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1. Separation of halogenated benzenes enabled by investigation of halogen–π interactions with carbon materials

   https://doi.org/10.1039/c9sc04906a  

   Kanao, Eisuke ; Morinaga, Takuya ; Kubo, Takuya ; Naito, Toyohiro ; Matsumoto, Takatoshi ; Sano, Tomoharu ; Maki, Hideshi ; Yan, Mingdi ; Otsuka, Koji ( January 2020 , Chemical Science)

   The halogen–π (X–π) interaction is an intermolecular interaction between the electron-poor region of bonded halogen atoms and aromatic rings. We report an experimental evaluation of the halogen–π (X–π) interaction using liquid chromatography with carbon-material coated columns providing strong π interactions in the normal phase mode. A C 70 -fullerene (C70)-coated column showed higher retentions for halogenated benzenes as the number of halogen substitutions increased as a result of X–π interactions. In addition, the strength of the X–π interaction increased in the order of F < Cl < Br < I. Changes to the UV absorption of C70 and the brominated benzenes suggested that the intermolecular interaction changed from the π–π interaction to X–π interaction as the number of bromo substitutions increased. Computer simulations also showed that the difference in dipole moments among structural isomers affected the strength of the π–π interaction. Furthermore, we concluded from small peak shifts in 1 H NMR and from computer simulations that the orbital interaction contributes to the X–π interactions. Finally, we succeeded in the one-pot separation of all isomers of brominated benzenes using the C70-coated column by optimizing the mobile phase conditions. 

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2. Investigations into mechanism and origin of regioselectivity in the metallaphotoredox-catalyzed α-arylation of N -alkylbenzamides

   https://doi.org/10.1039/d2sc01962k  

   Rand, Alexander W. ; Chen, Mo ; Montgomery, John ( September 2022 , Chemical Science)

   A mechanistic study on the α-arylation of N -alkylbenzamides catalyzed by a dual nickel/photoredox system using aryl bromides is reported herein. This study elucidates the origins of site-selectivity of the transformation, which is controlled by the generation of a hydrogen atom transfer (HAT) agent by a photocatalyst and bromide ions in solution. Tetrabutylammonium bromide was identified as a crucial additive and source of a potent HAT agent, which led to increases in yields and a lowering of the stoichiometries of the aryl bromide coupling partner. NMR titration experiments and Stern–Volmer quenching studies provide evidence for complexation to and oxidation of bromide by the photocatalyst, while elementary steps involving deprotonation of the N -alkylbenzamide or 1,5-HAT were ruled out through mechanistic probes and kinetic isotope effect analysis. This study serves as a valuable tool to better understand the α-arylation of N -alkylbenzamides, and has broader implications in halide-mediated C–H functionalization reactions. 

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3. Supramolecular Sandwiches: Halogen-Bonded Coformers Direct [2+2] Photoreactivity in Two-Component Cocrystals

   https://doi.org/10.3390/molecules25040907  

   Quentin, Jay ; C. Swenson, Dale ; R. MacGillivray, Leonard ( February 2020 , Molecules)

   null (Ed.)

   The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene (1,3-di-I-tFb and 1,4-di-I-tFb, respectively) form cocrystals with trans-1,2-bis(2-pyridyl)ethylene (2,2′-bpe) assembled by N···I X-bonds. In each cocrystal, 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-tFb)·(2,2′-bpe), the donor molecules support the C=C bonds of 2,2′-bpe to undergo an intermolecular [2+2] photodimerization. UV irradiation of each cocrystal resulted in stereospecific and quantitative conversion of 2,2′-bpe to rctt-tetrakis(2-pyridyl)cyclobutane (2,2′-tpcb). In each case, the reactivity occurs via face-to-face π-stacked columns wherein nearest-neighbor pairs of 2,2′-bpe molecules lie sandwiched between X-bond donor molecules. Nearest-neighbor C=C bonds are stacked criss-crossed in both cocrystals. The reactivity was ascribed to the olefins undergoing pedal-like motion in the solid state. The stereochemistry of 2,2′-tpcb is confirmed in cocrystals 2(1,3-di-I-tFb)·(2,2′-tpcb) and (1,4-di-I-tFb)·(2,2′-tpcb). 

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4. On the rank of a random binary matrix

   https://doi.org/10.1137/1.9781611975482.58  

   Cooper, Colin ; Frieze, Alan ; Pegden, Wesley ( January 2019 , Proceedings of the annual ACM-SIAM Symposium on Discrete Algorithms)

   We study the rank of a random n × m matrix An, m; k with entries from GF(2), and exactly k unit entries in each column, the other entries being zero. The columns are chosen independently and uniformly at random from the set of all (nk) such columns. We obtain an asymptotically correct estimate for the rank as a function of the number of columns m in terms of c, n, k, and where m = cn/k. The matrix An, m; k forms the vertex-edge incidence matrix of a k-uniform random hypergraph H. The rank of An, m; k can be expressed as follows. Let |C2| be the number of vertices of the 2-core of H, and | E (C2)| the number of edges. Let m* be the value of m for which |C2| = |E(C2)|. Then w.h.p. for m < m* the rank of An, m; k is asymptotic to m, and for m ≥ m* the rank is asymptotic to m – |E(C2)| + |C2|. In addition, assign i.i.d. U[0, 1] weights Xi, i ∊ 1, 2, … m to the columns, and define the weight of a set of columns S as X(S) = ∑j∊S Xj. Define a basis as a set of n – 1 (k even) linearly independent columns. We obtain an asymptotically correct estimate for the minimum weight basis. This generalises the well-known result of Frieze [On the value of a random minimum spanning tree problem, Discrete Applied Mathematics, (1985)] that, for k = 2, the expected length of a minimum weight spanning tree tends to ζ(3) ∼ 1.202. 

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5. Crystal structure of 8-hydroxyquinoline: a new monoclinic polymorph

   https://doi.org/10.1107/S1600536814016110  

   Castañeda, Raúl ; Antal, Sofia A. ; Draguta, Sergiu ; Timofeeva, Tatiana V. ; Khrustalev, Victor N. ( September 2014 , Acta Crystallographica Section E Structure Reports Online)

   In an attempt to grow 8-hydroxyquinoline–acetaminophen co-crystals from equimolar amounts of conformers in a chloroform–ethanol solvent mixture at room temperature, the title compound, C 9 H 7 NO, was obtained. The molecule is planar, with the hydroxy H atom forming an intramolecular O—H...N hydrogen bond. In the crystal, molecules form centrosymmetric dimers via two O—H...N hydrogen bonds. Thus, the hydroxy H atoms are involved in bifurcated O—H...N hydrogen bonds, leading to the formation of a central planar four-membered N 2 H 2 ring. The dimers are bound by intermolecular π–π stacking [the shortest C...C distance is 3.2997 (17) Å] and C—H...π interactions into a three-dimensional framework. The crystal grown represents a new monoclinic polymorph in the space group P 2 1 / n . The molecular structure of the present monoclinic polymorph is very similar to that of the orthorhombic polymorph (space group Fdd 2) studied previously [Roychowdhury et al. (1978). Acta Cryst. B 34 , 1047–1048; Banerjee & Saha (1986). Acta Cryst. C 42 , 1408–1411]. The structures of the two polymorphs are distinguished by the different geometries of the hydrogen-bonded dimers, which in the crystal of the orthorhombic polymorph possess twofold axis symmetry, with the central N 2 H 2 ring adopting a butterfly conformation. 

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