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Trans4-(trifluoromethyl) cinnamic acid: Structural characterisation and crystallographic investigation into the temperature induced phase transition
Abstract
The crystal structure of trans-4-(trifluoromethyl) cinnamic acid (1) has been determined in the triclinic space groupP. Differential scanning calorimetry showed that 1 undergoes a single fully reversible temperature induced phase transition at around 132/131 K (cooling/heating). Single crystal structure determinations, carried out at 200, 145 and 120 K, revealed that the volume of the unit cell quadruples as the crystal is cooled through the phase transition with Z′ increasing from 2 to 8. The structures are stabilised by the presence of O–HO hydrogen bonding and C–HO interactions. The results of DSC, single crystal and powder X-ray diffraction studies on 1 are reported.
... The prospect that polymorphic phase transitions may occur in trans-cinnamic acids arose from the seminal work of Schmidt and co-workers, 2 although this issue received little attention until relatively recently. 16,17 Transformations between two members of the same polymorphic classification (i.e., α, β, or γ) are of particular relevance to the present study; examples are the transformation of the triclinic α polymorph of 2-ethoxy-trans-cinnamic acid to a second triclinic form on heating above 333 K, 18 transformation of the triclinic β polymorph of 4-trifluoromethyl-trans-cinnamic acid to a second triclinic form on cooling below 132 K, 19 and transformation of the monoclinic β polymorph of 4-bromotrans-cinnamic acid to a second monoclinic form on cooling below ca. 250−260 K. 20 In all cases, the transformations are reported to be reversible. ...
We report the discovery of a rare case of polymorphism among trans-cinnamic acid derivatives in which both polymorphs are classified as β-type structures based on their solid-state structural properties and their solid-state photoreactivity. Specifically, for 3-fluoro-trans-cinnamic acid, crystallization from many solvent systems leads to formation of one polymorph (denoted β1), although, in a few cases, concomitant crystallization of another polymorph (denoted β2) is also observed. On heating the β1 polymorph, a solid-state phase transition occurs at ca. 119 °C to produce the β2 polymorph. This polymorphic phase transition is irreversible, and the β2 polymorph remains stable on subsequent cooling to ambient temperature. Both the β1 and the β2 polymorphs undergo a topochemical [2 + 2] photodimerization reaction upon UV irradiation to produce 3,3′-difluoro-β-truxinic acid as the photoproduct in almost 100% yield. However, these reactions are associated with complete loss of crystallinity, preventing the structural properties of the directly produced solid photoproduct from being determined by single-crystal or powder X-ray diffraction techniques.
... 156 The role of mechanical stress in the crystallization and relaxation behavior of amorphous indomethacin has been studied, 157 as has the physical stability of amorphous solid dispersions of nifedipine and felodipine containing poly-(vinylpyrrolidone) and small levels of water. 158 The 2008 literature also contained a number of additional reports that summarized investigations of phase transitions among polymorphs and solvatomorphs of various organic compounds, and aspects of these [159][160][161][162][163][164][165][166][167][168][169][170] have been summarized in Table 3. ...
Papers and patents that deal with polymorphism and solvatomorphism have been summarized in an annual review. The review is divided into sections that cover articles of general interest, computational and theoretical studies, preparative and isolation methods, structural characterization and properties of polymorphic and solvatomorphic systems, studies of phase transformations, effects associated with secondary processing, and United States patents issued during 2008. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association.
Solid state [2+2] photodimerization is one of the unique organic reactions that take place between unsaturated molecules under ambient conditions. Complex molecules that are difficult to obtain by traditional organic synthesis methods can be obtained in high yield without the use of solvents. In this work, solid-state [2+2] photodimerization of trans-4-trifluoromethyl cinnamic was investigated via X-ray diffraction and optical spectroscopy methods (UV-vis spectroscopy, FTIR spectroscopy). The crystal structure was determined from powder X-ray diffraction. Furthermore, optical spectroscopy results indicate that the trans-4-trifluoromethyl cinnamic acid undergoes [2+2] photodimerization reaction to yield a product with a cyclobutane framework. The photoconversion process is associated with a complete loss of the long-range order of the reactant and the resulting material is amorphous. Due to the utilization of broad-band light for the reaction, large amounts of product form on the surface and this leads to the precipitation of the product phase which disintegrates the crystal. Транс-4-трифлуорометил фенилпропионы хүчлийн [2+2] фотодимеризаци урвалын судалгаа Хураангуй: Тасалгааны орчинд гэрлийн үйлчлэлээр давхар холбоо бүхий ханаагүй молекул хооронд явагддаг органик химийн өвөрмөц урвалуудын нэг нь [2+2]фотодимеризацийн урвал юм. Энэ урвалыг хатуу төлөвт явуулснаар уламжлалт органик синтезийн аргуудаар гарган авахад хүндрэлтэй комплекс молекулуудыг хортой уусгагч ашиглалгүйгээр, өндөр гарцтайгаар гарган авах боломжтой. Бид энэ ажлаар транс-4-трифлуорометил фенилпропионы хүчлийн кристаллын тасалгааны температурт явагдах хатуу төлөвийн [2+2] фотодимеризацийн урвалын судалгааг нунтгийн рентген дифракци ба оптик спектроскопын аргуудаар (хэт ягаан туяа-үзэгдэх гэрлийн спектроскоп, нил улаан туяаны спектроскоп) хийж гүйцэтгэсэн. Транс-4-трифлуорометил фенилпропионы хүчлийн кристалл бүтцийг нунтгийн рентген дифракцийн аргаар тодорхойлсон. Цаашлаад, транс-4-трифлуорометил фенилпропионы хүчил нь [2+2] фотодимеризацийн урвалд орж циклобутан бүхий молекул үүсгэдэг болохыг оптик спектроскопын үр дүнгээр харуулав. Урвалын явцад дээжийн кристалл бүтэц алдагдаж шинэ аморф бүтэц үүсэж буйг нунтгийн рентген дифракцийн аргаар батлав. Энэ үзэгдлийг транс-4-трифлуорометил фенилпропионы хүчлийн кристаллыг өргөн мужийн гэрлээр үйлчилэх үед кристаллын гадаргууд үүссэн их хэмжээний бүтээгдэхүүн бодис нь тунадасжиж шинэ фаз үүссэнтэй холбон тайлбарлав. Түлхүүр үг: хатуу төлөв, димеризаци, рентген дифракци, фотохими
The molecular structural parameters and thermodynamic properties of trans–3,4–(methylenedioxy)cinnamic acid have been determined by B3LYP method by using 6–311++G** and cc–pVTZ basis sets. The vibrational frequencies of the compound have been precisely assigned, analysed and the theoretical results are compared with FT-IR and FT-Raman data. The energies of important MOs of the compound are also evaluated from DFT method. The frontier orbital energy gap (ELUMO–EHOMO) is found to be 4.1032 eV. The hydrogen and carbon chemical shifts are elaborately discussed. The dual descriptors Δfk, Δsk and Δωk reveals that the order of reactivity towards nucleophilic attack is C1 > O13 > O12 > C3 > C8 > C6. The minimum and maximum limits of the electrostatic potential observed in MDCA is –1.448e × 10–2(red) and +1.448e × 10–2 (blue). Thus, the present investigation provides complete structural, vibrational and reactivity characteristics of the compound.
In order to systematically investigate the influence of factors on the structures of coordination complexes, {[Cu(imta)2]•2.5H2O}n (1), [M(imta)2]n M=(Co, Mn, Cd) (2~4); [Pb(imta)2]n (5); [M(imta)2(H2O)4]•2H2O] M=(Co, Mn, Ni, Zn) (6~9) {H-imta=2-[4-(1H-imidazole-1-ylmethyl)-1H-1,2,3-triazol-1-yl] acetic acid}, have been prepared and characterized by single-crystal X-ray diffraction analyses. 1 is a 2-D MOF with one-dimensional open channels, 2, 3 and 4 are isostructural and display (3,6)-connected 3D frameworks which can be simplified into novel Schläfli symbols of (4•62)2(42•610•83). 5 features a (3,6)-connected topology with the Schläfli symbol (42•6)2(44•62•89). 6~9 are isostructural and mononuclear, which are further stretched to a 3D supramolecular structure through hydrogen bonding. Based on complexes 2 (or 3) and 6 (or 7), a systematic and comprehensive investigation of temperature, solvent, M/L ratios and anion on complex architectures were carried out. The present results may help unravel the mechanism for the roles that synthesis parameters play in the formation of complex structures, and provide insight into the discovery of new interesting compounds. In addition, the photoluminescent properties of 2~9 were also studied.
Four new lanthanide-carboxylate coordination polymers of the formula [Ln3L4(phen)4(H2O)4](ClO4)·2H2O, (Ln = Dy (1), Tb (2), Ho (3) and Er (4), L = 2,3-naphthalenedicarboxylate and phen = 1,10-phenanthroline) were solvothermally synthesized. Their structures as well as magnetic and photoluminescent properties were completely investigated. Complexes 1-4 are isostructural and exhibit 1D ribbon-like chains with envelope-like sixteen-membered rings. The DyIII and TbIII complexes show the corresponding characteristic luminescence in the visible region and the ErIII complex shows luminescence in the near-infrared region upon excitation with UV rays. Studies on the magnetic properties are reported. The Dy(III) analog complex 1 exhibits slow magnetic relaxation and could be a new example of a Dy(III) SMM (single molecule magnet).
The knowledge about the prevalence of weak interactions in terms of the nature and energetics associated with their formation is of significance in organic solids. In the current manuscript, we have systematically explored the existence of different types of intermolecular interactions in ten out of the fifteen newly synthesized trifluoromethyl derivatives of isomeric N-methyl-N-phenylbenzamides. The detailed analysis of all the crystalline solids was performed with quantitative inputs from interaction energy calculations using the PIXEL method. These studies revealed that in the absence of a strong hydrogen bond, the crystal packing is mainly stabilized by a co-operative interplay of weak C-H•••O=C, C-H•••π, C(sp2)/(sp3)-H•••F-C(sp3) hydrogen bonds along with other related interactions, namely π•••π and C(sp3)-F•••F-C(sp3). It is of interest to observe the presence of short and directional weak C-H•••O=C hydrogen bonds in the packing, having a substantial electrostatic (coulombic + polarization) contribution towards the total stabilization energy. The C(sp3)-F group were recognized in the formation of different molecular motifs in the crystal packing utilizing different intermolecular interactions. The contribution from electrostatics amongst the different weak hydrogen bonds was observed in decreasing order: C-H•••O=C > C-H•••F-C(sp3) > C-H•••π. Furthermore, there is an increase in the electrostatic component with concomitant decrease in the dispersion component for the shorter and directional hydrogen bonds.
Variable-temperature single-crystal X-ray diffraction analyses of 1,3-diethyl-spiro[imidazolidine-2,2′-[2H]indene]-1′,3′-dione (1) were performed at low temperatures. The temperature dependence of the cell parameters exhibited a discontinuous change between 158 and 153 K, indicating the existence of a phase transition and hence the occurrence of two polymorphs, I and II, which was also confirmed by DSC analysis: reversible exothermic and endothermic peaks with ΔH = 2.1 kJ mol–1 were observed in the cooling and heating processes, respectively. The molecular packing in I and II at 200 and 90 K, respectively, remained essentially unchanged, retaining the space group Pnma. I and II are conformationally isomorphic crystals, including two crystallographically independent molecules, A and B, with different conformations in a 1:2 ratio. Upon the phase transition from I to II with decreasing temperature, the dynamic disorder of the imidazolidine ring of A in I, which interconverts between quasi-half-chair conformations, is frozen to settle in an envelope conformation, whereas B is retained in a half-chair conformation in both I and II. The close intermolecular contacts in I and II were compared on the basis of Hirshfeld surface analysis. DFT calculations at the B3LYP/6-311G(d,p) level show that the half-chair structure is more stable than the envelope conformer by 2.6 kJ mol–1 in the gas phase. The quasi-half-chair conformation was not obtained as an energy-minimum geometry on the potential energy surface. Temperature-dependent IR spectroscopy along with theoretical calculation using the QST method gave assignments of the carbonyl stretching bands for each conformation. Compound 1 provides a unique example of the isostructural polymorphs in which the structural differences are small, being only at the order–disorder level in the ring conformation; nevertheless, the existence of the enantiotropic phase transition clearly indicates that they are polymorphs.
Five new Cu(II) complexes have been prepared by employing 2,3-naphthalenedicarboxylic acid (H(2)ndc) and different N-donor co-ligands, namely, [Cu(ndc)(H2O)](n) (1), [Cu(ndc)(H2O)(2)](n) (2), {[Cu-1.5(ndc)(OH)(H2O)(2)]center dot 2.5H(2)O}(n) (3), {[Cu(ndc)(phen)(H2O)]center dot CH3OH}(n) (4) (phen = 1,10-phenanthroline), and {[Cu-2(H(0.5)ndc)(2)(4bpy)(3)](ClO4)center dot 2H(2)O}(n) (5) (4bpy = 4,4'-bipyridine). Complex 1 displays a two-dimensional (2D) (4,6)-connected (3(2).4(4))(2)(3(4).4(2).6(4).7(5))(3(4).4(3).6(4).7(4)) coordination network. Complex 2 shows a 2D 3-connected hcb network which is further interlinked via the interlayered C-H center dot center dot center dot pi interactions to result in a three-dimensional (3D) supramolecular network. Complex 3 is a one-dimensional (1D) coordination chain which is further interlinked by the C-H center dot center dot center dot pi and pi center dot center dot center dot pi interactions to lead to the formation of the final 3D supramolecular network. When the N-donor co-ligands phen and 4,4'-bipyridine (4bpy) were introduced, the 1D complex 4 and the 4-connected sql layered complex 5 were constructed, respectively. The final supramolecular frameworks of 4 (2D) and 5 (3D) are extended by hydrogen-bonding and C-H center dot center dot center dot pi interactions. The structural diversity of the complexes was triggered by different reaction conditions and N-donor co-ligands. Moreover, the magnetic properties of the complexes have also been investigated and discussed.
A study of a solid state phase transition in dipivaloyl(3-cyclopentylpropanoyl)methane was undertaken by a combination of crystallographic and calorimetric methods. The high temperature polymorph with two molecules in the asymmetric unit and a disordered cyclopentyl group undergoes a reversible phase transition into an ordered structure with a single molecule in the asymmetric unit. The transition temperature is −20.80 °C with a minimal hysteresis of 1 °C. The nucleation of the new phase was followed by differential scanning calorimetry and occurs in a wide temperature range starting from ca. 0.5 °C above and below the transition temperature, and the crystallographic orientation of the crystals of the nascent phase is to a high degree in agreement with that of the parent crystal.
Solvothermal reactions of 2,2'-biphenyldicarboxylate (H(2)dpa) and 4,4'-azobispyridine (azpy) in the presence of transition metal ions produce six new complexes, namely, [Co(dpa)(bphy)](n) (1), [Co(dpa)(azpy)(H2O)](n)center dot 0.5nH(2)O (2), [Zn(dpa)(bphy](n) (3), [Zn(dpa)(azpy)(H2O)](n) (4), [Mn(dpa)(azpy)(H2O)]center dot 0.5nH(2)O (5), and [Mn(dpa)(azpy)(n)center dot nH(2)O center dot nMeOH (6) (bphy = 1,2-bis(4-pyridyl)hydrazine). The crystal structure analyses reveal that complexes 1 and 3 are isostructural which features a plywood-like structure, while complex 4 exhibits a parallel chains array. Complexes 2 and 5 afford a two-dimensional chiral (4,4) network, and complex 6 shows a (4,4) network. It was noteworthy that the in situ reduction of azpy to bphy was found in complexes 1 and 3, which were confirmed by single-crystal structures and LC-MS analyses. However, this in situ reduction was not found at a low reaction temperature in the syntheses of complexes 2, 4, and 5. The second-harmonic-generation (SHG) properties of 1 and 3 and the solid circular dichroism (CD) properties of 2 and 5 were investigated. In addition, luminescent properties of complexes 3 and 4 as well as magnetic properties of complexes 1, 2, 5, and 6 were also studied.
A crystallographic systematic study of a series of 6 anion receptor complexes has been performed, with the accurate atomic positions and displacement parameters of the hydrogen atoms for two complexes determined by neutron diffraction studies. The N-H center dot center dot center dot anion hydrogen bonding interaction is shown to be central to the geometry of the urea unit. Contributions of the intermolecular interactions to the packing of the molecule are highlighted by correlation to the Hirshfeld surfaces and their fingerprint plots, with alterations to the position and amount of nitro substitution on the receptor shown to affect the pi center dot center dot center dot pi stacking motifs observed across the structures.
Host-guest composites may exhibit abnormal and/or controllable physical properties that are unavailable for traditional solids. However, it is still very difficult to control or visualize the occupancy and motion of the guest. Here we report a flexible ultramicroporous coordination polymer showing exceptional guest-responsive thermal-expansion properties. The vacant crystal exhibits constant and huge thermal expansion over a wide temperature range not only in vacuum but also in air, as its ultramicroporous channel excludes air adsorption even at 77 K. More interestingly, as demonstrated by single-crystal X-ray crystallography, molecular dynamic simulations and solid-state nuclear magnetic resonance, it selectively responds to the molecular rearrangement of N,N-dimethylformamide, leading to conformation reversion of the flexible ligand, which transfers these actions to deform the whole crystal lattice. These results illustrate that combination of ultramicroporous channel and flexible pore surface could be an effective strategy for the utilization of external physical and chemical stimuli.
4-Bromo-trans-cinnamic acid (1) displays some interesting behaviour in the solid-state undergoing a photo-induced head-to-head [2 + 2] cycloaddition reaction in addition to a reversible phase transition. Differential scanning calorimetry established that the thermal phase transition shows a slight hysteresis occurring at 252 K upon cooling and 256 K upon warming. Interestingly it was noted that upon photoirradiation above the phase transition temperature the compound appears to undergo a conversion to the low temperature form, prior to undergoing a cycloaddition reaction. Crystal structures of 1 were determined at 120, 240 and 300 K and found to be in the monoclinic space group P21/n at all three temperatures. The crystal structure of 1i was obtained, after recrystallisation of a powdered sample of 1 that had been irradiated at room temperature, and was found to be in the triclinic space group P. In all cases, the structures were found to be stabilised by the presence of O–HO hydrogen bonding and C–HO interactions. The results of these investigations are presented herein.
Trifluoromethylated benzanilides, containing C(sp3)–F bonds, have been synthesized and their crystal and molecular structures have been investigated to highlight the significance of weak intermolecular interactions associated with the presence of organic fluorine in a molecule. It is observed that the molecular conformation and packing characteristics are a delicate interplay amongst different intermolecular interactions, namely strong N–HOC, weak C–HOC, C–HF–C, and C–Hπ H-bonds, along with C–FF–C and C–Fπ intermolecular interactions in the solid state. All the isomers are associated with variations in crystal packing due to the presence of different supramolecular variants introduced by the presence of a trifluoromethyl group at the ortho, meta and para positions of the phenyl ring. Furthermore, a comparison of the experimental molecular conformation with that obtained from DFT theoretical calculations delineates the significant role of the trifluoromethyl group on the phenyl ring, which results in variations in conformational flexibility associated with the molecule. It has been observed that the –CF3 group is associated with rotational disorder in some of the crystalline solids and the overall crystal structure is stabilized by a network of C(sp2)–HF–C(sp3) intermolecular H-bonds and C–(sp3)FF–C(sp3) contacts [the “fluorous effect”] in the crystalline lattice. It is also observed that there also exists a definite relationship between the acidity of the participating hydrogens with the hydrogen bonding characteristics of such weak C–HF–C H-bonds in the crystal.
Hydrothermal synthesis has afforded five divalent zinc coordination polymers containing 4-(4-carboxyphenyl)-2,2':6',2''-terpyridine (HL1) or its isomer 4-(4-carboxyphenyl)-2,2':4',4''-terpyridine (HL2), with or without the addition of auxiliary ligands, 1,3,5-benzenetricarboxylic acid (H3btc) and 1,4-benzenedicarboxylic acid (H2bdc). Their structures have been characterized by single crystal X-ray analyses and further characterized by infrared spectra, elemental analyses, powder X-ray diffraction, thermogravimetric analyses and photoluminescent spectra. Across this series, the ππ interactions have a dramatic impact on the self-assembly of these entanglement structures, in either case it can exert an important structure-directing role. In addition, the disposition of pyridine nitrogen atoms in ligands also plays a large role in structure direction in this system. Complex is a 2D + 2D→3D inclined polycatenated coordination polymer based on the resulting array of 2D (6,3) layers constructed by 1D→2D ππ directed self-assembly. Complex is assembled into a 3D framework by means of 1D + 1D→3D mutual interdigitation based on 1D→1D self-assembly driven by ππ stacking interactions. Complex shows a 2D + 2D→3D interdigital network involving 2D + 2D→2D parallel interpenetrated and 2D + 2D→2D interdigital (4,4) layer motifs. Complex displays a 2D + 2D→3D polythreaded framework based on a 2D (4,4) network comprised of alternating rings and rods. Complex is a (3,4)-connected 3D framework with topology (4.8(2).10(3))(4.8(2)). In comparison with covalently connected entanglements, such ππ directing self-assembly of entanglements are far less explored, especially, polycatenane based on 1D chain motifs and polythread based on 2D layer motifs are rarely reported. Furthermore, the luminescent properties of complexes at room temperature have also been studied in detail herein.
A simple pressure cell for single-crystal X-ray crystallography at pressures up to 1 kbar (105 kPa) has been developed. A special attachment for mounting the cell onto a goniometer and a new data collection procedure are described.
The polymorphic phase transition of 1,2,4,5-tetrachlorobenzene (TCB) has been investigated using neutron powder diffraction and single-crystal X-ray diffraction. The diffraction experiments show a reversible phase change that occurs as a function of temperature with no apparent loss of sample quality on transition between the two phases. Neutron powder diffraction gives detailed information on the molecular structural changes and lattice parameters from 2 K to room temperature. The structure of the low-temperature form has been elucidated for the first time using single-crystal X-ray diffraction. Comparison of the alpha and beta structures show that they are both based on the same sheet motif, with the differences between the two being very subtle, except in terms of crystal symmetry. Detailed analysis of the structures revealed the changes required for inter-conversion. A computational polymorph search showed that these two sheet structures are more thermodynamically stable than alternative herringbone-type structures.
Cristallisation dans P2 1 /n avec Z=4; affinement jusqu'a R=0,058. Les molecules presque planes s'empilent le long de l'axe b court pour former un empilement plan a plan parallele. Discussion de la structure en vue de clarifier les caracteristiques topochimiques des reactions de photodimerisation a l'etat solide et de formation d'acetal par une reaction heterogene solide-liquide de cette forme β
Oxygen atoms, pendant as substituents or occurring within the ring systems of planar aromatic molecules, have a pronounced tendency to direct crystallisation patterns of such compounds to the β-structure, characterised by a 4 Å short axis. They seem to perform this function by stabilising a critical number of hydrogen atoms, covalently bonded to carbon, through short and directionally specific intermolecular C–H O hydrogen bonds. Consequently, the number of ‘free’ hydrogen atoms which contribute to crystal stabilisation through non-β steering C H interactions is reduced. Both these factors result in the formation of C–H O stabilised two-dimensional entities such as sheets and ribbons. Such entities may be stacked at 4 Å translational separation to generate the entire structure. These concepts are illustrated for some methylenedioxy and alkoxy aromatic systems, quinones, and heterocycles. However, both intra- and inter-sheet C–H O interactions may sometimes be present and the unusual crystal structure of 7-acetoxycoumarin (5) shows how a significantly non-planar molecule may still adopt the β-structure if it is particularly well suited for the formation of C–H O bonds. Yet 4-acetoxycinnamic acid (6), the crystal structure of which was determined in this work and which has almost the same C:H:O ratio as (5), adopts a non-β structure because the number of oxygen atoms available for C–H O bond formation is greatly reduced. The crucial role of the number of such ‘available’ oxygen atoms and ‘H-bonded’ and ‘free’(C–)H atoms vis-à-vis the carbon content is exemplified by the β-steering behaviour of oxygen in some large fused-ring quinones and heterocycles. These trends may also be extended to nitrogen and sulphur heterocycles.
The crystal structure of [4-CH3C5H4NH][SbBr4] is determined at 297 K. The compound crystallizes in the monoclinic space group: P21/c. The structure consists of SbBr4− ions which form infinite chains through the crystal via halogen linkages. The anions are connected with the 4-methylpyridinium via weak NH⋯X hydrogen bonds. The phase situation was studied on two methyl-substituted pyridinium analogous: [4-CH3C5H4NH][SbBr4] and [4-CH3C5H4NH]2[SbBr5] with the differential scanning calorimetry and dilatometric methods. The former derivative undergoes two structural phase transitions: at 351/349 K (heating–cooling) and at 385/381. [4-CH3C5H4NH]2[SbBr5] reveals one phase transition at 426/419 K. The dielectric relaxation studies on [4-CH3C5H4NH]2[SbBr5] in the frequency range between 800 Hz and 6 MHz indicate a presence of relaxation process below room temperature. The dielectric response is well explained in terms of an existence of two dielectric relaxators. The mechanism of the phase transitions in the studied crystals is discussed.
The o-, m- and p-trifluoromethyl trans-cinnamic acids have been prepared by the Doebner modification of the Knoevenagel reaction. The apparent ionization constants of these acids and their fluoro analogs in 50% ethanol-water have been measured and compared with the ionization constant of trans-cinnamic acid. The ultraviolet spectra of these compounds have also been measured.
The alpha-polymorph of ortho-ethoxy-trans-cinnamic acid (OETCA) undergoes a reversible single-crystal-to-single-crystal phase transformation at 333 K. The new high-temperature polymorph (alpha'-OETCA) is stable between 333 and 393 K with three molecules in the asymmetric unit (Z' = 3), space group P1;. Unlike the other polymorphs (and solvate) of OETCA recently reported, two of the molecules in alpha'-OETCA deviate significantly from planarity. This conformational change results in the corrugated sheet-type structure of alpha'-OETCA. The sheets are made up of ribbons, each composed of R(2)(2)(8) hydrogen-bonded pairs (via the -COOH groups), which are further connected by CH.O interactions. When exposed to UV radiation the alpha'-OETCA polymorph can be stabilized below 333 K with ca 8% of the monomer converted into the photodimer. The crystal structures of alpha'-OETCA are reported at two temperatures above the phase transition point (at 345 and 375 K) as well as the stabilized forms at 173 and 293 K. A mechanism for the phase transition involving a cooperative conformational transformation coupled with a shift of layers of OETCA molecules is proposed. The alpha'-OETCA polymorph is also an example of a cinnamic acid derivative where two different potentially photoreactive environments exist in one crystal in which each unit cell has two non-centrosymmetric predimer sites and one centrosymmetric predimer site.
1,2-diaminoethane has been in-situ pressure- and temperature-frozen; apart from two known low-temperature phases, Ialpha and II, three new phases, Ibeta, Igamma and III, have been observed and their structures determined by X-ray diffraction. The measurements at 0.1 MPa were carried out at 274, 243 and 224 K, and 296 K measurements were made at 0.15 GPa (phase Ialpha), at 0.3 and 1.1 GPa (phase Ibeta), at 1.5 GPa (phase Igamma), and at 0.2, 0.3 and 0.5 GPa (phase III). All these phases are monoclinic, space group P2(1)/c, but the unit-cell dimension of phases Ialpha and III are very different at 296 K: aIalpha=5.078 (5), bIalpha=7.204 (8), cIalpha=5.528 (20) A, betaIalpha=115.2 (2) degrees at 0.15 GPa, and aIII=5.10 (3), bIII=5.212 (2), cIII=7.262 (12) A, betaIII=111.6 (4) degrees at 0.2 GPa, respectively; in both phases Z=2. An ambient-pressure low-temperature phase II has been observed below 189 K. Discontinuities in the unit-cell dimensions and in the N...N distance mark the isostructural transition between phases Ialpha and Ibeta at 0.2 GPa, which can be attributed to a damping process of the NH2 group rotations. In phase Igamma the unit-cell parameter a doubles and Z increases to 4. The molecule has inversion symmetry in all the structures determined. 1,2-Diaminoethane can be considered as a simple structural ice analogue, but with NH...N hydrogen bonds and with the H-atom donors (four in one molecule) in excess over H-atom acceptors (two per molecule). Thus, the transformations of 1,2-diaminoethane phases involving the conformational dynamics affect the hydrogen-bonding geometry and molecular association in the crystal. The 1,2-diaminoethane:1,2-dihydroxyethane mixture has been separated by pressure-freezing, and a solid 1,2-diaminoethane crystal in liquid 1,2-dihyroxyethane has been obtained.