Abstract
The ejecta velocities of Type Ia supernovae (SNe Ia), as measured by the Si
ii
λ
6355 line, have been shown to correlate with other supernova properties, including color and standardized ...luminosity. We investigate these results using the Foundation Supernova Survey, with a spectroscopic data release presented here, and photometry analyzed with the SALT2 light-curve fitter. We find that the Foundation data do not show significant evidence for an offset in color between SNe Ia with high and normal photospheric velocities, with Δ
c
= 0.004 ± 0.015. Our SALT2 analysis does show evidence for redder high-velocity SNe Ia in other samples, including objects from the Carnegie Supernova Project, with a combined sample yielding Δ
c
= 0.018 ± 0.008. When split on velocity, the Foundation SNe Ia also do not show a significant difference in Hubble diagram residual, Δ
HR
= 0.015 ± 0.049 mag. Intriguingly, we find that SN Ia ejecta velocity information may be gleaned from photometry, particularly in redder optical bands. For high-redshift SNe Ia, these rest-frame red wavelengths will be observed by the Nancy Grace Roman Space Telescope. Our results are in line with previous work that suggests SN Ia host-galaxy stellar mass is correlated with ejecta velocity: high-velocity SNe Ia are found nearly exclusively in high-stellar-mass hosts. However, host-galaxy properties alone do not explain velocity-dependent differences in supernova colors and luminosities across samples. Measuring and understanding the connection between intrinsic explosion properties and supernova environments, across cosmic time, will be important for precision cosmology with SNe Ia.
The bicyclo4.1.0heptane substructure, featured in a number of natural products, is economically formed via gold(III)-mediated cycloisomerization of a 5-acetoxy-1,6-enyne. This Ohloff–Rautenstrauch ...rearrangement takes place with high regio- and stereocontrol and purportedly proceeds through either of two pathways that differ in the order of major events: cyclization followed by ester migration (“cyclization first”) or its transpose (“migration first”). Implicit solvent-phase (dichloroethane) electronic structure calculations IEFPCM-B2PLYP-D3/def2-TZVP//IEFPCM-B2PLYP/6-31G(d)-LANL2DZ aimed at elucidation of the minimum energy pathway corresponding to the “cyclization first” and “migration first” pathways are presented herein. Both pathways feature multiple steps and are characterized by low-energy barriers, indicating that facile interconversion of structures on the surface is possible. In addition, the highest-energy structures for each of the two pathways are very close in energy (ΔΔE ⧧ < 3.0 kcal/mol). Relative turnover frequency (TOF) and degree of TOF control (X TOF) calculations indicate that although a cyclization first pathway may dominate, both cyclization and acyl migration processes influence the rate of this reaction.
ABSTRACT A growing number of supernovae (SNe) are now known to exhibit evidence for significant interaction with a dense, pre-existing, circumstellar medium (CSM). SNe Ibn comprise one such class ...that can be characterized by both rapidly evolving light curves and persistent narrow He i lines. The origin of such a dense CSM in these systems remains a pressing question, specifically concerning the progenitor system and mass-loss mechanism. In this paper, we present multiwavelength data of the Type Ibn SN 2020nxt, including HST/STIS ultraviolet spectra. We fit the data with recently updated CMFGEN models designed to handle configurations for SNe Ibn. The UV coverage yields strong constraints on the energetics and, when combined with the CMFGEN models, offer new insight on potential progenitor systems. We find the most successful model is a ≲4 M⊙ helium star that lost its $\sim 1\, {\rm M}_\odot$ He-rich envelope in the years preceding core collapse. We also consider viable alternatives, such as a He white dwarf merger. Ultimately, we conclude at least some SNe Ibn do not arise from single, massive (>30 M⊙) Wolf–Rayet-like stars.
Abstract
We fit the UV/optical lightcurves of the Seyfert 1 galaxy Mrk 817 to produce maps of the accretion disk temperature fluctuations
δ
T
resolved in time and radius. The
δ
T
maps are dominated ...by coherent radial structures that move slowly (
v
≪
c
) inward and outward, which conflicts with the idea that disk variability is driven only by reverberation. Instead, these slow-moving temperature fluctuations are likely due to variability intrinsic to the disk. We test how modifying the input lightcurves by smoothing and subtracting them changes the resulting
δ
T
maps and find that most of the temperature fluctuations exist over relatively long timescales (hundreds of days). We show how detrending active galactic nucleus (AGN) lightcurves can be used to separate the flux variations driven by the slow-moving temperature fluctuations from those driven by reverberation. We also simulate contamination of the continuum emission from the disk by continuum emission from the broad-line region (BLR), which is expected to have spectral features localized in wavelength, such as the Balmer break contaminating the
U
band. We find that a disk with a smooth temperature profile cannot produce a signal localized in wavelength and that any BLR contamination should appear as residuals in our model lightcurves. Given the observed residuals, we estimate that only ∼20% of the variable flux in the
U
and
u
lightcurves can be due to BLR contamination. Finally, we discus how these maps not only describe the data but can make predictions about other aspects of AGN variability.
Compounds of the rotenoid class are naturally occurring in the Leguminosae and Nyctaginacae families. Rotenoids have found a myriad of uses, for example, in the agricultural industry as an ...insecticide and piscicide, and as an anticancer therapeutic. The scientific literature questions whether cyclization of the rotenoid B-ring occurs via a pathway containing either cationic or free-radical intermediates. In this work, both propositions are analyzed using DFT (B3LYP and M06-2X) and the G3 composite method in gas- and (implicit) solution-phase. The accuracy of these methods is compared to several experimental C–H bond dissociation energies (BDEs). We find that of the methods surveyed M06-2X provides the most accurate BDEs. Further, there is a clear thermodynamic preference for the free-radical pathway.
Herein we describe our exploration, using density functional theory calculations, of a conjugate addition–rearrangement sequence that leads to medium-ring cyclic amines. On the basis of the results ...of our calculations, we conclude that the rearrangement step is rate determining. In addition, we analyze the role of a carbanion lone pair in the rearrangement step, concluding that it functions as a substituent on a 3,3 sigmatropic shift, rather than a nucleophile; thus, the Woodward–Hoffmann rules are not circumvented in this reaction via involvement of orthogonal orbitals on an atom involved in the rearrangement.
The sophomore organic chemistry sequence (Organic I and II) has a reputation: one that has prospective students worried. Their anxiety is well-founded, given estimates of the national success rate ...(defined as a grade of “C” or better) of 50–75%. In an attempt to improve both student success and student opinions about chemistry, we implemented a one-week “boot camp” (preparatory) course that we called Preparing for Organic Chemistry. This course reviewed the concepts of general chemistry required for success in Organic Chemistry I and II. We found improvements in student opinion of Organic Chemistry I (both before starting and after taking the course). Further, we have found that the boot camp had a measurable and statistically significant impact on student success rates.
Direct dynamics trajectory simulations were performed for two examples of the thiolate-disulfide exchange reaction, that is, HS– + HSSH and CH3S– + CH3SSCH3. The trajectories were computed for the ...PBE0/6-31+G(d) potential energy surface using both classical microcanonical sampling at the ion-dipole complex and quasi-classical Boltzmann sampling (T = 300 K) at the central transition state. The potential energy surface for these reactions involves a hypercoordinate sulfur intermediate. Despite the fact that the intermediate resides in a shallow well (less than 5 kcal/mol), very few trajectories follow a direct substitution path (the SN2 pathway). Rather, the mechanism is addition–elimination, with several trajectories sampling the intermediate for long times, up to 15 ps or longer.
Herein we use quantum chemical methods (B3LYP/LANL2DZ) to chronicle the path of the Pd(II)-promoted Cope rearrangement. It was discovered that although the mechanism can proceed through a stepwise ...associative pathway (forming a cyclohexyl cation-like intermediateif appropriately substituted with an electron-donating group), a simple change in substituent position or nature (e.g., an aptly placed electron-withdrawing group) makes the concerted pathway energetically favorable. The origins of rate acceleration in these systems upon complexation are discussed in terms of “transition-state complexation”. Since the Pd(II)-promoted Cope rearrangement, much like its metal-free counterpart, changes mechanism on the basis of the nature and position of appended substituents, both should share the distinctive classification “chameleonic.”
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![Conjugate Addition/[3,3] Si...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0022-3263/small "Conjugate Addition/[3,3] Sigmatropic Shift Processes for Formation of Medium-Ring Cyclic Amines – Do They Circumvent the Woodward–Hoffmann Rules?")
