RESUMEN
This article presents the synthesis and molecular dynamics investigation of three novel cyclic thioethers: 2,3-(4'-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2-ene (compound 1), 2,3,14,15-bis(4',4â³(5â³)-methylbenzo)-1,4,7,10,13,16,19,22,25-octathiacyclotetracosa-2,14-diene (compound 2), and 2,3,8,9-bis(4',4â³(5â³)-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2,8-diene (compound 3). The compounds exhibit relatively high glass transition temperatures (Tg), which range between 254 and 283 K. This characteristic positions them within the so-far limited category of crown-like glass-formers. We demonstrate that cyclic thioethers may span both the realms of ordinary and sizeable molecular glass-formers, each featuring distinct physical properties. Furthermore, we show that the Tg follows a sublinear power law as a function of the molar mass within this class of compounds. We also reveal multiple dielectric relaxation processes of the novel cyclic thioethers. Above the Tg, their dielectric loss spectra are dominated by a structural relaxation, which originates from the cooperative reorientation of entire molecules and exhibits an excess wing on its high-frequency slope. This feature has been attributed to the Johari-Goldstein (JG) process. Each investigated compound exhibits also at least one intramolecular secondary non-JG relaxation stemming from conformational changes. Their activation energies range from approximately 19 kJ/mol to roughly 40 kJ/mol. Finally, we analyze the high-pressure molecular dynamics of compound 1, revealing a pressure-induced increase in its Tg with a dTg/dp coefficient equal to 197 ± 8 K/GPa.
Asunto(s)
Vidrio , Simulación de Dinámica Molecular , Animales , Temperatura de Transición , Temperatura , Vidrio/químicaRESUMEN
A combination of structural, dielectric and calorimetric studies is used to describe a highly atypical behaviour of novel hybrid formate [NH3(CH2)3NH2(CH2)3NH3][Mn(HCOO)3]3, incorporating large triprotonated molecular cations. Two successive phase transitions, switching between fast multiple rotor modes, and the surprising probable coexistence of static and dynamic disorder are discussed for this compound.
RESUMEN
In the last decade, one of the most widely examined compounds of motal-organic frameworks was undoubtedly ((CH3)2NH2)(Zn(HCOO)3), but the problem of the importance of framework dynamics in the order-disorder phase change of the mechanism has not been fully clarified. In this study, a combination of temperature-dependent dielectric, calorimetric, IR, and Raman measurements was used to study the impact of ((CH3)2NH2)(Zn(DCOO)3) formate deuteration on the phase transition mechanism in this compound. This deuteration led to the stiffening of the metal-formate framework, which in turn caused an increase in the phase transition temperature by about 5 K. Interestingly, the energetic ordering of DMA+ cations remained unchanged compared to the non-deuterated compound.
RESUMEN
Two novel three-dimensional metal-organic compounds of formula FA2KM(CN)6, where M = Co, Fe and FA = formamidinium (CH(NH2)2+), have been found to crystallize in a perovskite-like architecture. They have been investigated by X-ray diffraction, dielectric and spectroscopic methods as a function of temperature in order to determine the interactions in the crystals and the mechanism of phase transitions occurring at ca. 321 K upon heating. The phase transitions in both crystals are of first order and originate from the ordering of the formamidinium cations below TC. Symmetry reduction (cubic-to-triclinic) seems to be the strongest upon temperature decrease among known metal-organic frameworks. These materials have been proposed as possible switchable dielectrics with a convenient near-room-temperature transition temperature.
RESUMEN
A set of five novel formate frameworks templated with assemblies comprising diprotonated 1,2-diaminoethane (DAE) and a water molecule of the formula: [NH3(CH2)2NH3]M2(HCOO)6·H2O, where M = Mg, Mn, Co, Ni, Zn, has been synthesized. Four compounds crystallize in the polar R3 space group and one in the chiral P6322 space group (Ni-analog) at room temperature. The polyammonium-water assemblies, mutually joined by hydrogen bonds, fill the cavities of the frameworks and are disordered in the three latter compounds. Additional disorder is found in the Ni-sample as the DAE2+-H2O couple is placed in a special position on the 63 screw axis. IR spectroscopy provides evidence of proton dynamic disorder within the assemblies, which turns into a static one at low temperatures. The crystals preserve their arrangement up to approximately 370 K as shown by differential calorimetric measurements and temperature-dependent IR spectroscopy. The ferroelectric nature of a representative of the family, DAEMgF, at room temperature has been confirmed by pyroelectric measurements. It has been found that the spontaneous polarization may be changed by an external electric field. The magnetic studies reveal a weak ferromagnetic behavior within 8.5-35 K for magnetically active ions: Mn, Co, and Ni.