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1. Structural transitions, octahedral rotations, and electronic properties of A3Ni2O7 rare-earth nickelates under high pressure

   https://doi.org/10.1038/s41535-024-00648-0  

   Geisler, Benjamin; Hamlin, James_J; Stewart, Gregory_R; Hennig, Richard_G; Hirschfeld, P_J (April 2024, npj Quantum Materials)

   Abstract Motivated by the recent observation of superconductivity withT_c ~ 80 K in pressurized La₃Ni₂O₇¹, we explore the structural and electronic properties ofA₃Ni₂O₇bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La₃Ni₂O₇at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations betweenT_cand thein-planeNi-O-Ni bond angles for La₃Ni₂O₇. Moreover, two structural phases with significantc⁺octahedral rotations and in-plane bond disproportionations are uncovered forA = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Nie_gmanifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb₃Ni₂O₇as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism. 

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2. A structural perspective on the temperature dependent activity of enzymes

   https://doi.org/10.1016/j.str.2025.02.013  

   McLeod, Matthew J; Barwell, Sarah AE; Holyoak, Todd; Thorne, Robert E (May 2025, Structure)

   Kuhnel, Karen (Ed.)

   Enzyme activity varies with temperature. Unlike small-molecule catalysts, the structural ensembles of enzymes can change substantially with temperature, but it is unclear how this modulates temperature dependent activity. Here, multi-temperature X-ray crystallography was used to record structural changes from -20C to 40C for a mesophilic enzyme in complex with inhibitors mimicking substrate-, intermediate-, and product-bound states, representative of major complexes on the reaction coordinate. Inhibitors, substrates and active site loops increasingly populated catalytically competent conformations as temperature increased. These changes occurred even in temperature ranges where kinetic measurements showed roughly linear Arrhenius/Eyring behavior, where parameters characterizing the system are assumed to be temperature independent. Simple analysis shows that linear Arrhenius/Eyring behavior can still be observed when the underlying activation energy/enthalpy values vary with temperature. Our results indicate a critical role for temperature dependent atomic-resolution structural data in interpreting temperature dependent kinetic data from enzymatic systems. 

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3. Dithioesters: simple, tunable, cysteine-selective H ₂ S donors

   https://doi.org/10.1039/c8sc04683b  

   Cerda, Matthew M.; Newton, Turner D.; Zhao, Yu; Collins, Brylee K.; Hendon, Christopher H.; Pluth, Michael D. (February 2019, Chemical Science)

   Dithioesters have a rich history in polymer chemistry for RAFT polymerizations and are readily accessible through different synthetic methods. Here we demonstrate that the dithioester functional group is a tunable motif that releases H 2 S upon reaction with cysteine and that structural and electronic modifications enable the rate of cysteine-mediated H 2 S release to be modified. In addition, we use (bis)phenyl dithioester to carry out kinetic and mechanistic investigations, which demonstrate that the initial attack by cysteine is the rate-limiting step of the reaction. These insights are further supported by complementary DFT calculations. We anticipate that the results from these investigations will allow for the further development of dithioesters as important chemical motifs for studying H 2 S chemical biology. 

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4. A theoretical investigation of the effect of Ga alloying on thermodynamic stability, electronic-structure, and oxidation resistance of Ti2AlC MAX phase

   https://doi.org/10.1038/s41598-022-17365-y  

   Sauceda, Daniel; Singh, Prashant; Arroyave, Raymundo (December 2022, Scientific Reports)

   Abstract We present a systematic investigation of thermodynamic stability, phase-reaction, and chemical activity of Al containing disordered Ti 2 (Al-Ga)C MAX phases using machine-learning driven high-throughput framework to understand the oxidation resistance behavior with increasing temperature and exposure to static oxygen. The A-site (at Al) disordering in  Ti 2 AlC MAX (M=Ti, A=Al, X=C) with Ga shows significant change in the chemical activity of Al with increasing temperature and exposure to static oxygen, which is expected to enable surface segregation of Al, thereby, the formation of Al 2 O 3 and improved oxidation resistance. We performed in-depth convex hull analysis of ternary Ti–Al–C, Ti–Ga–C, and Ti–Al–Ga–C based MAX phase, and provide detailed contribution arising from electronic, chemical and vibrational entropies. The thermodynamic analysis shows change in the Gibbs formation enthalpy (Δ G form ) at higher temperatures, which implies an interplay of temperature-dependent enthalpy and entropic contributions in oxidation resistance Ga doped Ti 2 AlC MAX phases. A detailed electronic structure and chemical bonding analysis using crystal orbital Hamilton population method reveal the origin of change in phases stability and in oxidation resistance in disorder Ti 2 (Al 1−x Ga x )C MAX phases. Our electronic structure analysis correlate well with the change in oxidation resistance of Ga doped MAX phases. We believe our study provides a useful guideline to understand to role of alloying on electronic, thermodynamic, and oxidation related mechanisms of bulk MAX phases, which can work as a precursor to understand oxidation behavior of two-dimensional MAX phases, i.e., MXenes (transition metal carbides, carbonitrides and nitrides). 

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5. Correlative Analysis of Metal Organic Framework Structures through Manifold Learning of Hirshfeld Surfaces

   https://doi.org/10.1039/C8ME00014J  

   Shen, Xiaozhou; Zhang, Tianmu; Broderick, Scott; Rajan, Krishna (January 2018, Molecular Systems Design & Engineering)

   We demonstrate the use of non-linear manifold learning methods to map the connectivity and extent of similarity between diverse metal-organic framework (MOF) structures in terms of their surface areas by taking into account both crystallographic and electronic structure information. The fusing of geometric and chemical bonding information is accomplished by using 3-dimensional Hirshfeld surfaces of MOF structures, which encode both chemical bonding and molecular geometry information. A comparative analysis of the geometry of Hirshfeld surfaces is mapped into a low dimensional manifold through a graph network where each node corresponds to a different compound. By examining nearest neighbor connections, we discover structural and chemical correlations among MOF structures that would not have been discernible otherwise. Examples of the types of information that can be uncovered using this approach are given. 

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