A generative artificial intelligence framework based on a molecular diffusion model for the design of metal-organic frameworks for carbon capture.

Metal-organic frameworks (MOFs) exhibit great promise for CO2 capture. However, finding the best performing materials poses computational and experimental grand challenges in view of the vast chemical space of potential building blocks. Here, we introduce GHP-MOFassemble, a generative artificial intelligence (AI), high performance framework for the rational and accelerated design of MOFs with high CO2 adsorption capacity and synthesizable linkers. GHP-MOFassemble generates novel linkers, assembled with one of three pre-selected metal nodes (Cu paddlewheel, Zn paddlewheel, Zn tetramer) into MOFs in a primitive cubic topology. GHP-MOFassemble screens and validates AI-generated MOFs for uniqueness, synthesizability, structural validity, uses molecular dynamics simulations to study their stability and chemical consistency, and crystal graph neural networks and Grand Canonical Monte Carlo simulations to quantify their CO2 adsorption capacities. We present the top six AI-generated MOFs with CO2 capacities greater than 2m mol g-1, i.e., higher than 96.9% of structures in the hypothetical MOF dataset.

How Accurate Can Crystal Structure Predictions Be for High-Energy Molecular Crystals?

Molecular crystals have shallow potential energy landscapes, with multiple local minima separated by very small differences in total energy. Predicting molecular packing and molecular conformation in the crystal generally requires ab initio methods of high accuracy, especially when polymorphs are involved. We used dispersion-corrected density functional theory (DFT-D) to assess the capabilities of an evolutionary algorithm (EA) for the crystal structure prediction (CSP) of well-known but challenging high-energy molecular crystals (HMX, RDX, CL-20, and FOX-7). While providing the EA with the experimental conformation of the molecule quickly re-discovers the experimental packing, it is more realistic to start instead from a naïve, flat, or neutral initial conformation, which reflects the limited experimental knowledge we generally have in the computational design of molecular crystals. By doing so, and using fully flexible molecules in fully variable unit cells, we show that the experimental structures can be predicted in fewer than 20 generations. Nonetheless, one must be aware that some molecular crystals have naturally hindered evolutions, requiring as many attempts as there are space groups of interest to predict their structures, and some may require the accuracy of all-electron calculations to discriminate between closely ranked structures. To save resources in this computationally demanding process, we showed that a hybrid xTB/DFT-D approach could be considered in a subsequent study to push the limits of CSP beyond 200+ atoms and for cocrystals.

Microkinetic insights into the role of catalyst and water activity on the nucleation, growth, and dissolution during COF-5 synthesis.

The chemical pathway for synthesizing covalent organic frameworks (COFs) involves a complex medley of reaction sequences over a rippling energy landscape that cannot be adequately described using existing theories. Even with the development of state-of-the-art experimental and computational tools, identifying primary mechanisms of nucleation and growth of COFs remains elusive. Other than empirically, little is known about how the catalyst composition and water activity affect the kinetics of the reaction pathway. Here, for the first time, we employ time-resolved in situ Fourier transform infrared spectroscopy (FT-IR) coupled with a six-parameter microkinetic model consisting of ∼10 million reactions and over 20 000 species. The integrated approach elucidates previously unrecognized roles of catalyst pKa on COF yield and water on growth rate and size distribution. COF crystalline yield increases with decreasing pKa of the catalysts, whereas the effect of water is to reduce the growth rate of COF and broaden the size distribution. The microkinetic model reproduces the experimental data and quantitatively predicts the role of synthesis conditions such as temperature, catalyst, and precursor concentration on the nucleation and growth rates. Furthermore, the model also validates the second-order reaction mechanism of COF-5 and predicts the activation barriers for classical and non-classical growth of COF-5 crystals. The microkinetic model developed here is generalizable to different COFs and other multicomponent systems.

Integrated High-Throughput and Machine Learning Methods to Accelerate Discovery of Molten Salt Corrosion-Resistant Alloys.

Insufficient availability of molten salt corrosion-resistant alloys severely limits the fruition of a variety of promising molten salt technologies that could otherwise have significant societal impacts. To accelerate alloy development for molten salt applications and develop fundamental understanding of corrosion in these environments, here an integrated approach is presented using a set of high-throughput (HTP) alloy synthesis, corrosion testing, and modeling coupled with automated characterization and machine learning. By using this approach, a broad range of CrFeMnNi alloys are evaluated for their corrosion resistances in molten salt simultaneously demonstrating that corrosion-resistant alloy development can be accelerated by 2 to 3 orders of magnitude. Based on the obtained results, a sacrificial protection mechanism is unveiled in the corrosion of CrFeMnNi alloys in molten salts which can be applied to protect the less unstable elements in the alloy from being depleted, and provided new insights on the design of high-temperature molten salt corrosion-resistant alloys.

Temperature-Transferable Coarse-Grained Model for Poly(propylene oxide) to Study Thermo-Responsive Behavior of Triblock Copolymers.

Thermo-responsive behavior of ethylene oxide (EO)-propylene oxide (PO) copolymers makes them suitable for many potential applications. Reproducing the origins of the tunable properties of EO-PO copolymers using coarse-grained (CG) models such as the MARTINI force field is critically important for building a better understanding of their behavior. In the present work, we have investigated the effects of coarse-graining on the water-polymer interaction across a temperature range. We compared the performance of different all-atom force fields to find the most appropriate one for the purpose of PO block parameterization in the MARTINI platform. We parameterized a CG temperature-dependent PO model based on the reproduction of the atomistic free energy of transfer of propylene oxide trimer from octane to water over a range of temperatures (20-60 °C) and compared the atomistic bond and angle distributions. Then, we used the model to study the effects of EO/PO ratio, molecular weight, and concentration on the thermo-responsive behavior of EO-PO copolymers in water. The results show an excellent agreement with experiments in different areas. Our temperature-dependent model reproduces (1) micellar phase above critical micelle temperature (CMT) and unimer phase below CMT for different Pluronics (a class of EO-PO triblock copolymers) spanning many EO/PO ratios and molecular weights; (2) spherical-to-rodlike micellar shape transition for Pluronics with 60 wt % of PO content or more; (3) diffusion coefficients for Pluronics with high PO content (P104 Pluronic with a PO mass of 3500 g mol-1) across a broad range of temperatures; and (4) micelle core size and micelle diameter similar to experimental results. Overall, our model improves the temperature sensitivity of EO-PO copolymers of existing models significantly, particularly for copolymers that are dominated by PO agents.

Uncertainty Quantification in Atomistic Modeling of Metals and Its Effect on Mesoscale and Continuum Modeling: A Review.

The design of next-generation alloys through the integrated computational materials engineering (ICME) approach relies on multiscale computer simulations to provide thermodynamic properties when experiments are difficult to conduct. Atomistic methods such as density functional theory (DFT) and molecular dynamics (MD) have been successful in predicting properties of never before studied compounds or phases. However, uncertainty quantification (UQ) of DFT and MD results is rarely reported due to computational and UQ methodology challenges. Over the past decade, studies that mitigate this gap have emerged. These advances are reviewed in the context of thermodynamic modeling and information exchange with mesoscale methods such as the phase-field method (PFM) and calculation of phase diagrams (CALPHAD). The importance of UQ is illustrated using properties of metals, with aluminum as an example, and highlighting deterministic, frequentist, and Bayesian methodologies. Challenges facing routine uncertainty quantification and an outlook on addressing them are also presented.

Multi-Shell Nano-CarboScavengers for Petroleum Spill Remediation.

Increasingly frequent petroleum contamination in water bodies continues to threaten our ecosystem, which lacks efficient and safe remediation tactics both on macro and nanoscales. Current nanomaterial and dispersant remediation methods neglect to investigate their adverse environmental and biological impact, which can lead to a synergistic chemical imbalance. In response to this rising threat, a highly efficient, environmentally friendly and biocompatible nano-dispersant has been developed comprising a multi-shelled nanoparticle termed 'Nano-CarboScavengers' (NCS) with native properties for facile recovery via booms and mesh tools. NCS treated different forms of petroleum oil (raw and distillate form) with considerable efficiency (80% and 91%, respectively) utilizing sequestration and dispersion abilities in tandem with a ~10:1 (oil: NCS; w/w) loading capacity. In extreme contrast with chemical dispersants, the NCS was found to be remarkably benign in in vitro and in vivo assays. Additionally, the carbonaceous nature of NCS broke down by human myeloperoxidase and horseradish peroxidase enzymes, revealing that incidental biological uptake can enzymatically digest the sugar based core.

Ultrathin, transferred layers of thermally grown silicon dioxide as biofluid barriers for biointegrated flexible electronic systems.

Materials that can serve as long-lived barriers to biofluids are essential to the development of any type of chronic electronic implant. Devices such as cardiac pacemakers and cochlear implants use bulk metal or ceramic packages as hermetic enclosures for the electronics. Emerging classes of flexible, biointegrated electronic systems demand similar levels of isolation from biofluids but with thin, compliant films that can simultaneously serve as biointerfaces for sensing and/or actuation while in contact with the soft, curved, and moving surfaces of target organs. This paper introduces a solution to this materials challenge that combines (i) ultrathin, pristine layers of silicon dioxide (SiO2) thermally grown on device-grade silicon wafers, and (ii) processing schemes that allow integration of these materials onto flexible electronic platforms. Accelerated lifetime tests suggest robust barrier characteristics on timescales that approach 70 y, in layers that are sufficiently thin (less than 1 µm) to avoid significant compromises in mechanical flexibility or in electrical interface fidelity. Detailed studies of temperature- and thickness-dependent electrical and physical properties reveal the key characteristics. Molecular simulations highlight essential aspects of the chemistry that governs interactions between the SiO2 and surrounding water. Examples of use with passive and active components in high-performance flexible electronic devices suggest broad utility in advanced chronic implants.

Prevalence of oral ulcers and its association with addictions in rural population of western Uttar Pradesh and eastern Rajasthan.

BACKGROUND: Head and neck cancer in Indian perspective predominantly relates to tobacco use. The present study explores the prevalence of oral ulcers and its association with addictions among the population of Uttar Pradesh and Rajasthan, India. METHODOLOGY: The screening method in early detection of head and neck cancer is broadly symptom based. 1399 subjects from Uttar Pradesh and Rajasthan were screened by trained personnel between April and June 2015. RESULTS: Study findings showed, mouth ulcers and trismus were common symptoms and tobacco chewing and smoking were common addictions. There were statistically significant associations among the symptoms and addictions as well as predominance in rural populations. The majority of smokers (27.1%) belonged to age ≥55 years whereas the tobacco chewers (29.2%) and alcohol abusers (45.8%) in the age group 25-34 years. Also the risk of developing mouth ulcers and trismus in this area are approximately 35 (MRR: 35.7, 95% CI: 15.5-81.9) and nearly eight (MRR: 7.7, 95% CI: 2.2-26.6) times higher respectively in males. However, joint use of smoked and smokeless tobacco increases nearly three times more risk of either mouth ulcers or trismus. CONCLUSION: Male individuals are more exposed to certain addictions such as tobacco (smoked and smokeless) and alcohol. The prevalence of oral ulcers is primarily associated with the addictions. Therefore, these persons are more at risk of further developing head neck cancer. A large level community screening and awareness are required especially among the rural population of India.

Mirrored continuum and molecular scale simulations of the ignition of high-pressure phases of RDX.

We present a mirrored atomistic and continuum framework that is used to describe the ignition of energetic materials, and a high-pressure phase of RDX in particular. The continuum formulation uses meaningful averages of thermodynamic properties obtained from the atomistic simulation and a simplification of enormously complex reaction kinetics. In particular, components are identified based on molecular weight bin averages and our methodology assumes that both the averaged atomistic and continuum simulations are represented on the same time and length scales. The atomistic simulations of thermally initiated ignition of RDX are performed using reactive molecular dynamics (RMD). The continuum model is based on multi-component thermodynamics and uses a kinetics scheme that describes observed chemical changes of the averaged atomistic simulations. Thus the mirrored continuum simulations mimic the rapid change in pressure, temperature, and average molecular weight of species in the reactive mixture. This mirroring enables a new technique to simplify the chemistry obtained from reactive MD simulations while retaining the observed features and spatial and temporal scales from both the RMD and continuum model. The primary benefit of this approach is a potentially powerful, but familiar way to interpret the atomistic simulations and understand the chemical events and reaction rates. The approach is quite general and thus can provide a way to model chemistry based on atomistic simulations and extend the reach of those simulations.

Intermolecular Energy Transfer Dynamics at a Hot-Spot Interface in RDX Crystals.

The phonon mediated vibrational up-pumping mechanisms assume an intact lattice and climbing of a vibrational ladder using strongly correlated multiphonon dynamics under equilibrium or near-equilibrium conditions. Important dynamic processes far from-equilibrium in regions of large temperature gradient after the onset of decomposition reactions in energetic solids are relatively unknown. In this work, we present a classical molecular dynamics (MD) simulation-based study of such processes using a nonreactive and a reactive potential to study a fully reacted and unreacted zone in RDX (1,3,5-trinitro-1,3,5-triazocyclohexane) crystal under nonequilibrium conditions. The energy transfer rate is evaluated as a function of temperature difference between the reacted and unreacted regions, and for different widths and cross-sectional area of unreacted RDX layers. Vibrational up-pumping processes probed using velocity autocorrelation functions indicate that the mechanisms at high-temperature interfaces are quite different from the standard phonon-based models proposed in current literature. In particular, the up-pumping of high-frequency vibrations are seen in the presence of small molecule collisions at the hot-spot interface with strong contributions from bending modes. It also explains some major difference in the order of decomposition of C-N and N-N bonds as seen in recent literature on initiation chemistry.

Graphene-Assisted Solution Growth of Vertically Oriented Organic Semiconducting Single Crystals.

Vertically oriented structures of single crystalline conductors and semiconductors are of great technological importance due to their directional charge carrier transport, high device density, and interesting optical properties. However, creating such architectures for organic electronic materials remains challenging. Here, we report a facile, controllable route for producing oriented vertical arrays of single crystalline conjugated molecules using graphene as the guiding substrate. The arrays exhibit uniform morphological and crystallographic orientations. Using an oligoaniline as an example, we demonstrate this method to be highly versatile in controlling the nucleation densities, crystal sizes, and orientations. Charge carriers are shown to travel most efficiently along the vertical interfacial stacking direction with a conductivity of 12.3 S/cm in individual crystals, the highest reported to date for an aniline oligomer. These crystal arrays can be readily patterned and their current harnessed collectively over large areas, illustrating the promise for both micro- and macroscopic device applications.

Reactive simulation of the chemistry behind the condensed-phase ignition of RDX from hot spots.

Chemical events that lead to thermal initiation and spontaneous ignition of the high-pressure phase of RDX are presented using reactive molecular dynamics simulations. In order to initiate the chemistry behind thermal ignition, approximately 5% of RDX crystal is subjected to a constant temperature thermal pulse for various time durations to create a hot spot. After application of the thermal pulse, the ensuing chemical evolution of the system is monitored using reactive molecular dynamics under adiabatic conditions. Thermal pulses lasting longer than certain time durations lead to the spontaneous ignition of RDX after an incubation period. For cases where the ignition is observed, the incubation period is dominated by intermolecular and intramolecular hydrogen transfer reactions. Contrary to the widely accepted unimolecular models of initiation chemistry, N-N bond dissociations that produce NO2 species are suppressed in the condensed phase. The gradual temperature and pressure increase in the incubation period is accompanied by the accumulation of short-lived, heavier polyradicals. The polyradicals contain intact triazine rings from the RDX molecules. At certain temperatures and pressures, the polyradicals undergo ring-opening reactions, which fuel a series of rapid exothermic chemical reactions leading to a thermal runaway regime with stable gas-products such as N2, H2O and CO2. The evolution of the RDX crystal throughout the thermal initiation, incubation and thermal runaway phases observed in the reactive simulations contains a rich diversity of condensed-phase chemistry of nitramines under high-temperature/pressure conditions.

Opening gates to oxygen reduction reactions on Cu(111) surface.

Electrocatalytic reduction of oxygen is composed of multiple steps, including the diffusion-adsorption-dissociation of molecular oxygen. This study explores the role of electrical double layer in aqueous medium in quantifying the rate of these coupled electrochemical processes at the electrode interface during oxygen reduction. The electronic, energetic, and configurational aspects of molecular oxygen diffusion and adsorption onto Cu(111) in water are identified through density functional theory based computations. The liquid phase on Cu(111) is modeled with hexagonal-ordered water bilayers, at two slightly different structures, with O-H bonds either facing the vacuum or the metal surface. The results indicate that the energetically preferred structure of water bilayers and adsorption configuration of O2 are different in cathodic and anodic potentials. The diffusion of O2 is found to be heavily hindered at the water/metal interface because of the ordering of water molecules in bilayers as compared to the bulk liquid. The unique correlations of diffusion and adsorption kinetics with water structure identified in this work can provide clues for improving oxygen reduction efficiency.

Density functional theory calculations of pressure effects on the structure and vibrations of 1,1-diamino-2,2-dinitroethene (FOX-7).

Pressure effects on the Raman vibrations of an energetic crystal FOX-7 (1, 1-diamino-2, 2-dinitroethene) were examined using density functional theory (DFT) calculations. High accuracy calculations were performed with a periodic plane-wave DFT method using norm-conserving pseudopotentials. Different exchange-correlation functionals were examined for their applicability in describing the structural and vibrational experimental data. It is shown that the PBE functional with an empirical dispersion correction by Grimme, PBE-D method, reproduces best the molecular geometry, unit cell parameters, and vibrational frequencies. Assignments of intramolecular Raman active vibrations are provided. The calculated pressure dependence of Raman shifts for the intramolecular and lattice modes were found to be in good agreement with the experimental data; in particular, the calculations predicted correctly a decrease of frequencies for the NH2 stretching modes with pressure. Also, in accord with experiments, the calculations indicated some instances of modes mixing/coupling with increasing pressure. This work demonstrates that the dispersion-corrected PBE functional can account for the structural and vibrational properties of FOX-7 crystal at ambient and high pressures.

Linking molecular level chemistry to macroscopic combustion behavior for nano-energetic materials with halogen containing oxides.

Coupling molecular scale reaction kinetics with macroscopic combustion behavior is critical to understanding the influences of intermediate chemistry on energy propagation, yet bridging this multi-scale gap is challenging. This study integrates ab initio quantum chemical calculations and condensed phase density functional theory to elucidate factors contributing to experimentally measured high flame speeds (i.e., >900 m∕s) associated with halogen based energetic composites, such as aluminum (Al) and iodine pentoxide (I2O5). Experiments show a direct correlation between apparent activation energy and flame speed suggesting that flame speed is directly influenced by chemical kinetics. Toward this end, the first principle simulations resolve key exothermic surface and intermediate chemistries contributing toward the kinetics that promote high flame speeds. Linking molecular level exothermicity to macroscopic experimental investigations provides insight into the unique role of the alumina oxide shell passivating aluminum particles. In the case of Al reacting with I2O5, the alumina shell promotes exothermic surface chemistries that reduce activation energy and increase flame speed. This finding is in contrast to Al reaction with metal oxides that show the alumina shell does not participate exothermically in the reaction.

Design of anti-icing coatings using supercooled droplets as nano-to-microscale probes.

A multiscale simulation-based approach is presented for predicting anti-icing properties of nanocomposite coatings. Development of robust anti-icing coatings is a challenging task. An anti-icing coating that can prevent in-flight icing is of particular interest to the aircraft industry. A multiscale simulations based approach is developed to provide insights into the complex effect of coating material and surface topology on the prevention of in-flight icing. Chemical properties of different coatings and kinetics of icing or inhibition of ice nucleation are calculated from nanoscale atomistic simulations. In addition, in-flight icing environments including impingement and rolling of supercooled microdroplet and nucleation of ice under wind shear have been implemented using fluid dynamics methodologies. A model for icing in nano-to-microscale for surfaces with known chemical composition and surface topology is used for developing predictive capabilities regarding anti-icing performance of potential coatings. In this work, fluorinated polyhedral oligomericsilsesquioxanes molecules have been used to increase nanoscale roughness when embedded in a polycarbonate polymeric matrix. The findings suggest that a successful anti-icing coating will require precise control over nanoscale and microscale roughness. The multiscale methodology presented therefore can potentially help in identifying coupled effects of material, surface topology, and icing environment for promising coatings before performing icing tunnel experiments.

Turning aluminium into a noble-metal-like catalyst for low-temperature activation of molecular hydrogen.

Activation of molecular hydrogen is the first step in producing many important industrial chemicals that have so far required expensive noble-metal catalysts and thermal activation. We demonstrate here that aluminium doped with very small amounts of titanium can activate molecular hydrogen at temperatures as low as 90 K. Using an approach that uses CO as a probe molecule, we identify the atomistic arrangement of the catalytically active sites containing Ti on Al(111) surfaces, combining infrared reflection-absorption spectroscopy and first-principles modelling. CO molecules, selectively adsorbed on catalytically active sites, form a complex with activated hydrogen that is removed at remarkably low temperatures (115 K; possibly as a molecule). These results provide the first direct evidence that Ti-doped Al can carry out the essential first step of molecular hydrogen activation under nearly barrierless conditions, thereby challenging the monopoly of noble metals in hydrogen activation.

Finite size effects on aluminum/Teflon reaction channels under combustive environment: a Rice-Ramsperger-Kassel-Marcus and transition state theory study of fluorination.

The effect of particle size on combustion efficiency is an important factor in combustion research. Gas-phase aluminum clusters in oxidizing environment constitute a relatively simple and extensively studied system. In an attempt to underscore the correlation between electronic structure, finite size effect, and reactivity in small aluminum clusters, reactions between aluminum, [Al(13)](-) cluster, and Teflon decomposition fragments were studied using theoretical calculations at the density functional theoretical level. The unimolecular rate constants calculated using transition state and Rice-Ramsperger-Kassel-Marcus theory show that reactions with COF and CF(2) species with aluminum are faster than those involving CF(3) and COF(2). The results show that the kinetic barriers along different exothermic reaction channels correlate with the trends in HOMO(R)-HOMO(TS) (HOMO denotes highest occupied molecular orbital) energy gap and related shifts of the HOMO levels of reactants. Overall reactions involving carbonyl fluoride species (COF and COF(2)) lead to CO elimination and fluorination of the Al cluster. The CF(3)/CF(2) fragments lead to stable multicenter Al-C bond formation on the fluorinated Al cluster surface. Temperature-, energy-, and pressure-dependent rate constants are provided for extrapolating the expected reaction kinetics to conditions similar to known combustion reactions.

Multiscale modeling of interaction of alane clusters on Al(111) surfaces: a reactive force field and infrared absorption spectroscopy approach.

We have used reactive force field (ReaxFF) to investigate the mechanism of interaction of alanes on Al(111) surface. Our simulations show that, on the Al(111) surface, alanes oligomerize into larger alanes. In addition, from our simulations, adsorption of atomic hydrogen on Al(111) surface leads to the formation of alanes via H-induced etching of aluminum atoms from the surface. The alanes then agglomerate at the step edges forming stringlike conformations. The identification of these stringlike intermediates as a precursor to the bulk hydride phase allows us to explain the loss of resolution in surface IR experiments with increasing hydrogen coverage on single crystal Al(111) surface. This is in excellent agreement with the experimental works of Go et al. [E. Go, K. Thuermer, and J. E. Reutt-Robey, Surf. Sci. 437, 377 (1999)]. The mobility of alanes molecules has been studied using molecular dynamics and it is found that the migration energy barrier of Al(2)H(6) is 2.99 kcal/mol while the prefactor is D(0)=2.82 x 10(-3) cm(2)/s. We further investigated the interaction between an alane and an aluminum vacancy using classical molecular dynamics simulations. We found that a vacancy acts as a trap for alane, and eventually fractionates/annihilates it. These results show that ReaxFF can be used, in conjunction with ab initio methods, to study complex reactions on surfaces at both ambient and elevated temperature conditions.