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1. N -(3,5-Dichloro-4-hydroxyphenyl)acetamide

   https://doi.org/10.1107/S2414314626000751  

   Uppu, Rao M; Fronczek, Frank R (January 2026, IUCrData)

   The title compound, C₈H₇Cl₂NO₂, crystallizes in the triclinic space groupP1with three molecules in the asymmetric unit. Two of them are essentially planar, with mean deviations of 13 non-hydrogen atoms of 0.029 and 0.030 Å, and differ only in the conformation of the O—H hydrogen atom. The third molecule is quite nonplanar, with the CCNO acetamide plane forming a dihedral angle of 67.56 (5)° with the remainder of the molecule. In the extended structure, the two almost planar molecules form antiparallel hydrogen-bonded chains through N—H...O interactions of the acetamide substituents. The N—H group of the nonplanar molecule donates a bifurcated hydrogen bond to the O—H group and a Cl substituent of one of the planar molecules. The O—H groups of all molecules form intermolecular hydrogen bonds to other O—H groups or carbonyl O atoms. Together, the hydrogen bonds generate a three-dimensional network. 

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2. Structure of 2,3,5-triphenyltetrazol-3-ium chloride hemipentahydrate

   https://doi.org/10.1107/S205698902400940X  

   Uppu, Rao M; Chikkula, Krishnaveni; Saneei, Soheil; Babu, Sainath; Fronczek, Frank R (October 2024, Acta Crystallographica Section E Crystallographic Communications)

   The title hydrated molecular salt, C₁₉H₁₅N₄⁺·Cl⁻·2.5H₂O, has two triphenyltetrazolium cations, two chloride anions and five water molecules in the asymmetric unit. The cations differ in the conformations of the phenyl rings with respect to the heterocyclic core, most notably for the C-bonded phenyl ring, for which the N—C—C—C torsion angles differ by 36.4 (3)°. This is likely a result of one cation accepting an O—H...N hydrogen bond from a water molecule [O...N = 3.1605 (15) Å], while the other cation accepts no hydrogen bonds. In the extended structure, the water molecules are involved in centrosymmetric (H₂O)₂Cl₂rings as well as (H₂O)₄chains. An unusual O—H...π interaction and weak C—H...O and C—H...Cl hydrogen bonds are also observed. 

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3. N-(3-Chloro-4-hydroxyphenyl)acetamide

   https://doi.org/10.1107/S2414314625006959  

   Uppu, Rao M; Fronczek, Frank R (August 2025, IUCrData)

   In the title compound, C₈H₈ClNO₂, the acetamide substituent is twisted out of the phenyl plane, forming a dihedral angle of 58.61 (7)°. In the extended structure, each molecule donates two hydrogen bonds [N—H...O(carbonyl) and O—H...O(carbonyl)] and thus also accepts two such hydrogen bonds. The chlorine atom is not involved in the hydrogen bonding. 

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4. Crystal engineering of a 1:1 5-fluorocytosine–4-hydroxybenzaldehyde cocrystal: insights from X-ray crystallography and Hirshfeld analysis

   https://doi.org/10.1107/S2056989025004463  

   Sangavi, Marimuthu; Mohana, Marimuthu; Butcher, Ray J; McMillen, Colin D (June 2025, Acta Crystallographica Section E Crystallographic Communications)

   The 1:1 cocrystal of 5-fluorocytosine (5FC) and 4-hydroxybenzaldehyde (4HB), C₄H₄FN₃O·C₇H₆O₂has been synthesized and its structure characterized by single-crystal X-ray diffraction and Hirshfeld surface analysis. The compound crystallizes in the monoclinicP2₁/cspace group. A robust supramolecular architecture is stabilized by N—H...O, N—H...N, C—H...O and C—H...F hydrogen bonds, formingR₂²(8),R₄⁴(22),R₆⁶(32), andR₈⁸(34) ring motifs. The N—H...O and N—H...N hydrogen bonds form strong directional interactions, contributing to theR₂²(8) andR₈⁸(34) motifs through dimeric and extended ring structures. O—H...O interactions link 5FC and 4HB molecules, generating anR₆⁶(32) ring that enhances the packing. Weaker C—H...F bonds help form theR₄⁴(22) tetrameric motif, supporting the overall three-dimensional supramolecular framework. Additionally, C—F...π interactions between the fluorine atom and the aromatic ring add further to the crystal cohesion. Hirshfeld surface analysis and two-dimensional fingerprint plots confirm that O...H/H...O contacts are the most significant, highlighting the central role of hydrogen bonding in the stability and organization of the crystal structure. 

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5. Conformational disorder in the crystal structure of methyl 2-acetamido-2-deoxy-β- D -glucopyranosyl-(1→4)–2-acetamido-2-deoxy-β- D -glucopyranoside (methyl β-chitobioside) methanol monosolvate

   https://doi.org/10.1107/S2053229624005199  

   Shit, Pradip; Tetrault, Timothy; Zhang, Wenhui; Yoon, Mi-Kyung; Oliver, Allen G; Serianni, Anthony S (July 2024, Acta Crystallographica Section C Structural Chemistry)

   Methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranoside (methyl β-chitobioside), (IV), crystallizes from aqueous methanol at room temperature to give a structure (C₁₇H₃₀N₂O₂₂·CH₃OH) containing conformational disorder in the exocyclic hydroxymethyl group of one of its βGlcNAc residues. As observed in other X-ray structures of disaccharides containing β-(1→4)O-glycosidic linkages, inter-residue hydrogen bonding between O3H of the βGlcNAc bearing the OCH₃aglycone and O5 of the adjacent βGlcNAc is observed based on the 2.79 Å internuclear distance between the O atoms. The structure of (IV) was compared to that determined previously for 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranose (β-chitobiose), (III). TheO-glycosidic linkage torsion angles,phi(ϕ) andpsi(ψ), in (III) and (IV) differ by 6–8°. TheN-acetyl side chain conformation in (III) and (IV) shows some context dependence, with the C1—C2—N—C_cartorsion angle 10–15° smaller for the βGlcNAc residue involved in the internalO-glycosidic linkage. In (IV), conformational disorder is observed in the exocyclic hydroxymethyl (–CH₂OH) group in the βGlcNAc residue bearing the OCH₃aglycone, and a fitting of the electron density indicates an approximate 50:50 distribution of thegauche–gauche(gg) andgauche–trans(gt) conformers in the lattice. Similar behavior is not observed in (III), presumably due to the different packing structure in the vicinity of the –CH₂OH substituent that affects its ability to hydrogen bond to proximal donors/acceptors. Unlike (IV), a re-examination of the previously reported electron density of (III) revealed conformational disorder in theN-acetyl side chain attached to the reducing-end βGlcNAc residue caused by rotation about the C2—N bond. 

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