Multi-decadal improvements in the ecological quality of European rivers are not consistently reflected in biodiversity metrics.

Humans impact terrestrial, marine and freshwater ecosystems, yet many broad-scale studies have found no systematic, negative biodiversity changes (for example, decreasing abundance or taxon richness). Here we show that mixed biodiversity responses may arise because community metrics show variable responses to anthropogenic impacts across broad spatial scales. We first quantified temporal trends in anthropogenic impacts for 1,365 riverine invertebrate communities from 23 European countries, based on similarity to least-impacted reference communities. Reference comparisons provide necessary, but often missing, baselines for evaluating whether communities are negatively impacted or have improved (less or more similar, respectively). We then determined whether changing impacts were consistently reflected in metrics of community abundance, taxon richness, evenness and composition. Invertebrate communities improved, that is, became more similar to reference conditions, from 1992 until the 2010s, after which improvements plateaued. Improvements were generally reflected by higher taxon richness, providing evidence that certain community metrics can broadly indicate anthropogenic impacts. However, richness responses were highly variable among sites, and we found no consistent responses in community abundance, evenness or composition. These findings suggest that, without sufficient data and careful metric selection, many common community metrics cannot reliably reflect anthropogenic impacts, helping explain the prevalence of mixed biodiversity trends.

Understanding temporal variability across trophic levels and spatial scales in freshwater ecosystems.

A tenet of ecology is that temporal variability in ecological structure and processes tends to decrease with increasing spatial scales (from locales to regions) and levels of biological organization (from populations to communities). However, patterns in temporal variability across trophic levels and the mechanisms that produce them remain poorly understood. Here we analyzed the abundance time series of spatially structured communities (i.e., metacommunities) spanning basal resources to top predators from 355 freshwater sites across three continents. Specifically, we used a hierarchical partitioning method to disentangle the propagation of temporal variability in abundance across spatial scales and trophic levels. We then used structural equation modeling to determine if the strength and direction of relationships between temporal variability, synchrony, biodiversity, and environmental and spatial settings depended on trophic level and spatial scale. We found that temporal variability in abundance decreased from producers to tertiary consumers but did so mainly at the local scale. Species population synchrony within sites increased with trophic level, whereas synchrony among communities decreased. At the local scale, temporal variability in precipitation and species diversity were associated with population variability (linear partial coefficient, ß = 0.23) and population synchrony (ß = -0.39) similarly across trophic levels, respectively. At the regional scale, community synchrony was not related to climatic or spatial predictors, but the strength of relationships between metacommunity variability and community synchrony decreased systematically from top predators (ß = 0.73) to secondary consumers (ß = 0.54), to primary consumers (ß = 0.30) to producers (ß = 0). Our results suggest that mobile predators may often stabilize metacommunities by buffering variability that originates at the base of food webs. This finding illustrates that the trophic structure of metacommunities, which integrates variation in organismal body size and its correlates, should be considered when investigating ecological stability in natural systems. More broadly, our work advances the notion that temporal stability is an emergent property of ecosystems that may be threatened in complex ways by biodiversity loss and habitat fragmentation.

Exploring the impact of alignment media on RDC analysis of phosphorus-containing compounds: a molecular docking approach.

Residual dipolar couplings (RDCs) are employed in NMR analysis when conventional methods, such as J-couplings and nuclear Overhauser effects (NOEs) fail. Low-energy (optimized) conformers are often used as input structures in RDC analysis programs. However, these low-energy structures do not necessarily resemble conformations found in anisotropic environments due to interactions with the alignment medium, especially if the analyte molecules are flexible. Considering interactions with alignment media in RDC analysis, we developed and evaluated a molecular docking-based approach to generate more accurate conformer ensembles for compounds in the presence of the poly-γ-benzyl-L-glutamate alignment medium. We designed chiral phosphorus-containing compounds that enabled us to utilize 31P NMR parameters for the stereochemical analysis. Using P3D/PALES software to evaluate diastereomer discrimination, we found that our conformer ensembles outperform moderately the standard, low-energy conformers in RDC analysis. To further improve our results, we (i) averaged the experimental values of the molecular docking-based conformers by applying the Boltzmann distribution and (ii) optimized the structures through normal mode relaxation, thereby enhancing the Pearson correlation factor R and even diastereomer discrimination in some cases. Nevertheless, we presume that significant differences between J-couplings in isotropic and in anisotropic environments may preclude RDC measurements for flexible molecules. Therefore, generating conformer ensembles based on molecular docking enhances RDC analysis for mildly flexible systems while flexible molecules may require applying more advanced approaches, in particular approaches including dynamical effects.

Unraveling actinide-actinide bonding in fullerene cages: a DFT versus ab initio methodological study.

Actinide-actinide bonding poses a challenge for both experimental and theoretical chemists because of both the scarcity of experimental data and the exotic nature of actinide bonding due to the involvement and mixing of actinide 7s-, 6p-, 6d-, and particularly 5f-orbitals. Only a few experimental examples of An-An bonding have been reported so far. Here, we perform a methodological study of actinide-actinide bonding on experimentally known Th2@C80 and U2@C80 systems. We compared selected GGA, meta-GGA, hybrid-GGA and range-separated hybrid-GGA functionals with the results obtained using a multireference CASPT2 method, which we consider as a reference point. We show that functionals such as BP86, PBE or TPSS perform well for predicting geometries, while range-separated hybrids are superior in the description of the chemical bonding. None of the tested functionals were deemed reliable regarding the correct electronic spin ground state. Based on the results of this methodological study, we re-evaluate selected previously studied diactinide fullerene systems using more reliable protocol.

Boron-based octahedral dication experimentally detected: DFT surface confirms its availability.

Borane and heteroborane clusters have been known as neutral or anionic species. In contrast to them, several ten-vertex monocationic nido and closo dicarbaborane-based systems have recently emerged from the reaction of the parent bicapped-square antiprismatic dicarbaboranes with N-heterocyclic carbenes followed by the protonization of the corresponding nido intermediates. The expansion of these efforts has afforded the very first closo-dicationic octahedral phosphahexaborane along with new closo-monocationic pnictogenahexaboranes of the same shapes. All are the products of the one-pot procedure that consists in the reaction of the same carbenes with the parent closo-1,2-Pn2B4Br4 (Pn = As, P). Whereas in the case of phosphorus such a monocation appears to be a mixture of stable intermediates, and arsenahexaboranyl monocation has occurred as the final product, all of them without using any subsequent reaction. The well-established DFT/ZORA/NMR approach has unambiguously confirmed the existence of these species in solution, and computed electrostatic potentials have revealed the delocalization of the positive charge in these monocations and in the very first dication, namely within the octahedral shapes in both cases.

European river typologies fail to capture diatom, fish, and macrophyte community composition.

Typology systems are frequently used in applied and fundamental ecology and are relevant for environmental monitoring and conservation. They aggregate ecosystems into discrete types based on biotic and abiotic variables, assuming that ecosystems of the same type are more alike than ecosystems of different types with regard to a specific property of interest. We evaluated whether this assumption is met by the Broad River Types (BRT), a recently proposed European river typology system, that classifies river segments based on abiotic variables, when it is used to group biological communities. We compiled data on the community composition of diatoms, fishes, and aquatic macrophytes throughout Europe and evaluated whether the composition is more similar in site groups with the same river type than in site groups of different river types using analysis of similarities, classification strength, typical species analysis, and the area under zeta diversity decline curves. We compared the performance of the BRT with those of four region-based typology systems, namely, Illies Freshwater Ecoregions, the Biogeographic Regions, the Freshwater Ecoregions of the World, and the Environmental Zones, as well as spatial autocorrelation (SA) classifications. All typology systems received low scores from most evaluation methods, relative to predefined thresholds and the SA classifications. The BRT often scored lowest of all typology systems. Within each typology system, community composition overlapped considerably between site groups defined by the types of the systems. The overlap tended to be the lowest for fishes and between Illies Freshwater Ecoregions. In conclusion, we found that existing broad-scale river typology systems fail to delineate site groups with distinct and compositionally homogeneous communities of diatoms, fishes, and macrophytes. A way to improve the fit between typology systems and biological communities might be to combine segment-based and region-based typology systems to simultaneously account for local environmental variation and historical distribution patterns, thus potentially improving the utility of broad-scale typology systems for freshwater biota.

A quest for ideal electric field-driven MX@C70 endohedral fullerene memristors: which MX fits the best?

Endohedral fullerenes with a dipolar molecule enclosed in the fullerene cage have great potential in molecular electronics, such as diodes, switches, or molecular memristors. Here, we study a series of model systems based on MX@D5h(1)-C70 (M = a metal or hydrogen, X = a halogen or a chalcogen) endohedral fullerenes to identify potential molecular memristors and to derive a general formula for rapid identification of potential memristors among analogous MX@Cn systems. To obtain sufficiently accurate results for switching barriers and encapsulation energies, we perform a benchmark of ten DFT functionals against ab initio SCS-MP2 and DLPNO-CCSD(T) methods at the complete basis set limit. The whole series is then investigated using the PBE0 functional which was found to be the most efficient vs. the ab initio methods. Nine of the 34 MX@C70 molecules studied are predicted to have suitable switching barriers to be considered as potential candidates for molecular switches and memristors. We have identified several structure-property relationships for the switching barrier and response of the systems to the electric field, in particular the dependence of the switching barrier on the available space for M-X switching and faster response of the system to the electric field with a larger dipole moment of MX and MX@C70.

The essential role of symmetry in understanding 3He chemical shifts in endohedral helium fullerenes.

The 3He atom is an excellent NMR probe, particularly when enclosed in endohedral helium fullerenes. The 3He chemical shift, δ(3He), in fullerenes spans a range from ca. -50 to +10 ppm, and changes sensitively between different cages, isomers, and external substituents. Reduction of the fullerenes to anions changes the δ(3He) dramatically and unexpectedly, particularly for the most symmetric and also the most abundant C60 and C70 cages. While the 3He atom is shielded by ∼43 ppm upon charging the He@C60 to He@C606-, it is correspondingly deshielded by ∼37 ppm in the He@C70/He@C706- pair. Here, we show that such puzzling differences in δ(3He) relate to the high symmetry of the host fullerene cages. While similar shielding is induced at the 3He atom by the core orbitals of different cages, the symmetry of the cage allows or quenches large paramagnetic, i.e., deshielding orbital interactions of frontier orbitals upon charging of the cage, which is directly responsible for the large observed chemical shift range of endohedral 3He.

Can Copper(I) and Silver(I) be Hydrogen Bond Acceptors?

Gold(I) centers can form moderately strong (Au⋅⋅⋅H) hydrogen bonds with tertiary ammonium groups, as has been demonstrated in the 3AuCl+ (3+ =1-(tert-butyl)-3-phenyl-4-(2-((dimethylammonio)methyl)phenyl)-1,2,4-triazol-5-ylidene) complex. However, similar hydrogen bonding interactions with isoelectronic silver(I) or copper(I) centers are unknown. Herein, we first explored whether the Au⋅⋅⋅H bond originally observed in 3AuCl+ can be strengthened by replacing Cl with Br or I. Experimental gas-phase IR spectra in the ∼3000 cm-1 region showed only a small effect of the halogen on the Au⋅⋅⋅H bond. Next, we measured the spectra of 3AgCl+ , which exhibited significant differences compared to its 3AuX+ counterparts. The difference has been explained by DFT calculations which indicated that the Ag⋅⋅⋅H interaction is only marginal in this complex, and a Cl⋅⋅⋅H hydrogen bond is formed instead. Calculations predicted the same for the 3CuCl+ complex. However, we noticed that for Ag and Cu complexes with less flexible ligands, such as dimethyl(2-(dimethylammonio)phenyl)phosphine (L7 H+ ), the computations predict the presence of the respective Ag⋅⋅⋅H and Cu⋅⋅⋅H hydrogen bonds, with a strength similar to the Au⋅⋅⋅H bond in 3AuCl+ . We, therefore, propose possible complexes where the presence of (Ag/Cu)⋅⋅⋅H bonds could be experimentally verified to broaden our understanding of these unusual interactions.

Cationic Gold(II) Complexes: Experimental and Theoretical Study.

Gold(II) complexes are rare, and their application to the catalysis of chemical transformations is underexplored. The reason is their easy oxidation or reduction to more stable gold(III) or gold(I) complexes, respectively. We explored the thermodynamics of the formation of [AuII (L)(X)]+ complexes (L=ligand, X=halogen) from the corresponding gold(III) precursors and investigated their stability and spectral properties in the IR and visible range in the gas phase. The results show that the best ancillary ligands L for stabilizing gaseous [AuII (L)(X)]+ complexes are bidentate and tridentate ligands with nitrogen donor atoms. The electronic structure and spectral properties of the investigated gold(II) complexes were correlated with quantum chemical calculations. The results show that the molecular and electronic structure of the gold(II) complexes as well as their spectroscopic properties are very similar to those of analogous stable copper(II) complexes.

On the origin of the inverted singlet-triplet gap of the 5th generation light-emitting molecules.

Excitation energies of the lowest singlet and triplet state of molecules whose first excited singlet state lies energetically below the first triplet state have been studied computationally at (time-dependent) density functional theory, coupled-cluster, and second-order multiconfiguration perturbation theory levels. The calculations at the ab initio levels show that the singlet-triplet gap is inverted as compared to the one expected from Hund's rule, whereas all density functionals yield the triplet state as the lowest excited state. Double excitations responsible for the inverted singlet-triplet gap have been identified. Employing the spin-flip and ΔSCF methods, singlet-triplet inversion was obtained at the density functional theory level for some of the studied molecules.

Evaluating the biological validity of European river typology systems with least disturbed benthic macroinvertebrate communities.

Humans have severely altered freshwater ecosystems globally, causing a loss of biodiversity. Regulatory frameworks, like the Water Framework Directive, have been developed to support actions that halt and reverse this loss. These frameworks use typology systems that summarize freshwater ecosystems into environmentally delineated types. Within types, ecosystems that are minimally impacted by human activities, i.e., in reference conditions, are expected to be similar concerning physical, chemical, and biological characteristics. This assumption is critical when water quality assessments rely on comparisons to type-specific reference conditions. Lyche Solheim et al. (2019) developed a pan-European river typology system, the Broad River Types, that unifies the national Water Framework Directive typology systems and is gaining traction within the research community. However, it is unknown how similar biological communities are within these individual Broad River Types. We used analysis of similarities and classification strength analysis to examine if the Broad River Types delineate distinct macroinvertebrate communities across Europe and whether they outperform two ecoregional approaches: the European Biogeographical Regions and Illies' Freshwater Ecoregions. We determined indicator and typical taxa for the types of all three typology systems and evaluated their distinctiveness. All three typology systems captured more variation in macroinvertebrate communities than random combinations of sites. The results were similar among typology systems, but the Broad River Types always performed worse than either the Biogeographic Regions or Illies' Freshwater Ecoregions. Despite reaching statistical significance, the statistics of analysis of similarity and classification strength were low in all tests indicating substantial overlap among the macroinvertebrate communities of different types. We conclude that the Broad River Types do not represent an improvement upon existing freshwater typologies when used to delineate macroinvertebrate communities and we propose future avenues for advancement: regionally constrained types, better recognition of intermittent rivers, and consideration of biotic communities.

Comprehensive Theoretical View of the [Cu2 O2 ] Side-on-Peroxo-/Bis-µ-Oxo Equilibria.

Coupled binuclear copper (CBC) sites are employed by many metalloenzymes to catalyze a broad set of biochemical transformations. Typically, the CBC catalytic sites are activated by the O2 molecule to form various [Cu2 O2 ] reactive species. This has also inspired synthesis and development of various biomimetic inorganic complexes featuring the CBC core. From theoretical perspective, the [Cu2 O2 ] reactivity often hinges on the side-on-peroxo-dicopper(II) (P) vs. bis-µ-oxo-dicopper(III) (O) isomerism - an equilibrium that has become almost iconic in theoretical bioinorganic chemistry. Herein, we present a comprehensive calibration and evaluation of the performance of various composite computational protocols available in contemporary computational chemistry, involving coupled-cluster and multireference (relativistic) wave function methods, popular density functionals and solvation models. Starting with the well-studied reference [Cu2 O2 (NH3 )6 ]2+ system, we compared the performance of electronic structure methods and discussed the relativistic effects. This allowed us to select several 'calibrated' DFT functionals that can be conveniently employed to study ten experimentally well-characterized [Cu2 O2 ] inorganic systems. We mostly predicted the lowest-energy structures (P vs. O) of the studied systems correctly. In addition, we present calibration of the used electronic structure methods for prediction of the spectroscopic features of the [Cu2 O2 ] core, mostly provided by the resonance Raman (rR) spectroscopy.

Phosphate linkers with traceable cyclic intermediates for self-immolation detection and monitoring.

Self-immolation (SI) is the key principle of ProTide nucleotide prodrugs such as remdesivir, which is currently used to treat COVID-19 patients. Developing novel tailor-made SI systems requires new analytical methods for the detection and monitoring of SI. We developed a robust method for SI analysis using novel phosphate-based SI linkers with NMR traceable cyclic intermediates to distinguish SI from alternative fragmentation pathways and to monitor cargo release in real time.

Al(III)-NTA-fluoride: a simple model system for Al-F binding with interesting thermodynamics.

Al(iii) complexes are extensively studied as [18F]fluoride carriers in positron emission tomography. However, our limited knowledge on their thermodynamic and kinetic properties has hindered efforts to easily prepare radiochemically pure compounds while simultaneously reducing the overall labeling time. Thus, to improve our understanding of fluoride binding to coordinatively unsaturated Al(iii) complexes, we investigated the ternary system Al(iii)-H3NTA-F- (H3NTA = nitrilo-triacetic acid) by NMR, potentiometry and X-ray diffraction. Our results show that the [Al(NTA)] complex binds two water molecules, which are replaced by fluorides. Individual species and isomers show separate 19F NMR signals and different stability constants. The two available positions on the [Al(NTA)] complex feature significantly different properties in terms of basicity of the coordinated water molecules and preferential binding of fluoride anions. Fluorides are effectively bound in weakly acidic or neutral solutions, whereas hydroxido species are preferentially formed in alkaline solutions. Our experimental observations were rationalized by theoretical calculations: predictions of the energy ordering of complexes and isomers, interpretation of 19F NMR chemical shifts, and natural bonding orbital analysis. Radiolabeling of [Al(NTA)] with [18F]fluoride gave low yields that confirmed a high affinity of the complex for hydroxide anions.

From π Bonds without σ Bonds to the Longest Metal-Metal Bond Ever: A Survey on Actinide-Actinide Bonding in Fullerenes.

Actinide-actinide bonds are rare. Only a few experimental systems with An-An bonds have been described so far. Recent experimental characterization of the U2@Ih(7)-C80 (J. Am. Chem. Soc. 2018, 140, 3907) system with one-electron two-center (OETC) U-U bonds as was predicted by some of us (Phys. Chem. Chem. Phys. 2015, 17, 24182) encourages the search for more examples of actinide-actinide bonding in fullerene cages. Here, we investigate actinide-actinide bonding in An2@D5h(1)-C70, An2@Ih(7)-C80, and An2@D5h(1)-C90 (An = Ac-Cm) endohedral metallofullerenes (EMFs). Using different methods of the chemical bonding analysis, we show that most of the studied An2@C70 and An2@C80 systems feature one or more one-electron two-center actinide-actinide bonds. Unique bonding patterns are revealed in plutonium EMFs. The Pu2@Ih(7)-C80 features two OETC Pu-Pu π bonds without any evidence of a corresponding σ bond. In the Pu2@D5h(1)-C90 with rPu-Pu = 5.9 Å, theory predicts the longest metal-metal bond ever described. Predicted systems are thermodynamically stable and should be, in principle, experimentally accessible, though radioactivity of studied metals may be a serious obstacle.

Experimental and Theoretical Evidence of Spin-Orbit Heavy Atom on the Light Atom 1 H†NMR Chemical Shifts Induced through H⋠⋠⋠I- Hydrogen Bond.

Invited for the cover of this issue is the group of Michal Straka and Martin Dracínský (IOCB Prague, Czech Academy of Sciences). The image depicts a neutron star, which is used to represent the relativistic effects between a heavy element and a hydrogen atom reported in this work. Read the full text of the article at 10.1002/chem.202001532.

Relativistic Heavy-Neighbor-Atom Effects on NMR Shifts: Concepts and Trends Across the Periodic Table.

Chemical shifts present crucial information about an NMR spectrum. They show the influence of the chemical environment on the nuclei being probed. Relativistic effects caused by the presence of an atom of a heavy element in a compound can appreciably, even drastically, alter the NMR shifts of the nearby nuclei. A fundamental understanding of such relativistic effects on NMR shifts is important in many branches of chemical and physical science. This review provides a comprehensive overview of the tools, concepts, and periodic trends pertaining to the shielding effects by a neighboring heavy atom in diamagnetic systems, with particular emphasis on the "spin-orbit heavy-atom effect on the light-atom" NMR shift (SO-HALA effect). The analyses and tools described in this review provide guidelines to help NMR spectroscopists and computational chemists estimate the ranges of the NMR shifts for an unknown compound, identify intermediates in catalytic and other processes, analyze conformational aspects and intermolecular interactions, and predict trends in series of compounds throughout the Periodic Table. The present review provides a current snapshot of this important subfield of NMR spectroscopy and a basis and framework for including future findings in the field.

Experimental and Theoretical Evidence of Spin-Orbit Heavy Atom on the Light Atom 1 H†NMR Chemical Shifts Induced through H⋠⋠⋠I- Hydrogen Bond.

Spin-orbit (SO) heavy-atom on the light-atom (SO-HALA) effect is the largest relativistic effect caused by a heavy atom on its light-atom neighbors, leading, for example, to unexpected NMR chemical shifts of 1 H, 13 C, and 15 N nuclei. In this study, a combined experimental and theoretical evidence for the SO-HALA effect transmitted through hydrogen bond is presented. Solid-state NMR data for a series of 4-dimethylaminopyridine salts containing I- , Br- and Cl- counter ions were obtained experimentally and by theoretical calculations. A comparison of the experimental chemical shifts with those calculated by a standard DFT methodology without the SO contribution to the chemical shifts revealed a remarkable error of the calculated proton chemical shift of a hydrogen atom that is in close contact with the iodide anion. The addition of the relativistic SO correction in the calculations significantly improves overall agreement with the experiment and confirms the propagation of the SO-HALA effect through hydrogen bonds.