One-dimensional Hydrogen-bonded Chains Research Articles

4-(1H-2,3-Dihydronaphtho-[1,8-de][1,3,2]di-aza-borinin-2-yl)-1-ethylpyridin-1-ium iodide monohydrate.

The cation of the title hydrated salt, C17H17BN3 +·I-·H2O, is a di-aza-borinane featuring substitution at the 1, 2, and 3 positions in the nitro-gen-boron six-membered heterocycle. The cation is approximately planar with a dihedral angle between the pyridyl ring and the di-aza-borinane ring system of 5.40 (5)°. In the crystal, the cations stack along [100] in an alternating head-to-tail manner, while the iodide ion and water mol-ecule form one-dimensional hydrogen-bonded chains beside the cation stack. The cation stacks and I--water chains are crosslinked by N-H⋯I and N-H⋯O hydrogen bonds.

Ethidium benzoate methanol monosolvate.

In the title salt solvate (systematic name: 8-amino-5-ethyl-6-phenyl-phenanthridin-5-ium benzoate methanol monosolvate), C21H20N3 +·C6H5CO2 -·CH3OH, two ethidium cations, C21H20N3 +, dimerize about a twofold axis through π-π inter-actions [inter-centroid separation = 3.6137 (4) Å]. The benzoate anions are connected through hydrogen bonding with the -NH2 groups of the ethidium cations and the -OH group of the MeOH mol-ecule. The MeOH mol-ecule also accepts a hydrogen bond from the -NH2 group of the ethidium cation. The result is a one-dimensional hydrogen-bonded chain along the b-axis direction.

Carrier Transport Switching of Ferroelectric BTBT Derivative.

Alkylamide-substituted [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative of BTBT-NHCOC14H29 (1), which has ferroelectric N-H···O= hydrogen-bonding network of alkylamide group and two-dimensional (2D) electric structure of BTBT π-cores, was prepared to design the external electric field-responsive organic semiconductors. The short-chain derivative of BTBT-NHCOC3H7 (1') revealed the coexistence of a 2D electronic band structure based on the herringbone BTBT arrangement and the one-dimensional (1D) hydrogen-bonding chain. 1 formed a smectic E (SmE) liquid crystal phase above 412 K and showed ferroelectric hysteresis in the electric field-polarization (P-E) curves at 403-433 K. The remanent polarization (Pr) and coercive electric field (Ec) of 1 at 408 K, 0.1 Hz were 24.0 μC cm-2 and 5.54 V μm-1, respectively. By thermal annealing of thin-film 1 at 443 K, the molecular assembly structure of 1 changed from a monolayer to a bilayer structure with high crystallinity, resulting in conducting layers of BTBT parallel to the substrate surface. The organic field-effect transistor (OFET) device with thermally annealed thin-film 1 showed p-type semiconducting behavior with the hole mobility of 1.0 × 10-3 cm2 V-1 s-1. Furthermore, device 1 showed switching behavior of semiconducting properties by electric field poling and thermal annealing cycle. The electric field response of ferroelectrics modulated the molecular orientation and conduction properties of organic semiconductors, resulting in external electric field control of carrier transport properties.

The Synthesis, Structure, and Dielectric Properties of a One-Dimensional Hydrogen-Bonded DL-α-Phenylglycine Supramolecular Crown-Ether-Based Inclusion Compound.

A novel hydrogen-bonded supramolecular crown-ether-based inclusion compound, [(DL-α-Phenylglycine)(18-crown-6)]+[(CoCl4)0.5]-(1), was obtained via evaporation in a methanolic solution at room temperature using DL-α-phenylglycine, 18-crown-6, cobalt chloride (CoCl2), and hydrochloric acid. Its structure, thermal properties, and electrical properties were characterized via elemental analysis, single-crystal X-ray diffraction, variable-temperature infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and variable temperature-variable frequency dielectric constant testing. The compound was a monoclinic crystal system in the C2 space group at low temperature (100 K) and room temperature (293 K). Analysis of the single crystal structure showed that [(CoCl4)0.5]- presented an edge-sharing ditetrahedral structure in the disordered state, while the protonated DL-α-phenylglycine molecule in the disordered state and intramolecular hydroxyl group (-OH) underwent dynamic rocking, causing a significant stretching motion of the O-H···Cl-type one-dimensional hydrogen bond chain. This resulted in dielectric anomalies in the three axes of the crystal, thus showing significant dielectric anisotropy.

Proton Conduction through Water Chain in a Hydrogen-Bonded Molecular Crystal of Hexaimidazole Ruthenium(II) Complex and Dicarboxylate

Abstract The development of molecular proton conductors has garnered considerable attention owing to their potential for use as solid-state electrolytes for fuel cells. Creating hydrogen-bonding networks between HIm (imidazole) and carboxylate is an attractive approach for constructing efficient proton conducting pathways. Herein, high proton conduction in a new hydrogen-bonded molecular crystal of [RuII(HIm)6](Glu)·4H2O (1·4H2O, H2Glu = glutaric acid) is demonstrated. 1·4H2O exhibited a proton conductivity of 1.33 × 10−4 S cm−1 at 318 K and 100% relative humidity. This is attributed to a three-dimensional (3D) hydrogen-bonding network via hydrogen-bonding between HIm and carboxylate group and water molecules, leading to produce a one-dimensional (1D) hydrogen-bonding water chain, which provides a proton conducting pathway. The calculated activation energy (Ea) value was 0.38 eV at 283–318 K, suggesting that proton transport in 1·4H2O is associated with the Grotthuss mechanism. The hydrogen-bonding networks between HIm and carboxylate may play a role to promote the proton hopping mechanism. This study will provide new molecular proton conductors using imidazole metal complexes.

Chloride salt of oligourea ligand: Synthesis, crystal structure, thermal analyses, and chloride anion binding properties

A novel oligourea ligand L, C33H27N11O8, was synthesized by the isocyanate method using 3, 5-diaminopyridine as the initial reactant. The chloride salt 1, TBA2(LCl2)·acetone, was prepared by the liquid-phase diffusion method of ligand L with tetrabutylammonium chloride (TBACl) at room temperature. The ligand L and salt 1 were characterized by 1H NMR, elemental analysis and Fourier transform infrared spectroscopy (FTIR). X-ray single crystal diffraction analysis showed that in the structure of salt 1, two ligands L formed a dimeric species with two appropriate cavities each trapping two chloride anions by multiple N-H···Cl hydrogen bonds. As a structural unit, the dimer was linked to a one-dimensional hydrogen bond chain through the C-H···O hydrogen bond interactions, which was further extended into two-dimensional network structures by C-H···O and C-H···Cl hydrogen bonds. Thermal decomposition properties of the ligand and salt 1 under nitrogen were studied by the TG-DTG method, and the apparent activation energy (Ea) of the main decomposition process was calculated. Furthermore, the selectivity of ligand L to different anions and the binding properties of Cl− anion were investigated by the method of the UV/Vis spectroscopy.

Homoleptic octahedral CoII complexes as precatalysts for regioselective hydroboration of alkenes with high turnover frequencies.

Homoleptic complexes adopting octahedral coordination modes are usually less active in catalysis due to the saturated coordination around metal centers that prevents substrate activation in a catalytic event. In this work, we demonstrated that a homoleptic octahedral cobalt complex (1) of 4'-pyridyl-2,2';6',2''-terpyridine that experienced monoprotonation at the non-coordinating pyridyl moiety upon crystallization could serve as a highly efficient precatalyst for the hydroboration of styrene derivatives with Markovnikov selectivity. The solid-state structure of this precatalyst along with relevant homoleptic CoII and FeII complexes has been characterized by X-ray crystallography. In the solid state, 1 features one-dimensional hydrogen-bonded chains that are further stacked by interchain π⋯π interactions. The newly synthesized complexes (1-3) along with several known analogues (4-6) were examined as precatalysts for the hydroboration of alkenes. The best-performing system, 1/KOtBu was found to promote Markovnikov hydroboration of substituted styrenes with high turnover frequencies (TOFs) up to ∼47 000 h-1, comparable to the most efficient polymeric catalyst [Co(pytpy)Cl2]n reported to date. Although some limitations in substrate scope as well as functional group tolerance exist, the catalyst shows good promise for several relevant hydrofunctionaliation reactions.

Deuteration triggered downward shift of dielectric phase transition temperature in a hydrogen-bonded molecular crystal

Deuteration of hydrogen-bonded phase transition crystals can increase the transition temperatures due to the isotope effect. But rare examples show the opposite trend that originates from the structural changes of the hydrogen bond, known as the geometric H/D isotope effect. Herein, we report an organic crystal, diethylammonium hydrogen 1,4-terephthalate, exhibits a reversible structural phase transition and dielectric switching. Structural study shows the cations reside in channels formed by one-dimensional hydrogen-bonded anionic chains and undergo an order-disorder transition at around 206 K. The deuterated counterpart shows an elongation of the O···O hydrogen bond by about 0.005 Å. This geometric isotope effect releases the internal pressure of the anionic host on the cation guests and results in a downward shift of the phase transition temperature by 10 K.

Hydrogen-bonding patterns in 2,2-bis-(4-methyl-phen-yl)hexa-fluoro-propane pyridinium and ethyl-enedi-ammonium salt crystals.

The crystal structures of two salt crystals of 2,2-bis-(4-methyl-phen-yl)hexa-fluoro-propane (Bmphfp) with amines, namely, dipyridinium 4,4'-(1,1,1,3,3,3-hexa-fluoro-propane-2,2-di-yl)dibenzoate 4,4'-(1,1,1,3,3,3-hexa-fluoro-propane-2,2-di-yl)di-benzoic acid, 2C5H6N+·C17H8F6O4 2-·C17H10F6O4, (1), and a monohydrated ethyl-enedi-ammonium salt ethane-1,2-diaminium 4,4'-(1,1,1,3,3,3-hexa-fluoro-propane-2,2-di-yl)dibenzoate monohydrate, C2H10N2 2+·C17H8F6O4 2-·H2O, (2), are reported. Compounds 1 and 2 crystallize, respectively, in space group P21/c with Z' = 2 and in space group Pbca with Z' = 1. The crystals of compound 1 contain neutral and anionic Bmphfp mol-ecules, and form a one-dimensional hydrogen-bonded chain motif. The crystals of compound 2 contain anionic Bmphfp mol-ecules, which form a complex three-dimensional hydrogen-bonded network with the ethyl-enedi-amine and water mol-ecules.

Transition Metal Coordination Polymers with Trans-1,4-Cyclohexanedicarboxylate: Acidity-Controlled Synthesis, Structures and Properties.

Five trans-1,4-cyclohexanedicarboxylate (chdc2−) metal–organic frameworks of transition metals were synthesized in aqueous systems. A careful control of pH, reaction temperature and solvent composition were shown to direct the crystallization of a particular compound. Isostructural [Co(H2O)4(chdc)]n (1) and [Fe(H2O)4(chdc)]n (2) consist of one-dimensional hydrogen-bonded chains. Compounds [Cd(H2O)(chdc)]n∙0.5nCH3CN (3), [Mn4(H2O)3(chdc)4]n (4) and [Mn2(Hchdc)2(chdc)]n (5) possess three-dimensional framework structures. The compounds 1, 4 and 5 were further characterized by magnetochemical analysis, which reveals paramagnetic nature of these compounds. A presence of antiferromagnetic exchange at low temperatures is observed for 5 while the antiferromagnetic coupling in 4 is rather strong, even at ambient conditions. The thermal decompositions of 1, 4 and 5 were investigated and the obtained metal oxide (cubic Co3O4 and MnO) samples were analyzed by X-ray diffraction and scanning electron microscopy.

Hydrogen-Bonded Polyrotaxane Cation Structure in Nickel Dithiolate Anion Radical Salts: Ferromagnetic and Semiconducting Behavior Associated with Structural Phase Transition.

The pseudo-polyrotaxane structure of [(H-bpy+ )- (DB-24-crown-8)]∞ (H-bpy+ = monoprotonated 4,4-bipyridinium; DB-24-crown-8 = dibenzo-24-crown-8) has been incorporated into the anion radical salt [Ni(dmit)2 ]- (dmit2- = 1,3-dithiole-2-thione-4,5-dithiolate). (H-bpy+ )(DB-24-crown-8)[Ni(dmit)2 ]- crystallized as two polymorphs, crystals 1 and 2. Crystal 1 was found to have a lower density and looser packing structure in which H-bpy+ forms a one-dimensional hydrogen-bonding chain that passes though the crown ether ring of DB-24-crown-8. DB-24-crown-8 adopts a U-shaped conformation in which two phenylene rings sandwich one of the pyridyl rings of H-bpy+ to stabilize the structure. The [Ni(dmit)2 ]- anions are arranged in a layer parallel to the (10) plane with uniform side-by-side interactions. A structural phase transition was observed at 235 K, accompanied by ordering of the polyrotaxane structure. In crystal 1, at 173 K, H-bpy+ is twisted around the central C-C bond, which perturbs the arrangement of [Ni(dmit)2 ]- through short C-H⋅⋅⋅S contacts. As a result, the semiconducting behavior, with an activation energy of 0.21 eV, becomes insulating below 235 K. The crystal exhibits ferromagnetic interactions because of the weak side-by-side interactions between [Ni(dmit)2 ]- anions. Crystal 2 has a similar pseudo-polyrotaxane structure but showed no phase transition. This suggests that the looser crystal packing in crystal 1 induces the structural change of the pseudo-polyrotaxane, perturbing the electron system of [Ni(dmit)2 ]- .

Salt formation of two BCS II drugs (indomethacin and naproxen) with (1R, 2R)-1,2-diphenylethylenediamine: Crystal structures, solubility and thermodynamics analysis

Indomethacin (Indo) and naproxen (Nap) belong to Biopharmaceutical Classification System (BCS) class II drugs, which permeate membranes well but are poorly soluble. Two pharmaceutical salts hydrate of Indo and Nap with (1R,2R)-1,2-diphenylethylenediamine (DPEN) were obtained using solution crystallization. Both salts, (DPEN)2·(Indo)2·(H2O)2 and (DPEN)1·(Nap)1·(H2O)0.5, were characterised using various solid-state characterisation techniques such as single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Raman spectra. The structure analysis shows that DPEN is monocation as only one amine is protonated into ammonium. The Indo, DPEN and water molecules form one-dimensional hydrogen bond chains through extensive NH⋯O, NH⋯N and OH⋯N interactions. The solubility study shows that the two salts are more soluble in pH 1.2, 4.5 and 6.8 buffer mediums compared with pure raw materials. Solubility thermodynamics calculation was carried out to further understand the solubility properties of two salts.

Synthesis and crystal structures of some bis(3-methyl-1H-indol-2-yl)(salicyl)methanes.

Four 2,2'-bisindolylmethanes (BIMs), a useful class of polyindolyl species joined to a central carbon, were synthesized using salicylaldehyde derivatives and simple acid catalysis; these are 2-[bis(3-methyl-1H-indol-2-yl)methyl]-6-methylphenol, (IIa), 2-[bis(3-methyl-1H-indol-2-yl)methyl]-4,6-dichlorophenol, (IIb), 2-[bis(3-methyl-1H-indol-2-yl)methyl]-4-nitrophenol, (IIc), and 2-[bis(3-methyl-1H-indol-2-yl)methyl]-4,6-di-tert-butylphenol, (IId). BIMs (IIa) and (IIb) were characterized crystallographically as the dimethyl sulfoxide (DMSO) disolvates, i.e. C26H24N2O·2C2H6OS and C25H20Cl2N2O·2C2H6OS, respectively. Both form strikingly similar one-dimensional hydrogen-bonding chain motifs with the DMSO solvent molecules. BIM (IIa) packs into double layers of chains whose orientations alternate every double layer, while (IIb) forms more simply packed chains along the a axis. BIM (IIa) has a remarkably long c axis.

High-density HNIW/TNT cocrystal synthesized using a green chemical method.

The main challenge for achieving better energetic materials is to increase their density. In this paper, cocrystals of HNIW (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, often referred to as CL-20) with TNT (2,4,6-trinitrotoluene) were synthesized using ethanol in a green chemical method. The cocrystal was formulated as C13H11N15O18 and possesses a higher density (1.934 g cm-3) than published previously (1.846 g cm-3). This high-density cocrystal possesses a new structure, which can be substantiated by the different types of hydrogen bonds. The predominant driving forces that connect HNIW with TNT in the new cocrystal were studied at ambient conditions using single-crystal X-ray diffraction, powder X-ray diffraction, Fourier transform-infrared spectroscopy and Raman spectroscopy. The results reveal that the structure of the new HNIW/TNT cocrystals consists of three one-dimensional hydrogen-bonded chains exploiting the familiar HNIW-TNT multi-component supramolecular structure, in which two hydrogen-bonded chains are between -NO2 (HNIW) and -CH (TNT), and one hydrogen-bonded chain is between -CH (HNIW) and -NO2 (TNT). The changes to the electron binding energy and type of element in the new cocrystal were traced using X-ray photoelectron spectroscopy. Meanwhile, the physicochemical characteristics alter after cocrystallization due to the hydrogen bonding. It was found that the new HNIW/TNT cocrystal is more thermodynamically stable than HNIW. Thermodynamic aspects of new cocrystal decomposition are investigated in order to explain this observation. The detonation velocity of new HNIW/TNT cocrystals is 8631 m s-1, close to that of HNIW, whereas the mechanical sensitivity is lower than HNIW.

Towards the synthesis of prenylated phloroglucinol derivatives: An X-ray crystallographic and DFT study of unexpected reaction products

Owing to their bioactivity and prevalence in medicinal plant extracts, prenylated phloroglucinols have garnered significant interest. Towards the synthesis of prenylated phloroglucinol derivatives, 2,4,6-trihydroxy-3-(3-methylbut-2-enyl)acetophenone is required as an intermediate. Herein, this was synthesised by a tandem Claisen-Cope rearrangement reaction on 2,4-bis(methoxymethoxy)-6-(3-methylbut-2-enyloxy)acetophenone and a subsequent hydrolysis to remove protecting groups. This reaction yielded the desired product as well as three by-products. Two of these by-products were isomeric chromane derivatives (2 and 3) and the third was a methoxy derivative (4). These compounds have been studied by single crystal X-ray crystallography and DFT methods. Compound (2) crystallised in the P21/c space group with two hydrogen-bonded molecules in the asymmetric unit (Z = 8). Compound (4) crystallised in the Pbca space group with a single molecule in the asymmetric unit (Z = 8). Both compounds formed extensive supramolecular structures supported by hydrogen bonds in the solid state. Compound (2) forms a simple one-dimensional hydrogen-bonded chain co-linear with the a-axis. Compound (4) forms a two-dimensional supramolecular structure comprising “pentameric” hydrogen-bonded motifs linked by additional H-bonds to form the supramolecular structure. Both structures showed intramolecular hydrogen bonds between the acetyl oxygen and adjacent OH group. DFT simulations were used to probe the relative energies of the molecules and hydrogen bonds. These simulations showed that the intramolecular hydrogen bond has a substantial stabilising effect with an interaction strength of 70.64 kJ mol−1. The formation of the hydrogen-bonded dimer of (2) from which the supramolecular structure is formed has a ΔHassoc constant of −42.32 kJ mol−1, illustrating that the formation of the hydrogen-bonded structure is energetically favourable.

Role of halogen-halogen contacts in the crystal structures of three new solvates of the drug oxyclozanide.

Halogen-halogen contacts are electrostatic in nature and exhibit directionality similar to hydrogen bonds. Oxyclozanide [systematic name: 2,3,5-trichloro-N-(3,5-dichloro-2-hydroxyphenyl)-6-hydroxybenzamide] is a drug used for the treatment of fascioliasis in domestic animals. The molecule carries five chlorine substituents and represents an ideal candidate for the study of halogen bonds in the crystal. Three new crystalline solvates of oxyclozanide, namely, oxyclozanide benzene hemisolvate, C13H6Cl5NO3·0.5C6H6, (I), oxyclozanide xylene hemisolvate, C13H6Cl5NO3·0.5C8H10, (II), and oxyclozanide toluene hemisolvate, C13H6Cl5NO3·0.5C7H8, (III), were structurally characterized. In this context, the crystal structure of oxyclozanide chlorobenzene hemisolvate, C13H6Cl5NO3·0.5C6H5Cl, (IV), was redetermined based on intensity data collected at 100 K. In all four solvates, the cocrystallized solvent molecules are located on crystallographic inversion centres. Solvates (I)-(IV) exhibit similar one-dimensional hydrogen-bonded chains generated by O-H...O, O-H...Cl and Cl...Cl interactions. The extension of these one-dimensional chains into two-dimensional layers is promoted by Cl...Cl and C-H...π contacts. Solvates (III) and (IV) are isostructural and differ from (I) and (II) with respect to subtle details concerning the intermolecular contacts.

Synthesis, characterization, and antimicrobial activity of two Schiff base silver(I) complexes derived from 4-carboxybenzaldehyde

Two new Schiff base silver(I) complexes, {[Ag(L1)][H2O]}∞ (1) and {[Ag(L2)][H2O]}∞ (2), where L1 = 4-((2-diethylaminoethylimino)methyl)phenecarboxylate and L2 = 4-([3-(morpholin-4-yl)propyl]iminomethyl)phenecarboxylate, have been synthesized and characterized. X-ray crystallographic analysis reveals that silver ions in 1 are bridged by L1 to form a one-dimensional zigzag chain, while three silver ions in 2 are linked by L2 to produce a two-dimensional coordination polymer. Guest water molecules in 1 generate a one-dimensional hydrogen-bonded chain. Both 1 and 2 offer high antibacterial activity against Gram-positive bacteria Staphylococcus aureus with MIC values of 0.26 and 0.24 mM and to exhibit good inhibitory activity against urease with the respective IC50 values of 3.5 ± 0.1 and 3.8 ± 0.2 μM.

Self-assembled structures of ferrocene-l-carnosine conjugates

Ferrocene-l-carnosine conjugates, Fc-l-Carnosine-OEt and Fc2-l-Carnosine-OEt, were synthesized and characterized by X-ray crystallographic analyses. The single-crystal X-ray structure determination of Fc-l-Carnosine-OEt was demonstrated to reveal the formation of antiparallel β-sheet-like 12-membered intermolecularly hydrogen-bonded ring between NH (β-alanine) and CO (β-alanine of another molecule). Furthermore, the self-assembled structure through antiparallel β-sheet-like hydrogen bonding network of β-alanyl moieties and homomeric association of the imidazole moieties through the one-dimensional hydrogen-bonded chain was formed in the crystal packing. The self-assembled structure based on antiparallel β-sheet-like hydrogen bonding between NH (histidine) and CO (the ferrocenoyl moiety of another molecule) was also observed in the crystal packing of Fc2-l-Carnosine-OEt. In the case of Fc2-l-Carnosine-OEt, which has the N-ferrocenoyl imidazole moiety, each molecule is connected through the intermolecular hydrogen bonds of CH⋯N pattern between the ferrocene and the imidazole moieties.

Isostructural Multicomponent Gliclazide Crystals with Improved Solubility

This study focused on pharmaceutical isostructural multicomponent crystals and compared their physicochemical properties. Gliclazide (GLI), a drug used for treating diabetes mellitus, formed isostructural multicomponent crystals with 4-aminopyridine and 3,4-diaminopyridine. The structures of these crystals were determined by single-crystal X-ray structure analysis. These crystals were categorized as salts because they showed proton transfer. The crystal structures revealed a robust one-dimensional hydrogen bond chain of GLI by the crystallographic 21 screw axis along the b-axis and the interaction between these two chains, which includes the coformer, to construct a dimeric structure were crucial to the isostructurality. These salts showed a promising dissolution rate that was higher relative to that of the raw material. Interestingly, although both are isostructural, 4-aminopyridine salt showed a faster dissolution rate than 3,4-diaminopyridine salt.

Structural phase transitions and dielectric transitions in a 1,4-diazabicyclo[2.2.2]octane (dabco) based organic crystal

1,4-Diazabicyclo[2.2.2]octane (dabco) based organic crystal [(Hdabco)(H2O)](PF6) (1) was synthesized and characterized. Its structure is featured by wavy-like one-dimensional supramolecular hydrogen bond chains built from the [(Hdabco)(H2O)] units from end to end. Compound 1 undergoes two reversible phase transitions at 226/268 K and 178/181 K, respectively. The disorder-order transition of water molecule in the hydrogen bond chain induces the phase transition at 226 K while the twisting of the [Hdabco]+ cation and displacements of the PF6− anion and H2O molecules trigger the phase transition at 178 K. Dielectric transitions are thus triggered in the crystal.