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.
A new in-situ NMR strategy (termed CLASSIC NMR) for mapping the evolution of crystallization processes is reported, involving simultaneous measurement of both liquid-state and solid-state NMR spectra as a function of time. This combined strategy allows complementary information to be obtained on the evolution of both the solid and liquid phases during the crystallization process. In particular, as crystallization proceeds (monitored by solid-state NMR), the solution state becomes more dilute, leading to changes in solution-state speciation and the modes of molecular aggregation in solution, which are monitored by liquid-state NMR. The CLASSIC NMR experiment is applied here to yield new insights into the crystallization of m-aminobenzoic acid.
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.
We report a strategy for structure
determination of organic materials in which complete solid-state nuclear
magnetic resonance (NMR) spectral data is utilized within the context
of structure determination from powder X-ray diffraction (XRD) data.
Following determination of the crystal structure from powder XRD data,
first-principles density functional theory-based techniques within
the GIPAW approach are exploited to calculate the solid-state NMR
data for the structure, followed by careful scrutiny of the agreement
with experimental solid-state NMR data. The successful application
of this approach is demonstrated by structure determination of the
1:1 cocrystal of indomethacin and nicotinamide. The 1H
and 13C chemical shifts calculated for the crystal structure
determined from the powder XRD data are in excellent agreement with
those measured experimentally, notably including the two-dimensional
correlation of 1H and 13C chemical shifts for
directly bonded 13C–1H moieties. The
key feature of this combined approach is that the quality of the structure
determined is assessed both against experimental
powder XRD data and against experimental solid-state
NMR data, thus providing a very robust validation of the veracity
of the structure.
To date, only one crystal structure of Lphenylalanine has been reported, with no confirmed report of polymorphism of this material. In the present work, we report the discovery of a new polymorph of L-phenylalanine, with the structural properties determined directly from powder X-ray diffraction data. The new polymorph of L-phenylalanine is stable only under rigorously dry conditions. In addition, two new solid hydrate phases of L-phenylalanine have been discovered: a monohydrate and a hemihydrate. The hemihydrate is susceptible to partial water deficiency. The crystal structures of the monohydrate and hemihydrate phases have also been determined directly from powder X-ray diffraction data. On the basis of results from dynamic vapor sorption and other experiments, we demonstrate that the three new solid forms are readily interconvertible as a function of relative humidity.
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