A comparative study of heavy-atom effects on the room-temperature phosphorescence of biologically important purines

Room-temperature phosphorescence (r.t.p.) of nine biologically important purines on filter paper is studied. The heavy-atom effect leads to a significant enhancement of r.t.p. signals, with the trend being Tl⁺ > Pb²⁺ ? I — ? Sm³⁺ for most purines. Absolute limits of detection range from 40 pg (purine) to 19 ng (theophylline).     

Study of complexation of phenylaza-15-crwon-5, 4-nitrobenzo- 15-crown-5, and benzo-15-crown-5 with Ag+, Tl+ and Pb2+ ions in methanol by competitive potentiometry

The complexation reaction of phenylaza-15-crwon-5, 4- nitrobenzo- 15-crown-5, and benzo-15-crown-5 with Ag⁺, Tl⁺ and Pb²⁺ ions in methanol solution have been studied by a competitive potentiometric method. The Ag⁺/Ag electrode used both as an indicator and reference electrode in a concentration cell. The emf of cell monitored as the crown ethers concentration varies through the titration. The stoichiometry and stability constants of resulting complexes have been evaluated by MINIQUAD. The stoichiometry for all resulting complexes was 1:1. The stability of these metal ions with derivatives of 15-crown-5 are in order phenylaza-15-crown-5 > Benzo-15-crown-5 > 4-nitrobenzo-15-crown-5, and for the each used crown ethers are as Pb²⁺ > Ag⁺ > Tl⁺. The effect of the substituted group on the stability of resulting complexes was considered. The obtained results are novel and interesting.     

Spectrophotometric Study of Complexation of Tri-Aza Dibenzosulfide and Dibenzosulfoxide Macrocyclic Compounds with Heavy Metal Ions

The complexation reactions between 7,10,13-triaza-1-thia-4,16-dioxa-20,24-dimethyl-2,3;17,18-dibenzo-cyclooctadecane-6,14-dione ( TTD ) and 7,10,13-triaza-1-sulfoxo-4,16-dioxa-20,24-dimethyl-2,3;17,18-dibenzo-cyclooctadecane-6,14-dione ( TSD ) macrocycles with Ag⁺, Cd²⁺, Cu²⁺, Pb²⁺, Sr²⁺, Tl⁺, and Zn²⁺ ions have been studied in ethanol and methanol solutions at 25°C. The complexes formed between macrocycles ( TTD ) and ( TSD ) with these metals cations had a stiochiometry of 1:1 and 1:2, respectively. The stability constants of the resulting complexes were determined and found to decrease in the order Cu²⁺ > Zn²⁺ > Ag⁺ > Tl⁺ > Cd²⁺ > Pb²⁺ > Sr²⁺ with macrocycle ( TTD ) and Tl⁺ > Zn²⁺ > Cd²⁺ > Pb²⁺ > Cu²⁺ > Ag⁺ > Sr²⁺ with macrocycle ( TSD ).     

Study of Complex Formation between 5,7‐Diiodo‐8‐hydroxyquinoline and Zn2+, Cd2+, Pb2+ and Tl+ Cations in Binary Non‐Aqueous Solvents Using Square Wave Polarography Technique (SWP)

The complexation reaction between Zn²⁺, Pb²⁺, Cd²⁺ and Tl⁺ cations by 5,7‐diiodo‐8‐hydroxyquinoline (IQN) was studied in the Dimethylformamide /Acetonitril (DMF‐AN) binary system using square wave polarography technique. The stoichiometry and stability of the complexes were determined by monitoring the shifts in half‐wave or peak potential of the polarographic waves of metal ions against the ligand concentration. The stoichiometry of the complexes was found to be 1:1. The results obtained show that there is an inverse relationship between the formation constant of the complexes and the donor number of solvent base on the Guttmann donocity scale. In all cases the formation constants increased with increasing amounts of AN in these binary systems. The selectivity order for IQN complexes with the cations is Zn²⁺ > Pb²⁺ > Cd²⁺ > Tl⁺.     

Electrochemistry in fluosol-43, a perfluorinated blood substitute: The influence of pluronic f-68 surfactant on some polarographic electrode processes

The inhibition of thaUium(I), lead(II) and cadmium(II) reductions at the dropping mercury electrode is studied in Fluosol-43 and explained as a result of surfactant adsorption. The adsorption of PIuronic F-68 on the DME, examined by electrocapillary, tensammetric and current—time curves, is diffusion-controlled. The similarity established between aqueous solutions of Pluronic F-68 at concentrations near the critical micelle concentration and Fluosol-43 confirms that the adsorption depends only on the monomer form of the surfactant. The magnitude of the inhibitory effect, evaluated from charge-transfer coefficients and rate constants, decreases in the order CD²⁺ > Pb²⁺ > Tl⁺; for the same cation, inhibition depends on the nature of the anion of the supporting electrolyte, the adsorbability of which decreases in the sequence ClO₄^- > NO₃^- > SO₄^2-.     

Determination of the stability constant and thermodynamic parameters between Tl<Superscript>+</Superscript>, Ag<Superscript>+</Superscript> and Pb<Superscript>2+</Superscript> cations with 2,6-di(furyl-2yl)-4-(4-methoxy phenyl)pyridine as a new synthesis ligand

The complexation reaction between Tl⁺, Ag⁺ and Pb²⁺ cations with 2,6-di(furyl-2yl)-4-(4-methoxy phenyl)pyridine as a new synthesis ligand in acetonitrile (ACN)–H₂O and methanol (MeOH)–H₂O binary solutions has been studied at different temperatures using conductometric method. The conductometric data show that the stoichiometry of the complexes is 1: 1 [M: L] and the stability constant of complexes changes with the binary solutions identity. Also, the structure of the resulting 1: 1 complexes was optimized using the LanL2dz basis set at the B3LYP level of theory using GAUSSIAN03 software. The results show that the change of logK_f for (DFMP.Pb)²⁺ and (DFMP.Ag)⁺ complexes with the mole ratio of acetonitrile and for (DFMP.Ag)⁺ and (DFMP.Tl)⁺ complexes with the mole ratio of methanol have a linear behavior, while the change of logK_f of (DFMP.Tl)⁺complex in ACN–H₂O binary solutions (with a minimum in X_ACN = 0.5) and (DFMP.Ag)⁺ complex in MeOH–H₂O binary solutions (with a minimum in X_MeOH = 0.75) show a non-linear behavior. The selectivity order of DFMP ligand for these cations in mol % CAN = 25 and 75 obtain Tl⁺ > Pb²⁺ > Ag⁺ but in mol % CAN = 50, the selectivity order observe Pb²⁺ > Tl⁺ > Ag⁺. Also, this selectivity sequence of DFMP in MeOH–H₂O (mol % MeOH = 75 and 100) and (mol % MeOH = 50) is obtained Pb²⁺ > Ag⁺ and Tl⁺ > Ag⁺ > Pb²⁺ respectively. The values of thermodynamic parameters show that these values are influenced by the nature and the composition of binary solution. In all cases, the resulting complexes are enthalpy stabilized and entropy destabilized. The TΔS_C^° versus ΔH_C^° plot of all obtained thermodynamic data shows a fairly good linear correlation which indicates the existence of enthalpy-entropy compensation in the complexation reactions.     

Thermodynamic modeling for the removal of Cs+, Sr2+, Ca2+and Mg2+ions from aqueous waste solutions using zeolite A</p> </p>

Summary The batch removal ofCs⁺, Sr²⁺, Ca²⁺and Mg²⁺ions from aqueous solutions using synthetic zeolite A was investigated. The influence of the initial ion concentration, pH and temperature was studied. The obtained isotherm data have been correlated with Langumir, Freundlich, and Dubinin-Radushkevich(D-R) isotherm models. The effect of the temperature on the equilibrium distribution values has been utilized to evaluate the standard thermodynamic parameters such as free energy (DG), enthalpy (DH) and entropy (DS). Based on the D-R isotherm expression, the maximum ion-exchange capacity and the mean free energy of each studied ion has been determined. The selectivity sequence, deduced from the equilibrium isotherm data is: Sr²⁺>Ca²⁺>Mg²⁺>Cs⁺>Na⁺.</p> </p>     

Versuche zur Anreicherung von Spurenelementen durch Ausf?llen des Hauptbestandteils

Zusammenfassung Am Beispiel von Silberhalogenidfällungen werden Möglichkeiten aufgezeigt, die adsorptive Mitfällung von Spurenelementen durch Konkurrenzreaktionen zu beeinflussen.Entgegen älteren Literaturangaben wurde eine Verringerung der Spurenadsorption durch Aciditätserhöhung bis zu 7 M salpetersauren Lösungen nicht beobachtet.Die von Silberhalogenid-Niederschlägen in Gegenwart überschüssigen Halogenids normalerweise weitgehend adsorbierten Kationenspuren Bi³⁺, Cd²⁺, Hg⁺, Pb²⁺, Pd²⁺, Tl⁺ und Tl³⁺ werden dann nicht mitgefällt, wenn sie bei der Matrixfällung in genügend stabile undissoziierte Moleküle oder in Komplexanionen übergeführt werden. Als Komplexbildner wirkt bei Cd²⁺, Hg²⁺, Pd²⁺, Tl⁺ und Tl³⁺ schon das im Überschuß vorhandene Bromid, während überschüssiges Jodid die Elemente Bi³⁺, Cd²⁺, Hg²⁺ und Pd²⁺ komplexiert. Mit Ausnahme von Tl⁺ können alle diese Ionen in neutraler Lösung auch mit ÄDTA maskiert und in Lösung gehalten werden, wobei die bekannte pH-Funktion der Komplexbildungskonstanten eine gezielte Selektivierung der Spurenanreicherung möglich macht. Solange die Probelösung bei der Fällungsreaktion noch Silber im Überschuß enthält, werden die ÄDTA-Komplexe zahlreicher Kationen zunächst adsorptiv mitgefällt, nach Überschreiten des Äquivalenzpunktes jedoch wieder desorbiert.Durch Belegen der aktiven Oberfläche der Silberhalogenid-Mederschläge mit stark adsorbierbaren Ionen kann die Mitfällung der übrigen Elemente verhindert werden. Die Untersuchungen ergaben die folgende Reihe in der Stärke der Adsorption von Kationen an AgCl: Tl⁺>Pb²⁺>(Hg²⁺; Pd²⁺)>Tl³⁺>Bi³⁺>Cd²⁺.

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Enrichment of trace elements by precipitation of the main componentIII. Investigations of the influence on trace adsorption at silver halogenide precipitates

The co-precipitation of trace elements by adsorption can be influenced by competetive reactions. This is demonstrated with silver halogenide precipitations. Opposed to former literature a diminution of trace adsorption by raising the acidity could not be observed up to 7 M solutions of nitric acid. Cation traces as Bi³⁺, Cd²⁺, Hg²⁺, Pb²⁺, Pd²⁺, Tl⁺, and Tl³⁺, which are normally adsorbed by silver halogenide precipitates in presence of surplus halogenids, are not co-precipitated if they are transformed into sufficiently stable undissociated molecules or complex anions.As complexing agent the surplus bromide acts with Cd²⁺, Hg²⁺, Pd²⁺, Tl⁺, and Tl³⁺; the surplus iodide with Bi³⁺, Cd²⁺, Hg²⁺, and Pd²⁺. With the exception of Tl⁺ all of these ions can be masked by EDTE in neutral medium and kept in solution. The known pH-function of the stability constants permits an aimed selection of the elements to be enriched. As long as the solution still contains a silver surplus during the precipitation, the EDTE complexes of numerous cations are co-precipitated at first, however desorbed after exceeding the equivalence point. Depositing of strongly adsorbable ions at the active surface of the silver halogenide precipitates prevents the co-precipitation of other elements. The strength of adsorption at silver chloride is given by the following order of cations: Tl⁺ > Pb²⁺ >(Hg²⁺; Pd²⁺)>Tl³⁺>Bi³⁺>Cd²⁺.

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II. Mitt.: diese Z. 235, 235 (1968).

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Herrn Professor Dr. E. Asmus zum 60. Geburtstag gewidmet.

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Wir danken Herrn J. Lohmar für die experimentelle Mitarbeit. Der Fa. E. Merck AG, Darmstadt, sei auch an dieser Stelle für die Überlassung reinsten Silbernitrats gedankt.     

Application of localized reactivity index in combination with periodic DFT calculation to rationalize the swelling mechanism of clay type inorganic material

Clays are layered alumino-silicates. Clays swell and expand in aqueous solution. This property governs the usage of these materials in synthesis of nano-composites and is a source of many of its catalytic applications. We used both localized and periodic calculations within the realm of density functional theory (DFT) on a series of monovalent (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), and divalent (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) cations, to monitor their effect on the swelling of clays. The activity order obtained for the exchangeable cations among all the monovalent and divalent series studied is: Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ > Cs⁺ > Rb⁺ > Na⁺ > Li⁺ > K⁺. We have studied two types of clays, montmorillonite and beidellite, with different surface structures and with/without water using periodic calculations. We have calculated the layer spacing at the first, second and third hydration shells of exchangeable cation, to compare with the experimentald-spacing values to correlate with humidity. A novel quantitative scale is proposed in terms of the intermolecular relative nucleophilicity of the active cation sites in their hydrated state through Fukui functions using hard-soft acid base (HSAB) principle. Finally, a swelling mechanism is proposed. This is a unique study where a combination of periodic and localized calculations has been performed to validate the capability of reactivity index calculations in material designing.     

Studies on the ion-exchange behaviour of chromium ferrocyanide

The sorption of univalent, bivalent and trivalent ions has been studied on chromium ferrocyanide gel. The studies reveal a high sorption capacity for Cs⁺, Tl⁺, Ag⁺, Cu²⁺, Zn²⁺, Cd²⁺, Fe³⁺ and Th⁴⁺. The sorption of monovalent cations show purely ion-exchange mechanism while the uptake of bivalent and trivalent cations is non-equivalent in nature. Single elution of Rb⁺, Cs⁺ and Tl⁺ has been performed from the columns of this exchanger and the recovery is almost complete in all the cases. Cu²⁺ and Ag⁺ get completely adsorbed on the gel column and their elution is not possible probably due to the formation of some new solid phases. Depending on the K_d values of the metal ions, a large number of separations of radiochemical as well as analytical importance can be performed on the columns of this exchanger material.     

Catalytic reduction of nitrate during electrodeposition of thallium from Tl3+ solution

The voltammetric response of vitreous carbon electrodes in nitrate solution in the presence of Tl³⁺ shows the reduction of Tl³⁺ in two stages, to Tl⁺ and to metallic thallium, respectively. Nitrate ions are reduced at high rates during the second stage, concurrently with Tl deposition. The catalytic current varies with the concentration of nitrate, but is virtually independent of the Tl³⁺ concentration. No nitrate reduction occurred when Tl deposition was carried out in a single stage from Tl⁺ solution, nor during reduction of Tl³⁺ to Tl⁺. The results obtained indicate that Tl²⁺, arising from disproportionation of Tl³⁺ and Tl⁰ to yield Tl⁺, mediates the catalytic reduction of nitrate to ammonia during Tl electrodeposition from Tl³⁺ solutions.     

Conductance Study of the Thermodynamics of Complexation of K+, Rb+, Cs+ and Tl+ Ions with Dibenzo-24-crown-8 in Binary Acetonitrile–Nitromethane Mixtures

The complexation reactions between dibenzo-24-crown-8 (DB24C8) and K⁺, Rb⁺, Cs⁺ and Tl⁺ ions were studied conductometrically in different acetonitrile–nitromethane mixtures at various temperatures. The formation constants of the resulting 1:1 complexes were calculated from the computer fitting of the molar conductance–mole ratio data at different temperatures. At 25 °C and in all solvent mixtures used, the stability of the resulting complexes varied in the order Tl⁺ > K⁺ > Rb⁺ > Cs⁺. The enthalpy and entropy changes of the complexation reactions were evaluated from the temperature dependence of formation constants. It was found that the stability of the resulting complexes increased with increasing nitromethane in the solvent mixture. The TΔS° vs. ΔH° plot of all thermodynamic data obtained shows a fairly good linear correlation indicating the existence of enthalpy–entropy compensation in the complexation reactions.     

A Conductometric Study of Complexation Reactions Between Dibenzo-18-Crown-6 (DB18C6) with Cu2+, Zn2+, Tl+ and Cd2+ Metal Cations in Dimethylsulfoxide–Ethylacetate Binary Mixtures

The complex formation between Cu²⁺, Zn²⁺, Tl⁺ and Cd²⁺ metal cations with macrocyclic ligand, dibenzo- 18-crown-6 (DB18C6) was studied in dimethylsulfoxide (DMSO)–ethylacetate (EtOAc) binary systems at different temperatures using conductometric method. In all cases, DB18C6 forms 1:1 complexes with these metal cations. The stability constants of the complexes were obtained from fitting of molar conductivity curves using a computer program, Genplot. The non-linear behaviour which was observed for variations of log K _f of the complexes versus the composition of the mixed solvent was discussed in terms of changing the chemical and physical properties of the constituent solvents when they mix with one another and, therefore, changing the solvation capacities of the metal cations, crown ether molecules and even the resulting complexes with changing the mixed solvent composition. The results show that the selectivity order of DB18C6 for the metal cations in pure ethylacetate and pure dimethylsulfoxide is: Tl⁺ > Cu²⁺ > Zn²⁺ > Cd²⁺ but the selectivity order is changed with the composition of the mixed solvents. The values of enthalpy changes (ΔH°_C) for complexation reactions were obtained from the slope of the van’t Hoff plots and the changes in standard enthalpy (ΔS°_C) were calculated from the relationship: ΔG°_C,298.15=ΔH°_C − 298.15 ΔS°_C. The obtained results show that in most cases, the complexes are enthalpy stabilized, but entropy destabilized and the values of ΔH°_C and ΔS°_C depend strongly on the nature of the medium.     

Thiophosphorylated cavitand: structure and affinity towards soft metal ions

The iiii stereoisomer of the tetrathiophosphonate-calix[4]resorcinarene host 1 exhibited excellent extraction properties towards soft metal ions, with a better affinity for Ag⁺ (91%), than for Tl⁺ (38%) and Hg²⁺ (16%). The extraction of other picrate salts (Cu²⁺, Ni²⁺, Co²⁺, Zn²⁺, Cd²⁺, Pb²⁺) was not detected. The stoichiometry and the structure of the Hg²⁺, Tl⁺ and Ag⁺ complexes were studied by NMR in solution and gave respectively 1:1, 1:1 and 1:2 host-guest complexes. The formation of the self-assembled 1₂·(AgPic)₄ complex was independent on the anion and only observed with silver(I) ion.     

A theoretical study of cation--π interactions: Li+, Na+, K+, Be2+, Mg2+ and Ca2+ complexation with mono- and bicyclic ring-fused benzene derivatives

DFT (B3LYP functional) and MP2 methods using 6-311+G(2d,2p) basis set have been employed to examine the effect of ring fusion to benzene on the cation--π interactions involving alkali metal ions (Li⁺, Na⁺, and K⁺) and alkaline earth metal ions (Be²⁺, Mg²⁺ and Ca²⁺). Our present study indicates that modification of benzene (π-electron source) by fusion of monocyclic or bicyclic (or mixture of these two kinds of rings) strengthens the binding affinity of both alkali and alkaline earth metal cations. The strength of interaction decreases in the following order: Be²⁺ > Mg²⁺ > Ca²⁺ > Li⁺ > Na⁺ > K⁺ for any considered aromatic ligand. The interaction energies for the complexes formed by divalent cations are 4–6 times larger than those for the complexes involving monovalent cations. The structural changes in the ring wherein metal ion binds are examined. The distance between ring centroid and the metal ion is calculated for all of the complexes. Strained bicyclo[2.1.1]hexene ring fusion has substantially larger effect on the strength of cation--π interactions than the monocyclic ring fusion for all of the cations due to the π-electron localization at the central benzene ring.     

Thermodynamic study for dicyclohexano-24-crown-8 complexes with K+, Rb+, Cs+ and Tl+ ions in binary acetonitrile–nitromethane mixtures by conductometric method

The complexation reactions between dicyclohexano-24-crown-8 (DC24C8) and K⁺, Rb⁺, Cs⁺ and Tl⁺ ions were studied conductometrically in the different acetonitrile-nitromethane mixtures at various temperatures. The formation constants of the resulting 1:1 complexes were calculated from the computer fitting of the molar conductance-mole ratio data at different temperatures. At 25 °C and in all solvent mixtures used, the stability of the resulting complexes varied in order of Tl⁺ > K⁺ > Rb⁺ ~ Cs⁺. The enthalpy and entropy changes of the complexation reactions were evaluated by the temperature dependence of formation constants. It was found that the stability of the resulting complexes increased with increasing nitromethane in the solvent mixture.     

Study of the complexation chemistry and speciation of thallium for on-line preconcentration with immobilised quinolin-8-ol

The effects of various buffering reagents and pH conditions were investigated by flame atomic absorption spectrometry to optimise complexation of T1³⁺ with quinolin-8-ol (8-Q), immobilised on controlled-pore glass beads in a 5-cm column. As Tl³⁺ is a softer acid than the other trivalent cations of the Group III elements, the effects of the buffers are different from those observed previously for Al³⁺, Ga³⁺ and In³⁺. A mixed buffer of 0.1 mol l^–1 acetate and 0.1 mol l^–1 ammonium chloride at pH 10 proved most successful, although 0.1 mol l^–1 maleate was also satisfactory over a pH range of 4–10. As thallium normally exists as Tl⁺ in solution, an oxidation method was developed to convert the ions to Tl³⁺, which is more efficiently complexed by 8-Q. Addition of 1–10 l of bromine per 100 ml of sample was sufficient to oxidise Tl⁺ without heating. Excess bromine was removed by addition of phenol. With a flow-rate of 6 ml min^–1, the detection limit of Tl³⁺ is 3 ng ml^–1, for a 3-miri preconcentration time. The enrichment factor under these conditions is 55 and the characteristic concentration is 2 ng ml^–1. The major ions in sea water did not interfere with Tl³⁺ preconcentration and the tolerable limits of Fe³⁺, Cu²⁺ and Al³⁺ are high enough to permit analysis of river and sea waters. The method was applied successfully to the determination of thallium in potassium-enriched table salt. It was also shown that the concentrations of Tl⁺ and Tl³⁺ in a solution can be derived using the described procedure, allowing speciation of inorganic thallium.     

Redox reactions involving molybdenum,tungsten and uranium hexafluorides in acetonitrile

The redox properties of molybdenum, tungsten and uranium hexafluorides in acetonitrile at 298 K have been compared with other redox couples using cyclic voltammetry, and by carrying out appropriate redox reactions under carefully controlled conditions. The order of oxidizing ability established is UF₆ > MoF₆ > NO⁺¹ > solvated Cu²⁺ ? WF₆. The position of the solvated Tl³⁺ cation probably lies between MoF₆ and Cu²⁺ Reactions which occur in the Cu metal/solvated Cuⁿ⁺ (n = 1 or 2)/WF₆ system are accounted for by redox and fluoride-ion-transfer equilibria.     

A new fluorogenic mono-ionizable calix[4]arene dansylcarboxamide as a selective chemosensor of soft metal ions, Tl and Hg

A new fluorogenic calix[4]arene containing one pendent N-dansylcarboxamide group has been synthesized. The ligand demonstrates selective optical recognition of Tl⁺ and Hg²⁺ in solvent extraction from aqueous solutions with high content of Na⁺. Complexation of Tl⁺ and Hg²⁺ produces contrasting changes in the fluorescence spectrum of this sensor. Partial cone is the dominant calixarene conformation in the complex with Tl⁺.     

Competitive Potentiometric Study of a Series of 18-crown-6 with Some Alkali and Alkaline Earth Metal Ions in Methanol Using an Ag+/Ag Electrode

The complexation of some alkali and alkaline earth cations with18-crown-6(18C6), dibenzo-18-crown-6 (DB18C6), dicyclohexyl-18-crown-6 (DCY18C6), and dibenzopyridino-18-crown-6 (DBPY18C6) in a methanol solution has been studied by a competitive potentiometric titration using Ag⁺/Ag electrode as a probe. The stoichiometry and stability constants of the resulting complexes have been evaluated by the MINIQUAD program. The stoichiometry for all resulting complexes was 1:1. The order of stability of Ag⁺ complexes with desired crown ethers varied as DBPY18C6 > DCY18C6 > 18C6 > DB18C6.The stability of the resulting complexes for each of these crown ethers varies in the order ofK⁺ > Na⁺ and Ba²⁺ > Sr²⁺ > Ca²⁺ > Mg²⁺.For each of the used metal ions the major sequence of the stability constants of the resulting complexes varies as DCY18C6 > 18C6 > DB18C6 > DBPY18C6 with minor exceptions.