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1. The role of defects in Fe(II)-goethite electron transfer

   Notini, L. ; Latta, D.E. ; Neumann, A.A. ; Pearce, C.I. ; Sassi, M. ; N’Diaye, A.T. ; Rosso, K.M. ; Scherer, M.M. ( February 2018 , Environmental science & technology)

   Despite substantial experimental evidence for Fe(II)–Fe(III) oxide electron transfer, computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited on structurally perfect surfaces. We used a combination of 57Fe Mössbauer spectroscopy, synchrotron X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectroscopies to investigate whether Fe(II)–goethite electron transfer is influenced by defects. Specifically, Fe L-edge and O K-edge XAS indicates that the outermost few Angstroms of goethite synthesized by low temperature Fe(III) hydrolysis is iron deficient relative to oxygen, suggesting the presence of defects from Fe vacancies. This nonstoichiometric goethite undergoes facile Fe(II)–Fe(III) oxide electron transfer, depositing additional goethite consistent with experimental precedent. Hydrothermal treatment of this goethite, however, appears to remove defects, decrease the amount of Fe(II) oxidation, and change the composition of the oxidation product. When hydrothermally treated goethite was ground, surface defect characteristics as well as the extent of electron transfer were largely restored. Our findings suggest that surface defects play a commanding role in Fe(II)–goethite redox interaction, as predicted by computational chemistry. Moreover, it suggests that, in the environment, the extent of this interaction will vary depending on diagenetic history, local redox conditions, as well as being subject to regeneration via seasonal fluctuations. 

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2. Geometrically frustrated interactions drive structural complexity in amorphous calcium carbonate

   https://doi.org/10.1038/s41557-023-01339-2  

   Nicholas, Thomas C. ; Stones, Adam Edward ; Patel, Adam ; Michel, F. Marc ; Reeder, Richard J. ; Aarts, Dirk G. ; Deringer, Volker L. ; Goodwin, Andrew L. ( September 2023 , Nature Chemistry)

   Amorphous calcium carbonate is an important precursor for biomineralization in marine organisms. Key outstanding problems include understanding the structure of amorphous calcium carbonate and rationalizing its metastability as an amorphous phase. Here we report high-quality atomistic models of amorphous calcium carbonate generated using state-of-the-art interatomic potentials to help guide fits to X-ray total scattering data. Exploiting a recently developed inversion approach, we extract from these models the effective Ca⋯Ca interaction potential governing the structure. This potential contains minima at two competing distances, corresponding to the two different ways that carbonate ions bridge Ca²⁺-ion pairs. We reveal an unexpected mapping to the Lennard-Jones–Gauss model normally studied in the context of computational soft matter. The empirical model parameters for amorphous calcium carbonate take values known to promote structural complexity. We thus show that both the complex structure and its resilience to crystallization are actually encoded in the geometrically frustrated effective interactions between Ca²⁺ions.

    

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3. Scrutinizing formally Ni IV centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory

   https://doi.org/10.1039/D3SC02001K  

   DiMucci, Ida M. ; Titus, Charles J. ; Nordlund, Dennis ; Bour, James R. ; Chong, Eugene ; Grigas, Dylan P. ; Hu, Chi-Herng ; Kosobokov, Mikhail D. ; Martin, Caleb D. ; Mirica, Liviu M. ; et al ( June 2023 , Chemical Science)

   Nickel K- and L 2,3 -edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L 2,3 -edge XAS reveals that the physical d-counts of the formally Ni IV compounds measured lie well above the d 6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF 6 2− is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d 6 Ni IV center. The reactivity of Ni IV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers. 

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4. A Computational Comparison of Ether and Ester Electrolyte Stability on a Ca Metal Anode

   https://doi.org/10.34133/2021/9769347  

   Liepinya, Diana ; Smeu, Manuel ( October 2021 , Energy Material Advances)

   Ca-ion batteries (CIBs) have the potential to provide inexpensive energy storage, but their realization is impeded by the lack of suitable electrolytes. Motivated by recent experimental progress, we perform ab initio molecular dynamics simulations to investigate early decomposition reactions at the anode-electrolyte interface. By examining different combinations of solvent—tetrahydrofuran (THF) or ethylene carbonate (EC)—and salt—Ca(BH 4 ) 2 , Ca(BF 4 ) 2 , Ca(BCl 4 ) 2 , and Ca(ClO 4 ) 2 —we identify a variety of behavioral trends between electrolyte solutions. Next, we perform a separate trajectory with pure THF and gradually increased negative charge; despite an addition of -32 e , no THF decomposition is detected. Charge analysis reveals that in a reductive environment, THF distributes excess charge evenly across its hydrocarbon backbone, while EC concentrates charge on its ester oxygens and carbonyl carbon, resulting in decomposition. Graphs of charge vs. time for both solvents reveal that EC decomposition products can be reduced by up to five electrons, while those of THF are limited to a single electron. Ultimately, we find Ca(BH 4 ) 2 and THF to be the most stable solution investigated herein, corroborating experimental evidence of its suitability as a CIB electrolyte. 

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5. Heterometal incorporation in NH 2 -MIL-125(Ti) and its participation in the photoinduced charge-separated excited state

   https://doi.org/10.1039/D0CC05339B  

   Hanna, Lauren ; Long, Conor L. ; Zhang, Xiaoyi ; Lockard, Jenny V. ( October 2020 , Chemical Communications)

   null (Ed.)

   Optical and X-ray spectroscopy studies reveal the location and role of Fe 3+ sites incorporated through direct synthesis in NH 2 -MIL-125(Ti). Fe K-edge XAS analysis confirms its metal–oxo cluster node coordination while time-resolved optical and X-ray transient absorption studies disclose its role as an electron trap site, promoting long-lived photo-induced charge separation in the framework. Notably, XTA measurements show sustained electron reduction of the Fe sites into the microsecond time range. Comparison with an Fe-doped MOF generated through post-synthetic modification indicates that only the direct synthesis approach affords efficient Fe participation in the charge separated excited state. 

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