logK Values Research Articles

Host–guest complexes of various cucurbit[n]urils with the hydrochloride salt of 2,4-diaminoazobenzene

The interaction products of normal cucurbit[n]urils (n = 7, 8; Q[7] Q[8]) and a sym- tetramethyl-substituted cucurbit[6]uril derivative (TMeQ[6]) with the hydrochloride salts of 2,4-diaminoazobenzene (g·HCl) were investigated in aqueous solution using 1H NMR spectroscopy, electronic absorption spectroscopy, as well as single crystal X-ray diffraction. The 1H NMR spectra analysis established a basic interaction model in which inclusion complexes with a host:guest ratio of 1:1 form for the TMeQ[6] and Q[7] cases, while they form with a host:guest ratio of 1:2 for the Q[8] case. Commonly, the hosts selectively bound to the phenyl moieties of the guests. Absorption spectrophotometric analysis in aqueous solution defined the stability of the host–guest inclusion complexes at pH 3.2. Quantitatively, at this pH, complexes with a host:guest ratio of 1:1—those with smaller hosts TMeQ[6] and Q[7]—formed with logK values between 6 and 7. That with host Q[8] and a host:guest ratio of 1:2 formed with a logK value of 10.8. Single crystal X-ray structures of the inclusion complexes TMeQ[6]–g·HCl and Q[8]–g·HCl showed the phenyl moiety of the guest inserted into the host cavity. This result supports the solution-based 1H NMR spectroscopic study.

Retention and release isotherm of arsenic in arsenic–humic/fulvic equilibrium study

Organic fractions from farm yard manure (FYM), vermicompost, municipal sludge, mustard cake, and surface soil of West Bengal, which was arsenic (As)-contaminated, were extracted and fractionated into fulvic and humic acid (FA and HA, respectively) fractions following standard procedures. These HA and FA samples were characterized by pH-potentiometric titrations, viscometric measurements and visible spectrophotometry. The stability constant (logK) of the complexes formed by these natural with As in aqueous phase was evaluated by the ion-exchange method. The logK values suggest that the organo-As complexes were quite stable. The release isotherm of As from the HA/FA complexes extracted from vermicompost and FYM was assayed in the presence of molybdate, nitrate, phosphate, sulfate and borate. The greatest tendency to displace As from the complexes was shown by sulfate, molybdite, and nitrate.

Metal Ion Coordinating Properties of the Highly Preorganized Tetradentate Ligand 1,10‐Phenanthroline‐2,9‐dicarboxaldehyde‐2,9‐dioxime

AbstractUse of UV/Vis spectroscopy to measure protonation and formation constants of PDOX (1,10‐phenanthroline‐2,9‐dicarboxaldehyde‐2,9‐dioxime) is reported. All complexes all show a protonation constant of an oxime that is quite close to pKa1 for the free ligand, that is, for ZnII log K for ZnPDOX + H+ [lrarr2] ZnPDOXH+. This is in agreement with literature crystal structures, which show that, for small metal ions such as ZnII, one oxime group of PDOX is left non‐coordinated. Only for PbII is the protonation constant of the PDOX complex significantly lowered, to a value of 8.2. In order to investigate the high log K1 value for the PbII/PDOX complex, and its relatively low protonation constant, the crystal structure of [Pb(PDOXH2)(DMSO)2Cl(ClO4)] (1) was determined.

Binding Constants of Mercury and Dissolved Organic Matter Determined by a Modified Ion Exchange Technique

Ion-exchange techniques have been widely used for determining the conditional stability constants (logK) between dissolved organic matter (DOM) and various metal ions in aqueous solution. An exception is mercuric ion, Hg2+, whose exceedingly strong binding with reduced sulfur or thiol-like functional groups in DOM makes the ion exchange reactions difficult. Using a Hg-selective thiol resin, we have developed a modified ion-exchange technique which overcomes this limitation. This technique allows not only the determination of binding constants between Hg2+ and DOM of varying origins, but also the discrimination of complexes with varying coordination numbers [i.e., 1:1 and 1:2 Hg:thiol-ligand (HgL) complexes]. Measured logK values of four selected DOM isolates varied slightly from 21.9 to 23.6 for 1:1 HgL complexes, and from 30.1 to 31.6 for 1:2 HgL(2) complexes. These results suggest similar binding modes that are likely occurring between Hg2+ and key thiolate functional groups in DOM particularly at a relatively low Hg to DOM ratio. Future studies should further elucidate the nature and precise stoichiometries of binding between Hg2+ and DOM at environmentally relevant concentrations.

Metal Ion Complexing Properties of Dipyridoacridine, a Highly Preorganized Tridentate Homologue of 1,10-Phenanthroline

DPA (dipyrido[4,3-b;5,6-b]acridine) may be considered as a tridentate homologue of phen (1,10-phenanthroline). In this paper some of the metal ion complexing properties of DPA in aqueous solution are reported. Using UV-visible spectroscopy to follow the intense π-π* transitions of DPA as a function of pH gave protonation constants at ionic strength (μ) = 0 and 25 °C of pK(1) = 4.57(3) and pK(2) = 2.90(3). Titration of 10(-5) M solutions of DPA with a variety of metal ions gave log K(1) values as follows: Zn(II), 7.9(1); Cd(II), 8.1(1); Pb(II), 8.3(1); La(III), 5.23(7); Gd(III), 5.7(1); Ca(II), 3.68; all at 25 °C and μ = 0. Log K(1) values at μ = 0.1 were obtained for Mg(II), 0.7(1); Sr(II), 2.20(1); Ba(II), 1.5(1). The log K(1) values show that the high level of preorganization of DPA leads to complexes 3 log units more stable than the corresponding terpyridyl complexes for large metal ions such as La(III) or Ca(II), but that for small metal ions such as Mg(II) and Zn(II) such stabilization is minimal. Molecular mechanics calculations (MM) are used to show that the best-fit M-N length for coordination with DPA is 2.60 Å, accounting for the high stability of Ca(II) or La(III) complexes of DPA, which are found to have close to this M-N bond length in their phen complexes.

Mechanism of (Salen)manganese(III)‐Catalyzed Oxidation of Aryl Phenyl Sulfides with Sodium Hypochlorite

AbstractThe oxidation of 4‐substituted phenyl phenyl sulfides was carried out with several oxo(salen)manganese(V) complexes in MeCN/H2O 9 : 1. The kinetic data show that the reaction is first‐order each in the oxidant and sulfide. Electron‐attracting substituents in the sulfides and electron‐releasing substituents in salen of the oxo(salen)manganese(V) complexes reduce the rate of oxidation. A Hammett analysis of the rate constants for the oxidation of 4‐substituted phenyl phenyl sulfides gives a negative ρ value (ρ=−2.16) indicating an electron‐deficient transition state. The log k2 values observed in the oxidation of each 4‐substituted phenyl phenyl sulfide by substituted oxo(salen)manganese(V) complexes also correlate with Hammett σ constants, giving a positive ρ value. The substituent‐, acid‐, and solvent‐effect studies indicate direct O‐atom transfer from the oxidant to the substrate in the rate‐determining step.

Metal ion selectivities of the highly preorganized tetradentate ligand 1,10-phenanthroline-2,9-dicarboxamide with lanthanide(III) ions and some actinide ions

Abstract Metal ion complexing properties of the ligand PDAM (1,10-phenanthroline-2,9-dicarboxamide) are reported in relation to its possible use as a functional group for solvent extractants in the separation of Am(III) from Ln(III) (lanthanide) ions. PDAM is only slightly water soluble, but variation of the intense π–π* transitions in the UV spectrum of 2×10−5 M PDAM solutions as a function of pH or metal ion concentration allowed for the determination of the protonation constant (pK) and logK 1 values with metal ions. The pK of PDAM is 0.6±0.1 in 1.0 M NaClO4, the lowest for any 1,10-phenanthroline (phen) derivative (in contrast, pK phen=5.1), which is attributed to the electron-withdrawing properties of the amide substituents of PDAM. The weak proton basicity of PDAM may be an important factor in its use as the functional group of a solvent extractant from acidic solutions. The formation constants are determined by UV-Visible spectroscopy for the Ln(III) ions from La(III) to Lu(III) (excluding Pm(III)), as well as for Y(III), Sc(III), Th(IV), and the UO2 2+ cation in 0.1 M NaClO4 at 25 °C. The logK 1 values for the Ln(III) ions show only small changes from La(III) to Lu(III) (both have logK 1 = 3.80). The amide O-donors (oxygen donors) of the amide groups of PDAM appear to cause considerable stabilization of the complexes of PDAM as compared to those of phen, consistent with the idea that the neutral O-donor is a strong Lewis base towards large metal ions such as the Ln(III) ions. A reviewer has pointed out that the amide groups would also stabilize the complexes of PDAM by virtue of the chelate effect, in that PDAM is tetradentate, while phen is only bidentate. The small change in complex stability for PDAM complexes in passing from La(III) to Lu(III) is rationalized in terms of the idea that neutral O-donors stabilize the complexes of the large La(III) ion more than the smaller Lu(III) ion, offsetting the greater affinity of Lu(III) than La(III) for N-donor ligands. The complexes of Th(IV), Sc(III), and the UO2 2+ cation with PDAM have logK 1 values slightly higher than those of the Ln(III) cations, while Y(III) forms a slightly less stable complex. The best-fit size of metal ion for coordination with PDAM is analyzed using molecular mechanics calculations. A fluorescence study shows that the free ligand PDAM fluoresces very strongly, but that the Ln(III) cations quench the fluorescence of PDAM rather than produce a chelation enhanced fluorescence (CHEF) effect as Ln(III) ions do with other phen-based tetradentate ligands. Possible reasons for the lack of a CHEF effect with PDAM are discussed.

Influence of pressure in the 0.1–100 MPa interval on the first dissociation constant of arsenous acid in water solutions at 298.15 K

The influence of pressure on the dissociation of arsenous acid H3AsO3 was studied at 298.15 K by the potentiometric method. In the pressure interval from 0.1 to 100 MPa the values of logK 1 o = −9.32 + 0.00246P. The change in the molar volume of the reaction of the dissociation of H3AsO3 from the first step (ΔV 1 o = −15.4 ± 1 cm3/mol) and the partial molar volume of its dissociation product, H2AsO 3 − (V o = 32.1 ± 1 cm3/mol) were determined.

Solvent Effects on Protonation Constants of Tryptophan in Some Aqueous Aliphatic Alcohol Solutions

The protonation constants of tryptophan (K1 and K2) were determined in binary mixtures of water with methanol, ethanol, and 1-propanol containing (0, 10, 20, 30, 40, 50, 60, 70, and 80) % (v/v) using a potentiometric method at 25 °C and constant ionic strength (0.1 mol·dm−3 sodium perchlorate). The autoprotolysis constant values (Kap) of the media were also determined in the same binary mixtures. The protonation constants of tryptophan and the autoprotolysis constant of the medium in different binary mixtures were analyzed in terms of Kamlet, Abboud, and Taft (KAT) parameters. Single-parameter correlations of the constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity), and π* (dipolarity/polarizability) are poor in all solutions. Multiparameter correlations show better results, but dual-parameter correlations represent significant improvements with regard to the single- and multiparameter models. Linear correlation is observed when the experimental log Kap, log K1, and log K2 values are plotted versus the calculated ones, while the KAT parameters are considered. Finally, the results are discussed in terms of the effect of the solvent on the protonation and autoprotolysis constants.

Synthesis and optical characterization of novel enantiopure BODIPY linked azacrown ethers as potential fluorescent chemosensors

Two novel enantiopure BODIPY linked azacrown ether chemosensors were prepared by reacting 3-chloro-5-methoxyBODIPY with new enantiopure monoaza-18-crown-6 ether ligands bearing two methyl and isobutyl groups on their chiral centres, respectively. The latter compounds were synthesized starting from optically active tetraethylene glycols in six steps. The operation of chemosensors is based on the intramolecular charge transfer (ICT) process. They exhibit pronounced off–on type fluorescent responses to some metal ions and chiral primary aralkyl ammonium ions, in particular to Ca2+ and Pb2+. Even in the case of 1:1 ratio of Ca2+ and Pb2+ ions to the ionophores, the fluorescence intensities and quantum yield values increased more than 10-fold, as well as both the absorption and emission bands were blue-shifted by about 20–30nm (hipsochromic effect) in acetonitrile. The formation of relatively stable complexes allowed the determination of logKs values by the mole-ratio method.

Noncovalent interactions and coordination reactions in the systems consisting of copper(II) ions, aspartic acid and diamines

Interactions of aspartic acid between 1,3-diaminopropane (tn) and 1,4-diaminobutane (Put) in metal-free systems as well as in the systems including copper(II) ions were studied. The composition and overall stability constants of the complexes formed were determined by the potentiometric method. The interaction centres and coordination sites were identified by spectroscopic methods. Each of the ligands has both negative and positive interaction centres. In aspartic acid such centres are carboxyl groups and amine group, while in the polyamine molecules - protonated amine groups. The centres are also the potential sites of the coordination of metal ions. Analysis of the logK(e) values of the adducts in the systems with polyamines has shown that the stability of the adducts in the metal-free systems depends on a significant degree on the steric factor that is the length of the polyamine. In some species the inversion effect, hitherto not reported in literature, was found. In the ternary systems including Cu(II) ions, only protonated species are formed, including molecular complexes with intermolecular interactions and metallation through the oxygen atoms of carboxyl groups and amine groups of the amino acid. In the adducts the protonated diamine is in the outer coordination sphere and is involved in noncovalent interactions with the anchoring CuH(Asp) or Cu(Asp) complexes.

QSAR for Inhibition of Pseudomonas Species Lipase by 1‐Acyloxy‐3‐N‐n‐octylcarbamyl‐benzenes

Abstract1‐Acyloxy‐3‐N‐n‐octylcarbamyl‐benzenes (1–9) are synthesized to characterize the Quantitative Structure–Activity Relationship (QSAR) for the Third Acyl Group Binding Site (TACS) of Pseudomonas species lipase. Inhibitors 1–9 are characterized as pseudo or alternate substrate inhibitors of the enzyme. The inhibition constant (Ki) and carbamylation constant (k2) for the enzyme inhibitions by inhibitors 1–9 are determined. The carbamate carbons of the n‐octylcarbamyl moieties of inhibitors 1–9 are nucleophilically attacked by the active site serine of the enzyme and the n‐octylcarbamyl groups of inhibitors 1–9 are bound to the Acyl Group Binding Site (ACS) of the enzyme. Both pKi and log k2 values are linearly corrected with the Hansch hydrophobicity π values of the substituents of the acyl moieties of inhibitors 1–7. The slopes for these corrections are 0.13 and 0.02, respectively. This result suggests that the enzyme inhibitions by inhibitors 1–7 have a common mechanism. Thus, all acyl moieties of inhibitors 1–7 should bind to the TACS of the enzyme since the acyl and carbamyl moieties of inhibitors 1–7 are meta to each other. This result also indicates that the major interaction between the acyl moiety of inhibitors 1–7 and the TACS of the enzyme is primarily the hydrophobic interaction. The more hydrophobic characters of inhibitors 1–7 are, the more tightly these inhibitors bind to the enzyme. In contrast, 1‐triphenylacetoxy‐3‐N‐n‐octylcarbamyl‐benzene (8) and 1‐trimethylacetoxy‐3‐N‐n‐octylcarbamyl‐benzene (9) do not bind to the TACS of the enzyme due to the fact that the inhibitions by both inhibitors are not linearly correlated with π. It is possible that these two inhibitors are too bulky to fit into the TACS of the enzyme.

Host–guest complexes of some cucurbit[n]urils with the hydrochloride salts of some imidazole derivatives

Interaction between the normal cucurbit[n]urils (n = 6,7,8; Q[6], Q[7], Q[8]) and a sym-tetramethyl-substituted cucurbit[6]uril derivative (TMeQ[6]) with the hydrochloride salts of some imidazole derivatives N-(4-hydroxylphenyl)imidazole (g1), N-(4-aminophenyl)imidazole (g2), 2-phenylimidazole (g3) in aqueous solution was investigated by using 1H NMR spectroscopy, electronic absorption spectroscopy and fluorescence spectroscopy, as well as by using a single crystal X-ray diffraction determination. The 1H NMR spectra analysis established a basic interaction model in which inclusion complexes with a host:guest ratio of 1:1 forms for the Q[6]s and Q[7] cases, while with a host:guest ratio of 1:2 form for the Q[8] cases. It was common that the hosts selectively bound the phenyl moiety of the guests. Absorption spectrophotometric and fluorescence spectroscopic analysis in aqueous solution defined the stability of the host–guest inclusion complexes at pH 5.8 with a host:guest ratio of 1:1 form quantitatively as logK values between 4 and 5 for the smaller hosts Q[6 or 7]s, while with a host:guest ratio of 1:2 form quantitatively as logK values between 11 and 12 for the host Q[8]. Two single crystal X-ray structures of the inclusion complexes TMeQ[6]-g2 · HCl and TMeQ[6]-g3 · HCl showed the phenyl moiety of these two guests inserted into the host cavity, which supported particularly the 1H NMR spectroscopic study in solution.

Solvent effects on kinetics of an aromatic nucleophilic substitution reaction in mixtures of an ionic liquid with molecular solvents and prediction using artificial neural networks

AbstractKinetics of the reaction between 1‐chloro‐2,4‐dinitrobenzene and aniline was studied in mixtures of 1‐ethyl‐3‐methylimidazolium ethylsulfate ([EMIM][EtSO4]) with methanol, chloroform, and dimethylsulfoxide at 25°C. Single‐parameter correlations of log kA versus normalized polarity parameter (ENT), hydrogen‐bond acceptor basicity (β), hydrogen‐bond donor acidity (α), and dipolarity/polarizability (π*) of media do not give acceptable results. Multiparameter linear regression (MLR) of log kA versus the solvatochromic parameters demonstrates that the reaction rate constant increases with ENT, π*, and β and decreases with α parameter. To predict accurately solvent effects on the rate constant, optimized artificial neural network with three inputs (including α, π*, and β parameters) was applied for prediction of the log kA values in the prediction set. It was found that properly selected and trained neural network could fairly represent the dependence of the reaction rate constant on solvatochromic parameters. Mean percent deviation of 5.023 for the prediction set by the MLR model should be compared with the value of 0.343 by the artificial neural network model. These improvements are due to the fact that the reaction rate constant shows nonlinear correlations with the solvatochromic parameters. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 153–159, 2009

Enantiodifferentiating Photoisomerization of Cyclooctene Included and Sensitized by Aroyl-β-cyclodextrins: A Critical Enantioselectivity Control by Substituents

A series of 6-O-benzoyl-beta-cyclodextrins (CDs) with methyl, methoxy, methoxycarbonyl, and bromo substituent(s) at the ortho-, meta-, and/or para-position(s) were synthesized as chiral sensitizing hosts for the use in supramolecular enantiodifferentiating photoisomerization of (Z)-cyclooctene (1Z) to its chiral (E)-isomer (1E). The complex stability constants (K(S)) of the modified beta-CDs with 1Z in aqueous methanol solutions were highly sensitive to the substituent(s) introduced to the benzoate moiety, and the log K(S) values decreased linearly with slightly different slopes with increasing methanol content. The enantiomeric excess (ee) of 1E obtained upon photosensitization with these hosts was a critical function of the size and position of the substituent, varying from almost zero to 46% ee. This indicates that even an apparently small structural difference between methyl and methoxy can critically affect the enantiodifferentiating photoisomerization of a guest included in the chiral cavity, probably through manipulation of both the orientation of ground-state 1Z and the subsequent rotational relaxation of excited 1Z inside the chiral cavity.

Influence of Chelate Ring Interactions on Copper(II) Chelate Stability Studied by Connectivity Index Functions

Linear models for estimation of the first (K1), second (K2), and overall stability constant (beta2) based on the valence connectivity index of the third order ((3)chi(v)) were developed and checked on four sets of copper(II) chelates (with diamines, N-alkylated glycines, and naturally occurring amino acids, including their mixed complexes). Univariate models were valid when log K1 and log K2 values were linearly correlated, i.e., when there was no interaction between chelate rings. The univariate models proved applicable for estimation of all three stability constants of complexes with diamines and N-alkylated glycines, but for complexes with amino acids additional terms were needed (bivariate models). Models reproduced stability constants with an error usually less than 0.3 log K units.

Phosphate Complexation Model and Its Implications for Chemical Phosphorus Removal

A phosphate complexation model is developed, in an attempt to understand the mechanistic basis of chemically mediated phosphate removal. The model presented here is based on geochemical reaction modeling techniques and uses known surface reactions possible on hydrous ferric oxide (HFO). The types of surface reactions and their reaction stoichiometry and binding energies (logK values) are taken from literature models of phosphate interactions with iron oxides. The most important modeling parameter is the proportionality of converting moles of precipitated HFO to reactive site density. For well-mixed systems and phosphate exposed to ferric chloride during HFO precipitation, there is a phosphate capacity of 1.18 phosphate ions per iron atom. In poorly mixed systems with phosphate exposed to iron after HFO formation, the capacity decreased to 25% of the well-mixed value. The same surface complexation model can describe multiple data sets, by varying only a single parameter proportional to the availability of reactive oxygen functional groups. This reflects the unavailability of reactive oxygen groups to bind phosphate. Electron microscope images and dye adsorption experiments demonstrate changes in reactive surface area with aging of HFO particles. Engineering implications of the model/mechanism are highlighted.

Affinity of the Highly Preorganized Ligand PDA (1,10-Phenanthroline-2,9-dicarboxylic acid) for Large Metal Ions of Higher Charge. A Crystallographic and Thermodynamic Study of PDA Complexes of Thorium(IV) and the Uranyl(VI) ion

The hydrothermal synthesis and structures of [UO2(PDA)] (1) and [Th(PDA)2(H2O)2].H2O (2) (PDA = 1,10-phenanthroline-2,9-dicarboxylic acid) are reported. 1 is orthorhombic, Pnma, a = 11.1318(7) A, b = 6.6926(4) A, c = 17.3114(12) A, V = 1289.71(14), Z = 4, R = 0.0313; 2 is triclinic, P1, a = 7.6190(15) A, b = 10.423(2) A, c = 17.367(4) A, alpha = 94.93(3) degrees , beta = 97.57(3) degrees , gamma = 109.26(3) degrees , V = 1278.3(4) A (3), Z = 2, R = 0.0654. The local geometry around the U in 1 is a pentagonal bipyramid with the two uranyl oxygens occupying the apical positions. The donor atoms in the plane comprise the four donor atoms from the PDA ligand (average U-N = 2.558 and U-O = 2.351 A) with the fifth site occupied by a bridging carboxylate oxygen from a neighboring UO2/PDA individual. The PDA ligand in 1 is exactly planar, with the U lying in the plane of the ligand. The latter planarity, as well as the near-ideal U-O and U-N bond lengths, and O-U-N and N-U-N bond angles within the chelate rings of 1 suggest that PDA binds to the uranyl cation in a low-strain manner. In 2, there are two PDA ligands bound to the Th (average Th-N = 2.694 and Th-O = 2.430 A) as well as two water molecules (Th-O = 2.473 and 2.532 A) to give the Th a coordination number of 10. The PDA ligands in 2 are bowed, with the Th lying out of the plane of the ligand. Molecular mechanics calculations suggest that the distortion of the PDA ligands in 2 arises because of steric crowding. UV spectroscopic studies of solutions containing 1:1 ratios of PDA and Th(4+) in 0.1 M NaClO4 at 25 degrees C indicate that log K1 for the Th(4+)/PDA complex is 25.7(9). The latter result confirms the previous prediction that complexes of PDA with metal ions of higher charge and an ionic radius of about 1.0 A such as Th(IV) would have remarkably high log K1 values with PDA. The origins of this very high stability are discussed in terms of a synergy between the pyridyl and the carboxylate donor groups of PDA. Metal ions of high charge normally bond poorly with pyridyl donors in aqueous solution because such metal ions require donor groups that are able to disperse charge to the solvent via hydrogen-bonding, which pyridyl groups are unable to do. In PDA, the carboxylates fulfill this need and so enable the high donor strength of the pyridyl groups of PDA to become apparent in the high log K1 for Th(IV) with PDA.

Determination of the conditional interaction constant between colloidal technetium(IV) and Gorleben humic substances

Varying pertechnetate (Tc(VII)) doses were reduced to Tc(IV) in the presence and absence of Gorleben humic substances with the aid of magnetite, a reducing Fe(II)-containing surface. In absence of humic substances dissolved Tc(IV) concentrations are over-saturated with respect to the known TcO2 · nH2O solubility and increase with increasing Tc(VII) dose due to the formation of a range of mononuclear to colloidal Tc(IV) species. In presence of dissolved humic substances, the Tc solubility is enhanced due to the additional interaction of dissolved Tc(IV) species with humic substances. Both in the absence and the presence of dissolved humic substances a sorption mechanism controls the distribution of the range of mononuclear to colloidal Tc(IV) species between the solid and the liquid phase. The proposed reaction mechanism between Tc(IV) and HS is represented by Σ[TcO(OH)2]n+HS = [TcO(OH)2]n − HS in which Σ[TcO(OH)2]n stands for the sum of monomeric and polynuclear (colloidal) Tc(IV) species present in the equilibrium solution. A log K-value of 2.9 (±0.3) was quantified from a modified Schubert approach which is based on the competition of HS and magnetite for all dissolved Tc(IV) species and was found independent of Tc–HS loading, Tc–magnetite loading and pH.

Solvent and acidity effects on the UV-visible spectra and protonation-deprotonation of free-base octaethylcorrole

The UV-visible spectrum of free-base octaethylcorrole, (OECor) H 3, was recorded in thirteen different nonaqueous solvents as well as in a mixed acetonitrile/acid solvent containing one of seven different acids. Spectra were also measured in seven different solutions of neat concentrated acid and in CH 3 CN containing piperidine or tetrabutylammonium hydroxide as an added base. The overall data was analyzed as a function of solvent acidity or basicity parameters and the number of protons on the central nitrogens of the macrocycle, the predominant form of the corrole in these solutions being respresented as (OECor)H 3, [ (OECor)H 4]+, [ (OECor)H 2]− or [ (OECor)H ]2− where OECor = trianion of octaethylcorrole. The mono-protonated corrole, [ (OECor)H 4]+, is formed in concentrated acetic acid or in CH 3 CN containing 0.10 M trifluoroacetic acid, H 2 SO 4, HCl , H 3 PO 4 or HClO 4. The mono-deprotonated corrole, [ (OECor)H 2]−, is generated in piperidine while doubly deprotonated [ (OECor)H ]2− exists in solutions of tetrabutylammonium hydroxide. An addition of protons to the macrocycle of [ (OECor)H 4]+ also occurs in the presence of concentrated strong acids and this results in a loss of the characteristic Soret band of the corrole leading presumably to [ (OECor)H 5]2+ where the second proton has been added to a meso-position of the macrocycle. The UV-visible spectral changes upon formation of [ (OECor)H 4]+, [ (OECor)H 2]− or [ (OECor)H ]2− in CH 3 CN were monitored during a titration with the relevent acid or base and equilibrium constants for protonation or deprotonation of (OECor)H 3 were determined using standard equations. The measured logK values are compared to protonation and deprotonation constants obtained for two related corroles and two related porphyrins under the same experimental conditions.