Isotopic Species Research Articles

The gas-phase structure determination of α-pinene oxide: An endo-cyclic epoxide of atmospheric interest.

The gas-phase rotational spectra of α-pinene oxide have been recorded using a chirped-pulse Fourier transform microwave spectrometer in the 6-18GHz frequency range. The parent species and all heavy atom isotopologues (13C and 18O) have been observed in their natural abundance. The experimental rotational constants of all isotopic species observed have been determined and used to obtain the substitution (rs) and the effective (r0) structures of the most stable conformer of α-pinene oxide. Calculations using the density functional theories B3LYP, M06-2X, and MN15-L and the abinitio method MP2 level of theory were carried out to check their performance against experimental results. The structure of the heavy atom's skeleton of α-pinene oxide has been compared to that of α-pinene and has shown that epoxidation does not overly affect the structure of the bicycle, validating its robustness. Furthermore, the structural features have been compared to those of other bicyclic molecules, such as nopinone and β-pinene.

Experimental results of deuterium/hydrogen co-current permeation through Nickel membrane, in non-steady-state surface limited regime

A better understanding of mutual influence among hydrogen isotope species in multi-isotope permeation is important for accurate assessments and predictions on tritium permeation through structural materials of fusion reactors. Moreover, this is important as the injection of H2 gas into the helium coolant of HCPB breeding blanket is proposed for mitigation of the permeation flux of tritium from the purge gas into the coolant. Although various modeling and experimental works have been carried out so far on this topic of multi-isotope permeation, unresolved issues or even controversies still persist. This work presents the results of some experimental tests of co-current permeation of deuterium with hydrogen through a very thin (0.075 mm) nickel membrane, performed in non-steady-state and surface limited regime (SLR), with the purposes to observe the effect of hydrogen on the permeation effective flux of deuterium, as well as to verify experimentally the predictions of a previous theoretical work. Thus, two deuterium/hydrogen gas mixtures of different ratios, as well as pure deuterium gas, were alternately introduced into the high-pressure side of the membrane by keeping the effective partial pressure of deuterium as close as possible to the same value (0.1 Pa), so that the comparison between multi- and mono-isotope permeation be relevant. The results indicate that the presence of (co-permeation) hydrogen does not lead to the decrease of deuterium permeation, but, on the contrary, to its increase. This effect is clearer revealed well before steady state is reached, where the effective permeation flux of deuterium has a constantly higher rise-rate when (co-permeation) hydrogen is present, and this rise-rate is even higher when the effective hydrogen pressure is higher. Also, a quite good consistency between experimental results and the theoretical predictions has been obtained, as well as a good repeatability of the results.

Visual Stratigraphy-Based Age Scale Developed for the Shallow Mount Siple Firn Core, Antarctica

Here we present a case study for using visual stratigraphy to date a shallow firn core from coastal West Antarctica. The Mount Siple ice core has the potential to reconstruct climate in this data-sparse region over recent decades. Line scanned images of the 24 m firn core were used to generate a grey-scale, which displays variability consistent with annual cycles. The resulting Mount Siple age scale spans from 1998 ± 1 to 2017 CE. This study demonstrates that the seasonal changes in the grey-scale record provide an independent method of dating firn cores. However, the presence of melt layers at this site has introduced an error of ±1 year. Visual line stratigraphy has the unique advantage over traditional annual layer counting, based on chemical or isotopic species, of being non-destructive and relatively inexpensive. Visual line stratigraphy has proved to be an effective dating method for this site.

High-level ab initio study of disulfur monoxide: Ground state potential energy surface and band origins for six isotopic species

In this work, a series of potential energy surfaces (PESs) of S2O was constructed in order to get the most accurate ab initio band origins of this massive molecule. The convergence of the coupled cluster energies with respect to both basis set size [aug-cc-pCVXZ, X = T, Q, 5, and 6] and the order of the excitation [CC(n), n = 2, 3, and 4] was analyzed. For the first time, the band origins of 32S216(18)O were variationally calculated with the error of 0.5 cm−1 using the ab initio PES. The vibrational energies of the most abundant six isotopologues of S2O were predicted.

Spectra of CO2-Rg2 and CO2-Rg-He trimers (Rg = Ne, Ar, Kr, and Xe): Intermolecular CO2 rock, vibrational shifts and three-body effects.

Weakly bound CO2-Rg2 trimers are studied by high-resolution (0.002cm-1) infrared spectroscopy in the region of the CO2 ν3 fundamental band (≈2350cm-1), using a tunable optical parametric oscillator to probe a pulsed supersonic slit jet expansion with an effective rotational temperature of about 2K. CO2-Ar2 spectra have been reported previously, but they are extended here to include Rg = Ne, Kr, and Xe as well as new combination and hot bands. For Kr and Xe, a unified scaled parameter scheme is used to account for the many possible isotopic species. Vibrational shifts of CO2-Rg2 trimers are compared to those of CO2-Rg dimers, and in all cases the trimer shifts are slightly more positive (blue-shifted) than expected on the basis of linear extrapolation from the dimer. Combination bands directly measure an intermolecular vibrational mode (the CO2 rock) and give values of about 32.2, 33.8, and 34.7cm-1 for CO2-Ar2, -Kr2, and -Xe2. Structural parameters derived for CO2-Rg2 trimers are compared with those of CO2-Rg and Rg2 dimers. Spectra of the mixed trimers CO2-Rg-He are also reported.

127I and 129I species in the English Channel and its adjacent areas: Uncovering impact on the isotopes marine pathways

Radioactive iodine-129 has been released from the La Hague nuclear fuel reprocessing facility (NRF) into the English Channel, but the distribution and transformation of the isotope species, and environmental consequences have not been fully characterized in the Channel. Here we present data on iodine isotopes (129I and 127I) species in surface water of the English Channel and the southern Celtic Sea. Compared to 127I species, the concentrations of 129I− and 129IO3− show more variations, but iodate is the major species for both 129I and 127I. Our data provide new information regarding iodide-iodate inter-conversion showing that water dilution and mixing are the main factors affecting the 127I and 129I species distribution in the Channel. Some reduction of iodate occurs within the English Channel and mainly in the west part because of biotic processes. The 129I species transformation is overall insignificant, especially in the eastern Channel, where a constant value of 129IO3−/129I is observed, which might characterize the La Hague wastewater signal. In the Celtic Sea, oxidation of iodide can be traced by 127I and 129I species. On a larger scale, 129I generally experienced an oxidation process in the Atlantic Ocean, while in the coast of shallow shelf seas, new produced 129I− can be identified, especially in the German Bight and the Baltic Sea. The data of 129I species in the English Channel can provide estimate of redox rates in a much broader marine areas if the transit time of 129I from La Hague is well-defined. Furthermore, estimate of inventories for 129I and its species in the Channel, and fluxes of 129I species from the English Channel to the North Sea add important information to the geochemical cycle of 129I.

Radiation Dosimetry of Theragnostic Pairs for Isotopes of Iodine in IAZA.

Theragnostic pairs of isotopes are used to infer radiation dosimetry for a therapeutic radiopharmaceutical from a diagnostic imaging study with the same tracer molecule labelled with an isotope better suited for the imaging task. We describe the transfer of radiation dosimetry from the diagnostic radioiodine isotope 123I, labelled for the hypoxia tracer molecule iodoazomycin arabinoside ([123I]IAZA), to isotopes 131I (therapeutic) and 124I (PET imaging). Uncertainties introduced by the dissimilar isotope half-lives are discussed in detail. Radioisotope dosimetries for [123I]IAZA were obtained previously. These data are used here to calculate residence times for 131I and 124I and their uncertainties. We distinguish two cases when extrapolating to infinity: purely physical decay (case A) and physical decay plus biological washout (case B). Organ doses were calculated using the MIRD schema with the OLIDNA/EXM code. Significant increases in some organ doses (in mSv per injected activity) were found for 131I and 124I. The most affected organs were the intestinal walls, thyroid, and urinary bladder wall. Uncertainty remained similar to 123I for case A but considerably greater for case B, especially for long biological half-lives (GI tract). Normal tissue dosimetries for IAZA must be considered carefully when substituting isotope species. A long biological half-life can significantly increase dosimetric uncertainties. These findings are relevant when considering PET imaging studies with [124I]IAZA or therapeutic administration of [131I]IAZA.

Isotopic Characterization of Cedrela to Verify Species and Regional Provenance of Bolivian Timber

With increasing concerns about sustainable exploitation of tropical timber, there is a need for developing independent tools to check their origin. We evaluated the potential of tree-ring stable isotopes for identifying four Cedrela species (C. balansae, C. fissilis, C. odorata, and C. saltensis) and for identifying geographic origin of C. fissilis and C. odorata, two of the most intensively exploited species. We studied differences in δ13C and δ18O of wood among 11 forest sites (163 trees). We quantified isotope composition of 10-year bulk samples, and for a subset we also evaluated isotopic annual fluctuations for the last 10 years. Although annual isotopic variability was not correlated to precipitation or elevation, we found a significant relationship between the 10-year bulk stable-isotope composition and average precipitation and elevation. However these relationships were not consistent across all sites. We also explored isotopic site and species differentiation using Kernel Discriminant Analyses. Site discrimination was low: 30% accuracy for C. odorata, and 40% for C. fissilis sites. However, species discrimination was 57.5% for C. odorata and 95.3% for C. fissilis. These results suggest that although δ13C and δ18O isotopic analyses hold potential to verify species identification, discrimination of geographical origin within a country may still be challenging.

High-order contact transformations of molecular Hamiltonians: general approach, fast computational algorithm and convergence of ro-vibrational polyad models

The paper describes methods and fast computational algorithm for building effective Hamiltonians in molecular physics using perturbative approach. Separations of fast and slow variables are considered in the framework of contact transformations (CT). The particular focus is on a systematic derivation of effective models for rovibrational spectroscopy from ab initio-based potential energy surfaces with an exhaustive review of previous studies in this field. We consider applications to several types of polyads coupled by Fermi, Coriolis, Darling-Dennison and other types of resonance interactions with examples for asymmetric top, symmetric top and spherical top molecules. A flexible choice of the modelling operator accounts for strong couplings of various types of nuclear motion in molecules among closely lying levels including vibrational resonance schemes (2:1:2 . ), (2:1:2:1), (4:2:6:3), (3:2:1:2:1:1), etc. that occur for C2v, C3v and Td molecules and their isotopic species. The method is implemented in the MOL_CT programme suite, which offers a complementary tool to variational methods in terms of convergence and computational time. The range of applications is also different. The goal of the CT method is providing mathematical models for analyses of molecular spectra with the high-resolution accuracy using physically meaningful parameters derived from ab initio functions.

Clumped isotope signatures of nitrous oxide formed by bacterial denitrification

Multiply substituted isotopic species of nitrous oxide (N2O), referred to as clumped isotopes, represent a promising new tool for distinguishing production pathways of this potent greenhouse gas. This work presents the first determination of enrichment factors of N2O clumped isotopes during bacterial denitrification. Samples of N2O obtained after 1-, 3-, and 7-day incubations of a pure culture of the denitrifier Pseudomonas aureofaciens at 20 °C and 30 °C were analysed by the recently developed quantum cascade laser absorption spectroscopy (QCLAS) method. Enrichment factors εp/s of the cumulative product (p) relative to the substrate (s) were determined using a Rayleigh model for the seven most abundant isotopically substituted molecules (isotopocules) of N2O. Values of the enrichment factors εp/s (with uncertainty expressed as expanded standard uncertainty at the 95% confidence interval) at the two incubation temperatures (20 °C/30 °C) are:14N15N16O (456): ε456 = (−40.3 ± 2.6)‰/(−35.1 ± 0.7)‰15N14N16O (546): ε546 = (−38.1 ± 3.4)‰/(−31.2 ± 0.6)‰14N14N17O (447): ε447 = (21.3 ± 1.2)‰/(24.5 ± 0.5)‰14N14N18O (448): ε448 = (38.8 ± 1.5)‰/(46.4 ± 1.2)‰14N15N18O (458): ε458 = (−8.9 ± 2.0)‰/(−11.7 ± 0.6)‰15N14N18O (548): ε548 = (−3.4 ± 1.1)‰/(−1.8 ± 0.5)‰15N15N16O (556): ε556 = (−85.9 ± 1.5)‰/(−63.9 ± 1.4)‰Temporal evolutions of the abundances of singly substituted N2O isotopocules during nitrate reduction agree with previously published experiments: there is normal isotope effect associated with the production of 14N15N16O and 15N14N16O; i.e., intermediates leading to 14N14N16O react faster than intermediates leading to 14N15N16O and 15N14N16O. However, the production of 14N14N17O and 14N14N18O is associated with inverse isotope effect; i.e., intermediates leading to 14N14N16O react slower than intermediates leading to 14N14N17O and 14N14N18O due to preferential cleavage of 16O during nitrate reduction to N2O. Isotopic fractionation at the incubation temperature of 30 °C was significantly lower compared to 20 °C. We observed a large kinetic isotope effect of the 15N site preference (SP) and the 15N–18O site preference (SP18) at the onset of the reaction. SP18 was found to be closer to 0‰ than SP, which is thought to arise from similar rates of breakage of the 15N–O and 14N–O bonds in the reaction intermediates. The 15N–18O clumped isotope anomalies in two isotopic isomers (isotopomers) 14N15N18O and 15N14N18O (Δ458+548avg) follow a temporal trend similar to those of SP and SP18. The 15N–15N clumped isotope anomalies in 15N15N16O are greater than 0‰ and show no clear temporal trend or influence of incubation temperature, suggesting no strong combinatorial effects involved during the N–N bond formation. Overall, our data illustrate that clumped N2O isotopes may be used as independent tracers for reaction mechanisms of N2O conversion and may establish themselves as a worthwhile tool to study the biogeochemical cycle of N2O.

The Shapes of Sulfonamides: A Rotational Spectroscopy Study.

Benzenesulfonamides are a class of molecules of extreme interest in the biochemical field because many of them are active against a variety of diseases. In this work, the pharmacophoric group benzensulfonamide, its derivatives para-toluensulfonamide and ortho-toluensulfonamide, and the bioactive molecule sulfanilamide, were investigated using rotational spectroscopy to determine their conformations and the influence of different substituents on their structures. For all species, the hyperfine structure due to the 14N atom was analyzed, and this provided crucial information for the unambiguous identification of the observed conformation of all molecules. In addition, for ortho-toluensulfonamide, the vibration–rotation hyperfine structure related to the methyl torsion was analyzed, and the methyl group rotation barrier was determined. For benzensulfonamide, partial rS and r0 structures were established from the experimental rotational constants of the parent and two deuterated isotopic species. In all compounds except ortho-toluensulfonamide, the amino group of the sulfonamide group lies perpendicular to the benzene plane with the aminic hydrogens eclipsing the oxygen atoms. In ortho-toluensulfonamide, where weak attractive interactions occur between the nitrogen lone pair and the methyl hydrogen atoms, the amino group lies in a gauche orientation, retaining the eclipsed configuration with respect to the SO2 frame. A comparison of the geometrical arrangements found in the PDB database allowed us to understand that the bioactive conformations are different from those found in isolated conditions. The conformations within the receptor are reached with an energy cost, which is balanced by the interactions established in the receptor.

Mean Amplitudes of Vibration in Some Isotopic Species of Disulfur Monoxide

Mean Amplitudes of Vibration in Some Isotopic Species of Disulfur Monoxide

Determining Water Transport Kinetics in Limestone by Dual-Wavelength Cavity Ring-Down Spectroscopy.

Water plays a major role in the deterioration of porous building materials such as those widely found in built heritage, influencing many physical, chemical, and biological decay processes. This article details a proof-of-principle study using near-infrared cavity ring-down spectroscopy (CRDS) to monitor the release of water and its artificially enriched isotopologues from small (ca. 25 × 25 × 5 mm) samples of limestone subject to drying by a fixed flow of nitrogen with varying levels of humidity and at room temperature and atmospheric pressure. Under low-humidity conditions, the drying kinetics are consistent with the well-established two-phase drying process exhibited by porous materials, namely, an initial constant drying rate period (phase I) followed by a falling drying rate period (phase II). The water diffusivity during phase II, DII, was measured (for Clipsham limestone) to be 3.0 × 10–9 ± 1 × 10–10 m2 s–1. The CRDS measurements allow spectroscopic determination of the total mass of water released by the sample, and the calculated values are in excellent agreement with gravimetric analysis. Importantly, the selectivity and sensitivity afforded by CRDS allows isotope analysis to be carried out, such that the flux of isotopically labeled water out of the sample can be determined under conditions of humidified flow where there may be a simultaneous ingress of water from the environment. Dual-wavelength CRDS distinguishes isotopic species, and it is demonstrated that the drying kinetics and physical properties of the samples are self-consistent when monitoring both HDO and H2O (for HDO, DII was 3.2 × 10–9 ± 4 × 10–10 m2 s–1). As the humidity levels in the flow increase, a departure from the distinct two-phase behavior is observed in the HDO drying curves. These new measurements of isotopically resolved mass fluxes will help refine models for drying mechanisms in porous media.

In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano.

Photochemical reactions are major pathways for the removal of Hg species from aquatic ecosystems, lowering the concentration of monomethylmercury (MMHg) and its bioaccumulation in foodwebs. Here, we investigated the rates and environmental drivers of MMHg photodegradation and inorganic Hg (IHg) photoreduction in waters of two high-altitude lakes from the Bolivian Altiplano representing meso- to eutrophic conditions. We incubated three contrasting waters in situ at two depths after adding Hg-enriched isotopic species to derive rate constants. We found that transformations mostly occurred in subsurface waters exposed to UV radiation and were mainly modulated by the dissolved organic matter (DOM) level. In parallel, we incubated the same waters after the addition of low concentrations of natural MMHg and followed the stable isotope composition of the remaining Hg species by compound-specific isotope analysis allowing the determination of enrichment factors and mass-independent fractionation (MIF) slopes (Δ199Hg/Δ201Hg) during in situ MMHg photodegradation in natural waters. We found that MIF enrichment factors potentially range from -11 to -19‰ and average -14.3 ± 0.6‰ (1 SE). The MIF slope diverged depending on the DOM level, ranging from 1.24 ± 0.03 to 1.34 ± 0.02 for the low and high DOM waters, respectively, and matched the MMHg MIF slope recorded in fish from the same lake. Our in situ results thus reveal (i) a relatively similar extent of Hg isotopic fractionation during MMHg photodegradation among contrasted natural waters and compared to previous laboratory experiments and (ii) that the MMHg MIF recorded in fish is characteristic for the MMHg bonding environment. They will enable a better assessment of the extent and conditions conducive to MMHg photodegradation in aquatic ecosystems.

Rotational spectroscopy of isotopic oxirane, [formula omitted]-C[formula omitted]H[formula omitted

We studied the rotational spectrum of oxirane in a sample of natural isotopic composition in selected regions between 158 GHz and 1093 GHz. Investigations of the isotopologs with one 13C or one 18O were the primary focus in order to facilitate searches for them in space. We also examined the main isotopic species mainly to look into the performance of Watson’s A and S reductions both in an oblate and in a prolate representation. Even though oxirane is a rather asymmetric oblate rotor, the A reduction in the IIIl representation did not yield a satisfactory fit, as was observed frequently earlier for other molecules. The other three combinations yielded satisfactory fits of similar quality among each other; the A reduction in the Ir representation required two parameters less than both S reduction fits.

Quantum cascade laser absorption spectrometer with a low temperature multipass cell for precision clumped CO2 measurement.

We present a quantum cascade laser-based absorption spectrometer deploying a compact (145 mL volume) segmented circular multipass cell (SC-MPC) with 6 m optical path length. This SC-MPC is embedded into an effective cooling system to facilitate operation at cryogenic temperatures. For CO2, the sample is cooled to 153 K, i.e. close to the sublimation point at 10 mbar. This enables efficient suppression of interfering hot-band transitions of the more abundant isotopic species and thereby enhances analytical precision. As a demonstration, the amount fractions of all three CO2 isotopologues involved in the kinetic isotope exchange reaction of 12C16O2 + 12C18O2⇌ 2·12C16O18O are measured. The precision in the ratios [12C18O2]/[12C16O2] and [12C16O18O]/[12C16O2] is 0.05 ‰ with 25 s integration time. In addition, we determine the variation of the equilibrium constant, K, of the above exchange reaction for carbon-dioxide samples equilibrated at 300 K and 1273 K, respectively.

Multinuclear NMR spectroscopy of ethanol isotopic forms in the liquid and gas phase

Several ethanol isotopic species were dissolved in the buffer gases, Xe and SF6, and carefully investigated by NMR spectroscopy. The 1H, 13C and 17O chemical shifts converted to magnetic shielding constants were obtained by extrapolation of the NMR parameters to the zero-density limit of the gaseous mixtures. These nuclear shielding constants are free from intermolecular interactions and can be directly compared with quantum calculation results. At the same time, the second virial coefficients of 1H, 13C and 17O chemical shielding were obtained from the slopes of density dependences.High resolution NMR spectra were also recorded for neat samples to allow measurements of appropriate gas-to-liquid shifts. They reflect the association processes which take place during condensation from gas to the liquid state. Several spin-spin coupling constants in the ethanol molecule were measured in the gas and liquid phase. The changes to the NMR parameters in different phases and isotopic compositions were given and briefly discussed.

Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran-Formaldehyde Complex.

The 1:1 benzofuran–formaldehyde complex has been chosen as model system for analyzing π→π* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint “rotational spectroscopy–quantum chemistry” strategy unveiled the dominant role of π→π* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low‐energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone‐pair→π interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi‐experimental equilibrium structure for a molecular complex of such a dimension.

From Molecular to Cluster Properties: Rotational Spectroscopy of 2-Aminopyridine and of Its Biomimetic Cluster with Water

We report the observation and analysis of the rotational spectrum of a 1:1 cluster between 2-aminopyridine and water (AMW) carried out with supersonic expansion Fourier transform microwave spectroscopy at 4.7–16.5 GHz. Measurements of the 2-aminopyridine monomer (AMP) were also extended up to 333 GHz for the room-temperature rotational spectrum and to resolved hyperfine splitting resulting from the presence of two 14N quadrupolar nuclei. Supersonic expansion measurements for both AMP and AMW were also carried out for two synthesized isotopic species with single deuteration on the phenyl ring. Nuclear quadrupole hyperfine structure has also been resolved for AMW and the derived splitting constants were used as an aid in structural analysis. The structure of the AMW cluster was determined from the three sets of available rotational constants and the hydrogen bonding configuration is compared with those for clusters with water of similarly sized single-ring molecules. Experimental results aided by quantum chemistry computations allow the conclusion that the water molecule is unusually strongly bound by two hydrogen bonds, OH...N and O...HN, to the NCNH atomic chain of AMP with the potential to replace hydrogen bonds to the identical structural segment in cytosine and adenine in CT and AT nucleic acid base pairs.

Three-dimensional Core-collapse Supernova Simulations with 160 Isotopic Species Evolved to Shock Breakout

Abstract We present three-dimensional simulations of core-collapse supernovae using the FLASH code that follow the progression of the explosion to the stellar surface, starting from neutrino radiation hydrodynamic simulations of the neutrino-driven phase performed with the Chimera code. We consider a 9.6 M ☉ zero-metallicity progenitor starting from both 2D and 3D Chimera models and a 10 M ☉ solar-metallicity progenitor starting from a 2D Chimera model, all simulated until shock breakout in 3D while tracking 160 nuclear species. The relative velocity difference between the supernova shock and the metal-rich Rayleigh–Taylor (R-T) “bullets” determines how the metal-rich ejecta evolves as it propagates through the density profile of the progenitor and dictates the final morphology of the explosion. We find maximum 56Ni velocities of ∼1950 and ∼1750 km s−1 at shock breakout from 2D and 3D 9.6 M ☉ Chimera models, respectively, due to the bullets’ ability to penetrate the He/H shell. When mapping from 2D, we find that the development of higher-velocity structures is suppressed when the 2D Chimera model and 3D FLASH model meshes are aligned. The development of faster-growing spherical-bubble structures, as opposed to the slower-growing toroidal structure imposed by axisymmetry, allows for interaction of the bullets with the shock and seeds further R-T instabilities at the He/H interface. We see similar effects in the 10 M ☉ model, which achieves maximum 56Ni velocities of ∼2500 km s−1 at shock breakout.