546 Search Results

Sugars are produced by living organisms, and are required building blocks for life as we know it, which raises the foundational question of how sugars formed in a prebiotic environment. The abiotic formose reaction produces sugars from formaldehyde, but our understanding of its initiation step remains murky, with chemists invoking the concept of an “activated aldehyde” to seed this reaction. Singlet hydroxycarbenes, high-energy isomers of aldehydes, were recently reported to facilitate sugar formation under cold, nonaqueous conditions relevant to interstellar environments. Here, we generate singlet methylhydroxycarbene (¹CH₃–C̈–OH) from the photodissociation of pyruvic acid and experimentally measure its gas-phase reaction withmore » d₄-acetaldehyde using multiplexed photoionization mass spectrometry. The C₄H₄D₄O₂ isomer d₄-acetoin is the sole product, which we kinetically link to the reactant CH₃–C̈–OH, and attribute to a carbonyl-ene formation mechanism. We see no evidence of 3-hydroxybutanal, the C–H insertion product expected in carbene chemistry. Using automated exploration we calculate stationary points on the potential energy surface and report master equation rate coefficients from T = 20–600 K, providing quantitative kinetics of this fast reaction for use in chemical models. The prereactive complex in this reaction is stabilized by both hydrogen bonding and electrophilic carbene C═O interactions. These effects create a short-range dynamical bottleneck for the reaction besides the long- and midrange barrierless bottlenecks. Combined with recent reports of ¹HC̈OH production from methanol photodissociation and pyruvic acid production in cold irradiated ices, this work provides evidence that singlet hydroxycarbene + aldehyde chemistry is a feasible path to prebiotic sugar formation.« less

Density functional theory (DFT) methods remain the most practical approach to calculating properties and reaction mechanisms of transition metal complexes. While the accuracy of DFT methods has been evaluated for some properties of mononuclear organometallic complexes there has been a general lack of evaluation for dinuclear organometallic complexes, in particular bonding changes related to reaction mechanisms. Here, this work evaluated DFT and coupled cluster methods for the accuracy of calculating the CpFe(CO)₂ radical (Fp•) generated from dinuclear [CpFe(CO)₂]₂ (Fp₂) and mononuclear [(Cp)(CO)₂Fe(H)] (Fp-H). This transition metal radical fragment was evaluated because dinuclear complexes built with it have recently shown amore » variety of unique reactions but has proven challenging to accurately calculate with DFT methods. Here we show that DFT methods provide a surprising wide range of fragmentation energies for Fp₂ and lower and mid rung DFT methods as well as DLPNO–CCSD(T) perform well for this dissociation energy. The highest rung double-hybrid methods have a large range in the Fp₂ dissociation energy, and the energy greatly depends on the amount of MP2 correlation energy included. For generating Fp• from Fp-H the lower and mid rung methods that worked well for Fp₂ showed significant error. Double-hybrid methods unfortunately are only accurate for the Fe–H bond if they are very inaccurate for the Fp₂ dissociation energy. While DLPNO–CCSD(T) is not perfect, and not close to chemically accurate for the Fe–H bond, it does provide reasonable accuracy for both Fp₂ and Fp-H dissociation energies.« less

Resonant two-photon ionization spectroscopy has been used to locate predissociation thresholds in the spectra of the actinide halides ThX and UX, where X = Cl, Br, and I. These predissociation thresholds are identified as the bond dissociation energies (BDEs) of the molecules. The resulting values show very similar BDEs for the corresponding ThX and UX species, with the thorium molecules being slightly more strongly bound: D₀(ThCl) = 5.077(6) eV, D₀(ThBr) = 4.391(4) eV, D₀(ThI) = 3.537(8) eV, D₀(UCl) = 4.989(3) eV, D₀(UBr) = 4.313(3) eV, and D₀(UI) = 3.449(8) eV. Here, the estimated error limit is given in parentheses inmore » units of the last reported digit. Spinor-based coupled cluster calculations have also been carried out on the halides of this work, including also ThF and UF. Here, the final D₀ values after including contributions due to basis set incompleteness, outer-core-correlation, picture-change, and QED effects are within 0.04 eV of the present experimental values in each case.« less

We present a formulation and implementation of an equation-of-motion (EOM) extension of the multireference driven similarity renormalization group (MR-DSRG) formalism for ionization potentials (IP-EOM-DSRG). The IP-EOM-DSRG formalism results in a Hermitian generalized eigenvalue problem, delivering accurate ionization potentials for strongly correlated systems. The EOM step scales as O(N⁵) with the basis set size N, allowing for efficient calculation of spectroscopic properties, such as transition energies and intensities. The IP-EOM-DSRG formalism is combined with three truncation schemes of the parent MR-DSRG theory: an iterative nonperturbative method with up to two-body excitations [MR-LDSRG(2)] and second- and third-order perturbative approximations [DSRG-MRPT2/3]. We benchmarkmore » these variants by computing (1) the vertical valence ionization potentials of a series of small molecules at both equilibrium and stretched geometries; (2) the spectroscopic constants of several low-lying electronic states of the OH, CN, N₂⁺, and CO⁺ radicals; and (3) the binding curves of low-lying electronic states of the CN radical. A comparison with experimental data and theoretical results shows that all three IP-EOM-DSRG methods accurately reproduce the vertical ionization potentials and spectroscopic constants of these systems. Notably, the DSRG-MRPT3 and MR-LDSRG(2) versions outperform several state-of-the-art multireference methods of comparable or higher cost.« less

Density functional embedding theory (DFET) enables use of electronic structure methods with higher accuracy than density functional theory in a local region, with applications thus far ranging from (photo/electro)catalysis to reactions in solution. DFET partitions a large collection of atoms into smaller groups that interact via a shared embedding (interaction) potential V_emb, determined via functional optimization. The optimized effective potential (OEP) process used to optimize V_emb is time-consuming and becomes a computational bottleneck due to sharp, oscillating features of V_emb near nuclei. Here, similar to pseudopotential theory, by reconstructing electron densities used in the OEP process from smoother pseudo-valence-only (PVO)more » electron densities as proxies for total densities of the full system and subsystems, we can retain accuracy in the embedded electronic structure calculations while potentially reducing the overhead of V_emb construction, within the projector augmented-wave (PAW) formalism. We explore three different chemical reactions as exemplars to test PVO–DFET, namely, H₂ dissociative adsorption on a Cu(111) surface, H₂O adsorption on a Pt(111) surface, and aqueous [Ca²⁺–SO₄^2–] ion-pair formation. The PVO approximation works well for all three systems with minimal loss of accuracy (∼10–70 meV error relative to the original exact-derivative (ED) approach) while accelerating V_emb generation for the Cu and Pt systems respectively by 20× and 5×. Given proper numerical convergence parameters, the spatial distributions of differences between PVO- and ED-based V_emb outside the core regions are small, explaining the exceptional agreement between the two approaches. Finally, we anticipate that this more efficient PVO–DFET approximation will be useful whenever computation of V_emb is much more expensive than subsequent embedded high-level electron correlation calculations.« less

Atmospheric iodine chemistry has garnered increasing attention as a result of increased iodine emissions. A key subset of this chemistry involves iodine oxides (I₂O_2–5), which serve as precursors to particle formation. Among these, I₂O₃ is the simplest iodine oxide involved in particle formation, but it has remained undetected in the atmosphere. Previous theoretical studies have characterized this peculiar molecule, primarily using energies to refine geometries obtained at low levels of theory. Due to the reemerging interest in I₂O₃, this study presents geometries optimized at the CCSD(T)/aug-cc-pwCVTZ-PP level of theory─marking the first instance, to the best of our knowledge, where thismore » system has been studied exclusively with CCSD(T). Harmonic vibrational frequencies were computed at the same level of theory. Final energetics were obtained using the very high level CCSDT(Q) method with basis sets up to quintuple-zeta cardinality (aug-cc-pwCV5Z-PP) and extrapolated to the CBS limit to yield CCSDT(Q)/CBS//CCSD(T)/aug-cc-pwCVTZ-PP energies. These energies include harmonic zero-point vibrational energy corrections and scalar relativistic energy corrections. Additionally, this study discovers new isomers along the I₂O₃ potential energy surface, a novel contribution to the field. The performance of different computational methods and DFT functionals commonly used in atmospheric chemistry is also assessed relative to high-level theoretical methods.« less

Here, we present MS25, a benchmark data set for evaluating machine learning interatomic potentials (MLIPs) across diverse materials-relevant systems including MgO surfaces, liquid water, zeolites, a catalytic Pt surface reaction, high-entropy alloys (HEAs), and disordered Zr-oxides. Five MLIP architectures (MACE, NequIP, Allegro, MTP, and Torch-ANI) are trained and tested, focusing not only on traditional metrics (energies, forces, and stresses) but also explicitly validating derived physical observables such as lattice constants, volumes, and reaction barriers. We find that most models reach comparable accuracy on standard error metrics across the simple systems, although equivariant MLIPs offer 1.5–2× improvements over nonequivariant MLIPs inmore » energy and force error for structurally complex or compositionally disordered environments such as HEAs and Zr–O systems. Our analysis highlights that low errors in energy and force predictions do not guarantee reliable observables, emphasizing the necessity of explicit validation. We demonstrate limitations in cross-framework transferability, as models trained on one zeolite framework (CHA) fail to reliably generalize to predictions of structurally distinct frameworks (e.g., MFI). Size-extensive tests show some dependence on system size for MgO, resulting from forced periodicity. The HEA and Zr–O data sets are identified as challenging tests for future benchmarks and MLIP model architecture developments as they show significant differentiation in error between MLIP architectures and are still relatively difficult at 1000 training images. Moving forward, we recommend that benchmarking efforts shift their focus from marginal accuracy improvements in energy and force errors toward identifying and understanding model failure modes, rigorously assessing transferability, and evaluating how their errors affect observable predictions. For researchers looking to choose an MLIP architecture, we suggest selecting equivariant MLIP architectures if the complexity of the system is a challenge. For simple materials problems, auxiliary features such as integration with molecular dynamics engines, trade-offs between computational data set generation cost vs MLIP inference speed, and framework integration may play a more important decision factor than small differences in error metrics that are unlikely to matter for production-level research.« less

Gas fermentation enables the production of fuels, chemicals, and foods from gaseous carbon sources and could serve as a technology for valorizing carbon that may otherwise be emitted to the atmosphere. In this review, we focus on upstream feedstock considerations: the supply of carbon and the supply of electrical power. Electrical power serves a dual role, providing both process energy and biochemical redox potential (via hydrogen or reduced intermediates). We define gas fermentation as bioprocesses involving gaseous feedstocks metabolized by microbes, distinct from microbial electrosynthesis. Trends in CO₂ point sources and low-carbon electricity systems are analyzed, highlighting opportunities and challengesmore » for future deployment. This review synthesizes current knowledge and identifies key R&D priorities for process integration at industrial scale.« less

Power is an often-cited reason for the move to advanced architectures on the path to Exascale computing. Here, this is due to practical considerations related to delivering enough power to successfully site and operate these machines, as well as concerns about energy usage while running large simulations. Since obtaining accurate power measurements can be challenging, it may be tempting to use the processor thermal design power (TDP) as a surrogate due to its simplicity and availability. However, TDP is not indicative of typical power usage while running simulations. Using commodity and advanced technology systems at Lawrence Livermore and Sandia Nationalmore » Labs, we performed a series of experiments to measure power and energy usage in running simulation codes. These experiments indicate that large scale Lawrence Livermore simulation codes are significantly more efficient than a simple processor TDP model might suggest.« less

Herein we report an electronic structure investigation of neutral and oxidized Ru complexes containing a redox noninnocent N₂S₂ ligand derived from o-phenylenediamide (L1). UV–vis spectroelectrochemistry (SEC) studies were conducted on the square pyramidal complex [Ru^II(L1)(PPh₃)] (1) and the six-coordinate complexes [Ru^II(μ-BH₃)(L1)(PPh₃)] (2) – which has BH₃ bound in a metal–ligand cooperative (MLC) fashion across Ru and L1 – and [Ru^II(L1)(PPh₃)(MeCN)] (3). The SEC results yielded spectra assigned to singly and doubly oxidized 1 and 3, revealing electronic structure changes as a function of oxidation state and in response to the presence and absence of bound MeCN. By contrast, the SECmore » results of 2 showed that it rapidly loses MLC-bound BH₃ upon oxidation. The SEC results for 1 and 3 were compared to single-crystal XRD data and UV–vis, EPR, and P K-edge, S K-edge, and Ru L₃-edge X-ray absorption spectroscopy (XAS) data collected on isolated samples of chemically oxidized 3. The data revealed that the first two oxidations are primarily localized on the ligand, which was supported by DFT and TDDFT calculations. DFT calculations for the doubly oxidized species revealed a singlet ground state with a singlet–triplet gap of 8.9 kcal/mol. CASPT2 calculations corroborated the DFT calculations and further revealed that the singlet ground state is multiconfigurational with 21% radical character. Collectively, the results establish redox formalisms and the underlying electronic structure of Ru complexes containing a noninnocent tetradentate ligand in different oxidation states.« less