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Substituent Effects Govern the Efficiency of Isoxazole Photoisomerization to Carbonyl-2H-Azirines
No full textThe photoisomerization of isoxazoles is an atom-economical route to carbonyl-2H-azirines, which are valuable in both synthetic and biological applications. However, isolation of the carbonyl-2H-azirine is challenged by reverse photoisomerization back to the isoxazole and irreversible rearrangement to an oxazole. In this work, we demonstrate that substituent selection on 3,5-disubstituted isoxazoles plays a critical role in driving the photochemical isoxazole–azirine equilibrium toward the carbonyl-2H-azirine while avoiding oxazole formation. We find that substituents affect the degree of overlap in the absorption spectra of isoxazole–azirine pairs, where reducing overlap increases the efficiency of photoisomerization. We use time-dependent density functional theory to predict absorption spectra for isomer pairs with varied 3,5-disubstituents, identifying tert-butyl- and trifluoromethyl-substituted 5-aminoisoxazoles as promising structures. We then tested these predictions experimentally, revealing efficient formation of carbonyl-2H-azirines in high yields with minimal oxazole formation. This is in contrast to a phenyl-substituted 5-aminoisoxazole, which was found to readily form oxazoles, precluding isolation of the carbonyl-2H-azirine. These results demonstrate the utility of substituent-driven design for tuning photoisomerization equilibria and provide an atom-economical option for generating carbonyl-2H-azirines on synthetically useful scales
Unsupervised learning reveals rapid gait adaptation after leg loss and regrowth in spiders
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The Atacama Cosmology Telescope: Quantifying atmospheric emission above Cerro Toco
No full textAt frequencies below 1 Hz, fluctuations in atmospheric emission in the Chajnantor region in northern Chile are the primary source of interference for bolometric millimeter-wave observations. This paper focuses on characterizing the statistics of these fluctuations using measurements from the Atacama Cosmology Telescope (ACT) and the Atacama Pathfinder Experiment (APEX) water vapor radiometer. We show that the total precipitable water vapor (PWV) is not in general an accurate estimator of the level of fluctuations in millimeter-wave atmospheric emission. We also show that the microwave frequency spectrum of atmospheric fluctuations is in good agreement with predictions by the am code for frequency bands above 90 GHz. We introduce a new method for separating atmospheric and systematic fluctuations, allowing us to fit a robust atmospheric flat field, as well as to study in the atmosphere in greater detail than previous works. We present a direct measurement of the temporal outer scale of turbulence of 0 ≈50 s corresponding to a spatial scale of 0 ≈500 m. Lastly, we show the variance of fluctuations in ACT’s mm-wave bands correlate with the variance of fluctuations in PWV measured by APEX, even though the observatories are 6 km apart and observe different lines of sight
Rigid rotor state-to-state cross-sections and rates of the PH3 + H2 collision
No full textAccurate interpretation of observational astronomical data requires reliable collisional rate coefficients for inelasting scattering events between interstellar molecules and the abundant buffer species. A five-dimensional potential energy surface (PES) for the PH3 ( 1A1) – H2 (1Σ+g) interaction was generated using the explicitly correlated CCSD(T)-F12 method in conjunction with the correlation-consistent triple-zeta aug-cc-pVTZ basis set, and averaged over H2 orientations to yield a reduced three-dimensional surface. Inelastic rotational cross-sections for collisions between ortho and para-PH3 with para-H2 (J = 0) are calculated using the close-coupling quantum scattering method. After Boltzmann thermal averaging, the rate coefficients are evaluated for temperatures up to 100 K. Our results reveal substantial discrepancies between computed PH3-para-H2 collisional rates and scaled PH3–He values, underlining the inadequacy of scaling approaches for reliable astrophysical modelling
To be Fussed Over: A Critical Phenomenological Examination of Black Feminist Practices of Care in the Midst of Polycrisis
No full textThis essay maps four Black feminist phenomenological reflections on care in the midst of polycrises. Linking the author’s series of Instagram stories ‘#MyBlackFeministCaronaChronicles’; an edited collection of West Philadelphia responses to the COVID-19 pandemic and uprisings, How We Stay Free; Toni Cade Bambara’s 1980 novel, The Salt Eaters; and bell hooks’s 1994 Sisters of the Yam: Black Women and Self-Recovery, the author seeks to describe how Black feminist modes of care are depicted via a Black feminist critical phenomenology that sees care as linked to intersubjectivity, intersectionality and liberation
