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DOE OSTI2026

Coordination-Induced Weakening of N–H Bonds Driven by Bimetallic Cooperativity in Zr/Co Compounds

The bond dissociation free energy (BDFE) of the element-hydrogen bonds of protic substrates have been found to decrease upon metal coordination. Herein, an early/late heterobimetallic complex is used to examine the impact on the BDFE N−H when the substrate binding site and the redox-active site are two different metals that are spatially separated. A tris- (phosphinoamide) framework is used to link a d 0 ZrIV center with an accessible substrate binding site to a coordinatively saturated redox-active Co center, which serves as an appended electron reservoir. A series of aniline, amido, and imido Zr/Co model compounds were synthesized starting from the Zr IV /Co −I aniline adduct PhH 2 N−Zr(MesNP i Pr 2 ) 3 CoCN t Bu (2). 2,4,6-tristert- butylphenoxyl radical ( t Bu 3 ArO • ) was used to abstract one or two H atoms and produce the amido and imido complexes PhHNZr( MesNP i Pr 2 ) 3 CoCN t Bu (3) and PhN≡Zr(MesNP i Pr 2 ) 3 CoCNtBu (4), respectively. Using open-circuit potential measurements, the BDFE N−H within 2 and 3 were determined to be 37 kcal/mol (2) and 55 kcal/mol (3). Cyclic voltammetry measurements were conducted to determine the Co I/0 and Co 0/−I redox potentials. The pK a s were then estimated using the Bordwell equation to provide further insight into the thermochemical aspects of the observed proton coupled electron transfer (PCET) reactions.

Bond dissociation free energy↗
DOE OSTI2022

The role of collisional energy transfer in the thermal and prompt dissociation of 1-methyl allyl

Collisional energy transfer plays a pivotal role in a-priori calculations of the pressure-dependent rate constants obtained from a master equation. However, accurate determinations of collisional energy transfer parameters are rare and so most kinetic studies rely on best-fits of these parameters to experimental measurements (if available) or estimates guided from literature studies. In this work, we have quantified the effect of the uncertainty in energy transfer parameters on the thermal and prompt dissociation kinetics of a resonance stabilized radical, 1-methyl allyl (1MA), of relevance to the combustion of 1- and 2-butene isomers. Simulations using literature kinetics models were performed to assess the impact of these uncertainties on flame propagation and speciation data in laminar flames of 1- and 2-butene. Analyses of the uncertainty propagated by the energy transfer parameters for 1MA dissociation to the flame simulations, in particular the laminar flame speed, indicate an intricate coupling between the kinetics of 1MA dissociation (chain propagation) and its reaction with H-atoms (chain-termination). Ab-initio based theoretical calculations were also performed to obtain pressure-dependent kinetics for the reaction of 1MA with H-atoms. Lastly, theoretically calculated energy transfer parameters were used to best characterize the kinetics and branching between the chain propagating 1MA dissociation and the chain-terminating reaction 1MA + H.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

QC-GN 2 oMS 2 : a Graph Neural Net for High Resolution Mass Spectra Prediction

Predicting the mass spectrum of a molecular ion is often accomplished via three generalized approaches: rules-based methods for bond breaking, deep learning, or quantum chemical (QC) modeling. Rules-based approaches are often limited by the conditions for different chemical subspaces and perform poorly under chemical regimes with few defined rules. QC modeling is theoretically robust but requires significant amounts of computational time to produce a spectrum for a given target. Among deep learning techniques, graph neural networks (GNNs) have performed better than previous work with fingerprint-based neural networks in mass spectra prediction. To explore this technique further, we investigate the effects of including quantum chemically derived information as edge features in the GNN to increase predictive accuracy. The models we investigated include categorical bond order, bond force constants derived from extended tight-binding (xTB) quantum chemistry, and acyclic bond dissociation energies. Throughout this work, we evaluated these models against a control GNN with no edge features in the input graphs. Bond dissociation enthalpies yielded the best improvement with a cosine similarity score of 0.462 relative to the baseline model (0.437). In this work we also apply dynamic graph attention which improves performance on benchmark problems and supports the inclusion of edge features. Between implementations, we investigate the nature of the molecular embedding for spectra prediction and discuss the recognition of fragment topographies in distinct chemistries for further development in tandem mass spectrometry prediction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Ultrafast infrared transient absorption spectroscopy of gas-phase Ni(CO) 4 photodissociation at 261 nm

Here, we employ ultrafast mid-infrared transient absorption spectroscopy to probe the rapid loss of carbonyl ligands from gas-phase nickel tetracarbonyl following ultraviolet photoexcitation at 261 nm. Here, nickel tetracarbonyl undergoes prompt dissociation to produce nickel tricarbonyl in a singlet excited state; this electronically excited tricarbonyl loses another CO group over tens of picoseconds. Our results also suggest the presence of a parallel, concerted dissociation mechanism to produce nickel dicarbonyl in a triplet excited state, which likely dissociates to nickel monocarbonyl. Mechanisms for the formation of these photoproducts in multiple electronic excited states are theoretically predicted with one-dimensional cuts through the potential energy surfaces and computation of spin–orbit coupling constants using equation of motion coupled cluster methods (EOM-CC) and coupled cluster theory with single and double excitations (CCSD). Bond dissociation energies are calculated with CCSD, and anharmonic frequencies of ground and excited state species are computed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

In-Situ Laser Diagnostics to Understand Plasma-Surface Interactions in Titanium Thin Film Deposition

During the current project period the following tasks were completed: (1) We demonstrated the feasibility of a new way of measuring CH3 radicals in a plasma environment using a single, femtosecond (fs) pulsed laser for the first time, in a CH4 + Ar flow. A single fs pulsed Ti:Sa laser was split into two beams and with beam conversion using crystals we generatedtwo beams of 275 nm and 205 nm. The 275 nm was used to predissociate the CH3 radicals to CH2+H, followed by 205 nm Two Photon Laser Induced Fluorescence (TALIF) detection of H atoms, with a 11 ns delay from 275 nm predissociation pulse. (2) We demonstrated preliminary CH3 concentration measurements with spatial (< 100 µm) and time resolution (< 10 ns) by image processing of H and Kr TALIF signal images. (3) The methodology to quantify CH3 concentrations was developed. (4) Preliminary measurements of laser dissociation cross sections for CH4 and CH3 to (CH3+H), and (CH2+H), respectively, were conducted. (5) 275 nm laser dissociation of CH4 is a multiphoton process, and the CH4 dissociation cross section is a more sensitive function of the laser pulse energy than CH4 concentration. In the coming months, we will make more precise measurements of the dissociation cross sections, 𝑘275, 𝐶𝐻4 and 𝑘275, 𝐶𝐻3, by varying the laser energy while keeping the flow composition and conditions constant. This will enable more precise quantification of CH3 radical concentrations. More detailed measurements of CH3 radicals will be conducted in the following months of 2025 (Sept-Dec) in a Titanium Isopropoxide (TIP) + H2 + Ar flow meant for pure phase Ti thin film deposition by Chemical Vapor Deposition (CVD). More precise measurements of 275 nm laser dissociation cross sections for CH4 (to CH3 + H) and CH3 (to CH2 + H) will be conducted in the future. Detailed 275 nm laser predissociation cross sections for CH4 and CH3 dissociation to (CH3+H), and (CH2+H), respectively, as a function of gas heating, plasma power, and laser pulse energy will be evaluated. This will enable quantification of CH3 concentrations in the precursor flow over a substrate with time and spatial resolution. Comparison of the concentrations with 2D numerical simulations can lead to understanding of plasma surface interactions during pure metallic Ti thin film deposition using environmentally safer non-halogen TIP precursor. Flow conditions for pure Ti thin film deposition using non-halogen precursor (TIP) will be identified in the following months.

Uddi, Mruthunjaya [Advanced Cooling Technologies, ↗
DOE OSTI2025

Quantifying the Decomposition Kinetics of Linear C 2 –C 4 Perfluoroalkanes

The pyrolysis of C 2 –C 4 linear perfluoroalkanes was investigated using laser schlieren densitometry (LS) in a diaphragmless shock tube. Mixtures of 1, 2, and 4% perfluoroethane (PFE), perfluoropropane (PFP), and perfluorobutane (PFB) in excess krypton were shock-heated to 1400–2500 K at pressures of 60, 120, and 240 Torr. The initiation step for all perfluoroalkanes involved carbon–carbon bond fission. The rate constants for unimolecular decomposition were computed using microcanonical rate theory, and these rate constants were combined into a detailed chemical kinetic mechanism. For PFE and PFP, there was one dissociation channel, while PFB dissociated via two competing bond-fission reactions. The perfluoroalkyl radicals subsequently recombined or underwent further unimolecular decomposition. In conclusion, the kinetic model accurately simulated the density gradient profiles of all perfluoroalkanes.

Guzman, Eduardo H. [Brown Univ., Providence, RI (U↗
DOE OSTI2022

The role of energy transfer and competing bimolecular reactions in characterizing the unimolecular dissociations of allylic radicals

Resonantly stabilized radicals (RSR's) such as allylic radicals typically have bond energies exceeding 45 kcal/mol and are therefore reasonably long-lived up to quite high temperatures. Consequently, kinetics of the unimolecular dissociations and bimolecular reactions of these long-lived RSR's assume significance in high temperature chemically reacting systems such as in flames. Our recent analyses (J. Cho et al., Proc. Comb. Inst., 2022) of the uncertainty propagated by the energy transfer parameters for 1-methylallyl (1MA) dissociation to the laminar flame speed, indicates an intricate coupling between the kinetics of 1MA dissociation (chain propagation) and its reaction with H-atoms (chain-termination). Here, this work extends such analyses to other C 3 – C 5 allylic radicals: allyl, 2-methylallyl, 1,1-dimethylallyl, 1,2-dimethylallyl, and 1,3-dimethylallyl. Theoretical kinetics for the C 3 – C 4 allylic radicals were taken from prior literature studies (J.A. Miller et al., J. Phys. Chem., 2008; J. Cho et al., Proc. Comb. Inst., 2022; R.S. Tranter et al., Proc. Comb. Inst., 2017). Potential energy surfaces were newly computed for the dissociations of the larger C5 allylic radicals and the reactions of these radicals with H-atoms. These PES's were then used to calculate rate constants for the relevant unimolecular and bimolecular reactions. Energy transfer has a crucial role to play in these pressure-dependent reactions and therefore the present kinetics calculations relied on a-priori predictions of collisional energy transfer parameters to best characterize the branching between chain propagating unimolecular dissociations of these allylic radicals and chain terminating bimolecular reactions of these RSR's with H-atoms. Prompt dissociations are now established to be a universal feature of free radical dissociations. Here we have also characterized the propensity for prompt dissociations in these more-stable allylic RSR's and their role in combustion simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Energetic and Electronic Properties of AcX and LaX (X = O and F)

The bonding and spectroscopic properties of LaX and AcX (X = O and F) diatomic molecules were studied by highlevel ab initio CCSD(T) and SO-CASPT2 electronic structure calculations. Bond dissociation energies (BDEs) were calculated at the Feller−Peterson−Dixon (FPD) level. Potential energy curves and spectroscopic constants for the lowest-lying spin−orbit Ω states were obtained at the SO-CASPT2/aQ-DK level. A dense manifold of excited states was described for the monofluorides with the ground states well separated from the excited states. The spectroscopic parameters were in good agreement with those reported experimentally for LaO and LaF. For the diatomic molecules containing actinides, no experimental data of these parameters was found, but the results were consistent with other high-level calculations. The BDEs calculated at the FPD level were 791.3 (LaO), 705.2 (AcO), 650.0 (LaF), and 678.6 (AcF) kJ/ mol. The NBO analysis showed that the monofluorides are essentially ionic, which explains why the BDE(AcF) is higher than BDE(LaF); for the monoxides, covalent contributions involving the d orbitals of the metal and the p orbitals of the oxygen are stronger for LaO than AcO, which explains the higher BDE for LaO. Finally, the bond orders are predicted to be 2 for LaF and AcF, 3 for AcO, and higher than 3 for LaO.

Basis sets↗
DOE OSTI2022

Enhancing exciton diffusion by reducing energy disorder in organic solar cells

A highly crystalline, highly emissive, and wide-bandgap polymer AC174 with an extremely small Stokes shift is designed and synthesized in water, and is used to reduce system energetic disorder and increase the exciton diffusion length of the classical PM6:Y6 blend system. AC174 is incompatible with PM6 and Y6, improves molecular packing, and reduces system energetic disorder. The long-range Förster resonance energy transfer between the donor and acceptor is enhanced, and the exciton diffusion constant and exciton lifetime are increased, leading to a longer exciton diffusion length and more efficient exciton dissociation and charge generation. The addition of AC174 simultaneously improves the open-circuit voltage, short-circuit current density (J SC ) and fill factor of PM6:Y6 devices; especially the highest internal quantum efficiency approaches 100%, and the highest J SC is 28.4 mA cm –2 . Ternary devices with 5% AC174 in the donors achieve a power conversion efficiency of 17.2%, higher than those of the parent binary devices based on PM6:Y6 (15.9%) and AC174:Y6 (3.24%).

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

Active Thermochemical Tables: the thermophysical and thermochemical properties of methyl, CH 3 , and methylene, CH 2 , corrected for nonrigid rotor and anharmonic oscillator effects

The thermophysical properties (isobaric heat capacity, entropy, enthalpy increment) of two prominent radicals, methyl, CH 3 , and methylene, CH 2 , were computed using the Nonrigid Rotor Anharmonic Oscillator (NRRAO) approach and compared to their RRHO counterparts, demonstrating significant differences between the results from the two approaches. Methylene presents a typical case in which the NRRAO thermophysical properties have significantly higher values than their RRHO counterparts at higher temperatures. In the case of methyl, the positive anharmonicity of the umbrella motion causes an opposite effect, and the NRRAO corrected thermophysical properties have lower values than their RRHO counterparts. The NRRAO corrected thermophysical properties, in turn, affect the resulting thermochemical properties. Here, two reactions important in combustion modelling were tested: the recombination of methyl radical with hydrogen atoms to form methane, and the recombination of two methyl radicals to form ethane. The related NRRAO equilibrium constants differ significantly from their RRHO analogs, and the consequences for chemical modelling are discussed. Also reported are the most current ATcT enthalpies of formation for CH n (n = 4-0) species and for C 2 H 6 , together with the tightly related sequential bond dissociation enthalpies along with the CHn series.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Hydrogen-Bonding Bottlebrush Networks: Self-Healing Materials from Super-Soft to Stiff

The impact of polymer architecture on network dynamics and self-healing is presented using bottlebrushes containing side chains that are end-functionalized with 2-ureido-4[1H]-pyrimidinone (UPy). The synthesis of these materials is straightforward through a three-step process: (1) synthesizing rubbery poly(4-methylcaprolactone) macromonomers (p4MCL–OH) with a norbornene-based initiator, (2) functionalizing the terminal hydroxyl group with UPy–isocyanate (p4MCL–UPy), and (3) statistically copolymerizing p4MCL–OH and p4MCL–UPy via ring-opening metathesis polymerization (ROMP) to form hydrogen-bonding bottlebrushes having a fraction (p) of side chains functionalized with UPy. Attaching UPy to the free end of bottlebrush side chains dilutes the impact of friction from complementary UPy interactions on segmental dynamics, leading to a much weaker dependence of the glass-transition temperature (T g ) on p than observed in linear analogues, while the activation energy to dissociate UPy–UPy bonds (41–47 kJ/mol) remains mostly unchanged. Longer side chains result in a competition between reducing T g and inducing entanglements that influence hydrogen-bonded network dynamics. Increasing the backbone length extends the sticky Rouse region without affecting the network modulus (G x ) or UPy–UPy dissociation time (τ s ). Gx scales linearly with p and ranges from 27 kPa to 1.6 MPa, while τs remains nearly constant in contrast to linear telechelic ionomers, implying a similar self-healability across bottlebrush networks containing different amounts of UPy. Furthermore, these stretchable networks with p ≤ 0.25 undergo self-healing upon repeated rupture and melt pressing at ≤100 °C while retaining similar tensile properties. In summary, decorating bottlebrush polymers with hydrogen bonds creates opportunities to independently manipulate associative network dynamics and mechanical moduli.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Photodissociation dynamics of the tert -butyl perthiyl radical

Here, the photodissociation dynamics of the tert-butyl perthiyl (t-BuSS) radical are investigated by fast-beam coincidence translational spectroscopy. A fast (6 keV-8 keV) beam of neutral t-BuSS radicals is produced via photodetachment of the corresponding anion, followed by photodissociation at 248 nm (5.00 eV) or 193 nm (6.42 eV) and coincident detection of the neutral products. Photofragment mass and translational energy distributions are obtained at both wavelengths. At 248 nm, the dominant product channel (90%) is found to be S loss, with a product translational energy distribution that peaks close to the maximum available energy and an anisotropic photofragment angular distribution, indicating dissociation along a repulsive excited state. A minor channel (10%) leading to the formation of S 2 + t-Bu is also observed. At 193 nm, both two- and three-body dissociation are observed. The formation of S 2 + t-Bu is the dominant two-body product channel, with multiple electronic states of the S 2 molecule produced via excited-state dissociation processes. The formation of S + t-BuS is a minor two-body channel at this dissociation energy. The three-body channels are S 2 + H + isobutene, S 2 + CH 3 + propene, and S + SH + isobutene. The first two of these channels result from a sequential dissociation process in which the loss of S 2 from t-BuSS results in ground-state t-Bu with sufficient internal energy to undergo secondary fragmentation. The third three-body channel, S + SH + isobutene, is attributed to the loss of internally excited HS 2 from t-BuSS, which then rapidly dissociates to form S + SH in an asynchronous concerted dissociation process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Conversion of acenaphthalene to phenalene via methylation: A theoretical study

Ab initio calculations of the C 13 H 10 and C 13 H 9 potential energy surfaces related to the reaction of 1-acenaphthyl and methyl radicals and secondary isomerization of C 13 H 9 primary radical products have been performed at the chemically accurate G3(MP2,CC)//B3LYP/6-311G** level of theory to unravel the mechanism of conversion of acenaphthalene to phenalene or phenalenyl radical + H. The computed energetics and molecular parameters were utilized in Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations of reaction rate constants and relative product yields. The 1-acenaphthyl + CH 3 reaction is predicted to proceed by a fast radical-radical recombination mechanism and to predominantly produce collisionally stabilized 1-methylacenaphthalene B1 or a C 13 H 9 benzylic radical A1 + H with exothermicities of 106.8 and 25.2 kcal/mol, respectively. The A1 + H channel is preferable at higher temperatures, whereas the stabilization of 1-methylacenaphthalene is favored at higher pressures. The radical A1 can nearly irreversibly interconvert to phenalenyl radical AP via a 22.7 kcal/mol exothermic isomerization process involving formal insertion of the CH 2 group into a C-C bond of the five-member ring leading to the expansion of this ring to a six-member ring. The rate constants for the H addition reaction to phenalenyl radical to form phenalene and the reverse H loss from phenalene were also computed and the strength of the weakest C-H bond in the CH 2 group of phenalene is evaluated as 62.2 kcal/mol. The analysis of the reaction kinetics allowed us to deduce a mechanism for the conversion of 1-acenaphthyl radical to phenalene or phenalenyl radical + H via methylation involving the formation of A1 via a well-skipping channel or stabilization and dissociation of (or H abstraction from) B1 followed by isomerization of A1 to phenalenyl AP, which can add an H atom producing phenalene. As a result, rate constants for the significant elementary reactions are fitted to modified Arrhenius expressions and are proposed for kinetic modeling of the expansion of a five-member ring on a zigzag edge of PAH to a six-member ring by methylation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Non-Abelian electric field correlator at NLO for dark matter relic abundance and quarkonium transport

We perform a complete next-to-leading order calculation of the non-Abelian electric field correlator in a SU(N c ) plasma, which encodes properties of the plasma relevant for heavy particle bound state formation and dissociation, and is different from the correlator for the heavy quark diffusion coefficient. The calculation is carried out in the real-time formalism of thermal field theory and includes both vacuum and finite temperature contributions. By working in the R ξ gauge, we explicitly show the results are gauge independent, infrared and collinear safe. The renormalization group equation of this electric field correlator is determined by that of the strong coupling constant. Our next-to-leading order calculation can be directly applied to any dipole singlet-adjoint transition of heavy particle pairs. For example, it can be used to describe dissociation and (re)generation of heavy quarkonia inside the quark-gluon plasma well below the melting temperature, as well as heavy dark matter pairs (or charged co-annihilating partners) in the early universe.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Redox-Induced Structural Reorganization Dictates Kinetics of Cobalt(III) Hydride Formation via Proton-Coupled Electron Transfer

Two-electron, one-proton reactions of a family of [CoCp(dxpe)(NCCH 3 )] 2+ complexes (Cp = cyclopentadienyl, dxpe = 1,2-bis(di(aryl/alkyl)phosphino)ethane) form the corresponding hydride species [HCoCp(dxpe)] + through a stepwise proton-coupled electron transfer process. For three [CoCp(dxpe)(NCCH 3 )] 2+ complexes (dxpe = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), and dcpe (1,2-bis(dicyclohexylphosphino)ethane)), peak shift analysis was employed to quantify apparent proton transfer rate constants from cyclic voltammograms recorded with acids ranging 22 pK a units. The apparent proton transfer rate constants correlate with the strength of the proton source for weak acids, but these apparent proton transfer rate constants curiously plateau (k pl ) as the reaction becomes increasingly exergonic. The absolute apparent proton transfer rate constants across both these regions correlate with the steric bulk of the chelating diphosphine ligand, with bulkier ligands leading to slower kinetics (k plateau,depe = 3.5 x 10 7 M -1 s -1 , k plateau,dppe = 1.7 x 10 7 M -1 s -1 , k plateau,dcpe = 7.1 x 10 4 M -1 s -1 ). Mechanistic studies were conducted to identify the cause of the aberrant $k_PT^{app}$–ΔpK a trends. When the deuterated acids are employed, deuterium incorporation in the Cp ring is observed, indicating protonation of the CoCp(dxpe) species to form the corresponding hydride proceeds via initial ligand protonation. Digital simulations of cyclic voltammograms show ligand loss accompanying initial reduction gates subsequent PCET activity at higher driving forces. Together, these experiments reveal the details of the reaction mechanism: reduction of the Co(III) species is followed by dissociation of the bound acetonitrile ligand, subsequent reduction of the unligated Co(II) species to form a Co(I) species is followed by protonation, which occurs at the Cp ring, followed by tautomerization to generate the stable Co(III)-hydride product [HCoCp(dxpe)] + . Analysis as a function of chelating disphosphine ligand, solvent, and acid strength reveals that the ligand dissociation equilibrium is directly influenced by the steric bulk of the phosphine ligands and gates protonation, giving rise to the plateau of apparent proton transfer rate constant with strong acids. The complexity of the reaction mechanism underpinning hydride formation, encompassing dynamic behavior of the entire ligand set, highlights the critical need to understand elementary reaction steps in proton-coupled electron transfer reactions.

14 SOLAR ENERGY↗
DOE OSTI2021

Ultraviolet photodissociation of gas-phase iron pentacarbonyl probed with ultrafast infrared spectroscopy

It is well known that ultraviolet photoexcitation of iron pentacarbonyl results in rapid loss of carbonyl ligands leading to the formation of coordinatively unsaturated iron carbonyl compounds. Here we employ ultrafast mid-infrared transient absorption spectroscopy to probe the photodissociation dynamics of gas-phase iron pentacarbonyl following ultraviolet excitation at 265 and 199 nm. After photoexcitation at 265 nm, our results show evidence for sequential dissociation of iron pentacarbonyl to form iron tricarbonyl via a short-lived iron tetracarbonyl intermediate. Photodissociation at 199 nm results in the prompt production of Fe(CO) 3 within 0.25 ps via several energetically accessible pathways. An additional 15 ps time constant extracted from the data is tentatively assigned to intersystem crossing to the triplet manifold of iron tricarbonyl or iron dicarbonyl. Mechanisms for formation of iron tetracarbonyl, iron tricarbonyl, and iron dicarbonyl are proposed and theoretically validated with one-dimensional cuts through the potential energy surface as well as bond dissociation energies. Ground state calculations are computed at the CCSD(T) level of theory and excited states are computed with EOM-EE-CCSD(dT).

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

Dissociation of salts in water under pressure

The investigation of salts in water at extreme conditions is crucial to understanding the properties of aqueous fluids in the Earth. We report first principles (FP) and classical molecular dynamics simulations of NaCl in the dilute limit, at temperatures and pressures relevant to the Earth's upper mantle. Similar to ambient conditions, we observe two metastable states of the salt: the contact (CIP) and the solvent-shared ion-pair (SIP), which are entropically and enthalpically favored, respectively. We find that the free energy barrier between the CIP and SIP minima increases at extreme conditions, and that the stability of the CIP is enhanced in FP simulations, consistent with the decrease of the dielectric constant of water. The minimum free energy path between the CIP and SIP becomes smoother at high pressure, and the relative stability of the two configurations is affected by water self-dissociation, which can only be described properly by FP simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Protein Electric Fields Enable Faster and Longer-Lasting Covalent Inhibition of β-Lactamases

The widespread design of covalent drugs has focused on crafting reactive groups of proper electrophilicity and positioning towards targeted amino-acid nucleophiles. In this work, we found that environmental electric fields projected onto a reactive chemical bond, an overlooked design element, play essential roles in the covalent inhibition of TEM-1 beta-lactamase by avibactam. Using the vibrational Stark effect, the magnitudes of the electric fields that are exerted by TEM active sites onto avibactam’s reactive C=O were measured and demonstrate an electrostatic gating effect that promotes bond formation yet relatively suppresses the reverse dissociation. These results suggest new principles of covalent drug design and off-target site prediction. Unlike shape and electrostatic complementary which address binding constants, electrostatic catalysis drives reaction rates, essential for covalent inhibition, and deepens our understanding of chemical reactivity, selectivity, and stability in complex systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗