Entropy changes and structural implications for crystalline phases of pyrazine

1979 ◽  
Vol 57 (23) ◽  
pp. 3056-3060 ◽  
Author(s):  
Robert K. Boyd ◽  
John Comper ◽  
George Ferguson
Keyword(s):  
Crystal Structures ◽  
Phase Ii ◽  
Adiabatic Calorimetry ◽  
Heat Capacities ◽  
Unit Cell ◽  
Phase Iii ◽  
Point Symmetry ◽  
Crystalline Phases ◽  
Spectroscopic Evidence ◽  
X Ray

Molar heat capacities of crystalline pyrazine have been measured by adiabatic calorimetry in the range 20–40 °C and interpreted to show that in the crystal structures of phases II and III half the molecules must be disordered. Together with previous X-ray studies, this allows possible structures for phase II and phase III to be deduced. Of the eight molecules in the phase III unit cell, four are disordered over two sites so that the point symmetry is effectively mmm; the remaining four molecules have 2/m symmetry and are not disordered. This structure is consistent with the available spectroscopic evidence. It is likely that the phase II structure is closely related to the phase III structure, for example by the molecules with 2/m symmetry adopting a slightly different orientation.

The structures of marialite (Me6) and meionite (Me93) in space groups P42/n and I4/m, and the absence of phase transitions in the scapolite series

2011 ◽  
Vol 26 (2) ◽  
pp. 119-125 ◽  
Author(s):  
Sytle M. Antao ◽  
Ishmael Hassan
Keyword(s):  
Phase Transitions ◽  
High Resolution ◽  
Crystal Structures ◽  
Unit Cell ◽  
X Ray Diffraction ◽  
Unit Cell Parameters ◽  
X Ray ◽  
Space Groups ◽  
Cell Parameters ◽  
The Mean

The crystal structures of marialite (Me6) from Badakhshan, Afghanistan and meionite (Me93) from Mt. Vesuvius, Italy were obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. Their structures were refined in space groups I4/m and P42/n, and similar results were obtained. The Me6 sample has a formula Ca0.24Na3.37K0.24[Al3.16Si8.84O24]Cl0.84(CO3)0.15, and its unit-cell parameters are a=12.047555(7), c=7.563210(6) Å, and V=1097.751(1) Å3. The average ⟨T1-O⟩ distances are 1.599(1) Å in I4/m and 1.600(2) Å in P42/n, indicating that the T1 site contains only Si atoms. In P42/n, the average distances of ⟨T2-O⟩=1.655(2) and ⟨T3-O⟩=1.664(2) Å are distinct and are not equal to each other. However, the mean ⟨T2,3-O⟩=1.659(2) Å in P42/n and is identical to the ⟨T2′-O⟩=1.659(1) Å in I4/m. The ⟨M-O⟩ [7]=2.754(1) Å (M site is coordinated to seven framework O atoms) and M-A=2.914(1) Å; these distances are identical in both space groups. The Me93 sample has a formula of Na0.29Ca3.76[Al5.54Si6.46O24]Cl0.05(SO4)0.02(CO3)0.93, and its unit-cell parameters are a=12.19882(1), c=7.576954(8) Å, and V=1127.535(2) Å3. A similar examination of the Me93 sample also shows that both space groups give similar results; however, the C–O distance is more reasonable in P42/n than in I4/m. Refining the scapolite structure near Me0 or Me100 in I4/m forces the T2 and T3 sites (both with multiplicity 8 in P42/n) to be equivalent and form the T2′ site (with multiplicity 16 in I4/m), but ⟨T2-O⟩ is not equal to ⟨T3-O⟩ in P42/n. Using different space groups for different regions across the series implies phase transitions, which do not occur in the scapolite series.

scholarly journals The crystal structures of two polyamides (‘nylons’)

1947 ◽  
Vol 189 (1016) ◽  
pp. 39-68 ◽  
Keyword(s):  
Crystal Structures ◽  
Unit Cell ◽  
Chain Molecule ◽  
X Ray Diffraction ◽  
X Ray ◽  
Cell Dimensions ◽  
C Fibre ◽  
Diffraction Patterns ◽  
Unit Cell Dimensions ◽  
Crystalline Forms

Detailed interpretations of the X -ray diffraction patterns of fibres and sheets of 66 and 6.10 polyamides (polyhexam ethylene adipamide and sebacamide respectively) are proposed. The crystal structures of the two substances are completely analogous. Fibres of these two polyam ides usually contain two different crystalline forms, α and β, which are different packings of geometrically similar molecules; most fibres consist chiefly of the α form. In the case of the 66 polymer, fibres have been obtained in which there is no detectable proportion of the β form. Unit cell dimensions and the indices of reflexions for the α form were determined by trial, using normal fibre photographs, and were checked by using doubly oriented sheets set at different angles to the X -ray beam. The unit cell of the a form is triclinic, with a — 4·9 A, b = 5·4 A, c (fibre axis) = 17·2A, α = 48 1/2º, β = 77º, γ = 63 1/2º for the 66 polymer; a = 4·95A, b = 5·4A, c (fibre axes) = 22·4A, α = 49º, β = 76 1/2º, γ = 63 1/2º for the 6.10 polymer. One chain molecule passes through the cell in both cases. Atomic coordinates in occrystals were determined by interpretation of the relative intensities of the reflexions. The chains are planar or very nearly so; the oxygen atoms appear to lie a little off the plane of the chain. The molecules are linked by hydrogen bonds between C = 0 and NH groups, to form sheets. A simple packing of these sheets of molecules gives the α arrangement.

Crystal Structures of the Pentacoordinate Bromo, Isocyanato, Iodo, Acetato and Isothiocyanato Complexes of the meso-Tetraphenylporphyrinatomanganese Cation

1998 ◽  
Vol 51 (9) ◽  
pp. 835 ◽  
Author(s):  
Peter Turner ◽  
Maxwell J. Gunter ◽  
Brian W. Skelton ◽  
Allan H. White
Keyword(s):  
Crystal Structures ◽  
Metal Ion ◽  
Crystal Packing ◽  
Carbonyl Oxygen ◽  
Unit Cell ◽  
X Ray Diffraction ◽  
Complex Molecules ◽  
X Ray ◽  
Rigid Body Model ◽  
Pyrrole Nitrogen

The room-temperature single-crystal X-ray diffraction determined structures of the Mn(tpp)Br.C7H8, Mn(tpp)(NCO), Mn(tpp)I.C7H8, Mn(tpp)(CO2CH3).0·5C7H8, and Mn(tpp)(NCS).0·5C7H8 complexs are described. The monoclinic P21/c unit cell of Mn(tpp)(NCO) has a 14·82(1), b 17·136(5), c 14·576(5) Å, β 111·41(5)°, V 3446(3) Å3, Z 4. The refinement converged with conventional R(|F|) 0·053 for No 3199 (I > 3·0σ(I)) ‘observed’ reflections. The monoclinic P 21/m unit cell of Mn(tpp)Br.C7H8 has a 9·984(1), b 15·453(6), c 13·583(3) Å, β 103·99(2)°, V 2033(1) Å3, Z 2, R 0·039 for No 2668. The Mn(tpp)I.C7H8 structure is triclinic, P-1, with a 22·28(1), b 14·466(4), c 13·555(6) Å, α 76·32(3), β 81·74(4), γ 74·75(3)°, V 4079(3) Å3, Z 4, R 0·050 for No 9039. The triclinic P-1 crystal structures of the Mn(tpp)(CO2CH3).0·5C7H8 and Mn(tpp)(NCS).0·5C7H8 complexes are isomorphous. The Mn(tpp)(CO2CH3).0·5C7H8 structure has a 26·18(1), b 13·503(3), c 12·074(6) Å, α 66·08(4), β 81·36(4), γ 86·71(5)°, V 3858(3) Å3, Z 4, R 0·075 for No 6388. Solvate disorder, requiring a rigid body model, islargely responsible for the relatively high residuals. The Mn(tpp)(NCS).0·5C7H8 structure has a 25·442(6), b 13·746(3), c 12·182(5) Å, α 66·97(3), β 78·59(3), γ 87·90(2)°, V 3839(2) Å3, Z 4, R 0·061 for No 5506. The asymmetric units of the iodo, acetato and isothiocyanato structures each contain two crystallographically independent complex molecules that are sensitive to crystal packing forces. The metal ion displacements from the least-squares planes formed by the pyrrole nitrogen atoms are 0·299(1) and 0·274(1) Å for the Mn(tpp)(NCO) and Mn(tpp)Br.C7H8complexes, and 0·240(1) and 0·252(1), 0·281(1) and 0·278(1), and 0·243(1) and 0·244(1) Å for the independent (a) and (b) complex molecules of Mn(tpp)I.C7H8, Mn(tpp)(CO2CH3).0·5C7H8, and Mn(tpp)(NCS).0·5C7H8 respectively. The axial Mn–X bond lengths are 2·029(5) and 2·490(1) Å for the Mn(tpp)(NCO) and Mn(tpp)Br.C7H8 complexes, and 2·767(1) and 2·730(1), 2·028(5) and 2·010(5), and 2·067(6) and 2·072(5) Å for the (a) and (b) complex molecules of Mn(tpp)I.C7H8, Mn(tpp)(CO2CH3).0·5C7H8, and Mn(tpp)(NCS).0·5C7H8. One of the independent complex molecules in the Mn(tpp)(CO2CH3).0·5C7H8 structure appears to exhibit acetate coordination through a carbonyl oxygen.

An explanation for the origin of hemihedrism in wulfenite: the single-crystal structures of I4̄1/a and I4̄ tungstenian wulfenites

2000 ◽  
Vol 64 (6) ◽  
pp. 1057-1062 ◽  
Author(s):  
D. E. Hibbs ◽  
C. M. Jury ◽  
P. Leverett ◽  
I. R. Plimer ◽  
P. A. Williams
Keyword(s):  
Single Crystal ◽  
Crystal Structures ◽  
San Francisco ◽  
Analytical Data ◽  
Site Occupancy ◽  
Unit Cell ◽  
Special Position ◽  
Space Group ◽  
X Ray ◽  
Single Crystal Structures

AbstractThe single-crystal X-ray structure of tungstenian wulfenite-I41/a containing 10 mol.% WO3 from the San Francisco mine, Sonora, Mexico, space group I41/a, a = 5.436(2), c = 12.068(8)Å and Z = 4, has been refined to R = 0.052. The Mo and W are disordered over special position 4a (0,0,0) in the lattice. Tungstenian wulfenite-I4̄ (‘chillagite’) from the Christmas Gift mine, Chillagoe, Queensland, Australia (Museum of Victoria specimen M16934), crystallizes in the closely related tetragonal space group I4̄, with a = 5.441(1), c = 12.068(6) Å and Z = 4. The structure was refined to R = 0.038. Refined site occupancy factors show that Mo and W are not distributed equally over the two crystallographically independent Mo/W positions, being 0.136(2) for Mo and 0.114(2) for W in special position 2a (0,0,0) and 0.184(2) for Mo and 0.066(2) for W in special position 2c (0,Ý,Ü). These give a composition corresponding to wulfenite64stolzite36, in agreement with analytical data. The Mo/W distributions in the unit cell provide one explanation for the origin of hemihedrism in the wulfenite-stolzite series.

scholarly journals VI. Tabulated data for the examination of the 230 space-groups by homogeneous X-rays

1924 ◽  
Vol 224 (616-625) ◽  
pp. 221-257 ◽  
Keyword(s):  
Crystal Structures ◽  
Chemical Society ◽  
Unit Cell ◽  
X Rays ◽  
Screw Axis ◽  
Monoclinic System ◽  
Space Group ◽  
X Ray ◽  
Space Groups ◽  
Relative Arrangement

The object of the present paper is to express the conclusions of mathematical crystallography in a form which shall be immediately useful to workers using homogeneous X-rays for the analysis of crystal structures. The results are directly applicable to such methods as the Bragg ionisation method, the powder method, the rotating crystal method, etc., and summarise in as compact a form as possible what inferences may be made from the experimental observations, whichever one of the 230 possible space-groups may happen to be under examination. It is only in certain cases that the spacings of crystal planes as determined by the aid of homogeneous X-rays agree with the values of those spacings which would be expected from ordinary crystallographic calculations. In the majority of cases the relative arrangement of the molecules in the unit cell leads to apparent anomalies in the experimental results, the observed spacings of certain planes or sets of planes being sub-multiples of the calculated spacings. The simplest case (fig. 8) of such an apparent anomaly is found in the space-group C 2 2 of the monoclinic system, where the presence of a two-fold screw-axis, because it interleaves halfway the (010) planes by molecules which are exactly like those lying in the (010) planes, except that they have been rotated through 180°, leads to an observed periodicity which is half the periodicity to be inferred from the dimensions of the unit cell, that is, leads to an observed spacing for (010) which is half the calculated. All screw-axes produce similar results, and, in general, a p -fold screw-axis leads to an observed spacing for the plane perpendicular to it which is 1/ p th that to be inferred from the dimensions of the cell. Besides those produced by the screw-axes, other abnormalities arise out of the presence of glide-planes. The simplest case of this is shown by the space-group C s 2 (fig. 4) of the monoclinic system, in which the second molecule is obtained from the first by a reflection in a plane parallel to (010) and half a primitive translation parallel to that plane. If we look along a direction perpendicular to this glide-plane, the projections of the two molecules on the (010) plane are indistinguishable except in position, which is equivalent to saying that, for the purposes of X-ray interference, certain planes perpendicular to this plane of projection are interleaved by an identical molecular distribution. Furthermore, since the translation associated with the glide-plane must always be half a primitive translation parallel to the glide-plane, we know that the interleaving is always a submultiple of the full spacing and the periodicity is again reduced in a corresponding manner. The use of this method for discriminating between the various space-groups of the monoclinic system was described by Sir Wm. Bragg in a lecture to the Chemical Society. In the present paper the method has been extended to the whole of the 230 space-groups possible to crystalline structures. In general, it may be said that if a crystal possesses a certain glide-plane, a certain set of planes lying in the zone whose axis is perpendicular to that glide-plane will have their periodicity reduced by one-half.

Structural Study of the Disordered RECd6 Quasicrystal Approximants (RE = Tb, Ho, Er, Tm and Lu)

2006 ◽  
Vol 61 (6) ◽  
pp. 644-649 ◽  
Author(s):  
Shu Ying Piao ◽  
Cesar P. Gömez ◽  
Sven Lidin
Keyword(s):  
Single Crystal ◽  
Crystal Structures ◽  
Structural Study ◽  
Diffraction Data ◽  
Unit Cell Dimension ◽  
Unit Cell ◽  
Cell Dimension ◽  
X Ray Diffraction ◽  
X Ray ◽  
Different Types

The crystal structures of approximants RECd6 (RE = Tb, Ho, Er, Tm and Lu) have been refined from single crystal X-ray diffraction data. This work is a continuation of a previous study of MCd6 approximants [1] in which the different types of disorder of the central Cd4 tetrahedra located in the dodecahedral cavities were examined. The structures of the title compounds are all similar to GdCd6 and disorder was observed in all these compounds. There is a correlation between the anisotropic displacement parameter and the unit cell dimension

Molecular and Crystal Structure of Sildenafil Base

2012 ◽  
Vol 67 (5) ◽  
pp. 491-494 ◽  
Author(s):  
Dmitrijs Stepanovs ◽  
Anatoly Mishnev
Keyword(s):  
Crystal Structure ◽  
Erectile Dysfunction ◽  
Crystal Structures ◽  
Unit Cell ◽  
Sildenafil Citrate ◽  
Unit Cell Parameters ◽  
Monoclinic System ◽  
Space Group ◽  
X Ray ◽  
Cell Parameters

Sildenafil citrate monohydrate, well known as Viagra®, is a drug for the treatment of erectile dysfunction. Here we present the X-ray crystal structure of the sildenafil base, C22H30N6O4S. The compound crystallizes in the monoclinic system, space group P21/c with the unit cell parameters a = 17:273(1), b=17:0710(8), c=8:3171(4) Å , b =99:326(2), Z = 4, V = 2420:0(3) Å3. A comparison with the known crystal structures of sildenafil citrate monohydrate and sildenafil saccharinate is also presented.

Temperature phase changes in solid bicyclo[3.3.1]nonane-2,6-dione and bicyclo[3.3.1]nonane-3,7-dione from powder X-ray diffraction data

2007 ◽  
Vol 40 (4) ◽  
pp. 702-709 ◽  
Author(s):  
Michela Brunelli ◽  
Marcus A. Neumann ◽  
Andrew N. Fitch ◽  
Asiloé J. Mora
Keyword(s):  
Simulated Annealing ◽  
Crystal Structures ◽  
Phase Ii ◽  
Diffraction Data ◽  
Direct Methods ◽  
Temperature Phase ◽  
Molecular Orbital Calculations ◽  
X Ray Diffraction ◽  
Space Group ◽  
X Ray

The crystal structures of bicyclo[3.3.1]nonane-2,6-dione and bicyclo[3.3.1]nonane-3,7-dione have been solved by direct methods and by direct-space simulated annealing, respectively, from powder synchrotron X-ray diffraction data. Both compounds have a transition to a face-centred-cubic orientationally disordered phase (phase I) near 363 K, as shown by differential scanning calorimetry and powder diffraction measurements. Phase II of bicyclo[3.3.1]nonane-2,6-dione, which occurs below 363 K, is monoclinic, space groupC2/c, witha= 7.38042 (4),b= 10.38220 (5),c= 9.75092 (5) Å and β = 95.359 (1)° at 80 K. Phase II of bicyclo[3.3.1]nonane-3,7-dione, which occurs below 365 K, is tetragonal, space groupP41212, witha= 6.8558 (1) andc= 16.9375 (1) Å at 100 K. This phase coexists in a biphasic mixture with a minor monoclinic phase II′ [a= 11.450 (6),b = 20.583 (1),c= 6.3779 (3) Å, β = 94.7555 (5)°, at 100 K] detected in the sample, which impeded indexing with standard programs. The crystal structures of phases II were solved by direct methods and by direct-space simulated annealing, employing powder synchrotron X-ray diffraction data of increased instrumental intensity and resolution from the ID31 beamline at the ESRF, and novel indexing algorithms.Ab initiomolecular orbital calculations on the two systems are reported. In the solid state, the molecules pack in chair–chair conformation; molecular structures and packing are discussed.

scholarly journals High-pressure polymorphism in pyridine

IUCrJ ◽  
2020 ◽  
Vol 7 (1) ◽  
pp. 58-70 ◽  
Author(s):  
Nico Giordano ◽  
Christine M. Beavers ◽  
Branton J. Campbell ◽  
Václav Eigner ◽  
Eugene Gregoryanz ◽  
...  
Keyword(s):  
High Pressure ◽  
Phase Ii ◽  
Density Functional ◽  
Equations Of State ◽  
Density Functional Theory Calculations ◽  
Phase Iii ◽  
Mode Analysis ◽  
Cubic Phase ◽  
X Ray ◽  
Lattice Mode

Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P212121 with Z′ = 1 and phase III in P41212 with Z′ = ½. Neutron powder diffraction experiments using pyridine-d5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C 2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH...π and CH...N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified.

Experimental and theoretical morphologies of diuron, N'-(3,4-dichlorophenyl)-N,N-dimethylurea

1996 ◽  
Vol 52 (4) ◽  
pp. 662-667 ◽  
Author(s):  
G. Pfefer ◽  
R. Boistelle
Keyword(s):  
Crystal Structures ◽  
Crystal Morphology ◽  
Unit Cell ◽  
X Ray Diffraction ◽  
X Ray ◽  
Good Agreement ◽  
Attachment Energy

Crystals of diuron, N′-(3,4-dichlorophenyl)-N,N-dimethylurea, C9H10Cl2N2O, were grown from ethanol at low supersaturation. The crystal faces were indexed using a two-circle optical goniometer and X-ray diffraction was used to orientate the crystal morphology with respect to the unit cell. The experimental morphologies were compared with the morphologies predicted by the BFDH (Bravais, Friedel, Donnay, Harker) and attachment energy (AE) methods and calculated from two crystal structures. Good agreement was obtained between experimental and theoretical habits, despite the fact that the crystals exhibit 27 faces belonging to 13 crystallographic forms.

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