cue, which is perhaps a form of rapid perceptual recalibration. Reversal rate reductions were found to generalize to untrained source locations, and persisted at least 4 months following the training procedure.     

Nucleation and collapse of scroll rings in excitable media

We describe a novel nucleation mechanism of scroll rings in three-dimensional reaction-diffusion systems with anomalous dispersion. The vortices form after the collision of two spherical wave fronts from a third, trailing wave that only partially annihilates in the wake of its predecessor. Depending on the relative positions of the three relevant wave sources, one obtains untwisted or twisted scroll rings. The formation of both vortex structures is demonstrated for a modified Belousov-Zhabotinsky reaction.     

Preferential excitation transfer in Cs1(6D32)- Cs(6S12) collisions

A cw dye laser beam, tuned near the Cs 8761 Å resonance (Cs¹(6P_{$\text{1}\text{2}$}) → Cs¹(6D_{$\text{3}\text{2}$}) transition) is focused into a Cs vapor. At a Cs density higher than 5 × 10¹⁵ cm^-3, we observe a greater 6P_{$\text{1}\text{2}$} population when the laser is on resonance than when it is off resonance. However, at a lower Cs density, the reverse is observed. This phenomenon is explained as due to the preferential excitation transfer process: Cs¹(6D_{$\text{3}\text{2}$}) + Cs(6S_{$\text{1}\text{2}$}) → Cs¹(6P) + Cs¹(6P), and the corresponding cross secti on is estimated to be (1.5^+1.5_-0.7) × 10^-14 cm² by fitting the experimental results to an approximate rate-equation analysis.     

On the disappearance of criticality in the theory of thermal ignition

Summary It is shown that a boundary value problem in the theory of thermal ignition has multiple solutions when a certain parameter is greater than 4, for the geometries of an infinite circular cylinder and a sphere.

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Zusammenfassung Es wird gezeigt, dass im Fall der unendlichen Platte, des unendlichen Zylinders und der Kugel, das Randwertproblem in der Theorie der thermischen Explosionen eine mehrfache Lösung hat, wenn ein gewisser Parameter grösser als 4 ist.

     

Concentration-induced nonuniform power in tunable erbium-doped fiber lasers

We report, for the first time to our knowledge, the presence of concentration-induced nonuniform power in tunable erbium-doped fiber lasers. A theoretical model is proposed with pair-induced quenching taken into account. We obtain good agreement between numerical and experimental results of a high-concentration erbium-doped fiber ring laser with a large tuning range of over 100 nm. These findings are useful for the design of lasers with doped fibers.     

Combined directed ortho metalation/cross-coupling strategies: synthesis of the tetracyclic A/B/C/D ring core of the antitumor agent camptothecin

A convergent synthesis of the A/B/C/D ring fragment 5 of camptothecin using a combination of directed ortho metalation and Negishi cross-coupling is described. The key features of the synthetic sequence are an anionic ortho-Fries rearrangement (10 --> 12), a Negishi cross-coupling (7 --> 6), and a terminal modified von Braun reaction (16 --> 5) that leads to tetracyclic derivative 5 in 7 steps and 11% overall yield.