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1. Increasing the structural boundary of quasiracemate formation: 4-substituted naphthylamides

   https://doi.org/10.1039/D0CE01331E  

   Craddock, Drew E.; Parks, McKenzie J.; Taylor, Lauren A.; Wagner, Benjamin L.; Ruf, Michael; Wheeler, Kraig A. (January 2021, CrystEngComm)

   null (Ed.)

   Quasiracemates – materials consisting of pairs of near enantiomers – form crystalline motifs that mimic the inversion relationships observed for their racemic counterparts. Recent investigations from our group explored a family of chiral ( N -benzoyl)methylbenzylamines to understand the structural boundary of cocrystallization. This investigation extends these earlier studies to include naphthylamide quasiracemates, where the molecular framework is ∼20% larger by volume than the previous diarylamides. A family of naphthylamides was prepared where the pendant functional group differs incrementally in size ( i.e. , H to C 6 H 5 ) to give 55 possible unique pairs of racemic and quasiracemic combinations. Data collected from these materials using X-ray crystallography, thermal analysis methods and lattice energy calculations offer important insight into how a spatially larger naphthylamide molecular framework promotes greater structural variance of substituents during the pairwise assembly of quasienantiomers. 

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2. Amino acid hydrogen oxalate quasiracemates – hydrocarbon side chains

   https://doi.org/10.1039/D1CE01213D  

   Wells, Russell G.; Sahlstrom, Katriel D.; Ekelem, Franklin I.; Wheeler, Kraig A. (November 2021, CrystEngComm)

   Amino acid quasiracemates – generated from the assembly of pairs of chemically distinct amino acids of opposite handedness – continue to provide important opportunities to understand how self-assembly can be promoted despite using components with drastically different sizes and molecular shapes. Previous studies by Görbitz et al. and others cataloged 32 crystal structures of amino acid quasiracemates, with each showing the building blocks aligned with near inversion symmetry similar to their racemic counterparts. This investigation examined the impact of using a secondary coformer molecule, hydrogen oxalate, on the cocrystalline landscape of amino acid quasiracemates with hydrocarbon side chains. Eight racemic (4) and quasiracemic (4) hydrogen oxalate structures were generated. Crystal structures of these systems show the hydrogen oxalate moieties assembled into C(5) molecular columns by the construction of robust O–H⋯O − hydrogen bonds with the amino acid enantiomers and quasienantiomers linked to these column motifs using a complex blend of N + –H⋯O − , O–H⋯O − , and N + –H⋯OC contacts. The racemates and quasiracemates form similar packing motifs; however, due to the chemically non-identical nature of the quasiracemic components, the outcome is that the amino acids organize with near inversion symmetry. Both the conformational similarity ( χ RMS ) and degree of inversion symmetry ( C i ) of related pairs of quasienantiomeric components have been systematically assessed using readily available structural tools. This study shows how coformer molecules such as hydrogen oxalate can provide new and critical insight into the molecular recognition process of quasiracemic materials. 

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3. Accessing Centnerszwer's quasiracemate – molecular shape controlled molecular recognition

   https://doi.org/10.1039/C6RA08131B  

   Spaniol, Jacqueline M.; Wheeler, Kraig A. (January 2016, RSC Advances)

   M. Centnerszwer's seminal 1899 report investigated the stereochemical relationship between optical antipodes of different substances using melting-point behavior. One intriguing melting-point phase diagram produced from this early investigation combined (+)-2-chlorosuccinic acid [(+)- 1 ] and (−)-2-bromosuccinic acid [(−)- 2 ]. While Centnerszwer's data clearly indicates the formation of a quasiracemic phase – i.e. , materials constructed from pairs of isosteric molecules of opposite handedness – at the 1 : 1 component ratio, this material is energetically less favorable than the chiral counterparts. The consequence of this crystal instability is significant as evident by the absence of literature sited crystal structures for the quasiracemic phase (+)- 1 /(−)- 2 and racemates (±)- 1 and (±)- 2 . This study circumvented this challenge by generating multi-molecular assemblies using additional crystallizing agents capable of complementing the hydrogen-bond abilities of succinic acids 1 and 2 . Both imidazole (Im) and 4,4′-bipyridyl- N , N ′-dioxide (BPDO) served as tailor-made additives that effectively modified the crystal packing landscape of quasiracemate of (+)- 1 /(−)- 2 . Combining imidazole with the quasiracemate, racemate, and enantiopure forms of 1 and 2 resulted in crystal structures characterized as molecular salts with layered motifs formed from highly directional N + –H⋯carboxylate and carboxyl⋯carboxylate interactions. In contrast to the enantiopure [(+)- 1 ·Im and (−)- 2 ·Im] and racemic [(±)- 1 ·Im and (±)- 2 ·Im] systems, neighboring molecular layers observed in quasiracemate (+)- 1 /(−)- 2 ·Im are organized by approximate inversion symmetry. Assessment of the crystal packing efficiency for this series of molecular salts via crystal densities and packing coefficients ( C k ) indicates imidazole greatly alters the crystal landscape of the system in favor of racemic and quasiracemic crystal packing. A similar desymmetrized crystal environment was also realized for the ternary cocrystalline system of (+)- 1 /(−)- 2 ·BPDO where the components organize via N + –O − ⋯carboxyl contacts. This study underscores the importance of molecular shape to molecular recognition processes and the stabilizing effect of tailor-made additives for creating new crystalline phases of previously inaccessible crystalline materials. 

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4. Amino acid hydrogen oxalate quasiracemates – sulfur containing side chains

   https://doi.org/10.1039/D1CE01214B  

   Wells, Russell G.; Sahlstrom, Katriel D.; Wheeler, Kraig A. (November 2021, CrystEngComm)

   New additions to quasiracemic materials have been developed by cocrystallizing a ternary component – hydrogen oxalate – with pairs of amino acid quasienantiomers where at least one of the side-chain R groups contains a sulfur atom. Of the eight quasiracemates investigated, six exhibit crystal packing that drastically deviates from the expected centrosymmetric alignment present in the racemic counterparts and the extant database of quasiracemic materials. These structures were quantitatively assessed for conformational similarity (CCDC-Mercury structure overlay) and the degree of inversion symmetry (Avnir's Continuous Symmetry Measures) for each quasienantiomeric pair. Despite the variance in quasienantiomeric components, these structures exhibit a high degree of isostructurality where the principal components assemble by a complex blend of common N + –H⋯O and O–H⋯O − interactions. These charge-assisted hydrogen-bonded networks form thermodynamically favored crystal packing that promotes cocrystallization of a structurally diverse set of quasienantiomeric components. 

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5. Effect of halogen substitution on energies and dynamics of reversible photomechanical crystals based on 9-anthracenecarboxylic acid

   https://doi.org/10.1039/D1CE00846C  

   Gately, Thomas J.; Sontising, Watit; Easley, Connor J.; Islam, Imadul; Al-Kaysi, Rabih O.; Beran, Gregory J.; Bardeen, Christopher J. (January 2021, CrystEngComm)

   null (Ed.)

   9-Anthracene carboxylic acid derivatives comprise a family of thermally reversible photomechanical molecular crystals. The photomechanical response relies on a [4 + 4] photodimerization followed by dissociation that occurs on timescales of seconds to minutes. A combined theoretical and experimental investigation is undertaken to better understand how chemical modification of the anthracene core influences energetics of both the isolated molecule and the crystal lattice. We use both density functional theory and dispersion-corrected Moller–Plesset perturbation theory computational methods to establish orbital energies, photodimerization reaction energies, and lattice energies for a set of substituted 9-anthracene carboxylic acid molecules. The calculations reveal that steric interactions play a dominant role in the ability to form photodimers and indicate an energetic threshold of 80–90 kJ per mole for the dimerization reaction. Examination of intermolecular bonding in a subset of fluorinated 9ACs revealed the absence of H⋯F intermolecular bond formation and energy differences that can explain observed trends in the dissociation kinetics and mechanical reset times. Fluorescence recovery after photobleaching experiments shows that the photodimer dissociation kinetics depend on the amount of initial photodimer, preventing a straightforward correlation between halogen atom substitution and dissociation rates using the Bell–Evans–Polanyi principle. The results clarify how molecular structure affects intermolecular interactions and photoreactivity in this family of molecular crystals, but the origin of the complex photodimer dissociation dynamics remains an open question. 

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