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Tobbens D.M.,University of Innsbruck | Tobbens D.M.,Helmholtz Center Berlin | Glinneman J.,Goethe University Frankfurt | Chierotti M.R.,University of Turin | And 2 more authors.
CrystEngComm | Year: 2012

The crystal structure of the high-temperature phase (HT-form or form III) of barbituric acid has been solved from X-ray powder diffraction data. It was found to be monoclinic, space group C2/c, a = 8.5302(3) Å, b = 6.8167(3) Å, c = 9.3304(4) Å, β = 89.865(2)°, volume = 542.54(4) Å 3 at T = 236(1) °C. The crystal structure is closely related to the also monoclinic structure of form II. A combined Rietveld refinement of X-ray and neutron powder diffraction data revealed fine details of the disorder aspects of the structure. 1H variable temperature and 13C MAS solid-state NMR confirmed those and proved the dynamic nature of the rotational disorder of the molecules. Energy optimizations of ordered subgroup structures with dispersion-corrected DFT gave insights into the underlying reason for the disorder. The reorientation of the unique 2-fold symmetry axis at the II → HT phase transition derives from both the anisotropy of the hydrogen bonds and the resulting anisotropy of the thermal rotational disorder. The unusual thermal expansion behavior of form II also finds its explanation in the incipient disorder. © The Royal Society of Chemistry 2012.


Minguez Espallargas G.,University of Sheffield | Minguez Espallargas G.,University of Valencia | Van De Streek J.,Avant garde Materials Simulation | Fernandes P.,University of Strathclyde | And 5 more authors.
Angewandte Chemie - International Edition | Year: 2010

Hydrogen bonding leads the way: Reaction of nonporous crystalline coordination compound 1 with HCl gas results in conversion into the crystalline salt 2 (see picture) following chemisorption and insertion of HCl into Cu - N bonds. Powder diffraction studies show that [CuBr2Cl 2]2- ions formed in 2 are reoriented to maximize the strength of hydrogen bonds and halogen bonds by preferentially involving Cl rather than Br ligands as acceptor sites. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Schmidt M.U.,Goethe University Frankfurt | Bruning J.,Goethe University Frankfurt | Glinnemann J.,Goethe University Frankfurt | Hutzler M.W.,Goethe University Frankfurt | And 7 more authors.
Angewandte Chemie - International Edition | Year: 2011

Stabilizing the unstable: In textbooks barbituric acid is always drawn in its keto tautomeric form, which is indeed preferred in solution and in most polymorphic phases. However, phase IV, obtained by grinding, consists of molecules in the enol form, as shown by neutron powder diffraction. This phase is found to be the most stable one at room temperature; the "unstable" enol tautomer is stabilized by a higher number of hydrogen bonds. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


Minguez Espallargas G.,University of Sheffield | Minguez Espallargas G.,University of Valencia | Florence A.J.,University of Strathclyde | Van De Streek J.,Avant garde Materials Simulation | Brammer L.,University of Sheffield
CrystEngComm | Year: 2011

Reaction of green crystalline solid trans-[CuBr2(3-Brpy) 2] 1 (3-Brpy = 3-bromopyridine) with HBr (aq) vapour yields brown crystalline salt (3-BrpyH)2[CuBr4] 2 with quantitative conversion. Notably 2 adopts a different crystal structure to the three mutually isostructural compounds (3-XpyH)2[CuCl4] (X = Cl, Br) and (3-BrpyH)2[CuBr2Cl2] which result from reaction with HCl. Crystalline product 2 has been characterised by X-ray powder diffraction and its conversion back to 1 at 370-400 K has been followed in situ by synchrotron X-ray powder diffraction. Crystalline 1 and 2 are further notable for the presence of intermolecular C-Br⋯Br-Cu halogen bonds and (only in the case of 2) N-H⋯Br-Cu hydrogen bonds. © 2011 The Royal Society of Chemistry.


Gorelik T.E.,Johannes Gutenberg University Mainz | Van De Streek J.,Avant garde Materials Simulation | Kilbinger A.F.M.,University of Fribourg | Brunklaus G.,Max Planck Institute for Polymer Research | And 2 more authors.
Acta Crystallographica Section B: Structural Science | Year: 2012

Ab-initio crystal structure analysis of organic materials from electron diffraction data is presented. The data were collected using the automated electron diffraction tomography (ADT) technique. The structure solution and refinement route is first validated on the basis of the known crystal structure of tri-p-benzamide. The same procedure is then applied to solve the previously unknown crystal structure of tetra-p-benzamide. In the crystal structure of tetra-p-benzamide, an unusual hydrogen-bonding scheme is realised; the hydrogen-bonding scheme is, however, in perfect agreement with solid-state NMR data. © 2012 International Union of Crystallography Printed in Singapore-all rights reserved.


Van De Streek J.,Avant Garde Materials Simulation | Neumann M.A.,Avant Garde Materials Simulation
Acta Crystallographica Section B: Structural Science | Year: 2010

This paper describes the validation of a dispersion-corrected density functional theory (d-DFT) method for the purpose of assessing the correctness of experimental organic crystal structures and enhancing the information content of purely experimental data. 241 experimental organic crystal structures from the August 2008 issue of Acta Cryst. Section E were energy-minimized in full, including unit-cell parameters. The differences between the experimental and the minimized crystal structures were subjected to statistical analysis. The r.m.s. Cartesian displacement excluding H atoms upon energy minimization with flexible unit-cell parameters is selected as a pertinent indicator of the correctness of a crystal structure. All 241 experimental crystal structures are reproduced very well: the average r.m.s. Cartesian displacement for the 241 crystal structures, including 16 disordered structures, is only 0.095 Å (0.084 Å for the 225 ordered structures). R.m.s. Cartesian displacements above 0.25 Å either indicate incorrect experimental crystal structures or reveal interesting structural features such as exceptionally large temperature effects, incorrectly modelled disorder or symmetry breaking H atoms. After validation, the method is applied to nine examples that are known to be ambiguous or subtly incorrect. © 2010 International Union of Crystallography.


Van De Streek J.,Avant garde Materials Simulation | Neumann M.A.,Avant garde Materials Simulation | Perrin M.-A.,Sanofi S.A.
CrystEngComm | Year: 2010

A crystal structure prediction study for the molecular salt pyridinium chloride was carried out with Z′ = 1 and Z′ = 2 in all 230 space groups. The predicted crystal structures were ranked by energy using dispersion-corrected density functional theory calculations, with the dispersion-correction parameterised against uncharged systems only. The experimental structures are ranked 1st and shared 2nd, which suggests that the dispersion-correction parameters are likely to be at least partially transferable to charged systems. For the structure generation step, a high-accuracy tailor-made force field was prepared for pyridinium chloride. The accuracy of the tailor-made force field is comparable to those for neutral molecules. A problem caused by spurious pseudo-symmetry due to the use of a cascade of energy potentials of different accuracies is described, as is its solution. Our calculations confirm that a previously reported P2 1/m structure for pyridinium chloride should have been reported in P1, our calculations suggest the existence of a high-pressure polymorph in Pnma, and our calculations suggest that the high-temperature phase of pyridinium chloride might be subtly different from the high-temperature phase of pyridinium iodide. © 2010 The Royal Society of Chemistry.


Van De Streek J.,Avant garde Materials Simulation | Neumann M.A.,Avant garde Materials Simulation
CrystEngComm | Year: 2011

A crystal-structure prediction study with up to four independent molecules in all 230 space groups was undertaken for pyridine with a dispersion-corrected density functional theory method. The two known polymorphs, P212 121 with Z′ = 1 and Pna21 with Z′ = 4, were found as ranks 1 and 2 in the correct stability order at low temperature. A pressure scan correctly predicts that the P212 121 form is favoured under pressure. The completeness of the structure generation and the number of hypothetical crystal structures per energy interval are analysed as a function of Z′. This journal is © The Royal Society of Chemistry.


Bond A.D.,University of Southern Denmark | Solanko K.A.,University of Southern Denmark | Van De Streek J.,Avant garde Materials Simulation | Neumann M.A.,Avant garde Materials Simulation
CrystEngComm | Year: 2011

Energy minimisation of 2,6-bis(2,4-dichlorobenzylidene)cyclohexanone using dispersion-corrected Density Functional Theory (DFT-D) suggested a subtle low-temperature phase transition that has been verified by experiment. The results demonstrate that DFT-D calculations are sufficiently reliable to guide experimental studies towards targets most likely to exhibit interesting temperature-dependent structural change. © 2011 The Royal Society of Chemistry.


Kendrick J.,University of Bradford | Leusen F.J.J.,University of Bradford | Neumann M.A.,Avant garde Materials Simulation | Van De Streek J.,Avant garde Materials Simulation
Chemistry - A European Journal | Year: 2011

The results of the application of a density functional theory method incorporating dispersive corrections in the 2010 crystal structure prediction blind test are reported. The method correctly predicted four out of the six experimental structures. Three of the four correct predictions were found to have the lowest lattice energy of any crystal structure for that molecule. The experimental crystal structures for all six compounds were found during the structure generation phase of the simulations, indicating that the tailor-made force fields used for screening structures were valid and that the structure generation engine, which combines a Monte Carlo parallel tempering algorithm with an efficient lattice energy minimiser, was working effectively. For the three compounds for which the experimental crystal structures did not correspond to the lowest energy structures found, the method for calculating the lattice energy needs to be further refined or there may be other polymorphs that have not yet been found experimentally. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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