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Diastereoselectivity Switch During Alkene Reductions: Diastereodivergent Syntheses of Molecular Fossils via MHAT or Homogeneous Catalytic Hydrogenation Reactions
No full textSixteen geosterane derivatives were synthesized in up to 57 % overall yields in four steps harnessing the olefin cross-metathesis (OCM) and Metal hydride H atom transfer (MHAT) or homogeneous hydrogenation reactions as key steps. Drawing on this strategy, the diastereomeric ratio (d. r.) reached up to 24 : 1 for the thermodynamic isomer and 7 : 1 for the other isomer in the hydrogenation step. In a geological sample from northeast Brazil, we confirmed the putative structures previously assumed as methyl 2-(3α-5αH-cholestan) acetate, methyl 2-(3β-5αH-cholestan)acetate, and methyl 6-(3β-5αH-cholestan)hexanoate, as well three new molecular fossils of approximately 120 million years old. We also proved the migration marking ability of those carboxylic acids derived from forerunner geosteranes during an oil migration event, which suggests their aptitudes as molecular odometers. Our approach demonstrated swiftness and effectiveness in preparing a molecular library of geological biomarkers would also be appropriate to generate stereochemical diversity in molecular libraries for medicinal chemistry and natural product anticipation
Mid-infrared cross-comb spectroscopy
No full textDual-comb spectroscopy has been proven beneficial in molecular characterization but remains challenging in the mid-infrared region due to difficulties in sources and efficient photodetection. Here we introduce cross-comb spectroscopy, in which a mid-infrared comb is upconverted via sum-frequency generation with a near-infrared comb of a shifted repetition rate and then interfered with a spectral extension of the near-infrared comb. We measure CO₂ absorption around 4.25 µm with a 1-µm photodetector, exhibiting a 233-cm⁻¹ instantaneous bandwidth, 28000 comb lines, a single-shot signal-to-noise ratio of 167 and a figure of merit of 2.4 × 10⁶ Hz^(1/2). We show that cross-comb spectroscopy can have superior signal-to-noise ratio, sensitivity, dynamic range, and detection efficiency compared to other dual-comb-based methods and mitigate the limits of the excitation background and detector saturation. This approach offers an adaptable and powerful spectroscopic method outside the well-developed near-IR region and opens new avenues to high-performance frequency-comb-based sensing with wavelength flexibility
Quantum Gravity in the Lab. II. Teleportation by Size and Traversable Wormholes
No full textIn Brown et al. [PRX Quantum, TBA, TBA (2023)], we discuss how holographic quantum gravity may be simulated using quantum devices and we give a specific proposal—teleportation by size and the phenomenon of size winding. Here, we elaborate on what it means to do quantum gravity in the lab and how size winding connects to bulk gravitational physics and traversable wormholes. Perfect size winding is a remarkable fine-grained property of the size wave function of an operator; we show from a bulk calculation that this property must hold for quantum systems with a nearly AdS₂ bulk. We then examine in detail teleportation by size in three systems—the Sachdev-Ye-Kitaev model, random matrices, and spin chains—and discuss prospects for realizing these phenomena in near-term quantum devices
Sequence Order in the Range 1 to 19 by Chimpanzees on a Touchscreen Task: Processing Two-Digit Arabic Numerals
No full textThe sequence of Arabic numerals from 1 to 19 was taught to six chimpanzees, three pairs of mother and child. Each chimpanzee participant sat facing a touchscreen on which the numerals appeared in random positions within an imaginary 5-by-8 matrix. They had to touch the numerals in ascending order. Baseline training involved touching the adjacent numerals from 1 to X or from the numeral X to 19. Systematic tests revealed the following results: (1) The range 1 to 9 was easier than 1 to 19. (2) Adjacent numerals were easier than nonadjacent ones. (3) The “masking” (memory task) caused deterioration of performance. All these factors depended on the number of numerals simultaneously presented on the screen. A chimpanzee named Pal mastered the skill of ordering two-digit numerals with 100% accuracy. Human participants were tested in the same experiment with the same procedure. Both species showed relative difficulty in handling two-digit numerals. Global–local information processing is known to be different between humans and other primates. The assessment of chimpanzee performance and comparison with humans were discussed in terms of the possible difference in the global–local dual information processing of two-digit numerals
A survey for variable young stars with small telescopes: VI – Analysis of the outbursting Be stars NSW 284, gaia 19eyy, and VES 263
No full textThis paper is one in a series reporting results from small telescope observations of variable young stars. Here, we study the repeating outbursts of three likely Be stars based on long-term optical, near-infrared, and mid-infrared photometry for all three objects, along with follow-up spectra for two of the three. The sources are characterized as rare, truly regularly outbursting Be stars. We interpret the photometric data within a framework for modelling light-curve morphology, and find that the models correctly predict the burst shapes, including their larger amplitudes and later peaks towards longer wavelengths. We are thus able to infer the start and end times of mass loading into the circumstellar discs of these stars. The disc sizes are typically 3 – 6 times the areas of the central star. The disc temperatures are ∼40 per cent, and the disc luminosities are ∼10 per cent of those of the central Be star, respectively. The available spectroscopy is consistent with inside-out evolution of the disc. Higher excitation lines have larger velocity widths in their double-horned shaped emission profiles. Our observations and analysis support the decretion disc model for outbursting Be stars
Honoring the career and contributions of neurosurgery founding member Walter Edward Dandy (1886-1946)
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Search for MeV electron recoils from dark matter in EXO-200
No full textWe present a search for electron-recoil signatures from the charged-current absorption of fermionic dark matter using the EXO-200 detector. We report an average electron recoil background rate of 6.8 × 10⁻⁴ cts kg⁻¹ yr⁻¹ keV⁻¹ above 4 MeV and find no statistically significant excess over our background projection. Using a total ¹³⁶Xe exposure of 234.1 kg yr we exclude new parameter space for the charged-current absorption cross-section for dark matter masses between m_χ = 2.6−11.6 MeV with a minimum of 6 × 10⁻⁵¹ cm² at 8.3 MeV at the 90% confidence level
Quantum-mechanics-based molecular dynamics simulations of the structure and performance of sulfur-enriched Li₃PS₄ cathodes
No full textSulfur-enriched Li₃PS₄ is a promising cathode for lithium-sulfur batteries, but there is not yet clear structural information about this material. Here, we use quantum-mechanics-based molecular dynamics to predict the structures of the fully charged Li₃PS₄₊₅ and various stages of discharge to nearly discharged Li₃₊₉PS₄₊₅ at 298 K. We predict that the fully charged cathode has a distorted orthorhombic structure similar to Li₃PS₄, but with additional S₄ and S₈ chains each linked to an S atom of a PS₄ anion. This leads to a density of 2.19 g/cm³ for this material. For the nearly discharged cathode, we find that the lithium atoms insert into all S−S bonds, with none of the aforementioned chains remaining, leading to a density of 1.71 g/cm³. We examined the discharge process as lithium ions from the electrolyte diffuse into the cathode and react with the S−S chains to form Li₂S. The predicted discharge curve agrees well with the experiment
Nanoelectromechanical Tuning of High-Q Slot Metasurfaces
No full textNanoelectromechanical devices have been used widely in many applications across photonics, electronics, and acoustics. Their incorporation into metasurface systems could be beneficial in designing new types of active photonic devices. Here, we propose a design of active metasurfaces using a nanoelectromechanical system (NEMS) composed of silicon bars which operates under CMOS-level voltage and achieves phase modulation with wavelength-scale pixel pitch. By introducing a perturbation to the slot mode propagating between the silicon bars, the device operates in a high-Q regime, making the optical mode highly sensitive to mechanical movement. An over 12 dB reflection modulation is observed by full-wave simulation, and over 10% is achieved in the proof-of-concept experiment under CMOS-level voltage. We also simulate a device with 1.8π phase response using a bottom gold mirror. Based on this device, a 3-pixel optical beam deflector is shown to have 75% diffraction efficiency
Compactly Restrictable Metric Policy Optimization Problems
No full textWe study policy optimization problems for deterministic Markov decision processes (MDPs) with metric state and action spaces, which we refer to as metric policy optimization problems (MPOPs). Our goal is to establish theoretical results on the well-posedness of MPOPs that can characterize practically relevant continuous control systems. To do so, we define a special class of MPOPs called compactly restrictable MPOPs (CR-MPOPs), which are flexible enough to capture the complex behavior of robotic systems but specific enough to admit solutions using dynamic programming methods such as value iteration. We show how to arrive at CR-MPOPs using forward-invariance. We further show that our theoretical results on CR-MPOPs can be used to characterize feedback linearizable control affine systems