Symmetry reduction for molecular dynamics simulation of an imploding gas bubble

We propose a symmetry reduction technique whereby molecular dynamics (MD) simulations for spherically symmetric gas bubbles can be accelerated. Results for an imploding Xenon bubble containing 50 million particles—the smallest measured sonoluminescing system—are presented.     

Extreme statistics of complex random and quantum chaotic states

Complex random states have the statistical properties of the Gaussian and circular unitary ensemble eigenstates of random matrix theory. Even though their components are correlated by the normalization constraint, it is nevertheless possible to derive compact analytic formulas for their extreme values' statistical properties for all dimensionalities. The maximum intensity result slowly approaches the Gumbel distribution even though the variables are bounded, whereas the minimum intensity result rapidly approaches the Weibull distribution. Since random matrix theory is conjectured to be applicable to chaotic quantum systems, we calculate the extreme eigenfunction statistics for the standard map with parameters at which its classical map is fully chaotic. The statistical behaviors are consistent with the finite-N formulas.     

Magnetic edge-state excitons in zigzag graphene nanoribbons

We present first-principles calculations of the optical properties of zigzag-edged graphene nanoribbons (ZGNRs) employing the GW-Bethe-Salpeter equation approach with the spin interaction included. Optical response of the ZGNRs is found to be dominated by magnetic edge-state-derived excitons with large binding energy. The absorption spectrum is composed of a characteristic series of exciton states, providing a possible signature for identifying the ZGNRs. The edge-state excitons are charge-transfer excitations with the excited electron and hole located on opposite edges; they moreover induce a spin transfer across the ribbon, resulting in a photoreduction of the magnetic ordering. These novel characteristics are potentially useful in the applications.     

Monaural level discrimination under dichotic conditions

The ability to make judgments about the stimulus at one ear when a stimulus is simultaneously presented to the other ear was tested. Specifically, subjects discriminated the level of a 600 Hz target tone presented at the left ear while an identical-frequency distractor was simultaneously presented at the other ear. When there was no distractor, threshold was 0.7 dB. Threshold increased to 1.1 dB when a distractor with a fixed phase and level was introduced contra-aurally to the target. Further increases in threshold were observed when an across-presentation variability was introduced into the distractor phase (threshold of 1.6 dB) or level (threshold of 5.8 dB). When both the distractor level and phase varied, the largest threshold of 7.3 dB was obtained. These increases in threshold cannot be predicted by common binaural models, which assume that a target stimulus at one ear can be processed without interference from the stimulus at the nontarget ear. The measured thresholds are consistent with a model that utilizes two binaural dimensions that roughly correspond to the loudness and the position of a fused binaural image. The results show that, with binaurally fused tonal stimuli, subjects are unable to listen to one ear.     

Towards the Design of Optically Active Thiophene S-Oxides using Quantum Chemistry

The importance of axially chiral biaryls has risen steeply in the recent decades. This structural motif proved to be successful in catalytic asymmetric synthesis and the configuration of the biaryl axis is decisive for the biological activity. A new approach for the atroposelective synthesis of biaryls would be through a cycloaddition between an enantiopure phenyl-substituted thiophene S-oxide and an alkyne. Importantly, the chiral center of the thiophene S-oxide needs to be stable enough to avoid pyramidal inversion during the cycloaddition. Considering that the racemization of thiophene monoxides has been scarcely investigated so far, we perform a thorough quantum chemical study on the inversion barriers of a large number of chiral thiophene S-oxide derivatives. Our main goal is to identify substitution patterns leading to stable atropisomers at room temperature. Appealingly, the role of stereoelectronic effects and the position of the substituents as well as the importance of aromaticity on the pyramidal inversion barrier are elucidated for the first time.     

Synthesis of High χ–Low N Diblock Copolymers by Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) enables the scalable synthesis of functional block copolymer nanoparticles with various morphologies. Herein we exploit this versatile technique to produce so-called “high χ–low N” diblock copolymers that undergo nanoscale phase separation in the solid state to produce sub-10 nm surface features. By varying the degree of polymerization of the stabilizer and core-forming blocks, PISA provides rapid access to a wide range of diblock copolymers, and enables fundamental thermodynamic parameters to be determined. In addition, the pre-organization of copolymer chains within sterically-stabilized nanoparticles that occurs during PISA leads to enhanced phase separation relative to that achieved using solution-cast molecularly-dissolved copolymer chains.     

A Mechanistically and Operationally Simple Route to Metal–N-Heterocyclic Carbene (NHC) Complexes

We have been puzzled by the involvement of weak organic and inorganic bases in the synthesis of metal–N-heterocyclic carbene (NHC) complexes. Such bases are insufficiently strong to permit the presumed required deprotonation of the azolium salt (the carbene precursor) prior to metal binding. Experimental and computational studies provide support for a base-assisted concerted process that does not require free NHC formation. The synthetic protocol was found applicable to a number of transition-metal- and main-group-centered NHC compounds and could become the synthetic route of choice to form M–NHC bonds.     

A direct route for the preparation of Fmoc/OtBu protected iodotyrosine

Herein the synthesis of an Fmoc/O^tBu orthogonally protected iodotyrosine derivative is reported. This has been achieved via a simple two-step process in an overall 58% yield from commercially available Fmoc-Tyr(^tBu)-OH. The Fmoc/O^tBu orthogonally protected iodotyrosine was also shown to be amenable to Suzuki-Miyaura cross-coupling to deliver a novel bi-aryl tyrosine derivative.     

Cage motion of a probe atom in a vacancy complex in Pt

The perturbation function of a Cd probe atom bound in a hydrogen-decorated vacancy complex in Pt is found to exhibit dynamical relaxation at 294 K, as measured by the technique of perturbed γ-γ angular correlations. Based on other characteristics of the complex, a model is proposed for the structure of the complex. In the undecorated state, it consists of a probe atom at the center of a tetrahedron of vacancies in the fcc structure. In the decorated state at low temperature, the probe atom returns to one of the four vacant lattice sites. The relaxation at room temperature is attributed to motion of the probe atom in a cage formed by the four vacant sites.