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      Structure determination of the theophylline–nicotinamide cocrystal: a combined powder XRD, 1D solid-state NMR, and theoretical calculation study

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          Abstract

          The crystal structure of the theophylline–nicotinamide cocrystal is determined for the first time by using a combined multi-technique approach.

          Abstract

          The crystal structure of a powder pharmaceutical cocrystal, theophylline–nicotinamide (1 : 1) crystal complex, is determined for the first time by using a combination of X-ray powder diffraction (XRPD), 1D solid state NMR, as well as density functional theory (DFT) calculations. With the aid of solid state NMR spectroscopy, a candidate structure can be determined from XRPD data by Rietveld refinement with acceptable residual variances. The structure was subjected to periodic geometry optimization, followed by NMR parameter calculations. The agreement between experimental and computed 13C and 15N NMR chemical shift values validates the refined structure as an accurate representation of the actual cocrystal structure. Intermolecular interactions existing in the cocrystal are further confirmed by the commonly used vibrational spectra. This study confirms that the straightforward synergistic approach offers a simple and credible way to solve the crystal structure of powder cocrystal samples.

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          Encoding and decoding hydrogen-bond patterns of organic compounds

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            Unit-cell refinement from powder diffraction scans

            G. Pawley (1981)
            A procedure for the refinement of the crystal unit cell from a powder diffraction scan is presented. In this procedure knowledge of the crystal structure is not required, and at the end of the refinement a list of indexed intensities is produced. This list may well be usable as the starting point for the application of direct methods. The problems of least-squares ill-conditioning due to overlapping reflections are overcome by constraints. An example, using decafluorocyclohexene, C6F10, shows the quality of fit obtained in a case which may even be a false minimum. The method should become more relevant as powder scans of improved resolution become available, through the use of pulsed neutron sources.
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              The salt-cocrystal continuum: the influence of crystal structure on ionization state.

              Salts and cocrystals are multicomponent crystals that can be distinguished by the location of the proton between an acid and a base. At the salt end of the spectrum proton transfer is complete, and on the opposite end proton transfer is absent in cocrystals. However, for acid-base complexes with similar pK(a) values, the extent of proton transfer in the solid state is not predictable and a continuum exists between the two extremes. For these systems, both the DeltapK(a) value (pK(a) of base - pK(a) of acid) and the crystalline environment determine the extent of proton transfer. A total of 20 complexes containing theophylline and guest molecules with DeltapK(a) values less than 3 have been prepared, resulting in 13 cocrystals, five salts, and two complexes with mixed ionization states based on IR spectroscopy and single-crystal diffraction data. We propose modifications to the DeltapK(a) rule for selecting salt screen counterions that focus on the discovery of solid forms with useful physical properties rather than an arbitrary cutoff value for DeltapK(a).
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                Author and article information

                Journal
                CRECF4
                CrystEngComm
                CrystEngComm
                Royal Society of Chemistry (RSC)
                1466-8033
                2014
                2014
                : 16
                : 15
                : 3141-3147
                Article
                10.1039/C4CE00012A
                d23f63ab-7599-4556-a994-3b8a283c26b1
                © 2014
                History

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