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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Structural evidence for intermediates during O 2 formation in photosystem II

In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn 4 CaO 5 cluster first stores four oxidizing equivalents, the S 0 to S 4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S 3 →[S 4 ]→S 0 transition where O 2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn 4 CaO 5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom O x , which was introduced as a bridging ligand between Ca and Mn1 during the S 2 →S 3 transition disappears or relocates in parallel with Y z reduction starting at approximately 700 μs after the third flash. The onset of O 2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Ultraintense, ultrashort pulse X-ray scattering in small molecules

In this work, we examine X-ray scattering from an isolated organic molecule from the linear to nonlinear absorptive regime. In the nonlinear regime, we explore the importance of both the coherent and incoherent channels and observe the onset of nonlinear behavior as a function of pulse duration and energy. In the linear regime, we test the sensitivity of the scattering signal to molecular bonding and electronic correlation via calculations using the independent atom model (IAM), Hartree–Fock (HF) and density functional theory (DFT). Finally, we describe how coherent X-ray scattering can be used to directly visualize femtosecond charge transfer and dissociation within a single molecule undergoing X-ray multiphoton absorption.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Au 3 -to-Ag 3 coordinate-covalent bonding and other supramolecular interactions with covalent bonding strength

An efficient strategy for designing charge-transfer complexes using coinage metal cyclic trinuclear complexes (CTCs) is described herein. Due to opposite quadrupolar electrostatic contributions from metal ions and ligand substituents, [Au(m-Pz-(i-C 3 H7) 2 )] 3 ∙[Ag(m-Tz-(n-C 3 F 7 ) 2 )] 3 (Pz = pyrazolate, Tz = triazolate) has been obtained and its structure verified by single crystal X-ray diffraction – representing the 1st crystallographically-verified M 3 @M' 3 stacked adduct of monovalent coinage metal CTCs. Abundant supramolecular interactions with aggregate covalent bonding strength arise from a combination of M–M' (Au / Ag), metal–π, π–π interactions and hydrogen bonding in this charge-transfer complex, according to density functional theory analyses, yielding a computed binding energy of 66 kcal mol -1 between the two trimer moieties – a large value for intermolecular interactions between adjacent d 10 centres (nearly doubling the value for a recently claimed Au(I) / Cu(I) polar-covalent bond: Proc. Natl. Acad. Sci. U.S.A., 2017, 114, E5042) – which becomes 87 kcal mol -1 with benzene stacking. Surprisingly, DFT analysis suggests that: (a) some other literature precedents should have attained a stacked M 3 @M' 3 product akin to the one herein, with similar or even higher binding energy; and (b) a high overall intertrimer bonding energy by inferior electrostatic assistance, underscoring genuine orbital overlap between M and M' frontier molecular orbitals in such polar-covalent M–M' bonds in this family of molecules. The Au / Ag bonding is reminiscent of classical Werner-type coordinate-covalent bonds such as H 3 N: → Ag in [Ag(NH 3 ) 2 ] + , as demonstrated herein quantitatively. Solidstate and molecular modeling illustrate electron flow from the p-basic gold trimer to the p-acidic silver trimer with augmented contributions from ligand-to-ligand’ (LL'CT) and metal-to-ligand (MLCT) charge transfer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

CASPT2 molecular geometries of Fe( II ) spin-crossover complexes

Using fully internally contracted (FIC)-CASPT2 analytical gradients, geometry optimizations of spin-crossover complexes are reported. This approach is tested on a series of Fe(II) complexes with different sizes, ranging from 13 to 61 atoms. A combination of active space and basis set choices are employed to investigate their role in determining reliable molecular geometries. The reported strategy demonstrates that a wave function-based level of theory can be used to optimize the geometries of metal complexes in reasonable times and enables one to treat the molecular geometry and electronic structure of the complexes using the same level of theory. For a series of smaller Fe(II) SCO complexes, strong field ligands in the LS state result in geometries with the largest differences between DFT and CASPT2; however, good agreement overall is observed between DFT and CASPT2. For the larger complexes, moderate sized basis sets yield geometries that compare well with DFT and available experimental data. Here we recommend using the (10e,12o) active space since convergence to a minimum structure was more efficient than with truncated active spaces despite having similar Fe–ligand bond distances.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Scalable ab initio fragmentation methods based on a truncated expansion of the non-orthogonal molecular orbital model

An alternative formulation of the non-orthogonal molecular orbital model of electronic structure theory is developed based on the expansion of the inverse molecular orbital overlap matrix. From this model, a hierarchy of ab initio fragment-based quantum chemistry methods, referred to as the nth-order expanded non-orthogonal molecular orbital methods, are developed using a minimal number of approximations, each of which is frequently employed in intermolecular interaction theory. These novel methods are compared to existing fragment-based quantum chemistry methods, and the implications of those significant differences, where they exist, between the methods developed herein and those already existing methods are examined in detail. Computational complexities and theoretical scaling are also analyzed and discussed. Future extensions for the hierarchy of methods, to account for additional intrafragment and interfragment interactions, are outlined.

Westheimer, Bryce M. (ORCID:0000000267645714)↗
DOE OSTI2024

Resonant Auger decay of dissociating CH 3 I near the I 4d threshold

Resonant Auger processes provide a unique perspective on electronic interactions and excited vibrational and electronic states of molecular ions. Here, new data are presented on the resonant Auger decay of excited CH 3 I in the region just below the I 4d -1 ionization threshold. The resonances include the Rydberg series converging to the five spin–orbit and ligand-field split CH 3 I (I 4d -1 ) thresholds, as well as resonances corresponding to excitation from the I 4d 5/2,3/2 orbitals into the σ* lowest unoccupied molecular orbital. This study focuses on participator decay that populates the lowest lying states of CH 3 I + , in particular, the X̃ 2 E 3/2 and 2 E 1/2 states, and on spectator decay that populates the lowest-lying (CH 3 I 2+ )σ* states of CH 3 I + . The CH 3 I (I 4d -1 )σ* resonances are broad, and dissociation to CH 3 + I competes with the autoionization of the core-excited states. Auger decay as the molecule dissociates produces a photoelectron spectrum with a long progression (up to v 3 + ~ 25) in the C–I stretching mode of the X̃ 2 E 3/2 and 2 E 1/2 states, providing insights into the shape of the dissociative core-excited surface. Further, the observed spectator decay processes indicate that CH 3 I + is formed on the repulsive wall of the lower-lying (CH 3 I 2+ )σ* potentials, and the photon-energy dependence of the processes provides insights into the relative slopes of the (4d -1 )σ* and (CH 3 I 2+ )σ* potential surfaces. Data are also presented for the spectator decay of higher lying CH 3 I (I 4d -1 )nl Rydberg resonances. Photoelectron angular distributions for the resonant Auger processes provide additional information that helps distinguish these processes from the direct ionization signal.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Efficient simulation of optical spectra via machine learning and physical decomposition of environmental effects

Simulations of optical spectra can provide key insights to aid experimental interpretation of electronic excitation phenomena. For chromophores in the condensed phase, these spectra, which incorporate the coupling between electronic excitation and molecular and solvent nuclear motions, can be simulated using excitation energies obtained from molecular dynamics simulations of the chromophore and solvent. Here, we present a hybrid scheme that exploits machine learning and physically informed spectral densities to show that as few as 25 ground and excited state energetic gradient calculations can be used to construct models that accurately predict environment-influenced vibronic coupling in optical spectra. We demonstrate our approach for the green fluorescent protein chromophore in water and the cresyl violet chromophore in methanol. We show that our hybrid approach, employing a machine learning model for the high-frequency spectral density and an ab initio parameterized Debye spectral density for the low-frequency, results in a systematic improvement of the optical absorption lineshape, leading to a simple machine learning scheme that can be used for the simulation of spectral densities and optical spectra.

Snider, Andrew [Univ. of California, Merced, CA (U↗
DOE OSTI2021

Photosynthetic reaction center variants made via genetic code expansion show Tyr at M210 tunes the initial electron transfer mechanism

Significance Reaction centers (RCs) are critical to photosynthetic energy conversion. RCs in all characterized photosynthetic organisms contain two symmetrically arranged branches of chromophores and enable light-induced electron transfer with high yield. We fine-tune the properties of a key bacterial RC symmetry-breaking tyrosine via its replacement with noncanonical tyrosine analogs and determine kinetic outcomes. Results are interpreted through energetic characterization made possible by resonance Stark spectroscopy. Analysis indicates this tyrosine modulates the mechanism of the initial light-induced electron transfer, affording an alternative functional pathway that maintains the RC’s robust electron transfer. Modern molecular biology, ultrafast spectroscopy, crystallography, and energetic characterization enable the mechanistic model we describe. Our results deepen understanding of RC function and may have implications for other photocatalysts and enzymes.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Momentum-resolved above-threshold ionization of deuterated water

We present momentum-resolved coincidence measurements of strong-field single and double ionization of deuterated water (D 2 O) using intense few-cycle laser pulses. We measure the vector momentum of electrons and ions resulting from the laser-molecule interaction. This enables us to measure the photoelectron spectrum for electrons in coincidence with fragment ions having different momenta, which can be related to different final states of the cation and dication. The photoelectron spectra for different fragment ion momenta show striking differences, which can be interpreted in terms of the removal of electrons from different molecular orbitals, including the highest occupied molecular orbital (HOMO), as well as more deeply bound ones (e.g., HOMO-2). Here, we discuss our measurements in light of calculations which model the strong-field light matter interaction.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Molecular-Level Exploration of the Structure-Function Relations Underlying Interfacial Charge Transfer in the Subphthalocyanine/C 60 Organic Photovoltaic System

The arrangement of organic molecules at the donor-acceptor interface in an organic photovoltaic (OPV) cell can have a strong effect on the generation of charge carriers and thereby cell performance. In this paper, we report the molecular-level exploration of the ensemble of interfacial donor-acceptor pair geometries and the charge-transfer (CT) rates to which they give rise. Our approach combines molecular-dynamics simulations, electronic structure calculations, machine learning, and rate theory. This approach is applied to the boron subphthalocyanine chloride (donor) and C 60 (acceptor) OPV system. In this work we find that the interface is dominated by a previously unreported donor-acceptor pair edge geometry, which contributes significantly to device performance in a manner that depends on the initial conditions. Quantitative relations between the morphology and CT rates are established, which can be used to advance the design of more efficient OPV devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Molecular Optoelectronics (Final Technical Report)

The project developed new theoretical tools for analysis of nanodevices subjected to external optical and bias driving. Traditional theoretical techniques for molecular optoelectronics are either based on kinetic schemes or utilize quasiparticle description. The former fails to account for quantum mechanics of the system-bath interface and may fail qualitatively when used in open interacting systems. The latter is inconvenient in (usual for molecular optical spectroscopy) many body states description of the system. We developed a new technique - the Hubbard nonequilibrium Green's functions method - which allows to avoid shortcomings of standard theoretical approaches. The Hubbard NEGF was applied to analysis of experimental measurements of photoinduced current and bias-induced electroluminescence in STM molecular junctions. We also demonstrated practical usefulness of the Hubbard NEGF within newly proposed universal theory of current-induced forces for nonadiabatic molecular dynamics which generalizes celebrated Head-Gordon and Tully expression for electronic friction. Finally, following latest trends in laser techniques we studied local responses (local fluxes and local noise spectroscopy) of biased junctions as a source of information not accessible by total responses (total fluxes and noises) of the system.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2021

Enskog Transport Calculations with Magpie

he revised Enskog theory for mixtures has been implemented, as a library, in the LANL institutional thermochemical code Magpie. This report provides the necessary physical background and several representative examples of transport calculations with this library. The particular attention is paid to how the contributions of non-translational degrees of freedom (molecular vibrations, rotations, electronic excitations) to the energy transport can be consistently accounted for via the so called Eucken correction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Quantum Computing Algorithms and Applications for Coherent and Strongly Correlated Chemical Systems

This project advanced quantum algorithms, quantum information theory, strongly correlated electronic structure methods, molecular quantum materials, and exciton transport imaging in coherent condensed phase systems. Across the award period, the team developed new methods for open-quantum-system simulation, Hamiltonian learning, state tomography, reduced-density-matrix and contracted-quantum eigensolver approaches, and quantum diagnostics for device capability and openness.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Editorial: Quantum Information and Quantum Computing for Chemical Systems

Quantum computing has emerged as an exciting inter-disciplinary research topic that cuts across the traditional fields of physics, computer science, and engineering. It is a revolutionary model of computation that has offered new insights into methods for modeling and simulation of chemical systems. Applications of quantum computing to chemistry have demonstrated rapid progress on both theoretical and experimental fronts. Past theoretical efforts have shown how to adapt quantum computation to a variety of problems including electronic structure and molecular dynamics. In addition, the development of quantum algorithms for quantum chemistry has been stimulated by the greater availability of more capable quantum computing devices. Advances in the number and quality of qubits continues to enable remarkable proof-of-concept demonstrations working towards a milestone of quantum computational advantage.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1968

Diurnal survey of the thermosphere. I.

Thermosphere neutral particles analyzed with rocket probe, measuring diurnal variations of molecular nitrogen and electron density and temperature altitude profiles

Ballance, J. O.↗