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  1. Atomically precise thiolate-protected gold nanomolecules have attracted interest due to their distinct electronic and chemical properties. The structure of these nanomolecules is important for understanding their peculiar properties. Here, we report the X-ray crystal structure of a 24-atom gold nanomolecule protected by 16 tert -butylthiolate ligands. The composition of Au 24 (S-C 4 H 9 ) 16 {poly[hexadecakis(μ- tert -butylthiolato)tetracosagold]} was confirmed by X-ray crystallography and electrospray ionization mass spectrometry (ESI–MS). The nanomolecule was synthesized in a one-phase synthesis and crystallized from a hexane–ethanol layered solution. The X-ray structure confirms the 16-atom core protected by two monomeric and two trimeric staples with four bridging ligands. The Au 24 (S-C 4 H 9 ) 16 cluster follows the shell-closing magic number of 8.
    Free, publicly-accessible full text available August 1, 2023
  2. Co-crystal engineering is a promising method to create new classes of advanced materials. Co-crystal structure prediction is more challenging when one or more of the lattice constituents (tectons) are flexible molecules. This study reports four co-crystals that were prepared by mixing HAuCl 4 or HAuBr 4 with C 3 -symmetric tectons based on a 1,3,5-(methylacetamide)benzene scaffold. X-ray analysis of the co-crystals revealed the presence of three dominant supramolecular interactions; (a) hydrogen bonding between tecton amide NH residues and the AuX 4 − anion, (b) electrostatic stacking of the Au center against the tecton's π-electrons, (c) very short hydrogen bonds within a proton-bridged-carbonyls motif. Within all four co-crystals, the sterically-geared tecton was trapped in a high energy molecular conformation, which increased the number of favorable intermolecular interactions in the lattice. We infer from the results that the likelihood of high energy molecular conformations within a co-crystal increases if there are multiple dominant intermolecular interactions. Application of this generalizable rule should lead to improved crystal structure prediction.
    Free, publicly-accessible full text available May 30, 2023
  3. D-Mannosamine hydrochloride (2-amino-2-deoxy-D-mannose hydrochloride), C 6 H 14 NO 5 + ·Cl − , (I), crystallized from a methanol/ethyl acetate/ n -hexane solvent mixture at room temperature in a 4 C 1 chair conformation that is slightly distorted towards the C3,O5 B form. A comparison of the structural parameters of (I) with the corresponding parameters in α-D-glucosamine hydrochloride, (II), and β-D-galactosamine hydrochloride, (III)/(III′), was undertaken to evaluate the effects of ionic hydrogen bonding on structural properties. Three types of ionic hydrogen bonds are present in the crystals of (I)–(III)/(III′), i.e. N + —H...O, N + —H...Cl − , and O—H...Cl − . The exocyclic structural parameters in (I), (II), and (III)/(III′) appear to be most influenced by this bonding, especially the exocyclic hydroxy groups, which adopt eclipsed conformations enabled by ionic hydrogen bonding to the chloride anion. Anomeric disorder was observed in crystals of (I), with an α:β ratio of 37:63. However, anomeric configuration appears to exert minimal structural effects; that is, bond lengths, bond angles, and torsion angles are essentially identical in both anomers. The observed disorder at the anomeric C atom of (I) appears to be caused by the presence of the chloride anion and atom O3 ormore »O4 in proximal voids, which provide opportunities for hydrogen bonding to atom O1 in both axial and equatorial orientations.« less
    Free, publicly-accessible full text available April 1, 2023
  4. Methyl β-lactoside [methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside] monohydrate, C 13 H 24 O 11 ·H 2 O, (I), was obtained via spontaneous transformation of methyl β-lactoside methanol solvate, (II), during air-drying. Cremer–Pople puckering parameters indicate that the β-D-Gal p (β-D-galactopyranosyl) and β-D-Glc p (β-D-glucopyranosyl) rings in (I) adopt slightly distorted 4 C 1 chair conformations, with the former distorted towards a boat form ( B C1,C4 ) and the latter towards a twist-boat form ( O5 S C2 ). Puckering parameters for (I) and (II) indicate that the conformation of the βGal p ring is slightly more affected than the βGlc p ring by the solvomorphism. Conformations of the terminal O -glycosidic linkages in (I) and (II) are virtually identical, whereas those of the internal O -glycosidic linkage show torsion-angle changes of 6° in both C—O bonds. The exocyclic hydroxymethyl group in the βGal p residue adopts a gt conformation (C4′ anti to O6′) in both (I) and (II), whereas that in the βGlc p residue adopts a gg ( gauche – gauche ) conformation (H5 anti to O6) in (II) and a gt ( gauche – trans ) conformation (C4 anti to O6) in (I). The latter conformational change is critical tomore »the solvomorphism in that it allows water to participate in three hydrogen bonds in (I) as opposed to only two hydrogen bonds in (II), potentially producing a more energetically stable structure for (I) than for (II). Visual inspection of the crystalline lattice of (II) reveals channels in which methanol solvent resides and through which solvent might exchange during solvomorphism. These channels are less apparent in the crystalline lattice of (I).« less
  5. Seven doubly 13 C-labeled isotopomers of methyl β- d -glucopyranoside, methyl β- d -xylopyranoside, methyl β- d -galactopyranoside, methyl β- d -galactopyranosyl-(1→4)-β- d -glucopyranoside and methyl β- d -galactopyranosyl-(1→4)-β- d -xylopyranoside were prepared, crystallized, and studied by single-crystal X-ray crystallography and solid-state 13 C NMR spectroscopy to determine experimentally the dependence of 2 J C1,C3 values in aldopyranosyl rings on the C1–C2–O2–H torsion angle, θ 2 , involving the C2 carbon of the C1–C2–C3 coupling pathway. Using X-ray crystal structures to determine θ 2 in crystalline samples and by selecting compounds that exhibit a relatively wide range of θ 2 values in the crystalline state, 2 J C1,C3 values measured in crystalline samples were plotted against θ 2 and the resulting plot compared to that obtained from density functional theory (DFT) calculations. For θ 2 values ranging from ∼90° to ∼240°, very good agreement was observed between the experimental and theoretical plots, providing strong validation of DFT-calculated spin-coupling dependencies on exocyclic C–O bond conformation involving the central carbon of geminal C–C–C coupling pathways. These findings provide new experimental evidence supporting the use of 2 J CCC values as non-conventional spin-coupling constraints in MA′AT conformational modeling of saccharides in solution, andmore »the use of NMR spin-couplings not involving coupled hydroxyl hydrogens as indirect probes of C–O bond conformation. Solvomorphism was observed in crystalline βGal-(1→4)-βGlcOCH 3 wherein the previously-reported methanol solvate form was found to spontaneously convert to a monohydrate upon air-drying, leading to small but discernible conformational changes in, and a new crystalline form of, this disaccharide.« less
  6. Isopropyl 3-deoxy-α-D- ribo -hexopyranoside (isopropyl 3-deoxy-α-D-glucopyranoside), C 9 H 18 O 5 , (I), crystallizes from a methanol–ethyl acetate solvent mixture at room temperature in a 4 C 1 chair conformation that is slightly distorted towards the C5 S C1 twist-boat form. A comparison of the structural parameters in (I), methyl α-D-glucopyranoside, (II), α-D-glucopyranosyl-(1→4)-D-glucitol (maltitol), (III), and 3-deoxy-α-D- ribo -hexopyranose (3-deoxy-α-D-glucopyranose), (IV), shows that most endocyclic and exocyclic bond lengths, valence bond angles and torsion angles in the aldohexopyranosyl rings are more affected by anomeric configuration, aglycone structure and/or the conformation of exocyclic substituents, such as hydroxymethyl groups, than by monodeoxygenation at C3. The structural effects observed in the crystal structures of (I)–(IV) were confirmed though density functional theory (DFT) calculations in computed structures (I) c –(IV) c . Exocyclic hydroxymethyl groups adopt the gauche – gauche ( gg ) conformation (H5 anti to O6) in (I) and (III), and the gauche – trans ( gt ) conformation (C4 anti to O6) in (II) and (IV). The O -glycoside linkage conformations in (I) and (III) resemble those observed in disaccharides containing β-(1→4) linkages.