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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. Crystal structure of a (carboxymethyl)triethylazanium bromide–2-(triethylazaniumyl)acetate (1/1) hydrogen-bonded dimer

   https://doi.org/10.1107/S2056989023006850  

   Carlson, Faith M; Staples, Richard J; Biros, Shannon M (September 2023, Acta Crystallographica Section E Crystallographic Communications)

   The title compound, C₈H₁₈NO₂⁺·Br⁻·C₈H₁₇NO₂, crystallizes as the bromide salt of a 50:50 mixture of (triethylazaniumyl)carboxylic acid and the zwitterionic (triethylazaniumyl)carboxylate. The two organic entities are linked by a half-occupied bridging carboxylic acid hydrogen atom that is hydrogen-bonded to the carboxylate group of the second molecule. The tetralkylammonium group adopts a nearly perfect tetrahedral shape around the nitrogen atom with bond lengths that agree with known values. The carboxylic acid/carboxylate group is orientedantito one of the ethyl groups on the ammonium group, and the carbonyl oxygen atom is engaged in intramolecular C—H...O hydrogen bonds. 

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3. 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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4. 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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5. Crystal structure of (1,4,7,10,13,16-hexaoxacyclooctadecane-κ ⁶ O )potassium-μ-oxalato-triphenylstannate(IV), the first reported 18-crown-6-stabilized potassium salt of triphenyloxalatostannate

   https://doi.org/10.1107/S2056989024007758  

   Song, Xueqing; Li, William; Torres, Yolanda; Greaves, Tazena (September 2024, Acta Crystallographica Section E Crystallographic Communications)

   The title complex, (1,4,7,10,13,16-hexaoxacyclooctadecane-1κ⁶O)(μ-oxalato-1κ²O¹,O²:2κ²O^1′,O^2′)triphenyl-2κ³C-potassium(I)tin(IV), [KSn(C₆H₅)₃(C₂O₄)(C₁₂H₂₄O₆)] or K[18-Crown-6][(C₆H₅)₃SnO₄C₂], was synthesized. The complex consists of a potassium cation coordinated to the six oxygen atoms of a crown ether molecule and the two oxygen atoms of the oxalatotriphenylstannate anion. It crystallizes in the monoclinic crystal system within the space groupP2₁. The tin atom is coordinated by one chelating oxalate ligand and three phenyl groups, forming acis-trigonal–bipyramidal geometry around the tin atom. The cations and anions form ion pairs, linked through carbonyl coordination to the potassium atoms. The crystal structure features C—H...O hydrogen bonds between the oxygen atoms of the oxalate group and the hydrogen atoms of the phenyl groups, resulting in an infinite chain structure extending alonga-axis direction. The primary inter-chain interactions are van der Waals forces. 

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