High- and low-temperature phases in isostructural 4-chloro-3-nitroaniline and 4-iodo-3-nitroaniline

2014 ◽  
Vol 70 (12) ◽  
pp. 1153-1160 ◽  
Author(s):  
Jan Fábry ◽  
Michal Dušek ◽  
Přemysl Vaněk ◽  
Iegor Rafalovskyi ◽  
Jiří Hlinka ◽  
...  
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
Low Temperature ◽  
Hydrogen Bonds ◽  
Primary Amine ◽  
Zigzag Chain ◽  
Temperature Phase ◽  
Scanning Calorimetry ◽  
Acta Cryst ◽  
Graph Set

The structures of 4-chloro-3-nitroaniline, C6H5ClN2O2, (I), and 4-iodo-3-nitroaniline, C6H5IN2O2, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single-crystal X-ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high-temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene-ring planes at two different orientations. In the low-temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, thebaxis doubles with respect to the room-temperature cell. Each of the low-temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallographic inversion centres. The molecules within each pair have the same absolute value of the inclination angle. The Flack parameter of the low-temperature phases is very close to 0.5, indicating inversion twinning. This can be envisaged as stacking faults in the low-temperature phases. It seems that competition between the primary amine–nitro N—H...O hydrogen bonds which form three-centred hydrogen bonds is the reason for the disorder of the nitro groups, as well as for the phase transition in both (I) and (II). The backbones of the structures are formed by N—H...N hydrogen bonding of moderate strength which results in the graph-set motifC(3). This graph-set motif forms a zigzag chain parallel to the monoclinicbaxis and is maintained in both the high- and the low-temperature structures. The primary amine groups are pyramidal, with similar geometric values in all four determinations. The high-temperature phase of (II) has been described previously [Gardenet al.(2004).Acta Cryst.C60, o328–o330].

Phase transitions in solid cycloheptene

1990 ◽  
Vol 68 (4) ◽  
pp. 604-611 ◽  
Author(s):  
Julian Haines ◽  
D. F. R. Gilson
Keyword(s):  
Phase Transition ◽  
High Temperature ◽  
Low Temperature ◽  
Lattice Relaxation ◽  
Proton Spin ◽  
Spin Lattice Relaxation ◽  
Temperature Phase ◽  
Scanning Calorimetry ◽  
Spin Lattice ◽  
Low Temperature Phase

The phase transition behaviour of cycloheptene has been investigated by differential scanning calorimetry, proton spin-lattice relaxation, and vibrational spectroscopy (infrared and Raman). Two solid–solid phase transitions were observed, at 154 and 210 K, with transition enthalpies and entropies of 5.28 and 0.71 kJ mol−1 and 34.3 and 3.4 JK−1, respectively. Cycloheptene melted at 217 K with an entropy of melting of 4.5 JK−1 mol−1. The bands in the vibrational spectra of the two high temperature phases were broad and featureless, characteristic of highly disordered phases. The presence of other conformers, in addition to the chair form, was indicated from bands in the spectra. The ring inversion mode was highly phase dependent and exhibited soft mode type behaviour prior to the transition from the low temperature phase. The low frequency Raman spectra (external modes) of these phases indicated that the molecules are undergoing isotropic reorientation. In the low temperature phase, the vibrational bands were narrow; the splitting of the fundamentals into two components and the presence of nine external modes are consistent with unit cell symmetry of either C2 or Cs with two molecules per primitive unit cell. A glassy state can be produced from the intermediate phase and the vibrational spectra were very similar to those of the high temperature phases, indicating that static disorder was present. The barriers to reorientation, as obtained from proton spin-lattice relaxation measurements, are 9.0 kJ mol−1 in both the high temperature phases, and 15.4 kJ mol−1 in the low temperature, ordered phase. Keywords: cycloheptene, phase transition, differential scanning calorimetry, NMR, vibrational spectroscopy.

Interrelation between Molecular Motions and Phase Transitions in Monomethylammonium Perchlorate. A Study by DSC, Proton, and Deuteron NMR

1985 ◽  
Vol 40 (6) ◽  
pp. 602-610 ◽  
Author(s):  
S. Jurga ◽  
H. W. Spiess
Keyword(s):  
Low Temperature ◽  
Hydrogen Bonds ◽  
Phase Ii ◽  
Intermediate Phase ◽  
Relaxation Times ◽  
Spin Lattice Relaxation ◽  
Phase Iii ◽  
Temperature Phase ◽  
Molecular Motions ◽  
Scanning Calorimetry

Differential scanning calorimetry, the temperature dependence of proton and deuteron lineshapes and spin-lattice relaxation times are reported for the isotopic species CH3NH3ClO4, CD3NH3ClO4 and CH3ND3ClO4 of monomethylammonium Perchlorate. The data confirmed the existence of three different phase modifications in monomethylammonium Perchlorate and its selectively deuterated analogues. In addition they were used to identify the molecular motions occurring in the respective phases and to determine their activation parameters. In the low temperature phase III, stable below 320 K. the CH3 and NH3 groups reorient about their threefold symmetry axes C3 with different frequencies. In the low-temperature range of this phase the deuteron quadrupole coupling constant indicates N - H ... O hydrogen bonds between the monomethylammonium and the Perchlorate ions. In the intermediate phase II, between 320 K and 451 K for CH3NH3ClO4 and CD3NH3ClO4 and 320 K and 437 K for CH3ND3ClO4 , the methylammonium ions reorient about an axis inclined at an angle of 18 degrees to the C3 axis. The analysis of the entropy changes, associated with the III- II transitions indicates that the ClO4− ions have a large motional freedom in phase II, presumably because of breaking or weakening of N - H . . . O hydrogen bonds. In phase I the monomethylammonium ions undergo isotropic motion along with translational diffusion between different sites of the primitive cubic unit cell.

Rigid-Body Disorder Models for the High-Temperature Phase of Ferrocene

1997 ◽  
Vol 53 (6) ◽  
pp. 928-938 ◽  
Author(s):  
C. P. Brock ◽  
Y. Fu
Keyword(s):  
High Temperature ◽  
Low Temperature ◽  
Rigid Body ◽  
High Temperature Phase ◽  
Body Motion ◽  
Temperature Phase ◽  
Acta Cryst ◽  
Disorder Transition ◽  
Simple Order ◽  
Low Temperature Phase

Ferrocene, [Fe(C5H5)2], which crystallizes at room temperature in space group P21/a with Z = 2, is described in many textbooks as having D 5d symmetry. Previous work has shown, however, that the librational amplitude associated with motion about the fivefold axis does not decrease with temperature and that the crystals are probably disordered. Ferrocene molecules in triclinic crystals grown below 169 K have approximate D 5 symmetry and an almost eclipsed conformation; the low- and high-temperature phases may be related by an order–disorder transition, during which the number of independent atoms changes by a factor of 4. The structure of the high-temperature phase has been reinvestigated with rigid-body refinements of the neutron diffraction data collected at 173 and 298 K by Takusagawa & Koetzle [Acta Cryst. (1979), B35, 1074–1081]. The C5H5 ring was treated as a rigid group of C 5 symmetry; C—C and C—H distances were allowed to vary, as was the displacement of the H atoms from the C5 plane. The rigid-body motion of the C5H5 ligand was described by the TLS model. All the rigid-body disorder models fit better than conventional independent-atom models. A disorder model that includes three sites for each C5H5 ring is the best of the models that were investigated, which indicates that the structure of the high-temperature phase cannot be described by the superposition of the two independent ferrocene molecules in the low-temperature phase. The phase transition between the high- and low-temperature phases is not a simple order–disorder transition.

scholarly journals Orientational order-disorder γ ↔ β ↔ α′ ↔ α phase transitions in Sr2B2O5 pyroborate and crystal structures of β and α phases

2017 ◽  
Vol 73 (6) ◽  
pp. 1056-1067 ◽  
Author(s):  
Sergey Volkov ◽  
Michal Dušek ◽  
Rimma Bubnova ◽  
Maria Krzhizhanovskaya ◽  
Valery Ugolkov ◽  
...  
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
Crystal Structures ◽  
Intermediate Phase ◽  
Orientational Order ◽  
High Temperature Phase ◽  
Basic Unit ◽  
Temperature Phase ◽  
Scanning Calorimetry ◽  
X Ray

Crystal structures of γ-, β- and α-Sr2B2O5 polymorphs resulting from the γ ↔ (at 565 K) β ↔ (at 637 K) α′ ↔ (at 651 K) α sequence of reversible first-order phase transitions are studied by high-temperature single-crystal X-ray diffraction, high-temperature X-ray powder diffraction, differential scanning calorimetry and impedance spectroscopy. Out of these phases, the structure of γ-Sr2B2O5 was already known whereas the structures of β- and α-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate β-Sr2B2O5 phase and monoclinic low-temperature γ-Sr2B2O5 as well as high-temperature α-Sr2B2O5 phase. Taking the α-Sr2B2O5 phase with space group P21/c as a parent structure, the γ-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the β-Sr2B2O5 phase was a sixfold superstructure with symmetry P{\overline 1}. To construct a unified structure model for all Sr2B2O5 modifications, phases of γ- and β-Sr2B2O5 were also refined as commensurately modulated structures using the basic unit cell of the parent α-Sr2B2O5. The phase transitions are related to the orientational order–disorder arrangement of B2O5 pyroborate groups, where the degree of disorder grows towards the high-temperature phase. Thermal expansion is strongly anisotropic and dictated by preferable orientations of BO3 triangles in the structure. The intermediate phase α′-Sr2B2O5, stable over a narrow temperature range (637–651 K), features the largest anisotropy of expansion for the known borates: α11 = 205, α22 = 57, α33 = −81 × 10−6 K−1.

scholarly journals The phase transitions of 4-aminopyridine-based indolocarbazoles: twinning, local- and pseudo-symmetry

2019 ◽  
Vol 75 (1) ◽  
pp. 97-106 ◽  
Author(s):  
Thomas Kader ◽  
Berthold Stöger ◽  
Johannes Fröhlich ◽  
Paul Kautny
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
High Temperature ◽  
Low Temperature ◽  
Group Symmetry ◽  
Temperature Phase ◽  
Space Group ◽  
Local Symmetries ◽  
Group Symmetries ◽  
Low Temperature Phase

The phase transitions and polymorphism of three 4-aminopyridine-based indolocarbazole analogues are analyzed with respect to symmetry relationships and twinning. Seven polymorphs were structurally characterized using single-crystal diffraction. 5NICz (the indolo[3,2,1-jk]carbazole derivative with the C atom in the 5-position replaced by N) crystallizes as a P21/a high-temperature (270 K) polymorph and as a Pca21 low-temperature (150 K) polymorph. Even though their space-group symmetry is not related by a group–subgroup relationship, the local symmetries of both belong to the same order–disorder (OD) groupoid family. Both are polytypes of a maximum degree of order and are twinned by point operations of the other polytype. 2NICz (C atom in the 2-position replaced by N) likewise crystallizes in a high-temperature (Pcca, 280 K) polymorph and a low-temperature (P21/c, 150 K) polymorph. Here, the space-group symmetries are related by a group–subgroup relationship. The low-temperature phase is twinned by the point operations lost on cooling. The crystal structure of bulk 2,5NICz (N-substitution at the 2- and 5-positions) was unrelated to 2NICz and 5NICz and no phase transition was observed. Isolated single crystals of a different polymorph of 2,5NICz, isotypic with 2NICz, were isolated. However, the analogous phase transition in this case takes place at distinctly higher temperatures (> 300 K).

Temperature-induced order–disorder structural phase transitions of two-dimensional isostructural hexamethylenetetramine co-crystals

2017 ◽  
Vol 73 (5) ◽  
pp. 879-890 ◽  
Author(s):  
Tze Shyang Chia ◽  
Ching Kheng Quah
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
High Temperature ◽  
Low Temperature ◽  
Benzoic Acid ◽  
Structural Phase ◽  
Temperature Phase ◽  
Two Dimensional ◽  
Transition Temperatures ◽  
Orthorhombic Crystal Structure

Hexamethylenetetramine-benzoic acid (1/2) (HBA) and hexamethylenetetramine-4-methylbenzoic acid (1/2) (HMBA) co-crystals undergo order–disorder structural phase transition from a low-temperature monoclinic crystal structure to a high-temperature orthorhombic crystal structure at the transition temperatures of 257.5 (5) K (Pn↔Fmm2) and 265.5 (5) K (P21/n↔Cmcm), respectively, using variable-temperature single-crystal X-ray diffraction analysis. The observed phase transitions were confirmed to be reversible first-order transitions as indicated by the sharp endothermic and exothermic peaks in the differential scanning calorimetry measurement. The three-molecule aggregate of HBA and HMBA consists of a hexamethylenetetramine molecule and two benzoic acid or two 4-methylbenzoic acid molecules, respectively. The acid molecules are ordered at the low-temperature phase and are equally disordered over two positions, which are related by a mirror symmetry, at the high-temperature phase. The two-dimensional supramolecular constructs common to both co-crystals are formed by three-molecule aggregatesviaweak intermolecular C—H...O and C—H...π interactions into molecular trilayers parallel to theacplane with smallXPacdissimilarity indices and parameters. ThePIXELinteraction energies of all corresponding molecular contacts were calculated and the results are comparable between HBA and HMBA co-crystals, resulting in similar lattice energies and transition temperatures despite their two-dimensional isostructural relationship. The observed phase transitions of these two energetically similar co-crystals are triggered by similar mechanisms,i.e.the molecular rotator ordering and structural order–disorder transformation, which induced non-merohedral twinning with similar twin matrices in the low-temperature crystal form of both co-crystals.

Low-temperature structure of V6O13

2003 ◽  
Vol 59 (6) ◽  
pp. 747-752 ◽  
Author(s):  
Jonas Höwing ◽  
Torbjörn Gustafsson ◽  
John O. Thomas
Keyword(s):  
Low Temperature ◽  
Mirror Plane ◽  
Temperature Phase ◽  
Temperature Structure ◽  
X Ray Diffraction ◽  
Scanning Calorimetry ◽  
Acta Cryst ◽  
Lithium Polymer Batteries ◽  
Space Group ◽  
Dsc Measurements

The structure of the transition metal oxide V6O13, a potential cathode material in lithium-polymer batteries, has been studied at 95 K using single-crystal X-ray diffraction (XRD). A phase transition has been determined by differential scanning calorimetry (DSC) measurements to occur at 153 K, with a heat of transition of −1.98 kJ mol−1. In this low-temperature phase, the V and O atoms move by up to 0.21 Å out of the mirror plane they occupy in the room-temperature structure. It is concluded that the earlier reported space group P21/a [Kawada et al. (1978). Acta Cryst. B34, 1037–1039] is incorrect and that a more appropriate choice of space group is Pc.

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