Vibrational spectroscopy of a compound with a CS7 ring

The product of the Asinger reaction between elemental sulfur, n-butylamine and acetophenone is 8-(n-butylaminophenylmethyliden)-1,2,3,4,5,6,7-heptathiocane which contains a CS7 ring. A combination of infrared, Raman and inelastic neutron scattering spectroscopies with periodic density functional theory calculations is used to provide a complete assignment of the vibrational spectra of this unusual species. The similarity between the Raman spectra of the compound and that of elemental sulfur is particularly striking. Copyright (C) 2009 John Wiley & Sons, Ltd.

Discovery of three polymorphs of 7-fluoroisatin reveals challenges in using computational crystal structure prediction as a complement to experimental screening

A combined computational and experimental polymorph search was undertaken to establish the crystal forms of 7-fluoroisatin, a simple molecule with no reported crystal structures, to evaluate the value of crystal structure prediction studies as an aid to solid form discovery. Three polymorphs were found in a manual crystallisation screen, as well as two solvates. Form I ( P2(1)/c, Z0 1), found from the majority of solvent evaporation experiments, corresponded to the most stable form in the computational search of Z0 1 structures. Form III ( P21/ a, Z0 2) is probably a metastable form, which was only found concomitantly with form I, and has the same dimeric R2 2( 8) hydrogen bonding motif as form I and the majority of the computed low energy structures. However, the most thermodynamically stable polymorph, form II ( P1 , Z0 2), has an expanded four molecule R 4 4( 18) hydrogen bonding motif, which could not have been found within the routine computational study. The computed relative energies of the three forms are not in accord with experimental results. Thus, the experimental finding of three crystalline polymorphs of 7- fluoroisatin illustrates the many challenges for computational screening to be a tool for the experimental crystal engineer, in contrast to the results for an analogous investigation of 5- fluoroisatin.

Solid phases of cyclopentane: combined experimental and simulation study

The phase diagram of cyclopentane has been studied by powder neutron diffraction, providing diffraction patterns for phases I, II, and III, over a range of temperatures and pressures. The putative phase IV was not observed. The structure of the ordered phase III has been solved by single-crystal diffraction. Computational modeling reveals that there are many equienergetic ordered structures for cyclopentane within a small energy range. Molecular dynamics simulations reproduce the structures and diffraction patterns for phases I and III and also show an intermediate disordered phase, which is used to interpret phase II.

Accurate molecular structures of chlorothiazide and hydrochlorothiazide by joint refinement against powder neutron and X-ray diffraction data

The compounds chlorothiazide and hydrochlorothiazide (crystalline form II) have been studied in their fully hydrogenous forms by powder neutron diffraction on the GEM diffractometer. The results of joint Rietveld refinement of the structures against multi-bank neutron and single-bank X-ray powder data are reported and show that accurate and precise structural information can be obtained from polycrystalline molecular organic materials by this route.

A catemer-to-dimer structural transformation in cyheptamide

A catemeric crystal structure of cyheptamide undergoes a transformation in the solid-state upon heating to produce a dimer-based form whose structure has been determined from laboratory X-ray powder diffraction ( XRPD) data, thereby providing the first conclusive evidence of a carbamazepine analogue crystallising in both hydrogen bonded motifs.

A differential thermal expansion approach to crystal structure determination from powder diffraction data

Differential thermal expansion over the range 90-210 K has been applied successfully to determine the crystal structure of chlorothiazide from synchrotron powder diffraction data using direct methods. Key to the success of the approach is the use of a multi-data-set Pawley refinement to extract a set of reflection intensities that is more 'single-crystal-like' than those extracted from a single data set. The improvement in reflection intensity estimates is quantified by comparison with reference single-crystal intensities. (C) 2008 International Union of Crystallography Printed in Singapore - all rights reserved

Carbamazepine trifluoroacetic acid solvate

Carbamazepine forms a 1:1 solvate with trifluoroacetic acid (systematic name: 5H-dibenzo[b,f] azepine-5-carboxamide trifluoroacetic acid solvate), C(15)H(12)N(2)O center dot C(2)HF(3)O(2). The compound crystallizes with one molecule of carbamazepine and one of trifluoroacetic acid in the asymmetric unit to form an R(2)(2)(8) motif. The solvent molecule is disordered over two sites, with site-occupancy factors 0.53 (1) and 0.47 (1).