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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

The Atacama Cosmology Telescope DR6 and DESI: structure formation over cosmic time with a measurement of the cross-correlation of CMB lensing and luminous red galaxies

We present a high-significance cross-correlation of CMB lensing maps from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) with luminous red galaxies (LRGs) from the Dark Energy Spectroscopic Instrument (DESI) Legacy Survey spectroscopically calibrated by DESI. We detect this cross-correlation at a significance of 38σ; combining our measurement with the Planck Public Release 4 (PR4) lensing map, we detect the cross-correlation at 50σ. Fitting this jointly with the galaxy auto-correlation power spectrum to break the galaxy bias degeneracy with σ 8 , we perform a tomographic analysis in four LRG redshift bins spanning 0.4 ≤ z ≤ 1.0 to constrain the amplitude of matter density fluctuations through the parameter combination S 8 × = σ 8 (Ω m / 0.3) 0.4 . Prior to unblinding, we confirm with extragalactic simulations that foreground biases are negligible and carry out a comprehensive suite of null and consistency tests. Using a hybrid effective field theory (HEFT) model that allows scales as small as k max = 0.6 h/ Mpc, we obtain a 3.3% constraint on S 8 × = σ 8 (Ω m / 0.3) 0.4 = 0.792 +0.024 -0.028 from ACT data, as well as constraints on S 8 × (z) that probe structure formation over cosmic time. Our result is consistent with the early-universe extrapolation from primary CMB anisotropies measured by Planck PR4 within 1.2σ. Jointly fitting ACT and Planck lensing cross-correlations we obtain a 2.7% constraint of S 8 × = 0.776 +0.019 -0.021 , which is consistent with the Planck early-universe extrapolation within 2.1σ, with the lowest redshift bin showing the largest difference in mean. The latter may motivate further CMB lensing tomography analyses at z < 0.6 to assess the impact of potential systematics or the consistency of the ΛCDM model over cosmic time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

$\mathrm{UPDATED}$ $\mathrm{U3SI2}$ thermal creep model and sensitivity analysis of the $\mathrm{U3SI2-SIC}$ accident tolerant $\mathrm{FUEL}$

U 3 Si 2 is a candidate accident tolerant fuel (ATF) replacement for UO 2 . U 3 Si 2 ’s high uranium density and high thermal conductivity are favorable properties in steady-state and accident conditions. Low power performance of this U3Si2-SiC concept fuel is compared to that of UO 2 -Zr4 fuels by implementing models that describe the properties of U 3 Si 2 and SiC-SiC into Idaho National Laboratory's (INL) fuel performance code, BISON. Included in these material models is a thermal creep model for U 3 Si 2 based on compressive creep data. The simulated results are in keeping with community knowledge that the U 3 Si 2 -SiC concept fuel may serve as a replacement for UO 2 -Zr4 fuels during steady-state operation, provided the mSiC layer remains under compression. Through a moderate power history and three 24-month fuel cycles, the mSiC layer remains under compressive stress through a burnup of 80 MWd/kgU. During low power operation, failure of the mSiC layer generally occurs prior to significant thermal creep in U 3 Si 2 . Generally, U 3 Si 2 creep is temperature sensitive and of little importance at the temperatures and stresses simulated during steady operation and during fuel-to-cladding contact. A parameter variation study including 11,520 individual simulations with variations in nominal fuel thermal creep rate, cladding thermal conductivity, cladding irradiation creep and swelling, cladding gap size, and cladding thickness demonstrated that research priorities for this ATF should revolve around reducing cladding thickness as a means to minimize cladding failure. Generally, despite advances in SiC-SiC compliance, the brittle nature of mSiC excludes U 3 Si 2 -SiC for use where fuel cladding contact may occur.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

Deep learning with plasma plume image sequences for anomaly detection and prediction of growth kinetics during pulsed laser deposition

Abstract Materials synthesis platforms that are designed for autonomous experimentation are capable of collecting multimodal diagnostic data that can be utilized for feedback to optimize material properties. Pulsed laser deposition (PLD) is emerging as a viable autonomous synthesis tool, and so the need arises to develop machine learning (ML) techniques that are capable of extracting information from in situ diagnostics. Here, we demonstrate that intensified-CCD image sequences of the plasma plume generated during PLD can be used for anomaly detection and the prediction of thin film growth kinetics. We develop multi-output (2 + 1)D convolutional neural network regression models that extract deep features from plume dynamics that not only correlate with the measured chamber pressure and incident laser energy, but more importantly, predict parameters of an auto-catalytic film growth model derived from in situ laser reflectivity experiments. Our results demonstrate how ML with in situ plume diagnostics data in PLD can be utilized to maintain deposition conditions in an optimal regime. Further, the predictive capabilities of plume dynamics on the kinetics of film growth or other film properties prior to deposition provides a means for rapid pre-screening of growth conditions for the non-expert, which promises to accelerate materials optimization with PLD.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Direct neutron-diffraction-based measurement of magnetic order in brownmillerite SrCoO 2.5 and La 0.5 Sr 0.5 CoO 2.5 thin films

Epitaxial cobaltites have emerged as exemplary materials for electrochemical gating, in large part due to their topotactic perovskite (P) ↔ brownmillerite (BM) transformations. SrCoO 3-δ , for example, can be cycled between metallic ferromagnetic P SrCoO 3 and insulating BM SrCoO 2.5 , realizing exceptional modulation of electronic, thermal, and optical properties. It is often presumed that such cycling also generates ferromagnetic–antiferromagnetic (F-AF) modulation due to the G-type AF order in bulk SrCoO 2.5 . Little is understood about magnetism in thin-film BM SrCoO 2.5 , however, meaning that the true magnetic property modulation is unclear. We address this here through a neutron diffraction study of BM La 1-x Sr x CoO 2.5 films at x = 0.5 and 1.0. Lightly compressively strained SrCoO 2.5 films are shown to retain G-type AF order, albeit with suppressed Néel temperature (~340 K). Of high interest for AF spintronics, room-temperature F–AF cycling is thus possible across the SrCoO 3-δ P ↔ BM transformation. At x = 0.5, however, BM La 0.5 Sr 0.5 CoO 2.5 films are found to exhibit no detectable G-type AF order but instead weak F order (Curie temperature ~115 K), unveiling a La 0.5 Sr 0.5 CoO 3-δ phase diagram with two distinct F phases. These results thus uncover new, unanticipated magnetic phase behavior in these materials, in addition to being directly relevant to cobaltite-based magnetoionics.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Experimental investigation of PDI bifurcation of lower hybrid waves during electron density ramp-up in EAST

Abstract The effect of parametric decay instability (PDI) on the current drive efficiency of 4.6 GHz lower hybrid (LH) waves in EAST is investigated experimentally, showing the PDI channel bifurcation of LH waves for the first time in EAST. First, experiments with three platforms of LH power were performed, achieving the LH power required for the PDI occurrence. Second, PDI bifurcation experiments were further carried out by ramping up the plasma electron density. The loop voltage increases with an increase in density, implying a decrease in the current driven by the LH wave. PDI bifurcation during electron density ramp-up was studied by analyzing the parallel refractive index ( n ∥ ) and the frequency spectrum broadening, which is measured with a radio frequency magnetic probe array recently installed close to the LH antenna. It is observed for the first time that they both first increase with density, then there is not much variation and a clear sideband in the frequency spectrum is also observed when the density is up to 4 × 10 19 m − 3 , suggesting a change in the PDI channel. Calculation of the mode growth rate driven by PDI shows that when the edge electron density is up to 1.9 × 10 18 m − 3 , the growth rate of the ion cyclotron quasi-mode (ICQM) will exceed that of the ion sound quasi-mode (ISQM), quantitively explaining that with an increase in density, the PDI channel partly transits from the ISQM to the ICQM channel. Studies provide a possible way to reduce the power deposition in the edge region and improve drive capability by means of mitigating PDI behavior.

Physics↗
DOE OSTI2023

Magnetoelastic interactions in SrCu 2 (BO 3 ) 2 studied by Raman scattering experiments and first principles calculations

Dynamic and static crystal lattice properties of SrCu 2 (BO 3 ) 2 are studied by means of Raman scattering, magnetostriction, and thermal expansion measurements in magnetic fields to 45 T. Raman experiments versus temperature reveal that some phonon modes show an unusual behavior: their frequencies soften (modes at 200 and 450 cm –1 ) while others harden (modes at 385 and 478 cm –1 ) when decreasing the temperature below 15 K. Magneto-Raman experiments show that their field dependence correlates with their respective temperature dependencies; e.g., modes that are hardened with increasing temperature also harden with applied magnetic fields and modes that become softer with temperature also soften with applied fields. We use density functional theory to successfully model and compute the energies of these modes, classifying them into two types: pantograph (modes that soften when decreasing the temperature) and nonpantograph. We conclude that the former involves the modification of the intradimer exchange interaction J and the latter the interdimer J'. Lastly, dilatometry is used to correlate field-dependent Raman modes to the closing of the spin gap as well as fractional-magnetization stripe states M = 1/4 M s and M = 1/3 M s , where M s is the saturation magnetization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Magnetic properties of the frustrated spin-$\frac{1}{2}$ capped-kagome antiferromagnet (CsBr)Cu 5 V 2 O 10

Here, the structural and magnetic properties of a spin-$\frac{1}{2}$ averievite (CsBr)⁢Cu 5⁢ V 2 ⁢O 10 are investigated by means of temperature-dependent x-ray diffraction, magnetization, heat capacity, and 51 V nuclear magnetic resonance (NMR) measurements. The crystal structure (trigonal, $P\bar{3}$) features a frustrated capped-kagome lattice of the magnetic Cu 2+ ions. Magnetic susceptibility analysis indicates a large Curie-Weiss temperature of $\theta$ CW ≃ -175K. Heat capacity signals the onset of a magnetic long-range-order (LRO) at T N ≃21.5K at zero magnetic field due to the presence of significant inter-planer coupling in this system. The magnetic LRO below 27 K is further evident from the drastic change in the 51 V NMR signal intensity and rapid enhancement in the 51 V spin-lattice relaxation rate in a magnetic field of 6.3 T. The frustration index ƒ=|⁢$\theta$ CW ⁢|/⁢T N ≃ 8 ascertains strong magnetic frustration in this compound. From the high-temperature value of the 51 V NMR spin-lattice relaxation rate, the leading antiferromagnetic exchange interaction between the Cu 2+ ions is calculated to be J/k B ≃ 136K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Long-range three-dimensional magnetic structures of the spin S =1 hexamer cluster fedotovite-like A 2 Cu 3 O ( SO 4 ) 3 ( A 2 = K 2 , NaK, Na 2 ): A neutron diffraction study

The crystal and magnetic structures of the spin S = 1 hexamer cluster fedotovite-like A 2 Cu 3 O(SO 4 ) 3 (A 2 = K 2 , NaK, Na 2 ) were studied by neutron powder diffraction at temperatures 1.6–290 K. The crystal structures in all compounds are well refined in the monoclinic space group C2/c. The basic magnetic units of the compounds are copper hexamers, which are coupled by weak superexchange interactions giving rise to three-dimensional long-range magnetic order below 3.0 < T N < 4.7 K. We have found that for A 2 = K 2 and NaK the propagation vector of the magnetic structure is k = [0,0,0], and the coupling of the Cu hexamers is ferromagnetic (FM) along the ab diagonal and antiferromagnetic along the bc diagonal. In contrast, for A = Na the propagation vector is k = [0,1,0], and the Cu hexamers are coupled antiferromagnetically (AFM) along the ab diagonal. Further, the hexamers are formed by three Cu pairs arranged along the b axis. The calculated spin expectation values $\langle s \rangle$ for the simplest symmetric spin Hamiltonian (obtained from inelastic neutron spectroscopy) of the isolated hexamers in the mean field amounted to $\langle s \rangle$ = 3/8 for the side spins Cu1 and Cu2 and $\langle s \rangle$ = 1/4 for Cu3 in the middle. The Cu spins are FM coupled in pairs and AFM between neighboring pairs. The experimental magnetic moments of the Cu 2+ ions turn out to be not completely collinear due to spin frustrations within the weak interhexamer interactions. The sizes of magnetic moments of Cu in the hexamers determined from the diffraction data are in fair agreement with the calculated values.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Diversity of Antibiotic Resistance genes and Transfer Elements-Quantitative Monitoring (DARTE-QM): a method for detection of antimicrobial resistance in environmental samples

Effective monitoring of antibiotic resistance genes and their dissemination in environmental ecosystems has been hindered by the cost and efficiency of methods available for the task. We developed the Diversity of Antibiotic Resistance genes and Transfer Elements-Quantitative Monitoring (DARTE-QM), a method implementing TruSeq high-throughput sequencing to simultaneously sequence thousands of antibiotic resistant gene targets representing a full-spectrum of antibiotic resistance classes common to environmental systems. In this study, we demonstrated DARTE-QM by screening 662 antibiotic resistance genes within complex environmental samples originated from manure, soil, and livestock feces, in addition to a mock-community reference to assess sensitivity and specificity. DARTE-QM offers a new approach to studying antibiotic resistance in environmental microbiomes, showing advantages in efficiency and the ability to scale for many samples. This method provides a means of data acquisition that will alleviate some of the obstacles that many researchers in this area currently face.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Acoustic waves from the 2023 April 20 SpaceX Starship rocket explosion travelling in the elevated ‘AtmoSOFAR’ channel

SUMMARY The ability to detect low-frequency sounds from distant energetic events depends on the temperature and wind structure of the atmosphere. Thus, from time to time surface-based acoustic detectors may not be able to capture sounds arriving from certain directions. However, the temperature minimum at the tropopause may create an acoustic duct called the ‘AtmoSOFAR’ channel that could transmit acoustic waves laterally—but perhaps not to the ground. If true, then elevated sensors such as those borne aloft by balloons would record the signatures even in regions where ground-based sensors cannot. This has been difficult to prove because high altitude acoustic sources are rare and balloon deployments are sporadic. This work describes the detection and characterization of powerful acoustic waves generated during the launch and terminal explosion of the SpaceX Starship rocket on 2023 April 20 using a pair of microbarometers on a stratospheric balloon. The signals travelled through the AtmoSOFAR channel, carrying information about the size and nature of their source. This channel also appears to leak some acoustic energy to the ground, in agreement with previous studies. The acoustic yield of the Starship terminal explosion was on the order of $10^3$ tons trinitrotoluene equivalent, which agrees with the estimated fuel load of the vehicle to about a factor of 2, but is two orders of magnitude larger than optical estimates. These results support an earlier study that claimed lateral transmission of sound from a smaller rocket through the AtmoSOFAR channel. The transmission of source information through the AtmoSOFAR channel motivates its use for monitoring other natural and anthropogenic events using balloon-borne sensors. This may become increasingly important as more and more private and government entities conduct spacecraft launches and reentries. It may also provide a means of monitoring clear air turbulence and other sound-generating atmospheric phenomena at a distance.

58 GEOSCIENCES↗
DOE OSTI2023

Saccharomycopsis praedatoria sp. nov., a predacious yeast isolated from soil and rotten wood in an Amazonian rainforest biome

Three yeast isolates were obtained from soil and rotting wood samples collected in an Amazonian rainforest biome in Brazil. Comparison of the intergenic spacer 5.8S region and the D1/D2 domains of the large subunit rRNA gene showed that the isolates represent a novel species of the genus Saccharomycopsis. A tree inferred from the D1/D2 sequences placed the novel species near a subclade containing Saccharomycopsis lassenensis, Saccharomycopsis fermentans, Saccharomycopsis javanensis, Saccharomycopsis babjevae, Saccharomycopsis schoenii and Saccharomycopsis oosterbeekiorum, but with low bootstrap support. In terms of sequence divergence, the novel species had the highest identity in the D1/D2 domains with Saccharomycopsis capsularis, from which it differed by 36 substitutions. In contrast, a phylogenomic analysis based on 1061 single-copy orthologs for a smaller set of Saccharomycopsis species whose whole genome sequences are available indicated that the novel species represented by strain UFMG-CM-Y6991 is phylogenetically closer to Saccharomycopsis fodiens and Saccharomycopsis sp. TF2021a (=Saccharomycopsis phalluae). The novel yeast is homothallic and produces asci with one spheroidal ascospore with an equatorial or subequatorial ledge. The name Saccharomycopsis praedatoria sp. nov. is proposed to accommodate the novel species. The holotype of Saccharomycopsis praedatoria is CBS 16589 T . The MycoBank number is MB849369. S. praedatoria was able to kill cells of Saccharomyces cerevisiae by means of penetration with infection pegs, a trait common to most species of Saccharomycopsis.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2023

Light-Induced Decoupling of Electronic and Magnetic Properties in Manganites

The strongly correlated material La 0.7 ⁢Sr 0.3 ⁢Mn⁢O 3 (LSMO) exhibits metal-to-insulator and magnetic transition near room temperature. Although the physical properties of LSMO can be manipulated by strain, chemical doping, temperature, or magnetic field, they often require large external stimuli. Further, to include additional flexibility and tunability, we developed a hybrid optoelectronic heterostructure that uses photocarrier injection from cadmium sulfide (Cd⁢ S) to an LSMO layer to change its electrical conductivity. LSMO exhibits no significant optical response; however, the Cd ⁢S/LSMO heterostructures show an enhanced conductivity, with a resistance drop of about 37%, at the transition temperature under light stimuli. This enhanced conductivity in response to light is comparable to the effect of a 9 T magnetic field in pure LSMO. Surprisingly, the optical and magnetic responses of Cd⁢ S/LSMO heterostructures are decoupled and exhibit different effects when both stimuli are applied. This unexpected behavior shows that heterostructuring strongly correlated oxides may require a new understanding of the coupling of physical properties across the transitions and provide the means to implement new functionalities.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Magnon heat transport in a two-dimensional Mott insulator

Whether or not anomalies in the thermal conductivity in insulating cuprates can be attributed to antiferromagnetic order and magnons in a 2D Mott insulator remains an intriguing open question. To shed light on this issue, we investigate the thermal conductivity $\kappa$ and its relationship with the specific heat $c_v$ in the half-filled 2D single-band Hubbard model, using the numerically exact determinant quantum Monte Carlo algorithm and maximum entropy analytic continuation. At low temperatures where the charge degrees of freedom are gapped-out and $c_v$ exhibits a clear magnon peak, we observe that thermal conductivity $\kappa$ also tends to form a peak at similar temperatures. Reducing temperature further produces a sharp upturn in $\kappa$, associated with an increasing mean-free path. Here we identify this as the high-temperature side of the anomalous peak in insulating cuprates, where the mean-free path eventually is cut-off by other scattering effects, including phonons, disorder, and physical size. Different scattering effects in our model are identified and analyzed in the thermal diffusivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Evaluating China's Road to Cyber Super Power

This report examines open source, non-classified qualitative analysis to evaluate China’s current cyber maturity. Evidence for this document draws on materials from academia, private cybersecurity companies, and national security research institutions. Private sector threat intelligence firms produce high quality analysis on Chinese APTs tactics, techniques, and procedures (TTPs), and investigation from companies such as FireEye illuminate China’s ability to wield cyber means for its security ends. None of the materials cited in this assessment originate from classified United States or foreign government sources. Any references to United States government sources are sourced entirely to unclassified information.

45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEF↗
DOE OSTI2022

Mechanistic Elucidation of Electronically Conductive PEDOT:PSS Tailored Binder for a Potassium‐Ion Battery Graphite Anode: Electrochemical, Mechanical, and Thermal Safety Aspects

Potassium-ion batteries (KIBs) are considered more appropriate for grid-scale storage than lithium-ion batteries (LIBs) due to similar operating chemistry, abundant precursors, and compatibility with low-cost graphite anodes. However, a larger ion reduces rate capabilities and exacerbates capacity fading from volumetric expansion. In this report, conductive polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), is substituted for standard insulating polyvinylidene fluoride (PVDF). Half-cells using carbon black (CB) in continuously conductive PEDOT:PSS/CB binder outperforms PVDF/CB by mitigating electrically isolated “dead” graphite, improving 100 cycle capacity retention at C/10 from 63 to 80%. Enhanced electrical contact with PEDOT:PSS/CB also reduces ion impedance and improves rate capabilities. Without CB however, PEDOT:PSS binder performs poorly in electrochemical studies despite promising ex situ electronic conductivity. This discrepancy is mechanistically elucidated through identification of redox activity between PEDOT:PSS and K + which results in high impedances in the anode operating voltage window. Additionally, the impact of conducting binder on mechanical properties and thermal safety of the anode is investigated. Brittleness and poor wettability of PEDOT:PSS are identified as issues, but greater stability against reactive KC 8 reduces overall heat generation. Binder substitution offers a promising means of mitigating issues with current KIB anodes regardless of active material, and the work herein addresses issues towards further improvement.

electrochemical impedance spectroscopy↗
DOE OSTI2020

Unexpected electrochemical behavior of an anolyte redoxmer in flow battery electrolytes: solvating cations help to fight against the thermodynamic–kinetic dilemma

Redoxmers are redox-active molecules that can store energy in electrolytes for redox flow batteries (RFBs), and their electrochemical properties are significantly affected by the choice of supporting electrolytes. Herein, we use 2,1,3-benzothiadiazole (BzNSN) as a model system to scrutinize the supporting electrolyte impact. By systemically varying the components of supporting salts, BzNSN not only shows substantial redox potential shifts but also exhibits varying electrochemical stabilities. Specifically, changing the size of cations can effectively alter the coordination between the supporting salt and BzNSN species. From Li + , Na + , K + , to NEt 4 + , the redox potential of BzNSN shifts negatively, from -1.63 V to -1.82 V vs. Ag/Ag + . Molecular dynamics and density functional theory simulations revealed that smaller cations, like Li + , are closer to the charged BzNSN when coordinated, implying stronger coordination, while larger cations, like K + and NEt 4 + , are farther away. Interestingly, the large cation electrolytes also lead to much improved electrochemical stability, evidenced by the extraordinarily enhanced kinetic lifetime from electron paramagnetic resonance measurement. This study demonstrates the first example of tuning an anolyte redoxmer toward a concurrent improvement of lowered redox potentials AND enhanced calendar lives via solvation means, which is usually constrained by the thermodynamic–kinetic relation.

2,1,3-benzothiadiazole↗
DOE OSTI2022

Charge transport and antiferromagnetic ordering in nitroxide radical crystals

Nonconjugated radical polymers and small molecules are employed as functional materials in organic electronic devices. Furthermore, the unpaired electrons on these materials have permanent magnetic moments, making these materials promising candidates for organic magnets. Through molecular design, strong antiferromagnetic and ferromagnetic ordering have been achieved in conjugated materials. However, the magnetic properties of nonconjugated radical polymers have only shown weak magnetic interactions among the open-shell sites due to the large mean separation between radicals in typical materials. Here, we have designed, synthesized, and crystalized two open-shell molecules that used molecular engineering to control the assembly of the open-shell sites into a strong antiferromagnetically ordered network. The strong antiferromagnetic interaction is evidenced by a high paramagnetic-to-antiferromagnetic transition temperature of ~40 K. This high transition temperature was a result of a high spin exchange coupling constant J of about –20 cm –1 , which was suggested by both experimental and computed coupling parameters given by the energy difference between high-spin and low-spin broken-symmetry structures. In addition, a single-crystal electrical conductivity of ~10 –3 S m –1 was achieved, which indicated the potential of this material in electronic applications. As a result, this work provides an insight into a design strategy for radical-based electronic and magnetic materials through proper molecular structure modifications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Potentials of mean force fail to describe chemical bond-breaking in solution

Many liquid phase studies assume that the potential energy surfaces of reacting molecules are the same as in the gas phase, neglecting complex solvent dynamics that can completely alter the nature of chemical reactivity. Even studies that include solvent effects typically only consider them in an average, equilibrium way as part of a potential of mean force (PMF). In this work, we use mixed quantum/classical simulations to compare how equilibrium and non-equilibrium solvent motions affect the photodissociation of a simple diatomic molecule, NaK + , in liquid tetrahydrofuran. A PMF analysis shows that as the excited-state molecule dissociates with the solvent at equilibrium, the bonding electron remains associated with K + at short bond distances but eventually localizes on Na + at the end of dissociation. When we examine non-equilibrium dynamical photodissociation trajectories, however, we find that they fall into three distinct categories: about a quarter of them have the bonding electron mainly associated with Na + , another quarter stay mainly associated with K + , and about half have the bonding electron shared roughly equally between the two ions. The results show that equilibrium PMFs cannot accurately describe the dynamics of bond-breaking chemical reactions in solution because there is insufficient time for the solvent to reach equilibrium on the time scale over which bond dissociation occurs. Furthermore, our analysis shows that the solvent coupling between the electronic energy surfaces is similar at and away from equilibrium, suggesting that other factors, such as solute velocity-driven solvent memory effects, play a more important role in explaining the failure of the equilibrium PMF to predict the non-equilibrium dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗