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1. A Convenient Approach for the Construction of Lewis Structures by Focusing on the Valence Requirements of Outer Atoms

   https://doi.org/10.1021/acs.jchemed.3c00702  

   Parkin, Gerard (December 2023, Journal of Chemical Education)

   A convenient approach to obtain Lewis structures for compounds of the type YXn involves first constructing a trial structure that satisfies the valence of the outer atoms (e.g. 1 bond for fluorine, 2 bonds for oxygen and 3 bonds for nitrogen) and placing the molecular charge (if any) on the central atom. The second step involves evaluating the electron count of the central atom, which can give rise to three possibilities: (i) if the central atom has an octet configuration, no change in the number of bonds is required, (ii) if the central atom (Y) exceeds the octet, a Y–X bond is relocated as a lone pair on X, which results in a formal positive charge on Y and a formal negative charge on X, and (iii) if the electron count on the central Y atom is less than an octet, a lone pair on the outer atom is relocated as a Y–X bond, which results in a formal negative charge on Y and a formal positive charge on X; these transformations modify the electron configuration around X such that it will adopt a correct Lewis structure. This approach differs considerably from other methods that require one to first calculate the total number of valence electrons. As such, the method described here, which focuses on using valence as a guiding chemical principle, is much less mathematically oriented and therefore less subject to errors from incorrect calculations. 

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2. Binding energies of hydrated cobalt( ii ) by collision-induced dissociation and theoretical studies: evidence for a new critical size

   https://doi.org/10.1039/C7CP05828D  

   Coates, Rebecca A.; Armentrout, P. B. (January 2018, Physical Chemistry Chemical Physics)

   The experimental sequential bond energies for loss of water from Co 2+ (H 2 O) x complexes, x = 5–11, are determined by threshold collision-induced dissociation (TCID) using a guided ion beam tandem mass spectrometer with a thermal electrospray ionization source. Kinetic energy dependent TCID cross sections are analyzed to yield 0 K thresholds for sequential loss of neutral water molecules. The thresholds are converted from 0 to 298 K values to give hydration enthalpies and free energies. Theoretical geometry optimizations and single point energy calculations at several levels of theory are performed for the reactant and product ion complexes. Theoretical bond energies for ground structures are used for direct comparison with experimental values to obtain structural information on these complexes. In addition, the dissociative charge separation process, Co 2+ (H 2 O) x → CoOH + (H 2 O) m + H + (H 2 O) x−m−1 , is observed at x = 4, 6, and 7 in competition with primary water loss products. Energies for the charge separation rate-limiting transition states are calculated and compared to experimental threshold measurements. Results suggest that the critical size for which charge separation is energetically favored over water loss is x crit = 6, in contrast to lower values in previous literature reports. 

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3. Structural considerations for charge‐enhanced Brønsted acid catalysts

   https://doi.org/10.1002/poc.4069  

   Payne, Curtis; Kass, Steven R. (March 2020, Journal of Physical Organic Chemistry)

   Abstract All threeN‐methylated andN‐protonated hydroxypyridinium BAr^F₄^–salt isomers were synthesized and their hydrogen bond donating abilities were investigated. DFT and G4 theory computations along with IR spectroscopic measurements were found to be effective methods for predicting the catalytic activities of these O–H and N–H Brønsted acids. A UV‐vis titration approach for rapidly quantifying hydrogen bond donating ability revealed that carbon‐hydrogen bonds also can participate in electrostatic interactions, but the presence of multiple equilibrium complexes results in a limitation of this method. In the methylated series of hydroxypyridines, the ortho and para isomers displayed modest rate enhancements relative to the meta derivative. Protonation introduces a new acidic site and the ortho hydroxypyridinium ion salt is a significantly more active catalyst than all of the other species examined. This is indicative of bidentate activation by the N–H and O–H acidic sites, and suggests a new design strategy for improving charge‐enhanced catalysts. 

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4. Hydrogen doping in wide-bandgap amorphous In–Ga–O semiconductors

   https://doi.org/10.1039/D0TC03370G  

   Medvedeva, Julia E.; Bhattarai, Bishal (January 2020, Journal of Materials Chemistry C)

   Microscopic mechanisms of the formation of H defects and their role in passivation of under-coordinated atoms, short- and long-range structural transformations, and the resulting electronic properties of amorphous In–Ga–O with In : Ga = 6 : 4 are investigated using computationally-intensive ab initio molecular dynamics simulations and accurate density-functional calculations. The results reveal a stark difference between H-passivation in covalent Si-based and ionic oxide semiconductors. Specifically, it is found that hydrogen doping triggers an extended bond reconfiguration and rearrangement in the network of shared polyhedra in the disordered oxide lattice, resulting in energy gains that outweigh passivation of dangling O-p-orbitals. The H-induced structural changes in the coordination and morphology favor a more uniform charge density distribution in the conduction band, in accord with the improved carrier mobility measured in H-doped In–Ga–O [W. Huang et al. , Proc. Natl. Acad. Sci. U. S. A. , 2020, 117 , 18231]. A detailed structural analysis helps interpret the observed wide range of infrared frequencies associated with H defects and also demonstrate that the room-temperature stability of OH defects is affected by thermal fluctuations in the surrounding lattice, promoting bond migration and bond switching behavior within a short picosecond time frame. 

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5. Competition between chalcogen and halogen bonding assessed through isostructural species

   https://doi.org/10.1107/S205322962201052X  

   De Silva, Viraj; Magueres, Pierre Le; Averkiev, Boris B.; Aakeröy, Christer B. (December 2022, Acta Crystallographica Section C Structural Chemistry)

   The amino group of 2-amino-5-(4-halophenyl)-1,3,4-chalcogenadiazole has been replaced with bromo/iodo substituents to obtain a library of four compositionally related compounds. These are 2-iodo-5-(4-iodophenyl)-1,3,4-thiadiazole, C₈H₄I₂N₂S, 2-bromo-5-(4-bromophenyl)-1,3,4-selenadiazole, C₈H₄Br₂N₂Se, 2-bromo-5-(4-iodophenyl)-1,3,4-selenadiazole, C₈H₄BrIN₂Se, and 2-bromo-5-(4-iodophenyl)-1,3,4-thiadiazole, C₈H₄BrIN₂S. All were isostructural and contained bifurcated Ch...N (Ch is chalcogen) andX...X(Xis halogen) interactions forming a zigzag packing motif. The noncovalent Ch...N interaction between the chalcogen-bond donor and the best-acceptor N atom appeared preferentially instead of a possible halogen bond to the same N atom. Hirshfeld surface analysis and energy framework calculations showed that, collectively, a bifurcated chalcogen bond was stronger than halogen bonding and this is more structurally influential in this system. 

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