Perchlorate Salts Research Articles

Rapid Radiolytic Degradation of Amino Acids in the Martian Shallow Subsurface: Implications for the Search for Extinct Life.

Amino acids are fundamental to life as we know them as the monomers of proteins and enzymes. They are also readily synthesized under a variety of plausible prebiotic conditions and are common in carbon-rich meteorites. Thus, they represent a reasonable class of organics to target in the search for prebiotic chemistry or chemical evidence of life on Mars. However, regardless of their origin, amino acids and other organic molecules present in near-surface regolith and rocks on Mars can be degraded by exposure to cosmic rays that can penetrate to a depth of a few meters. We exposed several pure amino acids in dry and hydrated silicate mixtures and in mixtures of silicates with perchlorate salts to gamma radiation at various temperatures and radiation doses representative of the martian near-subsurface. We found that irradiation of amino acids mixed with dry silica powder increased the rate of amino acid radiolysis, with the radiolysis constants of amino acids in silicate mixtures at least a factor of 10 larger compared with the radiolysis constants of amino acids alone. The addition of perchlorate salts to the silicate samples or hydration of silicate samples further accelerated the rate of amino acid destruction during irradiation and increased the radiolysis constants by a factor of ∼1.5. Our results suggest that even low-molecular-weight amino acids could degrade in just ∼20 million years in the top 10 cm of the martian surface regolith and rock, and even faster if the material contains elevated abundances of hydrated silicate minerals or perchlorates. We did not detect evidence of amino acid racemization after gamma radiation exposure of the samples, which indicates that the chirality of some surviving amino acids may still be preserved. Our experimental results suggest serious challenges for the search of ancient amino acids and other potential organic biosignatures in the top 2 m of the martian surface.

A supramolecular non centrosymmetric 2,6-diaminopyridinium perchlorate salt: Crystal structure and optoelectronic DFT study

The synthesis of a supramolecular hybrid perchlorate salt using the aromatic amine 2,6-diaminopyridine afforded an unexpected crystal structure with non centrosymmetric crystallographic symmetry. The synthesized material with formula (C5H8N3)ClO4 crystallize according to the monoclinic symmetry in the space group P21. The observed supramolecular crystal structure, consisting of only weak interactions in all the three dimensions, built from hydrogen bonds and through Anion…π+ interaction between (ClO4)- tetrahedrons of the mineral part and the aromatic rings of the organic molecules, is rarely observed for similar compounds. The electronic and optical properties are characterized at the theoretical level using DFT; these characterizations demonstrate a moderate direct band gap and a high optical anisotropy. Our results provide the essential information about structural, electronic and optical properties of (C5H8N3)ClO4, which are useful for guiding the future studies.

Process for the catalytic conversion of micro-carbon residue content of heavy hydrocarbon feedstocks and a low surface area catalyst composition for use therein

The synthesis of a supramolecular hybrid perchlorate salt using the aromatic amine 2,6-diaminopyridine afforded an unexpected crystal structure with non centrosymmetric crystallographic symmetry. The synthesized material with formula (C5H8N3)ClO4 crystallize according to the monoclinic symmetry in the space group P21. The observed supramolecular crystal structure, consisting of only weak interactions in all the three dimensions, built from hydrogen bonds and through Anion…π+ interaction between (ClO4)- tetrahedrons of the mineral part and the aromatic rings of the organic molecules, is rarely observed for similar compounds. The electronic and optical properties are characterized at the theoretical level using DFT; these characterizations demonstrate a moderate direct band gap and a high optical anisotropy. Our results provide the essential information about structural, electronic and optical properties of (C5H8N3)ClO4, which are useful for guiding the future studies.

Thermodynamic modelling of perchlorate/chloride and perchlorate/chlorate deliquescence at Mars-relevant temperatures

Perchlorate (ClO4−) salts were discovered on Mars and are known to absorb water vapor from the atmosphere and deliquesce into the aqueous phase. Other species such as chlorides (Cl−) and chlorates (ClO3−) were also identified; these species can affect the deliquescence of perchlorates. Here we generate phase diagrams of perchlorate/chloride and perchlorate/chlorate binary mixtures for K, Na, Mg and Ca in the temperature range 223–273 K. Using a new approach based on thermodynamic modelling of evaporation, we determined the deliquescence relative humidity (the minimum relative humidity at which a salt converts into a liquid by absorbing atmospheric water vapor) and the eutonic relative humidity (the minimum relative humidity at which two salts are in equilibrium with liquid) for binary salt mixtures. Our modelling results show that the deliquescence relative humidity values of all salt mixtures is always lower than that of each individual end-member salt at a fixed temperature, typically a few percent lower. The closer the eutonic is to one of the end-member, the smaller the decrease in relative humidity compared to the pure pole. Thus, only eutonics which are far from both poles exhibit a significant drop in relative humidity. Moreover, the eutonic relative humidity always increases with decreasing temperature, which does not favor liquids in the dry and cold Martian environment. Finally, the increased stability of water ice at the lowest temperatures always reduces or even eliminates the stability of liquids. Therefore, the favorable temperature and relative humidity conditions under which binary salt liquid mixtures exist are generally not significantly improved compared to single salts.

Cadmium(II) compounds of the bis-cyanoethyl derivative (LCX) of Me8[14]aneC (LC): characterization and antibacterial studies

The isomeric ligand LC, a saturated analogue of 2,9-C-meso-Me8[14]diene, on reflux with excess acrylonitrile afforded 1,8-N-pendant cyanoethyl derivative LCX. Interaction of LCX with cadmium(II) perchlorate, nitrate, acetate, and chloride salts produced six coordinated octahedral compounds, [Cd(LCX) (ClO4)2]∙2H2O, [Cd(LCX) (NO3)2], [Cd(LCX) (CH3COO)2], and [Cd(LCX)Cl2], respectively. Further, axial substitution reactions between [Cd(LCX) (ClO4)2]∙2H2O and KI, KBr, KCl, KSCN, and NaNO2 in a 1:2 ratio yielded six coordinated octahedral compounds, [Cd(LCX)I2]∙H2O, [Cd(LCX)Br2]∙2H2O, [Cd(LCX)Cl(ClO4)]∙2H2O, [Cd(LCX) (NCS)2]∙H2O, and [Cd(LCX) (NO2) (ClO4)]∙2H2O, respectively. All of the newly prepared compounds have been characterized by analytical, spectroscopic, molar conductivity, and magnetochemical data. The crystal structure of the ligand LCX was determined by x-ray crystallography which showed the 14-membered ring to adopt an extended chair conformation. Antibacterial activities of the newly formed cadmium(II) complexes against selected bacteria showed these to exhibit moderate and selective activity with 1-4 and 8 exhibiting greatest potency against the gram negative bacterium Salmonella typhi, and 5, 6, and 7 against the gram positive bacterium Bacillus wiedmannii.

Selective oxidation of benzene by an iron oxide carbonaceous nanocatalyst prepared from iron perchlorate salts and hydrogen peroxide in benzene and acetonitrile

Direct benzene hydroxylation under mild conditions using hydrogen peroxide can be essential in producing phenol. A new iron oxide-carbonaceous nanocatalyst (Fe-oxide CNC) is prepared in situ using a precursor of Fe(ClO4)2 and H2O2 in benzene and CH3CN. The obtained Fe-oxide CNC can efficiently catalyze benzene oxidation to phenol using H2O2(aq) in CH3CN. The highest phenol production was achieved with the turnover number (TON) of 122 in unit iron content and selectivity of 96% at 60 °C. The catalyst was characterized by SEM, TEM, XAS, XPS, Raman, XRD, BET, and TGA techniques.Interestingly, materials identification and characterization of the Fe-oxide CNC provided evidence to support the presence of Fe2O3. In addition, the carbonaceous moieties mainly consisted of branched aliphatic hydrocarbons on the iron oxide surface, which enhance the catalytic performance for benzene oxidation to phenol. The Fe-oxide CNC can be recycled three times with comparable catalytic efficiency. Using H218O2 as an oxidant, the phenol product with highly enriched 18O-atom indicates that the reactive oxygen species (ROS) activated by H2O2(aq) is crucial for aromatic oxidation. Time-resolved spin trapping experiments displayed the formation of the intermediate adducts consisting of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and reactive oxygenated radicals. For 3‒24 h durations, higher steady-state concentrations of the oxygenated intermediates observed at 25 °C than 60 °C indicate that the catalytic oxidation of benzene mediated by Fe-oxide CNC performs much faster kinetics at 60 °C than the ambient condition.

Time-Sensitive Aspects of Mars Sample Return (MSR) Science.

Time-Sensitive Aspects of Mars Sample Return (MSR) Science.

Helix Induction and Inversion of Polymeric Foldamer Regulated by the Single Enantiomers.

Generally, a single enantiomer can induce a foldamer into a preferred-handed helix, while another condition is required for the helical inversion. Herein, it is found that the helix induction and subsequent inversion of poly(m-phenylene diethynylene)-based foldamer bearing aza-18-crown-6 pendants (Poly-1) can be realized by increasing the concentration of sterically hindered l-amino acid perchlorate salts. When the amount of chiral enantiomers is small, one enantiomer tends to complex with two non-adjacent aza-18-crown-6 rings via three N+ H···O hydrogen bonds in a sandwich mode. Notably, the transition dipole moment is perpendicular to the aza-18-crown-6 ring, so that the induced helical chirality in Poly-1 backbone is opposite to the chirality of enantiomers. When the amount of chiral enantiomers is large enough, each aza-18-crown-6 is occupied by one enantiomer, which causes the transition dipole moment in a parallel direction to aza-18-crown-6 ring. In this case, the increased steric hindrance can facilitate the inversion of screw sense of Poly-1 backbone, which is directed by chiral center of enantiomers. As a result, a helix inversion has been achieved successfully. This work not only provides a novel strategy for regulating the two-stage folded helical conformations by the single enantiomers, but opens a window to develop chiral recognition materials.

Perchlorate: Its Sources, Toxicity, Removal and Environmental effects

In water, the perchlorate anion CIO4 - is formed as a result of the dissolution of the potassium, solid salts of ammonium and sodium perchlorate as well as the perchloric acid. Ammonium perchlorate, which is used in solid rocket engine fuel, has a short shelf life and should be replenished on a regular basis, thus the pollution is thought to come mainly from the military and supporting industries. Perchlorate pollution of groundwater and soil appears to be a contentious issue and is a popular issue in many fields including industries, government, defence and the general public. The pollution is difficult to remove or remediate due to the perchlorate anion's basic chemical and physical characteristics. The health consequences of drinking low-dose perchlorate-contaminated water are unknown, although it interacts with the body's iodine intake, leading human thyroid development to be inhibited. Various major assessment studies of its risk in terms of environmental and health impact can be found in literature. However, information about its sources, contamination, removal or detection of the perchlorate is sparse. This review provides knowledge on perchlorate amounts in the atmosphere and how perchlorate remediation is performed.

Constructing Planar C–N Bond-Linked Bi-Triazole High-Energy Explosives via the Formation of Salts

Abstract Energy and safety are the two most concerning properties of energetic materials (EMs). Design and synthesis of planar molecules is considered to be an effective way to obtain high-energy low-sensitivity EMs. In this study, a new C–N bond-linked energetic bi-triazole compound, 3,4′,5-triamine-1,3′-bi(1,2,4-triazole) (1), and its nitrate (1a) and perchlorate salt (1b), as well as a nitrification product, 5-amino-3,4′-dinitramide-1,3′-bi(1,2,4-triazole) (2), and corresponding sodium (2a) and potassium salt (2b) were designed and synthesized. All compounds were fully characterized. The molecular structures of 1, 1a, 1b, 2a, and 2b were confirmed by single-crystal X-ray structure analyses. It was found that 1 exhibits non-planar loose molecular stacking, while 1a, 1b, and 2b exhibit compact face-to-face layered molecular packing structures, and salt 2a exhibits compact wave-like stacking. The physicochemical properties of all compounds, such as density, thermal stability, and mechanical sensitivity, were studied. 1, 1a, 1b, 2a, and 2b have low impact and friction sensitivity and good thermal stability. In addition, Hirshfeld surfaces (HS), independent gradient model (IGM) analyses combined with single-crystal analyses revealed intermolecular interactions in 1, 1a, 1b, 2a, and 2b and clarified the relationship between the structures and molecular properties. The sophisticated hydrogen bonds between the bi-triazole molecules and nitrate/perchlorate ions are the driving force of formation of planar structures. Detonation properties calculated with the professional software package EXPLO5 showed that nitrate 1a (D = 8872 m·s−1, P = 34.8 GPa) and perchlorate 1b (D = 8956 m·s−1, P = 35.1 GPa) exhibit good detonation performance.

Imidazolium‐Based Energetic Materials

AbstractWe show that 1‐(2‐hydroxyethyl)‐1H‐imidazol‐3‐ium and 1‐(2‐hydroxyethyl)‐3‐methyl‐1H‐imidazol‐3‐ium salts, as well as the salts of their nitrite esters, that include oxidizing anions (so‐called explosphores) such as nitrate, picrate and perchlorate„ are endowed with high enthalpies of decomposition, high decomposition temperatures, and high densities. This combined with detonation velocities and pressures calculated using Explo5, indicates that especially the perchlorate salts have potential as energetic materials. All the studied salts are easily preparable in up to gram scale through two or three synthetic steps from simple imidazole. Many of these salts are ionic liquids, which may render them useful as energetic plasticizers or gel filled materials.

Bimetallic Organoplatinum(II)‐Ag(I) Cluster Cations with Ag−Pt Interactions Unsupported by Conventional Bridging Ligands

AbstractThe organoplatinum complex of a primary amine and AgClO4 aggregate in methanol to oligonuclear mixed‐metal species. A perchlorate salt 3 with the empirical formula C96H146Ag7Cl13N12O62Pt6 could be isolated and structurally characterized with the help of X‐ray diffraction at the DESY synchrotron. The product contains both a penta‐ and a tetra‐nuclear complex cation in which Ag(I) and Pt(II) alternate, held together without any conventional bridge. The cationic species feature short Ag⋅⋅⋅Pt distances between 2.7 and 2.9 Å and contacts longer than 2.4 Å between Ag and C atoms in aromatic rings. In addition to the interactions with Ag(I) cations, Pt(II) is in a square‐planar arrangement, coordinated by a chelating and a terminal amine and an aqua ligand. The central Ag(I) in the pentanuclear cation is located on a twofold crystallographic axis and not involved in any obvious coordinative bond; it exclusively shows short contacts to the neighboring Pt(II) ions and the Pt‐bonded, formally anionic carbon atoms of the cyclometallated organic ligand. Powder diffraction shows that 3 melts and re‐solidifies without decomposition.

Mechanical properties of polyvinylpyrrolidone/polyvinyl alcohol‐based solid electrolytes

AbstractThe demand for flexible, lightweight, and long‐lasting energy devices has stimulated interest in solid polymer blends. The conventional lithium‐ion batteries use liquid electrolytes that are chemically unstable due to the presence of carbonates, which are highly volatile and flammable, creating a significant safety risk. Therefore, the need for the development of solid‐state electrolytes that are safe, environmentally friendly, with robust mechanical properties. In this work, the solid polymer blend is explored using a mixture of a polymer matrix of polyvinylpyrrolidone/polyvinyl alcohol and lithium perchlorate salt. The produced films are characterized using a scanning electron microscopy, X‐ray diffraction, and Fourier transform infrared. The mechanical properties of the flexible films are also measured using nanoindentation techniques, statistical deconvolution mapping, tensile tests, and fracture toughness measurements. (Young's modulus of 6.87 GPa, hardness of 1.3 GPa, tensile strength of 4.3 MPa, and fracture toughness of 0.81 MPa.m0.5) The implications of the results are then discussed for potential applications of robust solid polymer blends‐based electrolytes.

Instigating the In Vitro Anticancer Activity of New Pyridine-Thiazole-Based Co(III), Mn(II), and Ni(II) Complexes: Synthesis, Structure, DFT, Docking, and MD Simulation Studies.

The perchlorate salt of (4-(4-methoxy phenyl)-2-(2-(1-pyridine-2-yl)ethylidene)hydrazinyl)thiazole (PytH·ClO4) and its metal perchlorate derivatives, namely, [Co(Pyt)2]ClO4 (1), [Mn(PytH)2](ClO4)2 (2), and [Ni(PytH)2](ClO4)2 (3), have been synthesized and characterized through single X-ray crystallography and spectroscopic methods. The ligand crystallizes in a space group P21/n in a nearly planar structure. The overall geometry of the complex salts is described as a distorted octahedron with a MN6 chromophore. The ligand (PytH·ClO4) behaves as a neutral N,N,N-tridentate donor toward the "soft" Mn(II) and Ni(II) centers, whereas the deprotonated ligand stabilizes the "hard" Co(III) center. The DNA binding constant (Kb) values of PytH·ClO4, 1, 2, and 3 are determined using the UV-vis spectroscopic method, and the Kb values are 9.29 × 105, 7.11 × 105, 8.71 × 105, and 7.82 × 105 mol-1, respectively, indicating the intercalative mode of interactions with CT-DNA. All the derivatives show effective antiproliferative activity against U-937 human monocytic tumor cells with IC50 values 4.374 ± 0.02, 5.583 ± 0.12, 0.3976 ± 0.05, and 11.63 ± 0.01 μM for PytH·ClO4, 1, 2, and 3, respectively. The best apoptosis mode of cell death is shown by 2 followed by PytH·ClO4 and 1 at an equivalent concentration of IC50 values. The combined molecular docking and dynamics simulation study evaluates the binding energies of anticancer agents, providing groove binding property with DNA. The 20 ns molecular dynamics simulation study reveals the maximum DNA binding stability of 2 corroborating the experimental results. The new class of metal derivatives of pyridine-thiazole can be used for advanced cancer therapeutics.

Synthesis, characterization and properties of amphoteric heat-resistant explosive materials: Fused [1,2,5]oxadiazolo [3′,4′:5,6]pyrido[4,3-d][1,2,3]triazines

• The tricyclic triazine oxide was synthesized as the first fused amphoteric energetic molecule. • These compounds exhibited high decomposition temperatures (310–344 °C). • All the non-metallic compounds have better detonation properties than HNS and PYX and low sensitivities. • The perchlorate salt is a potential candidate for heat-resistant explosive. Fused cyclic energetic compounds are a unique class of large conjugated structures containing two or more rings, and have been identified as promising competitors for traditional energetic materials. In the present work, we investigated 5-amino-6-oxo-4,6-dihydro-[1,2,5]oxadiazolo[3′,4′:5,6]pyrido[4,3- d ][1,2,3]triazine 8-oxide ( 3 ) in detail as a fused amphoteric energetic molecule that can be protonated and deprotonated by acids and bases to obtain its cationic ( 11 – 13 ) and anionic ( 4 – 10 ) salts. All the new compounds were thoroughly characterized by infrared and multinuclear NMR ( 1 H and 13 C) spectroscopy, and elemental analysis. The structural features of 3 – 6 , 9 , 10 , 12 , and 13 were investigated by single-crystal X-ray diffractions and different theoretical techniques (Hirshfeld surfaces analyses, and noncovalent interactions). The aromaticity of compound 3 was studied via localized orbital locator- π and iso -chemical shielding surfaces (ICSS). Their thermal decomposition temperatures were investigated by differential scanning calorimetry and thermogravimetry to be 310–344 °C (onset). In addition, impact and friction tests revealed that these compounds were insensitive. Densities, heats of formation and detonation properties were measured by a gas pycnometer, calculated using the Born-Haber energy cycle and EXPLO5 v6.05.04 program, respectively. The neutral compound 3 and all its non-metallic salts have better detonation properties than HNS and PYX and lower sensitivities. In particular, salt 11 (N + O: 61.74 %, T d : 330 °C, Δ H f : 1.68 kJ g −1 , D : 8.795 km s −1 , P : 36.0 GPa, IS: > 40 J, FS: > 360 N) is a potential candidate for heat-resistant explosives.

Robustness of hydrogen bonding in the construction of supramolecular architectures in protonated perchlorate salts

Five new multicomponent salts of perchloric acid with a series of substituted anilines and N-heterocyclic amines namely Diphenylaminium perchlorate (DPAPC) (1), 2, 5-dichloroanilinium perchlorate (25DAP) hydrate (2), 4-Methylanilinium perchlorate (4MAPC) (3), 4-diamino-6-methyl-1, 3, 5-triazin-1-ium hydrogen perchlorate (24DAMTHP) (4), and 8-hydroxyquinolinium hydrogen perchlorate (8HQP) (5) were prepared and structurally characterized. The entire complexes were subjected to FTIR and elemental analysis. A vast family of intermolecular contacts N-H…O, O-H…O, N-H…N, and C-H…O was observed, which are key ingredient in the generation of privileged supramolecular self-assemblies appeared as one-dimensional chain, two-dimensional ladder, and helix. Cambridge Structural Database (CSD) analysis of 52 hits revealed the perchloric acid display higher propensity of ladder architectures. Molecular stability of the complexes was studied by quantum chemical calculations using DFT/B3LYP method with 6-31G(d,p) basis set. Furthermore, their relative charge distributions were identified using molecular electrostatic potential map. The use of Hirshfeld surfaces in combination with fingerprint plots was visualized in order to study the closer contacts within the molecule. The relative contribution of the whole percentage of interactions associated is highlighted.

Synthesis of Ionizable Calix[4]arenes for Chelation of Selected Divalent Cations.

Two sets of functionalised calix[4]arenes, either with a 1,3-crown ether bridge or with an open-chain oligo ether moiety in 1,3-position were prepared and further equipped with additional deprotonisable sulfonamide groups to establish chelating systems for selected cations Sr2+, Ba2+, and Pb2+ ions. To improve the complexation behaviour towards these cations, calix[4]arenes with oligo ether groups and modified crowns of different sizes were synthesized. Association constants were determined by UV/Vis titration in acetonitrile using the respective perchlorate salts and logK values between 3.2 and 8.0 were obtained. These findings were supported by the calculation of the binding energies exemplarily for selected complexes with Ba2+.

Atmospheric, Geomorphological, and Compositional Analysis of Martian Asimov and Hale Craters: Implications for Recurring Slope Lineae

Recurring slope lineae (RSL) are small, dark, seasonal albedo features lengthening down “warm” Martian steep slopes. Their origin has been attributed to both liquid and dry processes, hence representing one of the major open science questions on present day Mars. In the present study, we report a catalog of previous literature and newly added RSL sites making a total of 940 sites globally on Mars along with the detailed geological and compositional investigation of the Hale and Asimov craters with their RSL features. We also estimate temperature and atmospheric water abundances in the study area, which are two of the main factors to explain the origin and formation of RSL. The study found that the Asimov crater’s local temperatures are high enough to allow either the melting of brines or deliquescence of calcium perchlorate and other salts during the HiRISE observation period and found the water vapor column to be nearly five times higher than those measured “before RSL appearance.” This supports the theory of deliquescence as one of the mechanisms for the regolith-atmosphere interaction and RSL formation in the studied crater, which suggests that minerals absorb moisture from the environment until the minerals dissolve in the absorbed water and yield a solution. We also used compact reconnaissance imaging spectrometer for Mars–derived browse products for a compositional study associated with RSL features hosting craters and surface characteristics of Mars.

Dielectric and ionic transport properties of bio-based polyurethane acrylate solid polymer electrolyte for application in electrochemical devices

Solid polymer electrolyte has been extensively studied as an alternative to liquid electrolyte that is often affected by the leakage, deformation and limited range of operating temperature issues. The present study was conducted in an attempt to synthesize polyurethane acrylate (PUA) as a host polymer to evaluate polymer performance supplemented with Li salt as polymer electrolytes. PUA was prepared by the reaction of jatropha oil polyol with toluene 2,4-diisocyanate and hydroxylethylmethylacrylate. Lithium perchlorate (LiClO4) salt with different percentage of weight in the range of 5 wt. % to 25 wt. % was added to PUA to produce the PUA electrolyte. The mixtures were cured under UV radiation to obtain thin polymeric films with good thermal stability and ionic conductivity. PUA with 25 wt. % lithium salt has the highest conductivity of 6.40 × 10−5 S cm−1 at room temperature. The finding was supported by Fourier transform infrared (FTIR). Electrode polarization occurrence was interpreted by the complex dielectric constant (Ɛr), dielectric loss (Ɛi), real electrical modulus (Mr), imaginary electrical modulus (Mi) and tan σ. The effect of lithium salt addition was investigated by differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. Additionally, the ionic transport properties, transference number measurement and electrochemical stability were presented.

Thermotropic chirality enhancement of nanoparticles constructed from foldamer/bis(amino acid) complexes

Herein, chiral nanoparticles are constructed by mixing an artificial foldamer bearing aza-18-crown-6 pendants with l-homocystine perchlorate salt, showing a thermotropic chirality enhancement due to the binding mode changes in the heating process.