The Advantages of Flexibility: The Role of Entropy in Crystal Structures Containing C-H···F Interactions.

Molecular crystal structures are often interpreted in terms of strong, structure directing, intermolecular interactions, especially those with distinct geometric signatures such as H-bonds or π-stacking interactions. Other interactions can be overlooked, perhaps because they are weak or lack a characteristic geometry. We show that although the cumulative effect of weak interactions is significant, their deformability also leads to occupation of low energy vibrational energy levels, which provides an additional stabilizing entropic contribution. The entropies of five fluorobenzene derivatives have been calculated by periodic DFT calculations to assess the entropic influence of C-H···F interactions in stabilizing their crystal structures. Calculations reproduce inelastic neutron scattering data and experimental entropies from heat capacity measurements. C-H···F contacts are shown to have force constants which are around half of those of more familiar interactions such as hydrogen bonds, halogen bonds, and C-H···π interactions. This feature, in combination with the relatively high mass of F, means that the lowest energy vibrations in crystalline fluorobenzenes are dominated by C-H···F contributions. C-H···F contacts occur much more frequently than would be expected from their enthalpic contributions alone, but at 150 K, the stabilizing contribution of entropy provides, at -10 to -15 kJ mol-1, a similar level of stabilization to the N-H···N hydrogen bond in ammonia and O-H···O hydrogen bond in water.

Cation Dynamics as Structure Explorer in Hybrid Perovskites-The Case of MAPbI3.

Hybrid organic-inorganic perovskites exhibit remarkable potential as cost-effective and high-efficiency materials for photovoltaic applications. Their exceptional chemical tunability opens further routes for optimizing their optical and electronic properties through structural engineering. Nevertheless, the extraordinary softness of the lattice, stemming from its interconnected organic-inorganic composition, unveils formidable challenges in structural characterization. Here, by focusing on the quintessential methylammonium lead triiodide, MAPbI3, we combine first-principles modeling with high-resolution neutron scattering data to identify the key stationary points on its shallow potential energy landscape. This combined experimental and computational approach enables us to benchmark the performance of a collection of semilocal exchange-correlation functionals and to track the local distortions of the perovskite framework, hallmarked by the inelastic neutron scattering response of the organic cation. By conducting a thorough examination of structural distortions, we introduce the IKUR-PVP-1 structural data set. This data set contains nine mechanically stable structural models, each manifesting a distinct vibrational response. IKUR-PVP-1 constitutes a valuable resource for assessing thermal behavior in the low-temperature perovskite phase. In addition, it paves the way for the development of accurate force fields, enabling a comprehensive understanding of the interplay between the structure and dynamics in MAPbI3 and related hybrid perovskites.

A combined inelastic neutron scattering and simulation study of the 3He@C60 endofullerene.

The 3He@C60 endofullerene consists of a single 3He atom entrapped inside a C60 fullerene cage. The confining potential, arising from the non-covalent interaction between the enclosed He atom and the C atoms of the cage, is investigated by inelastic neutron scattering. These measurements allow us to obtain information in both energy (ω) and momentum (Q) transfers in the form of the dynamical structure factor S (Q, ω). Simulations of the S (Q, ω) maps are performed for a spherical anharmonic oscillator model. Good agreement between the experimental and simulated data sets is achieved.

Experimental and Modeling Studies of Local and Nanoscale para-Cresol Behavior: A Comparison of Classical Force Fields.

The dynamics of bulk liquid para-cresol from 340-390 K was probed using a tandem quasielastic neutron scattering (QENS) and molecular dynamics (MD) approach, due to its relevance as a simple model component of lignin pyrolysis oil. QENS experiments observed both translational jump diffusion and isotropic rotation, with diffusion coefficients ranging from 10.1 to 28.6 × 10-10 m2s-1 and rotational rates from 5.7 to 9.2 × 1010 s-1. The associated activation energies were 22.7 ± 0.6 and 10.1 ± 1.2 kJmol-1 for the two different dynamics. MD simulations applying two different force field models (OPLS3 and OPLS2005) gave values close to the experimental diffusion coefficients and rotational rates obtained upon calculating the incoherent dynamic structure factor from the simulations over the same time scale probed by the QENS spectrometer. The simulations gave resulting jump diffusion coefficients that were slower by factors of 2.0 and 3.8 and rates of rotation that were slower by factors of 1.2 and 1.6. Comparing the two force field sets, the OPLS3 model showed slower rates of dynamics likely due to a higher molecular polarity, leading to greater quantities and strengths of hydrogen bonding.

Using acoustic telemetry to quantify potential contaminant exposure of Vermilion Rockfish (Sebastes miniatus), Hornyhead Turbot (Pleuronichthys verticalis), and White Croaker (Genyonemus lineatus) at wastewater outfalls in southern California.

Contaminant Exposure Models (CEMs) were developed to predict population-level tissue contaminant concentrations in fishes by pairing sediment-bound contaminant concentrations (DDTs, PCBs) and fine-scale acoustic telemetry data from a habitat-associated species (Vermilion Rockfish, Sebastes miniatus), nomadic flatfish species (Hornyhead Turbot, Pleuronichthys verticalis), and nomadic benthic/midwater schooling species (White Croaker, Genyonemus lineatus) tagged near wastewater outfalls in southern California. Model results were compared to contaminant concentrations in tissue samples. The CEMs developed require further refinement before implementation into management efforts but may act as steppingstones to help shift primary monitoring methods away from the regular field collection of fish for tissue contaminant analyses and towards behavioral modeling and habitat mapping. We also developed Kernel Density Estimates that can be used by managers immediately to identify regions that contribute most to contaminant exposure in species of concern. Prioritizing remediation efforts in these areas are likely to be most effective at improving fish health.

Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites.

The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites (FAPbI3) has been investigated by high-resolution neutron spectroscopy. For the first time, we provide an exhaustive and accurate analysis of the cation vibrations and underlying local structure around the organic moiety in these materials using first-principles electronic-structure calculations validated by the neutron data. Inelastic neutron scattering experiments on FAPbI3 provide direct evidence of the formation of a low-temperature orientational glass, unveiling the physicochemical origin of phase metastability in the tetragonal structure. Further analysis of these data provides a suitable starting point to explore and understand the stabilization of the perovskite framework via doping with small amounts of organic cations. In particular, we find that the hydrogen-bonding interactions around the formamidinium cations are strengthened as a result of cage deformation. This synergistic effect across perovskite cages is accompanied by a concomitant weakening of the methylammonium interactions with the surrounding framework.

Vibrational Motions Make Significant Contributions to Sequential Methyl C-H Activations in an Organometallic Complex.

[(Pentamethylcyclopentadienyl)Rh(III)(bipyridine)(chloride)]+ (Cp*Rh-Cl) undergoes sequential deuteriation of its 15 Cp* CH groups in polar deuterated solvents. Vibrational spectra of H14-Cp*Rh-Cl and D14-Cp*Rh-Cl were captured via inelastic neutron spectroscopy (INS) and assigned using density functional theory (DFT) phonon calculations. These calculations were precisely weighted to the spectrometer's neutronic response. The Cp* ring behaves as a moving carousel, bringing each CH3 close to the Rh-OH/D center where proton abstraction occurs. Vibrations relevant for carousel movement and proximal positioning for H transfer were identified. DFT modeling uncovered changes in vibrations along the reaction path, involving a Rh(I)-fulvene intermediate. Vibronic energy contributions are large across the entire transition. Remarkably, they amount to over a 400-fold increase in the proton transfer rate. The inclusion of vibrational degrees of freedom could be applied more widely to catalysts and molecular machines to harness the energetics of these vibrations and increase their effective rates of operation.

Ecological effects of wastewater treatment modifications on infauna communities off the coast of Orange County, California.

The Orange County Sanitation District (OCSD) is a public agency that provides treatment of wastewater for residents in Orange County, California. The final effluent characteristics at OCSD have been altered in the last 18 years due to the ~20-fold increase of sodium hypochlorite usage for disinfecting waste streams from 2002 to 2010, implementation of the Ground Water Replenishment System (GWRS) in 2008, and conversion to full secondary treatment, together with a major reduction of sodium hypochlorite usage in 2011. This study analyzed infauna data gathered from 1994 to 2016 at the zone of initial dilution (ZID) and reference stations located on the San Pedro Shelf to assess the effects of these treatment process changes on biota in the receiving water. Impacts to the infauna community within the ZID were observed during the period of high chlorine usage, including the increased proportion of the pollution tolerant polychaete Capitella capitata complex from 6.2% in 2003 to nearly 60% in 2010, the lowest Infaunal Trophic Index score in 2009, and the highest Benthic Response Index score in 2006. The degradation of the infauna community in the ZID was correlated with chlorination and was coincidental with the initiation of the GWRS. The infauna community at the ZID recovered rapidly after the implementation of full secondary treatment as indicated by the significant reduction of the relative abundance of C. capitata complex from 59.8% in 2010 to less than 0.1% after 2012, and by the markedly improved community health index scores. This study demonstrated the composition and biointegrity of the infauna community at the ZID varied in response to changes in the wastewater treatment process. Caution should be exercised at wastewater treatment plants when relatively higher dosages of sodium hypochlorite are used over a multi-year period, as this may negatively impact aquatic biota in the receiving water.

Differentiating the role of organic additives to assemble open framework aluminosilicates using INS spectroscopy.

Presently, there is little clarity concerning how organic additives control structure formation in the synthesis of zeolite catalysts. Such ambiguity is a major obstacle towards synthesis design of new bespoke zeolites with intended applications. Herein, we have applied inelastic neutron scattering (INS) spectroscopy to experimentally probe the nature of organic-framework interactions, which are crucial in understanding structure direction. With this technique we have studied the dynamics of 18-crown-6 ether, which can be used as an additive to direct the formation of four zeolites: Na-X, EMC-2, RHO and ZK-5. We observed significant softening of the 18-crown-6 ether molecule's dynamics upon occlusion within a zeolite host, with a strong influence on both the circular and radial vibrational modes. Furthermore, there is a strong correlation between the size/geometry of the zeolite framework cages and perturbations in the dynamics of the 18C6 oxyethylene chain. We propose that the approach used herein can be used to study other zeolites, and hence gain a more comprehensive view of organic-framework interactions.

Deep-Glassy Ice VI Revealed with a Combination of Neutron Spectroscopy and Diffraction.

The recent discovery of a low-temperature endotherm upon heating hydrochloric-acid-doped ice VI has sparked a vivid controversy. The two competing explanations aiming to explain its origin range from a new distinct crystalline phase of ice to deep-glassy states of the well-known ice VI. Problems with the slow kinetics of deuterated phases have been raised, which we circumvent here entirely by simultaneously measuring the inelastic neutron spectra and neutron diffraction data of H2O samples. These measurements support the deep-glassy ice VI scenario and rule out alternative explanations. Additionally, we show that the crystallographic model of D2O ice XV, the ordered counterpart of ice VI, also applies to the corresponding H2O phase. The discovery of deep-glassy ice VI now provides a fascinating new example of ultrastable glasses that are encountered across a wide range of other materials.

Affecting an Ultra-High Work Function of Silver.

An ultra-high increase in the WF of silver, from 4.26 to 7.42 eV, that is, an increase of up to circa 3.1 eV is reported. This is the highest WF increase on record for metals and is supported by recent computational studies which predict the potential ability to affect an increase of the WF of metals by more than 4 eV. We achieved the ultra-high increase by a new approach: Rather than using the common method of 2D adsorption of polar molecules layers on the metal surface, WF modifying components, l-cysteine and Zn(OH)2 , were incorporated within the metal, resulting in a 3D architecture. Detailed material characterization by a large array of analytical methods was carried out, the combination of which points to a WF enhancement mechanism which is based on directly affecting the charge transfer ability of the metal separately by cysteine and hydrolyzed zinc(II), and synergistically by the combination of the two through the known Zn-cysteine finger redox trap effect.

Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors.

Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron-phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high-mobility molecular semiconductors, state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electron-phonon coupling constants are combined with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way, the long-axis sliding motion is identified as a "killer" phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high-mobility molecular semiconductors is suggested.

Examining the role of estrogenic activity and ocean temperature on declines of a coastal demersal flatfish population near the municipal wastewater outfall of Orange County, California, USA.

Wastewater treatment plant effluent introduces a mixture of pollutants into marine environments; however, the impacts of chronic sublethal exposures on populations are often unclear. Presence of estrogenic agents in sediments and uptake of these compounds by demersal flatfishes has been reported at the Orange County Sanitation District (OCSD) wastewater outfall. Furthermore, estrogenic activity has been identified in male flatfish in the area, potentially contributing to observed population declines in the OCSD region. Rising ocean temperatures may further contribute to flatfish declines as relationships between temperature and abundance have been reported in the Southern California Bight. To investigate declines, sex ratios, condition factor, organ health indices, hormones, and vitellogenin were quantified in flatfish collected at OCSD outfall and reference sites. Additionally, historical temperature data was examined for trends with population abundances. Rather than being linked to estrogenic activity, results indicated that population declines were more correlated to increases in ocean temperature.

New Insights into Hydride Bonding, Dynamics, and Migration in La2LiHO3 Oxyhydride.

Hydride anion-conducting oxyhydrides have recently emerged as a brand new class of ionic conductors. Here we shed a first light onto their local vibrations, bonding mechanisms, and anion migration properties using the powerful combination of high-resolution inelastic neutron scattering and a set of rigorously experimentally validated density functional theory calculations. By means of charge-density analysis we establish the bonding to be strongly anisotropic; ionic in the perovskite layer and covalent in the rock salt layer. Climbing nudged elastic band calculations allow us to predict the hydride migration paths, which crucially we are able to link to the observed exotic ionic-covalent hybrid bonding nature. In particular, hydride migration in the rock salt layer is seen to be greatly hindered by the presence of covalent bonding, forcing in-plane hydride migration in the perovskite layer to be the dominant transport mechanism. On the basis of this microscopic insight into the transport and bonding, we are able to propose future candidates for materials that are likely to show enhanced hydride conductivity.

Thermal and Structural Aspects of the Hydride-Conducting Oxyhydride La2LiHO3 Obtained via a Halide Flux Method.

Oxyhydrides, in which oxide and hydride anions share the same anionic lattice, are relatively rare compounds. La2LiHO3 belongs to this family. We report the synthesis of La2LiHO3 by means of an alkali halide flux method, which allows the production of larger quantities of material relative to the usually adopted synthesis routes. Powder X-ray and neutron diffraction studies show that La2LiHO3 adopts an n = 1 Ruddlesden-Popper (RP)-type structure with an orthorhombic distortion (Immm) due to hydride and oxide anion ordering. No sign of polymorphism is observed. La2LiHO3 is seen to decompose in an oxygen atmosphere at ∼450 °C into La2LiO3.5. We show that the high mobility of hydride anions close to the decomposition temperature is likely the main factor in inducing the oxidation. The crystal structure of La2LiO3.5 is also determined and takes an n = 1 RP-type structure with an orthorhombic distortion (Fmmm). This newly reported large-scale synthesis approach, combined with the proven high thermal stability, is a key factor for potential practical applications of this oxyhydride in real devices.

Thermal transport across nanoparticle-fluid interfaces: the interplay of interfacial curvature and nanoparticle-fluid interactions.

We investigate the general dependence of the thermal transport across nanoparticle-fluid interfaces using molecular dynamics computations. We show that the thermal conductance depends strongly both on the wetting characteristics of the nanoparticle-fluid interface and on the nanoparticle size. Strong nanoparticle-fluid interactions, leading to full wetting states in the host fluid, result in high thermal conductances and efficient interfacial transport of heat. Weak interactions result in partial drying or full drying states, and low thermal conductances. The variation of the thermal conductance with particle size is found to depend on the fluid-nanoparticle interactions. Strong interactions coupled with large interfacial curvatures lead to optimum interfacial heat transport. This complex dependence can be modelled using an equation that includes the interfacial curvature as a parameter. In this way, we rationalise the existing experimental and computer simulation results and show that the thermal transport across nanoscale interfaces is determined by the correlations of both interfacial curvature and nanoparticle-fluid interactions.

The rich phase behavior of the thermopolarization of water: from a reversal in the polarization, to enhancement near criticality conditions.

We investigate using non-equilibrium molecular dynamics simulations the polarization of water induced by thermal gradients using the accurate TIP4P/2005 water model. The full dependence of the polarization covering a wide range of thermodynamic states, from near supercritical to ambient conditions, is reported. Our results show a strong dependence of the thermo-polarization field with the thermodynamic state. The field features a strong enhancement near the critical point, which can be rationalized in terms of the large increase and ultimately the divergence of the thermal expansion of the fluid at the critical temperature. We also show that the TIP4P/2005 model features a reversal in the sign of the thermal polarization at densities ∼1 g cm(-3). The latter result is consistent with the recent observation of this reversal phenomenon in SPC/E water and points the existence of this general physical phenomenon in water.

Heads or tails: how do chemically substituted fullerenes melt?

We address the question as to whether the melting of chemically substituted fullerenes is driven by the dynamics of the fullerene moiety (the head) or the substituted sub-unit (the tail). To this end, we have performed quasielastic neutron-scattering experiments and classical molecular-dynamics simulations as a function of temperature on the prototypical fullerene derivative phenyl-C61-butyric acid methyl ester. To enable a direct and quantitative comparison between experimental and simulation data, dynamic structure factors for the latter have been calculated from atomic trajectories and further convolved with the known instrument response. A detailed analysis of the energy- and momentum-transfer dependence of this observable in the quasielastic regime shows that melting is entirely driven by temperature-activated tail motions. We also provide quantitative estimates of the activation energy for this process as the material first enters a plastic-crystalline phase, followed by the emergence of a genuine liquid at higher temperatures.

Temperature inversion of the thermal polarization of water.

Temperature gradients polarize water, a nonequilibrium effect that may result in significant electrostatic fields for strong thermal gradients. Using nonequilibrium molecular dynamics simulations, we show that the thermal polarization features a significant dependence with temperature that ultimately leads to an inversion phenomenon, whereby the polarization field reverses its sign at a specific temperature. Temperature inversion effects have been reported before in the Soret coefficient of aqueous solutions, where the solution changes from thermophobic to thermophilic at specific temperatures. We show that a similar inversion behavior is observed in pure water. Microscopically, the inversion is the result of a balance of dipolar and quadrupolar contributions and the strong temperature dependence of the quadrupolar contribution, which is determined by the thermal expansion of the liquid.