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Organic trisradicals featuring threefold symmetry have attracted significant interest because of their unique magnetic properties associated with spin frustration. Herein, we describe the synthesis and characterization of a triangular prismshaped organic cage for which we have coined the name PrismCage6+ and its trisradical trication—TR3(•+). PrismCage6+ is composed of three 4,4'bipyridinium dications and two 1,3,5phenylene units bridged by six methylene groups. In the solid state, PrismCage6+ adopts a highly twisted conformation with close to C3 symmetry as a result of encapsulating one PF6− anion as a guest. PrismCage6+ undergoes stepwise reduction to its mono, di and trisradical cations in MeCN on account of strong electronic communication between its 4,4'bipyridinium units. TR3(•+), which is obtained by reduction of PrismCage6+ employing CoCp2, adopts a triangular prismshaped conformation with close to C2v symmetry in the solid state. Temperaturedependent continuouswave and nutation frequencyselective EPR spectra of TR3(•+) in frozen N,Ndimethylformamide indicate its doublet ground state. The doubletquartet energy gap of TR3(•+) is estimated to be −0.06 kcal mol−1 and the critical temperature of spinstate conversion is found to be ca. 50 K, suggesting that it displays pronounced spinfrustration at the molecular level. To the best of our knowledge, this example is the first organic radical cage to exhibit spin frustration. The trisradical trication of PrismCage6+ opens up new possibilities for fundamental investigations and potential applications in the fields of both organic cages and spin chemistry.more » « lessFree, publiclyaccessible full text available June 1, 2024

The manifold scattering transform is a deep feature extractor for data defined on a Riemannian manifold. It is one of the first examples of extending convolutional neural networklike operators to general manifolds. The initial work on this model focused primarily on its theoretical stability and invariance properties but did not provide methods for its numerical implementation except in the case of twodimensional surfaces with predefined meshes. In this work, we present practical schemes, based on the theory of diffusion maps, for implementing the manifold scattering transform to datasets arising in naturalistic systems, such as single cell genetics, where the data is a highdimensional point cloud modeled as lying on a lowdimensional manifold. We show that our methods are effective for signal classification and manifold classification tasks.more » « less

Few techniques are available for elucidating the nature of forces that drive subcellular behaviors. Here we develop two complementary ones: 1) femtosecond stereotactic laser ablation (FESLA), which rapidly creates complex cuts of subcellular structures, thereby allowing precise dissection of when, where, and in what direction forces are generated; and 2) assessment of subcellular fluid flows, by comparing direct flow measurements, using microinjected fluorescent nanodiamonds, to largescale fluidstructure simulations of different models of force transduction. We apply these to study centrosomes in Caenorhabditis elegans early embryos, and use the data to construct a biophysicallybased model of centrosome dynamics. Taken together, we demonstrate that cortical pulling forces provide a general explanation for many behaviors mediated by centrosomes, including pronuclear migration/centration and rotation, metaphase spindle positioning, asymmetric spindle elongation and spindle oscillations. In sum, this work establishes new methodologies for disentangling the forces responsible for cell biological phenomena.more » « less

Abstract We select 48 multiflare gammaray bursts (GRBs) (including 137 flares) from the Swift/XRT database and estimate the spectral lag with the discrete correlation function. It is found that 89.8% of the flares have positive lags and only 9.5% of the flares show negative lags when fluctuations are taken into account. The median lag of the multiflares (2.75 s) is much greater than that of GRB pulses (0.18 s), which can be explained by the fact that we confirm that multiflare GRBs and multipulse GRBs have similar positive lag–duration correlations. We investigate the origin of the lags by checking the E peak evolution with the two brightest bursts and find the leading models cannot explain all of the multiflare lags and there may be other physical mechanisms. All of the results above reveal that Xray flares have the same properties as GRB pulses, which further supports the observation that Xray flares and GRB promptemission pulses have the same physical origin.more » « less

Meila, M. ; Zhang, T. (Ed.)In this paper, we propose conjugate energybased models (CEBMs), a new class of energybased models that define a joint density over data and latent variables. The joint density of a CEBM decomposes into an intractable distribution over data and a tractable posterior over latent variables. CEBMs have similar use cases as variational autoencoders, in the sense that they learn an unsupervised mapping from data to latent variables. However, these models omit a generator network, which allows them to learn more flexible notions of similarity between data points. Our experiments demonstrate that conjugate EBMs achieve competitive results in terms of image modelling, predictive power of latent space, and outofdomain detection on a variety of datasets.more » « less

III, H.D. ; Singh, A. (Ed.)We develop amortized population Gibbs (APG) samplers, a class of scalable methods that frame structured variational inference as adaptive importance sampling. APG samplers construct highdimensional proposals by iterating over updates to lowerdimensional blocks of variables. We train each conditional proposal by minimizing the inclusive KL divergence with respect to the conditional posterior. To appropriately account for the size of the input data, we develop a new parameterization in terms of neural sufficient statistics. Experiments show that APG samplers can be used to train highlystructured deep generative models in an unsupervised manner, and achieve substantial improvements in inference accuracy relative to standard autoencoding variational methods.more » « less

Abstract The present study uses eddy‐resolving numerical simulations to investigate how bed roughness affects flow and turbulence structure around an isolated, partially‐buried mussel (
Unio elongatulus ) aligned with the incoming flow. The rough‐bed simulations resolve the flow past the exposed part of a gravel bed, whose surface is obtained from a laboratory experiment that also provides some additional data for validation of the numerical model. Results are also discussed for the limiting case of a horizontal smooth bed. Additionally, the effects of varying the level of burial of the mussel inside the substrate and the discharge through the two mussel siphons are investigated via a set of simulations in which the ratio between the median diameter of the (gravel) particles forming the rough bed,d _{50}, and the height of the exposed part of the mussel,h , varies between 0.10 and 0.22. The increase of the bed roughness is associated with a strong amplification of the turbulence kinetic energy in the near‐wake region. Increasing the bed roughness and/or reducingh intensifies the interactions of the eddies generated by the bed particles with the base and tip vortices induced by the active filtering and by the mussel shell, respectively, which, in turn, induces a more rapid dissipation of these vortices. Increasing the bed roughness also reduces the strength of the main downwelling flow region forming in the wake. The strong downwelling near the symmetry plane is the main reason why the symmetric wake shedding mode dominates in the smooth bed simulations with negligible active filtering. By contrast, the anti‐symmetric wake shedding mode dominates in the simulations conduced with a high value of the bed roughness. The mean streamwise drag force coefficient for the emerged part of the shell and the dilution of the excurrent siphon jet increase with increasing bed roughness. 
Wim Uijttewaal, Mário J. (Ed.)Freshwater mussels are bivalve mollusks that inhabit the substrates of rivers. Fully threedimensional large eddy simulations are used to investigate flow, turbulence and the capacity of the flow to dislocate an isolated, partiallyburied, isolated freshwater mussel placed in a fullydeveloped incoming turbulent open channel flow. The mussel is aligned with the flow direction, which corresponds to normal conditions in rivers containing mussel beds. Its submergence depth is about 60% of the mussel height. The paper focuses on quantifying the effect of the active filtering flow through the incurring and excurring siphons. Simulation results are discussed for two limiting cases with no active filtering and with a filtering flow discharge that is close to the maximum value recorded for the investigated freshwater mussel species. It is shown that the active filtering increases the turbulent kinetic energy in the wake and slightly decreases the mean streamwise drag acting on the mussel shell. The paper also discusses the main types of largescale coherent structures generated by partiallyburrowed mussels aligned with the flow, how they are affected by the filtered flow and the effects of these eddies on the bed shear stress, sediment entrainment/deposition phenomena and nutrient transportmore » « less