Ground-state Electronic Structure Research Articles

Nature of the Nickel-Halogen Bond in NiX+ Compounds (X = F, Cl, Br, I): A Multireference Configuration Interaction and Ab Initio Valence Bond Study.

In a recent study (Inorg. Chem. 2024, 63, 11812-11820), gas-phase cationic NiX+ compounds (X = F, Cl, Br, and I) were probed by nickel L-edge XAS, from which it was concluded that NiF+ was best described as an ionic [Ni2+F-]+ complex while the remaining three compounds were described as covalent Ni(3d9)L species (L depicts a ligand-based hole). An abrupt transition from a classical to an inverted ligand field was suggested as responsible for the change in ground-state electronic structures. Herein, the NiX+ series is investigated by using MRCI and ab initio VB calculations. Nickel L-edge X-ray absorption spectra were also modeled within a ROCIS formalism. It was found that all four compounds possess normal ligand fields and nominal Ni(3d9) electron counts. Furthermore, it was found that the dominant bonding interactions in NiF+ and NiCl+ are two 2-center/3-electron (2c/3e) bonds, while in NiBr+ and NiI+ the dominant bonding interactions are one Ni-X covalent bond and one 2c/3e bond. The change in bonding interactions between NiF+/NiCl+ vs NiBr+/NiI+ was rationalized by considering the marked decrease in resonance energy that stabilizes the 2c/3e bond for the heavier halide congeners, which results from the disparity in electronegativities between the nickel-center and the heavier halides.

Simulating Real-Time Molecular Electron Dynamics Efficiently Using the Time-Dependent Density Matrix Renormalization Group.

Compared to ground-state electronic structure optimizations, accurate simulations of molecular real-time electron dynamics are usually much more difficult to perform. To simulate electron dynamics, the time-dependent density matrix renormalization group (TDDMRG) has been shown to offer an attractive compromise between accuracy and cost. However, many simulation parameters significantly affect the quality and efficiency of a TDDMRG simulation. So far, it is unclear whether common wisdom from ground-state DMRG carries over to the TDDMRG, and a guideline on how to choose these parameters is missing. Here, in order to establish such a guideline, we investigate the convergence behavior of the main TDDMRG simulation parameters, such as time integrator, the choice of orbitals, and the choice of matrix-product-state representation for complex-valued nonsinglet states. In addition, we propose a method to select orbitals that are tailored to optimize the dynamics. Lastly, we showcase the TDDMRG by applying it to charge migration ionization dynamics in furfural, where we reveal a rapid conversion from an ionized state with a σ character to one with a π character within less than a femtosecond.

Preparation and Ground-State Electronic Structure of Heterobimetallic Yb-PtIV-Alkyl Complexes.

This article focuses on the synthesis of heterobimetallic complexes of lanthanide and platinum. It describes the synthesis of the Cp*Yb(bipym)PtMe2 complex and its characterization, followed by its reactivity with oxidants, giving access to various Pt + IV compounds of trismethyl (PtMe3) and tetramethyl (PtMe4) fragments. Characterization of the electronic properties of the complexes by magnetic measurements demonstrated that the tetramethyl complex possesses a singlet ground state. The trismethyl fragments, on the other hand, have a ground state that evolves as a function of the ligand saturating the coordination sphere: a singlet for triflate and pyridine and a triplet for iodine, demonstrating the capacity for simple tuning of the electronic structure of these complexes. While the addition of B(C6F5)3 to the platinum + II bis methyl complex leads to FLP-like reactivity triggering THF opening, reactivity with [Ph3C]+[BPh4]- leads to oxidation of the bipym ligand. Furthermore, the light reactivity of the tetramethyl complex indicated the possible transfer of a methyl group, leading to functionalization of the bridging bipym ligand.

Predicting ELNES/XANES spectra by machine learning with an atomic coordinate-independent descriptor and its application to ground-state electronic structures

ELNES/XANES spectra can be observed using TEM or synchrotron radiation and can elucidate the unoccupied state electronic structures of an excited states. The computation of their features is usually demanding substantial computational resources due to the requisite structure optimization and electronic structure calculations. Herein, we leverage a machine learning technique alongside an atomic-coordinate-independent descriptor, SMILES, to yield the ELNES/XANES spectra, directly, with heightened precision. Moreover, our approach extends to obtain ground state electronic structure, namely PDOS at both occupied and unoccupied ground states, underscoring its viability for a ground-state spectroscopy. Our study revealed that incorporation of long-SMILES molecules into the training dataset enhances prediction accuracy for such molecular structures. This study's direct derivation of spectroscopy from SMILES strings holds promise for expediting spectroscopic inquiries.

A comprehensive analysis of the spectroscopic and drug-like properties of dimethyl 5-hydroxybenzene-1,3-dicarboxylate: insights from DFT and MD simulations

ABSTRACT An in-depth investigation was undertaken to explore the ground-state molecular and electronic structure of Dimethyl 5-hydroxybenzene-1,3-dicarboxylate (D5HD). The potential energy scan of D5HD at various dihedral angles identified the lowest energy conformer, which was re-optimised at the higher basis set. The recorded FT-Raman and FT-IR spectra were compared with corresponding theoretical spectra, demonstrating a high degree of coherence. A comprehensive Natural Bond Orbital analysis was done for D5HD to gain insights for the overall electronic distribution within the molecule. Additionally, nonlinear optical (NLO) properties along with various DFT-based descriptors were calculated. Molecular docking studies against human acetylcholinesterase enzyme (PDB 2JF0), D5HD displayed good binding affinity. The study revealed that hydrogen bonded as well as van der Waals interactions exist between D5HD and different residues in the binding cavity of the target protein. The 100 ns MD simulation confirms the conformational stability of protein (2JF0)–ligand (D5HD) complex. The binding energy and residue decomposition analysis performed using MM-GBSA approach showed positive results with binding energy of −17.76 kcal/mol. Furthermore, D5HD effectively passed all pharmacokinetic filters, establishing its potential as a promising candidate in the quest for novel acetylcholinesterase inhibitors. Highlights Systematic exploration of the structure and spectroscopic properties of D5HD. Molecular docking between D5HD and acetylcholinesterase (PDB 2JF0) reveals the establishment of hydrogen bonds and Van der Waals interactions. D5HD successfully complies with all relevant pharmacokinetic filters. MD simulation analysis on 2JF0–D5HD complex confirms the conformational stability. Energy and residue decomposition based on MMGBSA approach.

Design of Charge Transfer Donor-Acceptor Co-Crystals with Long Lived Charge Carriers

Organic donor-acceptor co-crystals with long exciton lifetimes have emerged as a promising class of semiconductors for their unique photophysical properties. These are crystalline, single-phase materials composed of two or more different compounds in a stoichiometric ratio, providing a platform for unveiling the structure–property relationships at a molecular level. Importantly, co-crystals that are packed through π stacking interactions have the ability to form charge transfer (CT) excitons within donor-acceptor pairs.1,2,3 We hypothesize that co-crystals with a high degree of CT will have longer exciton lifetimes. Therefore, we design and synthesized of new co-crystals using derivatives of N,N-bis(3-pentyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI, acceptor) with a series of donor molecules, including peri-xanthenoxanthene (PXX), perylene, and triphenylene. High quality single crystals were isolated by slow evaporation at room temperature from suitable solvent mixtures. Their crystal structures were successfully refined by single-crystal X-ray diffraction and the phase purity was validated by powder diffraction. Crystal structures revealed that the driving force for forming these co-crystals is π...π stacking. We measured diffuse reflectance UV/visible spectra of each of these co-crystals and compared with computed spectra to assign the observed electronic transitions. DFT calculations provide additional insights into the nature of the excited state CT interactions which can be derived from ground state electronic structure calculations. Transient absorption measurements were conducted to understand their long exciton lifetime for each of as synthesized co-crystals. We conclude that this study supported our previous hypothesis and such materials are potentially useful in applications ranging from photovoltaics and opto-electronics to photocatalysis. References 1) A. Abou Taka, J. E. Reynolds Iii, N. C. Cole-Filipiak, M. Shivanna, C. J. Yu, P. L. Feng, et al. Phys. Chem. Chem. Phys. 2023, 25, 27065.2) L. Sun, Y. Wang, F. Yang, X. Zhang and W. Hu. Adv. Mater. 2019, 31, 1902328.3) I. Schlesinger, N. E. Powers-Riggs, J. L. Logsdon, Y. Qi, S. A. Miller, R. Tempelaar, et al. Chem. Sci., 2020, 11, 9532–9541.

Toward the Atomic-Level Analysis of Ground-State Electronic Structures of Solid Materials via Prediction from Core-Loss Spectra: A Computational Study in Si

Toward the Atomic-Level Analysis of Ground-State Electronic Structures of Solid Materials via Prediction from Core-Loss Spectra: A Computational Study in Si

An under-approximation of entropy for elemental multiconfigurational ground state electronic structures

An under-approximation of entropy for elemental multiconfigurational ground state electronic structures

Stability and Electronic Structure of Nitrogen-Doped Graphene-Supported Cun (n = 1-5) Clusters in Vacuum and under Electrochemical Conditions: Toward Sensor and Catalyst Design.

Here, we present a detailed computational study of the stability and the electronic structure of nitrogen-doped graphene (N4V2) supported Cun (n = 1-5) clusters, which are promising carbon-dioxide electroreduction catalysts. The binding of the clusters to the nitrogen-doped graphene and the electronic structure of these systems were investigated under vacuum and electrochemical conditions. The stability analysis showed that among the systems, the nitrogen-doped graphene bound Cu4 is the most stable in vacuum, while in an electrolyte, and at a negative potential, the N4V2-Cu3 is energetically more favorable. The ground state electronic structure of the nitrogen-doped graphene substrate undergoes topological phase transition, from a semimetallic state, and we observed a metallic and topologically trivial state after the clusters are deposited. The electrode potential adjusts the type and density of the charge carriers in the semimetallic models, while the structures containing copper exhibit bands which are deformed and relaxed by the modified number of electrons.

Ground state property calculations of LiHn complexes using IBM Qiskit’s quantum simulator

In this study, the variational quantum eigensolver (VQE) on a quantum simulator is used in calculating ground state electronic structure properties of the LiHn, n = 1–3, complexes including their singly charged ions. Results calculated using classical electronic structure algorithms are also included. We investigate the use of the unitary coupled cluster with singles and doubles (UCCSD) Ansatz using VQE within Qiskit and compare results to full configuration interaction (FCI) calculations. Computed ground state energies, electron affinities, ionization potentials, and dipole moments are considered. We report the first-of-its-kind simulated quantum computing results of selected LiHn species and use the parity orbital to qubit mapping scheme. We find that VQE/UCCSD results are comparable to classical coupled clusters with singles and doubles for all considered systems with respect to FCI. A VQE calculation cost evaluation is included in which we evaluate performance using both Jordan–Wigner and parity orbital to qubit mapping schemes. We also discuss some of the current limitations of utilizing VQE for the study of chemical systems.

Mechanistic insights into diversified photoluminescence behaviours of BF2 complexes of N-benzoyl 2-aminobenzothiazoles.

Many BF2 complexes of heteroaromatics are well known for their dual-state emission (DSE) properties. However, AIE and ACQ effects have also been observed in certain cases. To date, no rational explanations have been proposed for these uncommon photoluminescence (PL) behaviours. The current research prepared four BF2 complexes of N-benzoyl 2-aminobenzothiazoles with diversified photoluminescence (PL) properties as model compounds and utilized quantum chemical calculation tools to address this issue. Theoretical calculations revealed that the electron-donating groups (EDGs) at the para-position of the exocyclic phenyl ring exert significant influence on their ground-state electronic structures and vertical excitation features. Potential energy curve (PEC) analysis showed that the exocyclic phenyl ring and NMe2 could not function as effective rotors due to elevated energy barriers. Only the NPh2 of BFBB-3 could spontaneously rotate ∼60° to induce the formation of an emissive twisted intramolecular charge transfer (TICT) state. The two-channel model involving both vibronic relaxation and S0/S1 surface crossing revealed that the drastic narrowing of the S1/S0 energy gap in the region approaching minimun energy conical intersection (MECI) led to the generation of a dark state in BFBB-1. The small energy barrier to access the dark-state region makes the resulting fast internal conversion a competitive channel for excited-state deactivation. In contrast, the presence of EDGs in BFBB-2 and 4 inhibits this pathway, thereby resulting in intense fluorescence emissions in solution. In addition, crystallographic analysis illustrated that the F atoms perpendicular to the polyheterocycle promoted a slipped face-to-face packing mode and enhanced intermolecular interactions. The efficiencies of their solid-state emissions are mainly affected by the degree of π-π overlaps.

Towards reliable and efficient modeling of [Cu2O2]2+-based compound electronic structures with the partially fixed reference space protocols.

This work reports a computationally efficient approach for reliable modeling of complex electronic structures based on [Cu2O2]2+ moieties. Specifically, we explore the recently developed partially fixed reference space (PFRS) protocol to minimize the active space size, taking into account the double d-shell effects. We show that the ground-state electronic structure of the core [Cu2O2]2+ model system is dominated by the d9/d10 occupations. The PFRS-crafted active spaces are further used to generate the reference wave functions for the multi-reference coupled cluster, configuration interaction, and multi-reference perturbation theory calculations. Specifically, we demonstrate that the bare [Cu2O2]2+ core can be modeled qualitatively using active spaces as small as CAS(2,2)PFRS. To obtain quantitative agreement with the reference DMRG(32,62)CI calculations, the CAS(4,4) has to be used in conjunction with the MRCCSD correction on top of it. This reliable and computationally efficient protocol is further used to model the electronic structure and properties of ammonia coordinated [Cu2O2]2+ complexes. Finally, based on the large amount of available experimental data regarding the oxo-peroxo equilibrium of [Cu2O2]2+-based systems, it is possible to formulate educated guesses regarding the effect of each experimental variable over each d-occupancy-specific state. With a large sample size of d-occupancy-specific state dependence with ligands and solvents, it should be possible to propose new ligands with specific d-occupancy and, therefore, oxidative properties based on the d-occupancy energy gaps of relatively low-cost calculations.

(Invited) DFT Studies on Interactions between Heteropolyacids, Noble Metal Ions, and TiO2 Support

Heteropolycompounds of Keggin structure have been used as homogeneous and heterogeneous catalysts for decades. Recently, they gained popularity in photo- and electrocatalysis where they can be used as components of complex materials e.g., composed of metal ions/heteropolyacids/oxide-based supports.The aim of the current theoretical studies is to characterize systems comprising of free and TiO2-supported heteropolyanions (PW12O40 3-, PMo12O40 3-) with embedded noble metal ions (Ag, Au, Cu, Pd, and Pt) acting as single atom catalysts. Their ground-state geometric and electronic structures were elucidated. Next, their redox and photochemical properties were determined: frontier (HOMO, LUMO) orbitals were defined, energy gaps and UV-VIS spectra were calculated, total and partial density of states were plotted and analysed. The interactions between components of the systems, in particular the influence of the support on the heteropolycompound electronic structure, were investigated. Next, the reactivity of the noble metal cations towards typical reagents (CO, H2, CH3, and C6H6) was studied.The performed calculations demonstrated that the type of the metal and its oxidation state determines the energetics and character of the frontier orbitals, bond lengths and bond orders in their first coordination sphere, incl. the strength of their interactions with common reagents. The presence of TiO2 as the support is manifested by the emergence of new metal ion sitting sites, usually located at the interface between the heteropolyacid and the support, and modification of the redox properties of the systems.Acknowledgements: This work was supported by the National Science Centre, Poland within Solar-Driven Chemistry project no 2019/01/Y/ST4/00024.

DFT Studies on Cun/TiO2 (n=1-7) System for Water Splitting

Copper deposited on titania is active in numerous catalytic reactions, incl. water splitting [1]. The activity of the system depends on the size of the copper phase, whether it is present as single atoms, sub-nanometer clusters, or nanoparticles.The aim of the current contribution is to elucidate the relationship between the size of the copper active phase and its electronic and geometric properties by performing Density Functional Theory (DFT) calculations with PBE functional and the def2-TZVP basis set.Theoretical model of the system comprised of a cluster model of anatase phase of titania – (TiO2)34 - with clusters of copper of varying sizes on top of it. The ground-state geometric and electronic structures were elucidated. Next, their redox and photochemical properties were determined: frontier (HOMO, LUMO) orbitals were defined, energy gaps, total and partial density of states were plotted and analysed. The interactions between components of the systems, in particular the influence of the support on the copper electronic structure, were investigated. Next, the reactivity of the metal phase towards typical intermediates observed during water splitting (H2O, OH, O, and H) was studied.The performed calculations demonstrated that the presence of metal determines the energetics and character of the frontier orbitals of the catalyst. The size of copper active phase influences the bond lengths and bond orders with studied reagents.Acknowledgements: This work was supported by the National Science Centre, Poland within Solar-Driven Chemistry project no 2019/01/Y/ST4/00024. This publication is based upon work from COST Action CA 18234 “Computational materials sciences for efficient water splitting with nanocrystals from abundant elements”, supported by COST (European Cooperation in Science and Technology). We gratefully acknowledge Polish high-performance computing infrastructure PLGrid (HPC Centers: ACK Cyfronet AGH) for providing computer facilities and support within computational grant no. PLG/2020/013927.Literature:[1] Cheng Cheng, Wei-Hai Fang, Run Long, and Oleg V. Prezhdo, JACS Au 2021, 1, 5, 550–559

Understanding LiNiO2 Electronic Structure and Redox Mechanism by Raman and X-Ray Techniques

LiNiO2 (LNO) is considered a very promising and high energy density alternative to Co-containing Li-ion battery cathodes. It is also the benchmarked cathode material of the European BIGMAP project, which aim is to combine advance operando characterization, computational modelling and artificial intelligence in order to accelerate the research and development of new generation batteries.However, the material suffers from several degradation issues resulting in fading cell performance. In particular, the presence of highly reactive Ni4+ species at the electrode/electrolyte interface1, Ni dissolution from the cathode leading to the loss of active material 2, and O2 release at high voltage are the major drawbacks precluding commercial applications. A recent in situ study1 has shown that O2 release compromises the structural stability and triggers reactions with ethylene carbonate (EC), prompting CO2evolution3,4.In parallel, solid state physics community is also largely interested into LiNiO2 in the framework of a larger investigation around the electronic and atomic structure of nickelates, in particular rare-earth nickelates. This interest rises from the fact that there is still no consensus on the atomic/electronic structure of LiNiO2, while different descriptions exist in literature. Local probe techniques demonstrate the distortion of NiO6 octahedra, and there are two on-going theories for explaining the presence of this distortion: (i) Ni3+ (t2g6 eg1) Jahn-Teller (JT) distortion, (ii) 2 Ni3+ → Ni2+ + Ni4+ bond disproportionation (BD)5. Surprisingly, the local order distortion does not result in the long-range distortion, the mechanism behind this phenomenon is not completely understood.In this contribution we present a detailed study of LNO cathode material by operando Raman and X-ray absorption spectroscopy (XAS) at the Ni K-edge, ex situ X-ray emission spectroscopy (XES) and resonant Inelastic X-ray scattering (RIXS). Distinguishing between JT and BD models is challenging, and therefore NaNiO2 reference is used, which is isoelectronic material to LiNiO2, and is known to feature long-range JT distortion.While XAS is widely established technique to probe local structure and transition metal oxidation state, RIXS is useful for assessing d-d transitions and bond disproportionation. Operando Raman spectroscopy is an excellent tool for studying phase transitions in Li-ion battery cathodes and is therefore complementary to the employed X-ray techniques6. It provides information on two characteristic for layered oxides crystalline phonon modes A1g and Eg, and sheds light on both short-range order and cationic/anionic redox. The penetration depth is estimated as top few hundred nanometers of around five micrometres sized secondary particle, therefore near-surface area is probed. The results show a complex behaviour of band positions and intensities during cycling (figure 1), corresponding to four phases transformations. Notably the peaks intensities increase considerably, while the widths are decreased, which is evidence of increased order upon material delithiation, and is in agreement with the Extended X-ray Absorption Fine Structure (EXAFS). Raman is also used to evaluate the contribution of anionic redox, since all the reduced oxygen species (superoxide, peroxide and molecular oxygen) have well-known Raman-active modes.To conclude, we demonstrate how a combination of multiple vibrational and X-rays based spectroscopies provides a better understanding of both LNO ground state electronic structure and redox mechanism. This increased fundamental understanding will stimulate the design of better strategies for prevention of the material degradation and improved cycle retention.

Effect of anion and methylation on the interionic interactions and reactivity of 1-butyl-2,3-dimethyl imidazolium-based ionic liquids

The study presents the investigation of anion and methylation effects on the interionic interactions, thus explaining the variations in the reactivity, electronic, and spectroscopic features in 1-butyl-2,3-dimethyl imidazolium-based ionic liquids with chloride, tetrafluoroborate, and hexafluorophosphate anions. Ground state electronic structures, IR and Raman spectra, electronic transition characteristics, and chemical shifts of the ionic liquids were obtained using DFT calculations at the B3LYP/6–311++G(d,p) level of theory. The calculated electron densities were analyzed to determine intra- and interionic interactions at critical points using Reduced Density Gradient and topological analysis through the atoms-in-molecules approach. Experimental spectroscopic characterizations were performed using the FT-IR, FT-Raman, NMR, and UV–vis techniques. These techniques and DFT calculations provide essential contributions to interpreting spectral features concerning the structure and dynamics of ionic liquids. Furthermore, molecular docking studies were performed to determine the binding status of studied ionic liquids with human serum albumin to explore their activity, thus reporting their potential due to their demand in pharmacology.

Mass-selected ion-molecule cluster beam apparatus for ultrafast photofragmentation studies.

We describe an apparatus for investigating the excited-state dissociation dynamics of mass-selected ion-molecule clusters by mass-resolving and detecting photofragment-ions and neutrals, in coincidence, using an ultrafast laser operating at high repetition rates. The apparatus comprises a source that generates ion-molecule clusters, a time-of-flight spectrometer, and a mass filter that selects the desired anions, and a linear-plus-quadratic reflectron mass spectrometer that discriminates the fragment anions after the femtosecond laser excites the clusters. The fragment neutrals and anions are then captured by two channeltron detectors. The apparatus performance is tested by measuring the photofragments: I-, CF3I-, and neutrals from photoexcitation of the ion-molecule cluster CF3I·I- using femtosecond UV laser pulses with a wavelength of 266nm. The experimental results are compared with our ground state and excited state electronic structure calculations as well as the existing results and calculations, with particular attention to the generation mechanism of the anion fragments and dissociation channels of the ion-molecule cluster CF3I·I- in the charge-transfer excited state.

Computational and photophysical probing interactions of newly synthesized benzothiazole azo-bonded molecules acting as potent antitubercular agents

In the present work, we have presented a very detailed study of unprecedented mono-azo benzothiazole derivatives. The targeted molecules were synthesized by a single-step involving diazotizing the primary aromatic amines followed by resultant diazonium salts coupling with N, N-Diethyl-m-toluidine (DEMT) through electrophilic substitution reaction influenced by positive mesomeric effect (+M effect) under suitable experimental conditions. The FT-IR, UV–vis, Photoluminescence (PL), 1H NMR, 13C NMR, and HRMS techniques were used to investigate the synthesized mono-azo benzothiazole dye derivatives. Additionally, for the first time, calculations based on density functional theory have been used to examine the ground state geometry, electronic structure, and molecular characteristics potential region maps of four benzothiazole mono-azo architectures (CR1-CR4). The effectiveness of the synthesized dye colourants against M. tuberculosis, H37Rv was assessed by utilizing the agar well diffusion assay method (ATCC 27,294), amongst the studied mono-azo derivatives, CR3 and CR4 exhibited a good zone of inhibition compared to CR1 and CR2. The compound CR1 demonstrated sensitivity at 20 µM, compared to 48.2 µM for the reference gentamicin. The antibacterial activity of the CR1 and CR4 compounds was revealed to have a 19 mm zone of inhibition. The derivative CR1 displayed 42.86 and CR3 displayed 38.75 of the concentration IC50 in 100 μl/ml, respectively, based on their cytotoxic potentiality.

Second-sphere tuning of analogues for the ferric-hydroperoxoheme form of Mycobacterium tuberculosis MhuD

Mycobacterium tuberculosis MhuD catalyzes the oxygenation of heme to mycobilin; experimental data presented here elucidates the novel hydroxylation reaction catalyzed by this enzyme. Analogues for the critical ferric–hydroperoxoheme (MhuD–heme–OOH) intermediate of this enzyme were characterized using UV/Vis absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectroscopies. In order to extract electronic transition energies from these spectroscopic data, a novel global fitting model was developed for analysis of UV/Vis Abs, CD, and MCD data. A variant of MhuD was prepared, N7S, which weakens the affinity of heme-bound enzyme for a hydroperoxo analogue, azide, without significantly altering the protein secondary structure. Global fitting of spectroscopic data acquired in this study revealed that the second-sphere N7S substitution perturbs the electronic structure of two analogues for MhuD–heme–OOH: azide-inhibited MhuD (MhuD–heme–N3) and cyanide-inhibited MhuD (MhuD–heme–CN). The ground state electronic structures of MhuD–heme–N3 and MhuD–heme–CN were assessed using variable-temperature, variable-field MCD. Altogether, these data strongly suggest that there is a hydrogen bond between the Asn7 side-chain and the terminal oxygen of the hydroperoxo ligand in MhuD–heme–OOH. As discussed herein, this finding supports a novel hydroxylation reaction mechanism where the Asn7 side-chain guides a transient hydroxyl radical derived from homolysis of the OO bond in MhuD–heme–OOH to the β- or δ-meso carbon of the porphyrin ligand yielding β- or δ-meso-hydroxyheme, respectively.

An over-approximation of entropy for elemental multiconfigurational ground state electronic structures

The ability to predict the chemical and physical properties of a material is directly related to the structure and interactions of its electrons. For materials comprised of f-block elements (the lanthanides and actinides found in the last two rows of the periodic table), the complexity of electronic structure has presented great difficulty in understanding, modeling, and predicting material properties. The complexity of multiconfigurational ground state electronic structures is illustrated herein by the combinatorics of electron permutations within individual and cumulative occupancy configurations. A non-integer orbital occupancy representation of multiconfigurational ground states is described for superposition mixing between multiple near-energy degenerate occupancy configurations and generalized in such a way that established ground states are returned by approximation for elements with less-complex electronic structures. By considering the occupancy configurations as statistical mechanics macrostates, and the permutations of electrons as statistical mechanics microstates within those macrostates, an over-approximation of entropy for multiconfigurational elemental ground state electronic structures has been calculated.