Given the importance of the phenomenon of polymorphism
from both
fundamental and applied perspectives, there is considerable interest
in the discovery of new systems that exhibit abundant polymorphism.
In the present article, the preparation strategies and structural
properties of three new polymorphs (denoted Forms III, IV, and V)
of m-aminobenzoic acid (m-ABA) are
reported, elevating this system to the rare class of polymorphic systems
with at least five known polymorphs. The crystal structures of the
three new polymorphs have been determined directly from powder X-ray
diffraction data, using the direct-space genetic algorithm technique
for structure solution followed by Rietveld refinement, demonstrating
the opportunities that now exist for determining crystal structures
when crystals of sufficient size and quality for single-crystal X-ray
diffraction are not available. In two of the new polymorphs (Forms
III and IV), the m-ABA molecules exist in the zwitterionic
form (as in the previously known Form I), while the m-ABA molecules in the other new polymorph (Form V) are nonzwitterionic
(as in the previously known Form II). Furthermore, disorder of the
molecular orientation, and hence disorder in the intermolecular hydrogen-bonding
arrangement, is revealed in Form V. The assignment of the tautomeric
form in each polymorph is confirmed by X-ray photoelectron spectroscopy.
Issues relating to the relative stabilities of the five polymorphs
of m-ABA are discussed.
We report the crystal structure of L-arginine, one of the last remaining natural amino acids for which the crystal structure has never been determined; structure determination was carried out directly from powder X-ray diffraction (XRD) data, exploiting the direct-space genetic algorithm technique for structure solution followed by Rietveld refinement.
Bis(I2) adducts of hexamethonium dihalides are pre-organized to respond dynamically to heating and reach a functional structure that favors the formation of the poorly stable and virtually unknown [I2Br2]2− and [I2Cl2]2− tetrahalides which could not be obtained in solution (see picture). The cavity-directed reactivity affords new opportunities for synthesis and interconversion of polyhalogen anions
The aluminum complex Alq3 (q = 8-hydroxyquinolinate),
which has important applications in organic light-emitting diode materials,
is shown to be readily synthesized as a pure phase under solvent-free
mechanochemical conditions from Al(OAc)2OH and 8-hydroxyquinoline
by ball milling. The initial product of the mechanochemical synthesis
is a novel acetic acid solvate of Alq3, and the α
polymorph of Alq3 is obtained on subsequent heating/desolvation
of this phase. The structure of the mechanochemically prepared acetic
acid solvate of Alq3 has been determined directly from
powder X-ray diffraction data and is shown to be a different polymorph
from the corresponding acetic acid solvate prepared by solution-state
crystallization of Alq3 from acetic acid. Significantly,
the mechanochemical synthesis of Alq3 is shown to be fully
scalable across two orders of magnitude from 0.5 to 50 g scale. The
Alq3 sample obtained from the solvent-free mechanochemical
synthesis is analytically pure and exhibits identical photoluminescence
behavior to that of a sample prepared by the conventional synthetic
route.
Ab initio and density functional theory (DFT) calculations on some model systems are presented to assess the extent to which intermolecular hydrogen bonding can affect the planarity of amide groups. Formamide and urea are examined as archetypes of planar and non-planar amides, respectively. DFT optimisations suggest that appropriately disposed hydrogen-bond donor or acceptor molecules can induce non-planarity in formamide, with OCNH dihedral angles deviating by up to ca. 201 from planarity. Ab initio energy calculations demonstrate that the energy required to deform an amide molecule from the preferred geometry of the isolated molecule is more than compensated by the stabilisation due to hydrogen bonding. Similarly, the NH 2 group in urea can be made effectively planar by the presence of appropriately positioned hydrogen-bond acceptors, whereas hydrogen-bond donors increase the non-planarity of the NH 2 group. Small clusters (a dimer, two trimers and a pentamer) extracted from the crystal structure of urea indicate that the crystal field acts to force planarity of the urea molecule; however, the interaction with nearest neighbours alone is insufficient to induce the molecule to become completely planar, and longer-range effects are required. Finally, the potential for intermolecular hydrogen bonding to induce non-planarity in a model of a peptide is explored. Inter alia, the insights obtained in the present work on the extent to which the geometry of amide groups may be deformed under the influence of intermolecular hydrogen bonding provide structural guidelines that can assist the interpretation of the geometries of such groups in structure determination from powder X-ray diffraction data.
Bis(I2) adducts of hexamethonium dihalides are pre‐organized to respond dynamically to heating and reach a functional structure that favors the formation of the poorly stable and virtually unknown [I2Br2]2− and [I2Cl2]2− tetrahalides which could not be obtained in solution (see picture). The cavity‐directed reactivity affords new opportunities for synthesis and interconversion of polyhalogen anions.
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