Charge equilibration (Qeq) methods can estimate the electrostatic potential of molecules and periodic frameworks by assigning point charges to each atom, using only a small fraction of the resources ...needed to compute density functional (DFT)-derived charges. This makes possible, for example, the computational screening of thousands of microporous structures to assess their performance for the adsorption of polar molecules. Recently, different variants of the original Qeq scheme were proposed to improve the quality of the computed point charges. One focus of this research was to improve the gas adsorption predictions in metal–organic frameworks (MOFs), for which many different structures are available. In this work, we review the evolution of the method from the original Qeq scheme, understanding the role of the different modifications on the final output. We evaluated the result of combining different protocols and set of parameters, by comparing the Qeq charges with high quality DFT-derived DDEC charges for 2338 MOF structures. We focused on the systematic errors that are attributable to specific atom types to quantify the final precision that one can expect from Qeq methods in the context of gas adsorption where the electrostatic potential plays a significant role, namely, CO2 and H2S adsorption. In conclusion, both the type of algorithm and the input parameters have a large impact on the resulting charges, and we draw some guidelines to help the user to choose the proper combination of the two for obtaining a meaningful set of charges. We show that, considering this set of MOFs, the accuracy of the original Qeq scheme is often still comparable with the most recent variants, even if it clearly fails in the presence of certain atom types, such as alkali metals.
Both polyesters and polycarbonates have been proposed as alternatives to polyethers as host materials for future polymer electrolytes for solid-state lithium-ion batteries. While being comparatively ...similar functional groups, the electron density on the coordinating carbonyl oxygen is different, thereby rendering different coordinating strength towards lithium ions. In this study, the transport properties of poly( -caprolactone) and poly(trimethylene carbonate) as well as random copolymers of systematically varied composition of the two have been investigated, in order to better elucidate the role of the coordination strength. The cationic transference number, a property well-connected with the complexing ability of the polymer, was shown to depend almost linearly on the ester content of the copolymer, increasing from 0.49 for the pure poly( -caprolactone) to 0.83 for pure poly(trimethylene carbonate). Contradictory to the transference number measurements that suggest a stronger lithium-to-ester coordination, DFT calculations showed that the carbonyl oxygen in the carbonate coordinates more strongly to the lithium ion than that of the ester. FT-IR measurements showed the coordination number to be higher in the polyester system, resulting in a higher total coordination strength and thereby resolving the paradox. This likely originates in properties that are specific of polymeric solvent systems,
e.g.
steric properties and chain dynamics, which influence the coordination chemistry. These results highlight the complexity in polymeric systems and their ion transport properties in comparison to low-molecular-weight analogues, and how polymer structure and steric effects together affect the coordination strength and transport properties.
A linear dependence of the lithium transference number and the composition of the polyester-polycarbonate copolymer electrolyte host material was discovered, and the coordination properties causing the trend were investigated with DFT calculations.
While solid polymer electrolytes (SPEs) have great potential for use in future lithium-based batteries, they do, however, not display conductivity at a sufficient level as compared to liquid ...electrolytes. To reach the needed requirements of lithium batteries it is therefore necessary to explore new materials classes to serve as novel polymer hosts. In this work, SPEs based on the polyketone poly(3,3-dimethylpentane-2,4-dione) were investigated. Polyketones are structurally similar to several polycarbonate and polyester SPE hosts investigated before but have, due to the lack of additional oxygen atoms in the coordinating motif, even more electron-withdrawing carbonyl groups and could therefore display better properties for coordination to the salt cation. In electrolyte compositions comprising 25−40 wt% LiTFSI salt, it was observed that this polyketone indeed conducts lithium ions with a high cation transference number, but that the ionic conductivity is limited by the semi-crystallinity of the polymer matrix. The crystallinity decreases with increasing salt content, and a fully amorphous SPE can be produced at 40 wt% salt, accompanied by an ionic conductivity of 3 × 10−7 S cm−1 at 32 °C. This opens up for further exploration of polyketone systems for SPE-based batteries.
Metal–organic frameworks (MOFs) are promising materials for the photocatalytic H2 evolution reaction (HER) from water. To find the optimal MOF for a photocatalytic HER, one has to consider many ...different factors. For example, studies have emphasized the importance of light absorption capability, optical band gap, and band alignment. However, most of these studies have been carried out on very different materials. In this work, we present a combined experimental and computation study of the photocatalytic HER performance of a set of isostructural pyrene-based MOFs (M-TBAPy, where M = Sc, Al, Ti, and In). We systematically studied the effects of changing the metal in the node on the different factors that contribute to the HER rate (e.g., optical properties, the band structure, and water adsorption). In addition, for Sc-TBAPy, we also studied the effect of changes in the crystal morphology on the photocatalytic HER rate. We used this understanding to improve the photocatalytic HER efficiency of Sc-TBAPy, to exceed the one reported for Ti-TBAPy, in the presence of a co-catalyst.
Declining ocean health, increasing human demands on marine ecosystems, and a history of management focused on individual activities, species or sectors has led to calls for more comprehensive, ...integrated management that considers entire coupled social-ecological systems. This transition to ecosystem-based management (EBM) for the oceans will certainly face a number of hurdles, and many practitioners struggle with how to move forward with EBM. In this paper, we assess whether the necessary science exists to support EBM. Specifically, we evaluate the state of the social and natural sciences for three research areas that are critical to EBM: (1) ecosystem services, (2) cumulative impacts, and (3) ecosystem variability and change. For each of the three research areas, we describe its importance to EBM and assess existing and emerging information and application of this knowledge, focusing on the US West Coast. We conclude that available science is not the bottleneck for moving forward with comprehensive EBM for this region, although we highlight important remaining knowledge gaps, particularly within the social sciences. Given imperfect and uncertain knowledge, EBM calls for an adaptive management approach, starting with readily available information, and continuously adapting as new information emerges. This synthesis can serve as a basis for comparison for other regions; it provides guidance for organizing information in support of EBM and outlines many novel and broadly applicable scientific approaches.
Clathrate hydrates with the cubic structure II (CS-II) form typically with large guest molecules, such as tetrahydrofuran, trimethylamine oxide, or propane. However, CS-II is also realized for argon ...hydrate despite the comparatively small van der Waals diameter of the guest (around 3.8 Å). Here, the structure of deuterated argon hydrate was studied at ambient pressure in the temperature range 20–95 K using neutron diffraction and comparing natural Ar with 36Ar, which scatters neutrons more than 13 times more efficiently. The procedure allowed to unambiguously establish the positional disorder within the large cages of CS-II, while simultaneously refining host and guest structures. These cages are singly occupied and off-centered argon atoms distribute on two tetrahedron-shaped split positions with a ratio 3:1. Molecular dynamics (MD) simulations revealed that the crystallographic positional disorder structure is due to mobile argon atoms even at 20 K. The MD potential energy distribution confirmed the diffraction model. It is noted that the unit cell volumes of argon hydrate in the investigated temperature range are virtually identical to N2 hydrate, which has a similar composition at ambient pressure, indicating a very similar (slightly attractive) host-guest interaction.
Disorder model for Ar in Ar clathrate hydrate according to isotope substitution neutron diffraction. Display omitted
•Study of the structure and thermal expansion of deuterated argon hydrate at ambient pressure at 20–95 K•Establishment of the positional disorder within the large cages of CS-II argon hydrate.•NatAr and 36Ar isotope substitution in the clathrate hydrate structure.•Molecular dynamics simulation in agreement with the neutron diffraction model.
Oceanic currents and larval accumulation potentially have large impacts on the choice of locations for marine reserves. Larval settlement of benthic invertebrates was greater in the lee than on the ...windward side of a small headland during the height of upwelling in central California during 2001 and 2002. Strong upwelling during the study was indicated by mean seasonal Bakun indices of 149 to 176 m³ s–1per 100 m of coastline in 2001–2002. Weekly sampling of near-surface and near-bottom settlement in the lee of Bodega Head from August 2000 to September 2001 revealed that most larvae of 7 crab taxa settled during spring and summer, which coincides with the upwelling season. Comparison of sites in the protected (lee) and exposed (windward) sides of the headland (2 sites each) during the peak settlement season in 2001 showed that most larvae settled in the lee of the headland, including 91% of crabs, 89% of barnacles, and 80% of mussels in weekly samples. During 2002, weekly sampling at 1 protected and 1 exposed site also demonstrated that most settlement occurred in the lee of the headland, including 74% of crabs, 82% of barnacles, and 65% of mussels. Crabs settled mostly at the surface, whereas barnacles and mussels primarily settled near the bottom, indicating that postlarvae in both surface and bottom waters accumulate in the lee of the headland. Larval accumulation zones should be included in networks of marine reserves to supply adult populations with propagules in recruitment-limited upwelling regions.
The interaction of alongshore coastal currents with large headlands has been shown to increase the retention of planktonic organisms through the formation of headland eddies or upwelling shadows in ...their lee. This study investigates the circulation within Bodega Bay (in the lee of a small headland), in an upwelling region, and the potential for retention of plankton. During the upwelling season of 2004, time series of temperature and velocity were recorded throughout Bodega Bay, conductivity‐temperature‐depth (CTD) surveys were conducted, and surface drifters were released during upwelling, downwelling, and relaxation conditions. Postlarval settlement was monitored daily over two periods coinciding with CTD surveys. Under strong upwelling favorable conditions, wind‐driven surface currents were equatorward both offshore and throughout the bay. However, there was significant current shear along the eastern shore of the bay where cold bottom waters move poleward, counter to the direction of the wind‐driven surface flow. During downwelling and relaxation conditions, flow was poleward throughout the water column along the eastern shore of the bay. Postlarvae settled during all wind conditions, but greatest settlement was observed at the onset of upwelling favorable conditions. While no “typical” upwelling shadow is evident in the lee of the Bodega headland, subsurface recirculation driven by the alongshore flow past Bodega Head may facilitate the retention of plankton in the bay. Previous studies have generally focused on large headlands; however, it is likely that other small embayments in the lee of small headlands may also provide retention opportunities for planktonic organisms in upwelling regions.
Larvae are thought to be highly vulnerable to offshore transport of upwelling regions, limiting recruitment to infrequent wind relaxation and downwelling events. However, larvae could also be ...transported onshore by upwelled bottom waters, onshore wind-forcing or internal tides throughout the water column. We determined the relative importance of these hypothetical mechanisms for the timing of recruitment of 8 invertebrate taxa during the peak upwelling season in a region of strong, persistent upwelling. Recruitment was determined for 5 yr at an open embayment near the surface and bottom of the water column to examine the interaction of behavioral and physical processes regulating larval recruitment. Postlarvae consistently recruited near the surface or near the bottom depending on the species. Onshore delivery of larvae during wind relaxations and reversals did not best explain recruitment patterns in our area for most taxa. Only mussels consistently recruited more during these events, and they recruited in bottom rather than surface waters. Six crab taxa recruited primarily during upwelling. Recruitment of 7 taxa was intermittently correlated with the maximum tidal range, suggesting that internal waves also may deliver larvae onshore. Thus larvae may recruit by multiple processes in upwelling regions rather than being limited to infrequent relaxation events, leaving the mechanisms responsible for observed spatial patterns in larval recruitment and adult densities unexplained. Comprehensive studies of the behavioral and physical processes underlying larval recruitment and post-settlement mortality are needed to explain observed temporal and spatial variation in population dynamics and community structure in upwelling regions.
The high pressure structural behavior of H2 and Ne clathrate hydrates with approximate composition H2/Ne·~4H2O and featuring cubic structure II (CS-II) was investigated by neutron powder diffraction ...using the deuterated analogues at ~95 K. CS-II hydrogen hydrate transforms gradually to isocompositional C1 phase (filled ice II) at around 1.1 GPa but may be metastably retained up to 2.2 GPa. Above 3 GPa a gradual decomposition into C2 phase (H2·H2O, filled ice Ic) and ice VIII’ takes place. Upon heating to 200 K the CS-II to C1 transition completes instantly whereas C1 decomposition appears sluggish also at 200 K. C1 was observed metastably up to 8 GPa. At 95 K C1 and C2 hydrogen hydrate can be retained below 1 GPa and yield ice II and ice Ic, respectively, upon complete release of pressure. In contrast, CS-II neon hydrate undergoes pressure-induced amorphization at 1.9 GPa, thus following the general trend for noble gas clathrate hydrates. Upon heating to 200 K amorphous Ne hydrate crystallizes as a mixture of previously unreported C2 hydrate and ice VIII’.
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