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1. 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 ( χmore »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.« less
2. Chemical synthesis of perfectly isotactic and high melting bacterial poly(3-hydroxybutyrate) from bio-sourced racemic cyclic diolide

   https://doi.org/10.1038/s41467-018-04734-3  

   Tang, Xiaoyan ; Chen, Eugene Y. -X. ( June 2018 , Nature Communications)

   Bacterial poly(3-hydroxybutyrate) (P3HB) is a perfectly isotactic, crystalline material possessing properties suitable for substituting petroleum plastics, but high costs and low volumes of its production are impractical for commodity applications. The chemical synthesis of P3HB via ring-opening polymerization (ROP) of racemicβ-butyrolactone has attracted intensive efforts since the 1960s, but not yet produced P3HB with high isotacticity and molecular weight. Here, we report a route utilizing racemic cyclic diolide (rac-DL) derived from bio-sourced succinate. With stereoselective racemic catalysts, the ROP ofrac-DL under ambient conditions produces rapidly P3HB with perfect isotacticity ([mm] > 99%), high melting temperature (T_m = 171 °C), and high molecular weight (M_n = 1.54 × 10⁵ g mol⁻¹,Đ = 1.01). With enantiomeric catalysts, kinetic resolution polymerizations ofrac-DL automatically stops at 50% conversion and yields enantiopure (R,R)-DL and (S,S)-DL with >99%e.e. and the corresponding poly[(S)-3HB] and poly[(R)-3HB] with highT_m = 175 °C.
3. 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.
4. Molecular Recognition and Shape Studies of 3- and 4-Substituted Diarylamide Quasiracemates

   https://doi.org/10.3390/cryst11121596  

   Brandt, Ali K. ; Boyle, Derek J. ; Butler, Jacob P. ; Gillingham, Abigail R. ; Penner, Scott E. ; Spaniol, Jacqueline M. ; Stockdill, Alaina K. ; Vanderwall, Morgan M. ; Yeraly, Almat ; Schepens, Diana R. ; et al ( December 2021 , Crystals)

   Families of quasiracemic materials constructed from 3- and 4-substituted chiral diarylamide molecular frameworks were prepared, where the imposed functional group differences systematically varied from H to CF3–9 unique components for each isomeric framework. Cocrystallization from the melt via hot stage thermomicroscopy using all possible racemic and quasiracemic combinations probed the structural boundaries of quasiracemate formation. The crystal structures and lattice energies (differential scanning calorimetry and lattice energy calculations) for many of these systems showed that quasienantiomeric components organize with near inversion symmetry and lattice energetics closely resembling those found in the racemic counterparts. This study also compared the shape space of pairs of quasienantiomers using an in silico alignment-based method to approximate the differences in molecular shape and provide a diagnostic tool for quasiracemate prediction. Comparing these results to our recent report on related 2-substituted diarylamide quasiracemates shows that functional group position can have a marked effect on quasiracemic behavior and provide critical insight to a more complete shape space, essential for defining molecular recognition processes.
5. Tuning the melting point of selected ionic liquids through adjustment of the cation's dipole moment

   https://doi.org/10.1039/D0CP01214A  

   Rabideau, Brooks D. ; Soltani, Mohammad ; Parker, Rome A. ; Siu, Benjamin ; Salter, E. Alan ; Wierzbicki, Andrzej ; West, Kevin N. ; Davis, James H. ( June 2020 , Physical Chemistry Chemical Physics)

   In previous work with thermally robust salts [Cassity et al., Phys. Chem. Chem. Phys. , 2017, 19 , 31560] it was noted that an increase in the dipole moment of the cation generally led to a decrease in the melting point. Molecular dynamics simulations of the liquid state revealed that an increased dipole moment reduces cation–cation repulsions through dipole–dipole alignment. This was believed to reduce the liquid phase enthalpy, which would tend to lower the melting point of the IL. In this work we further test this principle by replacing hydrogen atoms with fluorine atoms at selected positions within the cation. This allows us to alter the electrostatics of the cation without substantially affecting the sterics. Furthermore, the strength of the dipole moment can be controlled by choosing different positions within the cation for replacement. We studied variants of four different parent cations paired with bistriflimide and determined their melting points, and enthalpies and entropies of fusion through DSC experiments. The decreases in the melting point were determined to be enthalpically driven. We found that the dipole moment of the cation, as determined by quantum chemical calculations, is inversely correlated with the melting point of the given compound. Molecular dynamicsmore »simulations of the crystalline and solid states of two isomers showed differences in their enthalpies of fusion that closely matched those seen experimentally. Moreover, this reduction in the enthalpy of fusion was determined to be caused by an increase in the enthalpy of the crystalline state. We provide evidence that dipole–dipole interactions between cations leads to the formation of cationic domains in the crystalline state. These cationic associations partially block favourable cation–anion interactions, which are recovered upon melting. If, however, the dipole–dipole interactions between cations is too strong they have a tendency to form glasses. This study provides a design rule for lowering the melting point of structurally similar ILs by altering their dipole moment.« less