Cyclodextrin and modified cyclodextrin complexes of E-4-tert-butylphenyl-4'-oxyazobenzene: UV-visible, 1H NMR and ab initio studies

alpha-Cyclodextrin, beta-cyclodextrin, N-(6(A)-deoxy-alpha-cyclodextrin-6(A)-yl)-N'6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea and N,N-bis(6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea (alphaCD, betaCD, 1 and 2) form inclusion complexes with E-4-tert-butylphenyl-4'-oxyazobenzene, E-3(-). In aqueous solution at pH 10.0, 298.2 K and I = 0.10 mol dm(-3)(NaClO(4)) spectrophotometric UV-visible studies yield the sequential formation constants: K(11) = (2.83 +/- 0.28) x 10(5) dm(3) mol(-1) for alphaCD.E-(-), K(21) = (6.93 +/- 0.06) x 10(3) dm(3) mol(-1) for (alphaCD)(2).E-3(-), K(11) = (1.24 +/- 0.12) x 10(5) dm(3) mol(-1) for betaCD.E-(-), K(21) = (1.22 +/- 0.06) x 10(4) dm(3) mol(-1) for (betaCD)(2).E-(-), K(11) = (3.08 +/- 0.03) x 10(5) dm(3) mol(-1) for .E-3(-), K(11) = (8.05 +/- 0.63) x 10(4) dm(3) mol(-1) for .E-3(-) and K(12) = (2.42 +/- 0.53) x 10(4) dm(3) mol(-1) for .(E-3(-))(2). (1)H ROESY NMR studies show that complexation of E-3(-) in the annuli of alphaCD, betaCD, 1 and 2 occurs. A variable-temperature (1)H NMR study yields k(298 K)= 6.7 +/- 0.5 and 5.7 +/- 0.5 s(-1), DeltaH = 61.7 +/- 2.7 and 88.1 +/- 4.2 kJ mol(-1) and DeltaS = -22.2 +/- 8.7 and 65 +/- 13 J K(-1) mol(-1) for the interconversion of the dominant includomers (complexes with different orientations of alphaCD) of alphaCD.E-3(-) and (alphaCD)(2).E-3(-), respectively. The existence of E-3(-) as the sole isomer was investigated through an ab initio study.     

Hydroxypropyl-beta-cyclodextrin enhanced fluorimetric method for the determination of melatonin and 5-methoxytryptamine

The effects of native cyclodextrins (alpha, beta or gamma), hydroxypropyl-beta-cyclodextrin, beta-cyclodextrin solubilized in urea, soluble starch and glucose in water solution on the fluorescence behaviour of melatonin (N-acetyl-5-methoxytryptamine) (M) and 5-methoxytryptamine [5-methoxy-3-(2-aminoethyl)indole] (5M) were determined. In addition, the effect of methanol and propanol with and without beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin was assessed. From the fluorescence changes with pH, the values of the pKa for the ground (9.9 +/- 0.2) and the excited state (7.7 +/- 0.2) for 5M were determined. From the fluorescence changes with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin, the association constants of M, 5MH [5-methoxy-3-(2-ammoniumethyl)indole] and 5M with the two hosts were determined. The values with beta-cyclodextrin were KAssoc5MH = (1.4 +/- 0.4) x 10(2) mol-1 dm3, KAssoc5M = (1.6 +/- 0.1) x 10(2) mol-1 dm3 and KAssocM = (1.1 +/- 0.2) x 10(2) mol-1 dm3, and with hydroxypropyl-beta-cyclodextrin KAssoc5MH = (1.1 +/- 0.3) x 10(2) mol-1 dm3, KAssoc5M = (2.5 +/- 0.1) x 10(2) mol-1 dm3 and KAssocM = (1.51 +/- 0.07) x 10(2) mol-1 dm3. The ratios of the fluorescence quantum yields for the bound and free substrate (phi b/phi f) were in the range 1.15-1.48. The detection limits under the optimum conditions were 0.381 +/- 0.001 ng cm-3 for the complex 5MH-hydroxypropyl-beta-cyclodextrin in water and 0.290 +/- 0.001 ng cm-3 for the complex M-hydroxypropyl-beta-cyclodextrin in water with 5% of methanol. The recovery of melatonin from pharmaceutical preparations was 98-103% with an RSD of 2%. The recovery from rat pineals was also good. The method is direct, simple and accurate.     

Room temperature and shock tube study of the reaction HCO+O2 using the photolysis of glyoxal as an efficient HCO source

The rate of the reaction 1, HCO+O2-->HO2+CO, has been determined (i) at room temperature using a slow flow reactor setup (20 mbarH2+HCO+CO, into additional HCO radicals. The rate constants of reaction 4 were determined from unperturbed photolysis experiments to be k4(295 K)=(3.6+/-0.3)x10(10) cm3 mol-1 s-1 and k4(769-1107 K)=5.4x10(13)exp(-18 kJ mol-1/RT) cm3 mol-1 s-1(Delta log k4=+/-0.12).     

Triggering water exchange mechanisms via chelate architecture. Shielding of transition metal centers by aminopolycarboxylate spectator ligands

Paramagnetic effects on the relaxation rate and shift difference of the (17)O nucleus of bulk water enable the study of water exchange mechanisms on transition metal complexes by variable temperature and variable pressure NMR. The water exchange kinetics of [Mn(II)(edta)(H2O)](2-) (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMR data. The results revealed a rapid water exchange reaction for the [Mn(II)(edta)(H2O)](2-) complex with a rate constant of k(ex) = (4.1 +/- 0.4) x 10(8) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) are 36.6 +/- 0.8 kJ mol(-1), +43 +/- 3 J K(-1) mol(-1), and +3.4 +/- 0.2 cm(3) mol(-1), which are in line with a dissociatively activated interchange (I(d)) mechanism. To analyze the structural influence of the chelate, the investigation was complemented by studies on complexes of the edta-related tmdta (trimethylenediaminetetraacetate) chelate. The kinetic parameters for [Fe(II)(tmdta)(H2O)](2-) are k(ex) = (5.5 +/- 0.5) x 10(6) s(-1) at 298.2 K, DeltaH(double dagger) = 43 +/- 3 kJ mol(-1), DeltaS(double dagger) = +30 +/- 13 J K(-1) mol(-1), and DeltaV(double dagger) = +15.7 +/- 1.5 cm(3) mol(-1), and those for [Mn(II)(tmdta)(H2O)](2-) are k(ex) = (1.3 +/- 0.1) x 10(8) s(-1) at 298.2 K, DeltaH(double dagger) = 37.2 +/- 0.8 kJ mol(-1), DeltaS(double dagger) = +35 +/- 3 J K(-1) mol(-1), and DeltaV(double dagger) = +8.7 +/- 0.6 cm(3) mol(-1). The water containing species, [Fe(III)(tmdta)(H2O)](-) with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [Fe(III)(tmdta)](-). The kinetic parameters for [Fe(III)(tmdta)(H2O)](-) are k(ex) = (1.9 +/- 0.8) x 10(7) s(-1) at 298.2 K, DeltaH(double dagger) = 42 +/- 3 kJ mol(-1), DeltaS(double dagger) = +36 +/- 10 J K(-1) mol(-1), and DeltaV(double dagger) = +7.2 +/- 2.7 cm(3) mol(-1). The data for the mentioned tmdta complexes indicate a dissociatively activated exchange mechanism in all cases with a clear relationship between the sterical hindrance that arises from the ligand architecture and mechanistic details of the exchange process for seven-coordinate complexes. The unexpected kinetic and mechanistic behavior of [Ni(II)(edta')(H2O)](2-) and [Ni(II)(tmdta')(H2O)](2-) is accounted for in terms of the different coordination number due to the strong preference for an octahedral coordination environment and thus a coordination equilibrium between the water-free, hexadentate [M(L)](n+) and the aqua-pentadentate forms [M(L')(H2O)](n+) of the Ni(II)-edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [Ni(II)(edta')(H2O)](2-) (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 +/- 0.2) x 10(5) s(-1) at 298.2 K and ambient pressure was measured, and the activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) were found to be 34 +/- 1 kJ mol(-1), -27 +/- 2 J K(-1) mol(-1), and +1.8 +/- 0.1 cm(3) mol(-1), respectively. For [Ni(II)(tmdta')(H2O)](2-), we found k = (6.4 +/- 1.4) x 10(5) s(-1) at 298.2 K, DeltaH(double dagger) = 22 +/- 4 kJ mol(-1), and DeltaS(double dagger) = -59 +/- 5 J K(-1) mol(-1). The process is referred to as a water substitution instead of a water exchange reaction, since these observations refer to the intramolecular displacement of coordinated water by the carboxylate moiety in a ring-closure reaction.     

New functional model complexes of intradiol-cleaving catechol dioxygenases: properties and reactivity of CuII(L)(O2Ncat)

Complexes Cu(O2Ncat)(tbeda) (1) and Cu(O2Ncat)(tmeda) (2) (tbeda = N,N,N',N'-tetrabenzylethylenediamine, tmeda=N,N,N',N'-tetramethylethylenenediamine, O2NcatH2=4-nitrocatechol) have been prepared by the reaction of copper(II) perchlorate with 4-nitrocatechol in the presence of triethylamine and the appropriate bidentate ligand. These compounds represent structural and functional model systems for the copper-containing catechol 1,2-dioxygenase. Both complexes have been structurally characterized by X-ray crystallography and by UV-vis, IR, and EPR spectroscopies. Upon protonation of 1 and 2 with perchloric acid, the bidentate coordination of O2Ncat could be reversible converted to the monodentate coordination of O2NcatH. The equilibrium constants were found to be 4200 and 3500, respectively, by measuring the UV-vis spectra in DMF. Back-titration with morpholine proved the reversibility of both reactions. Kinetic data on the oxygenation of 1 and 2 revealed overall second-order rate equations with kinetic parameters: ktbeda=(4.63+/-0.23)x10(-2) mol-1 dm3 s-1, DeltaH=51+/-6 kJ mol-1, DeltaS=-137+/-16 J mol-1 K-1; ktmeda=(0.89+/-0.23) mol-1 dm3 s-1, DeltaH=85+/-7 kJ mol-1, DeltaS=-57+/-19 J mol-1 K-1 at 365.16 K. Oxygenation of 1, 2, and [Cu(O2NcatH)(L)]ClO4 (L=tbeda, tmeda) in DMF solution at ambient conditions gives the corresponding intradiol ring-cleaved (2-nitro-muconato)copper(II) complexes. These data support the assumption that the reaction of the differently coordinated catecholate ligand with dioxygen shows only 1,2-dioxygenase activity.     

Formation of Metallo-6(A)-((2-(bis(2-aminoethyl)amino)ethyl)amino)-6(A)-deoxy-beta-cyclodextrins and Their Complexation of Tryptophan in Aqueous Solution

A pH titration study shows that 6(A)-((2-(bis(2-aminoethyl)amino)ethyl)amino)-6(A)-deoxy-beta-cyclodextrin (betaCDtren) forms binary metallocyclodextrins, [M(betaCDtren)](2+), for which log(K/dm(3) mol(-)(1)) = 11.65 +/- 0.06, 17.29 +/- 0.05, and 12.25 +/- 0.03, respectively, when M(2+) = Ni(2+), Cu(2+), and Zn(2+), where K is the stability constant in aqueous solution at 298.2 K and I = 0.10 mol dm(-)(3) (NaClO(4)). The ternary metallocyclodextrins [M(betaCDtren)Trp](+), where Trp(-) is the tryptophan anion, are characterized by log(K/dm(3) mol(-)(1)) = 8.2 +/- 0.2 and 8.1 +/- 0.2, 9.5 +/- 0.3 and 9.4 +/- 0.2, and 8.1 +/- 0.1 and 8.3 +/- 0.1, respectively, where the first and second values represent the stepwise stability constants for the complexation of (R)- and (S)-Trp(-), respectively, when M(2+) = Ni(2+), Cu(2+), and Zn(2+). From comparisons of stabilities and UV-visible spectra, the binary and ternary metallocyclodextrins appear to be six-coordinate when M(2+) = Ni(2+) and Zn(2+) and five-coordinate when M(2+) = Cu(2+). The factors affecting the stoichiometries and stabilities of the metallocyclodextrins, are discussed and comparisons are made with related systems.     

Ozonolysis of vinyl compounds,CH2=CH-X,in aqueous solution--the chemistries of the ensuing formyl compounds and hydroperoxides

Reactions of ozone with some vinyl compounds of the general structure CH2=CH-X were studied in aqueous solution. Rate constants (in brackets, unit: dm3 mol-1 s-1) were determined: acrylonitrile (670), vinyl acetate (1.6 x 10(5)), vinylsulfonic acid (anion, 8.3 x 10(3)), vinyl phenylsulfonate (ca. 200), vinyl diethylphosphonate (3.3 x 10(3)), vinylphosphonic acid (acid, 1 x 10(4); mono-anion, 2.7 x 10(4); di-anion, 1 x 10(5)), vinyl bromide (1 x 10(4)). The main pathway leads to the formation of HOOCH2OH and HC(O)X. As measured by stopped flow with conductometric detection, the latter one may undergo rapid hydrolysis by water, e.g. HC(O)CN (3 s-1). Other HC(O)X hydrolyse much slower, e.g. HC(O)PO3(Et)2 (7 x 10(-3) s-1) and HC(O)P(OH)O2- (too slow to be measured). The OH(-)-induced hydrolyses range from ca. 5 dm3 mol-1 s-1 [HC(O)PO(3)2-] to 3.8 x 10(5) dm3 mol-1 s-1 [HC(O)CN]. HC(O)Br mainly decomposes rapidly (too fast for the determination of the rate) into CO and Br- plus H+, and the competing hydrolysis is of minor importance (3.7%). The slow hydrolysis of HC(O)PO(3)2- at pH 10.2, where HOOCH2OH is rapidly decomposed into CH2O plus H2O2, allows an H2O2-induced decomposition (k = 260 dm3 mol-1 s-1) to take place. Formate and phosphate are the final products.     

Ionic association of hydroperoxide anion HO2- in the binding mean spherical approximation. Spectroscopic study of hydrogen peroxide in concentrated sodium hydroxide solutions

The ultraviolet-visible (UV-vis) spectroscopy of hydrogen peroxide in concentrated sodium hydroxide solutions was studied. The peroxide band in the UV range shifts from approximately 214 nm to approximately 236 nm as the NaOH concentration increases from 0.338 mol dm-3 to 13.1 mol dm-3. The band originates from an intramolecular electronic transition of the hydroperoxide anion HO2-, as indicated by the negligible temperature effect on the band position and confirmed by ab initio quantum mechanical calculations. It is postulated that the bathochromic shift of the peroxide band that accompanies the increase in NaOH concentration originates from the formation of the ion pair (Na+HO2-). The equilibrium constant for the ion association reaction (0.048 mol-1 dm3) and the characteristics of the individual absorption bands of the hydroperoxide anion and its associate with Na+ were determined from the numerical modeling of the absorbance data, using the binding mean spherical approximation (BIMSA).     

Coordination and fluorescence of the intracellular Zn2+ probe [2-methyl-8-(4-toluenesulfonamido)-6-quinolyloxy]acetic acid (Zinquin A) in ternary Zn2+ complexes

A potentiometric study of the coordination of the fluorophore, 2-methyl-8-(4-toluenesulfonamido)-6-quinolyloxyacetic acid, (1)LH(2) (the intracellular Zn(2+) probe, Zinquin A) in its deprotonated form, (1)L(2)(-), in Zn(2+) ternary complexes, [Zn(n)L(1)L](n) (where n is the charge of (n)L) at 298.2 K in 50% aqueous ethanol (v/v) and I = 0.10 (NaClO(4)), shows that the formation of [Zn(n)L(1)L](n) from [Zn(n)L]((2+)(n)(+) is characterized by log(K(5)/dm(3) mol(-1)) = 8.23 +/- 0.05, 4.36 +/- 0.18, 8.45 +/- 0.10, 10.00 +/- 0.06, 11.53 +/- 0.06 and 5.92 +/- 0.15, respectively, where (n)L = (2)L - (6)L and (7)L(3-) are 1,4,7,10-tetraazacyclododecane, 1,4,8,11-tetraazacyclotetradecane, 1,4,7-triazacyclononane, 1,5,9-triazacyclododecane, tris(2-aminoethyl)amine and nitrilotriacetate, respectively, and K(5) is the stepwise complexation constant. Dissociation of a hydroxo proton from triethanolamine, (8)L, occurs in the formation of [Zn(8)LH(-1)](+) that subsequently forms [Zn(8)LH(-1)(1)L](-) for which log(K(5)/dm(3) mol(-1)) = 9.87 +/- 0.08. The variation of K(5) and the 5-fold variation of quantum yield of (1)L(2)(-) as its coordination environment changes in Zn(2+) ternary complexes are discussed with reference to the use of (1)L(2-) in the detection of intracellular Zn(2+).     

Some Thermodynamic Data on Copper-Chitin and Copper-Chitosan Biopolymer Interactions

Chitin and chitosan are good removers of cations from aqueous solution and wastewater. The interactive effect of cation with both biopolymers in aqueous medium was studied by the batch method at 298 +/- 1 K. The results were fitted to the modified Langmuir equation. The same adsorption was followed by calorimetric titration. In this process, 50.0 mg of each polymer was suspended in 19.0 cm3 of bidistilled water at 298.15 +/- 0.02 K, maintained under mechanical turbine stirring. The titration was performed by adding increments of 10 μL of 0.10 mol dm-3 Cu(NO3)2 aqueous solution to the system. The resulting isotherm was also adjusted to a modified Langmuir equation. From the thermal effects K and DeltaH values were determined, enabling the calculation of DeltaG and DeltaS for the interaction of copper cations with chitin and chitosan, giving the enthalpic values of -19.85 +/- 0.34 and -41.27 +/- 1.57 kJ mol-1, respectively. The spontaneity of this interaction is shown from DeltaG values of -35.9 +/- 0.1 and -36.8 +/- 0.1 kJ mol-1, which are followed by DeltaS values of +54 and of -15 J mol-1 K-1, respectively. The complexation is probably associated with the lack of order of the chitin polymeric chain or with the freedom of water molecules initially bonded to cations. The copper ion is coordinated to the pendant groups of the polymeric chain to form stable complexes. Copyright 1999 Academic Press.     

The impact of tripodal chelates on water exchange kinetics and mechanisms: a variable temperature and pressure 17O NMR study to clarify the structure-reactivity relationship in [Fe(II)(L)(H2O2](-) complexes

The effect of temperature and pressure on the water exchange reaction of [Fe(II)(NTA)(H2O)2](-) and [Fe(II)(BADA)(H2O)2](-) (NTA = nitrilotriacetate; BADA = beta-alanindiacetate) was studied by 17O NMR spectroscopy. The [Fe(II)(NTA)(H2O)2](-) complex showed a water exchange rate constant, k(ex), of (3.1 +/- 0.4) x 10(6) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH( not equal), DeltaS( not equal) and DeltaV( not equal) for the observed reaction are 43.4 +/- 2.6 kJ mol(-1), + 25 +/- 9 J K(-1) mol(-1) and + 13.2 +/- 0.6 cm(3) mol(-1), respectively. For [Fe(II)(BADA)(H2O)2](-), the water exchange reaction is faster than for the [Fe(II)(NTA)(H2O)2](-) complex with k(ex) = (7.4 +/- 0.4) x 10(6) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH( not equal), DeltaS( not equal) and DeltaV( not equal) for the water exchange reaction are 40.3 +/- 2.5 kJ mol(-1), + 22 +/- 9 J K(-1) mol(-1) and + 13.3 +/- 0.8 cm(3) mol(-1), respectively. The effect of pressure on the exchange rate constant is large and very similar for both systems, and the numerical values for DeltaV( not equal) suggest in both cases a limiting dissociative (D) mechanism for the water exchange process.     

Thermochemistry and gas-phase ion energetics of 2-hydroxy-4-methoxy-benzophenone (oxybenzone)

We have investigated the thermochemistry and ion energetics of the oxybenzone (2-hydroxy-4-methoxy-benzophenone, C14H12O3, 1H) molecule. The following parameters have been determined for this species: gas-phase enthalpy for the of neutral molecule at 298.15K, (Delta(f)H0(m)(g) = -303.5 +/- 5.1 kJ x mol-1), the intrinsic (gas-phase) acidity (GA(1H) = 1402.1 +/- 8.4 kJ x mol-1), enthalpy of formation for the oxybenzone anion (Delta(f)H0(m)(1-,g) = -402.3 +/- 9.8 kJ x mol-1). We also have obtained the enthalpy of formation of, 4-hydroxy-4'-methoxybenzophenone (Delta(f)H0(m)(g) = -275.4 +/- 10 kJ x mol-1) and 3-methoxyphenol anion (Delta(f)H0(m)(C7H7O2-,g) = -317.7 +/- 8.7 kJ x mol-1). A reliable experimental estimation of enthalpy related to intramolecular hydrogen bonding in oxybenzone has also been obtained (30.1 +/- 6.3 kJ x mol-1) and compared with our theoretical calculations at the B3LYP/6-311++G** level of theory, by means of an isodesmic reaction scheme. In addition, heat capacities, temperature, and enthalpy of fusion have been determined for this molecule by differential scanning calorimetry.     

A coated wire-type lead(II) ion-selective electrode based on a phosphorylated calix[4]arene derivative.

The preparation of a lead-selective electrode based on 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(diphenylphosphinoylmethoxy)calix[4]arene (1) as an ionophore is reported. The plasticized PVC membrane containing 30% PVC, 57% ortho-nitrophenyloctylether (NPOE), 4% sodium tetraphenylborate (NaTPB) and 9% ionophore 1 was directly coated on a graphite electrode. It exhibits a nearly Nernstian slope of 28.0 +/- 0.2 mV decade(-1) over a concentration range of 1 x 10(-5) - 1 x 10(-2) mol dm(-3) with a detection limit of 1.4 x 10(-6) mol dm(-3). The response time of the electrode was found to be ca. 17 s. The potential of the sensor was independent of the pH variation in the range 3.5 - 5.0. The selectivity of the electrode performance towards lead ions over Th4+, La3+, Sm3+, Dy3+, Y3+, Ca2+, Sr2+, Cd2+, Mn2+, Zn2+, Ni2+, Co2+, NH4+ Ag+, Li+, Na+ and K+ ions was investigated. The prepared electrode was used successfully as an indicator electrode for a potentiometric titration of a lead solution using a standard solution of EDTA. The applicability of the sensor for Pb2+ measurements in various synthetic water samples spiked with lead nitrate was also checked.     

Adsorption of Trivalent Cations on Silica

The temperature effect on the magnitude of adsorption was used to explain the mechanism of adsorption of gadolinium on silica at very low concentrations. Standard enthalpy of adsorption of gadolinium equals 36 kJ mol-1 for a total Gd concentration of 2 x 10(-8) mol dm-3 and 67 kJ mol-1 for 2 x 10(-5) mol dm-3. This result confirms the hypothesis that the Gd adsorption at low initial concentration is governed by formation of strong ternary surface complexes involving anionic impurities. Copyright 1999 Academic Press.     

Solvent effects on the oxidation of RuIV=O to O=RuVI=O by MnO4-. hydrogen-atom versus oxygen-atom transfer

The kinetics of the oxidation of trans-[RuIV(tmc)(O)(solv)]2+ to trans-[RuVI(tmc)(O)2]2+ (tmc is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, a tetradentate macrocyclic tertiary amine ligand; solv = H2O or CH3CN) by MnO4- have been studied in aqueous solutions and in acetonitrile. In aqueous solutions the rate law is -d[MnO4]/dt = kH2O[RuIV][MnO4-] = (kx + (ky)/(Ka)[H+])[RuIV][MnO4-], kx = (1.49 +/- 0.09) x 101 M-1 s-1 and ky = (5.72 +/- 0.29) x 104 M-1 s-1 at 298.0 K and I = 0.1 M. The terms kx and ky are proposed to be the rate constants for the oxidation of RuIV by MnO4- and HMnO4, respectively, and Ka is the acid dissociation constant of HMnO4. At [H+] = I = 0.1 M, DeltaH and DeltaS are (9.6 +/- 0.6) kcal mol-1 and -(18 +/- 2) cal mol-1 K-1, respectively. The reaction is much slower in D2O, and the deuterium isotope effects are kx/kxD = 3.5 +/- 0.1 and ky/kyD = 5.0 +/- 0.3. The reaction is also noticeably slower in H218O, and the oxygen isotope effect is kH216O/kH218O = 1.30 +/- 0.07. 18O-labeled studies indicate that the oxygen atom gained by RuIV comes from water and not from KMnO4. These results are consistent with a mechanism that involves initial rate-limiting hydrogen-atom abstraction by MnO4- from coordinated water on RuIV. In acetonitrile the rate law is -d[MnO4-]/dt = kCH3CN[RuIV][MnO4-], kCH3CN = 1.95 +/- 0.08 M-1 s-1 at 298.0 K and I = 0.1 M. DeltaH and DeltaS are (12.0 +/- 0.3) kcal mol-1 and -(17 +/- 1) cal mol-1 K-1, respectively. 18O-labeled studies show that in this case the oxygen atom gained by RuIV comes from MnO4-, consistent with an oxygen-atom transfer mechanism.     

Primary photophysical properties of ofloxacin

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.     

The study of complex equilibria of uranium(VI) with selenate

Uranyl (M)-selenate (L) complex equilibria in solution were investigated by spectrophotometry in visible range and potentiometry by means of uranyl ion selective electrode. The formation ML and ML(2) species was proved and the corresponding stability constants calculated were: log beta(1) = 1.57(6) +/- 0.01(6), log beta(2) = 2.42(3) +/- 0.01(3) (I = 3.0 mol 1(-1) Na(ClO(4), SeO(4)) (spectrophotometry) at 298.2 K. Using potentiometry the values for infinite dilution (I --> 0 mol 1(-1)) were: log beta(1) = 2.64 +/- 0.01, log beta(2) 3.4 at 298.2 K. Absorption spectra of the complexes were calculated and analysed by deconvolution technique. Derivative spectrophotometry for the chemical model determination has also been successfully applied.     

Kinetics and mechanism of complex formation of nickel(II) with tetra-N-alkylated cyclam in N,N-dimethylformamide (DMF): comparative study on the reactivity and solvent exchange of the species Ni(DMF)6(2+) and Ni(DMF)5Cl+

13C NMR was used to study the rate of DMF exchange in the nickel(II) cation Ni(DMF)6(2+) and in the monochloro species Ni(DMF)5Cl+ with 13C-labeled DMF in the temperature range of 193-395 K in DMF (DMF = N,N-dimethylformamide). The kinetic parameters for solvent exchange are kex = (3.7 +/- 0.4) x 10(3) s-1, delta H++ = 59.3 +/- 5 kJ mol-1, and delta S++ = +22.3 +/- 14 J mol-1 K-1 for Ni(DMF)6(2+) and kex = (5.3 +/- 1) x 10(5) s-1, delta H++ = 42.4 +/- 4 kJ mol-1, and delta S++ = +6.7 +/- 15 J mol-1 K-1 for Ni(DMF)5Cl+. Multiwavelength stopped-flow spectrophotometry was used to study the kinetics of complex formation of the cation Ni(DMF)6(2+) and of the 100-fold more labile cation Ni(DMF)5Cl+ with TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and TEC (1,4,8,11-tetraethyl-1,4,8,11-tetraazacyclotetradecane) in DMF at 298 K and I = 0.6 M (tetra-n-butylammoniumperchlorate). Equilibrium constants K for the addition of the nucleophiles DMF, Cl-, and Br- to the complexes Ni(TMC)2+ and Ni(TEC)2+ were determined by spectrophotometric titration. Formation of the complexes Ni(TMC)2+ and Ni(TEC)2+ was found to occur in two stages. In the initial stage, fast, second-order nickel incorporation with rate constants k1(TMC) = 99 +/- 5 M-1 s-1 and k1 (TEC) = 235 +/- 12 M-1 s-1 leads to the intermediates Ni(TMC)int2+ and Ni(TEC)int2+, which have N4-coordinated nickel. In the second stage, these intermediates rearrange slowly to form the stereochemically most stable configuration. First-order rate constants for the one-step rearrangement of Ni(TMC)int2+ and the two-step rearrangment of Ni(TEC)int2+ are presented. Because of the rapid formation of Ni(DMF)5Cl+, the reactions of Ni(DMF)6(2+) with TMC and TEC are accelerated upon the addition of tetra-n-butylammoniumchloride (TBACl) and lead to the complexes Ni(TMC)Cl+ and Ni(TEC)Cl+, respectively. For initial concentrations such that [TBACl]o/[nickel]o > or = 20, intermediate formation is 230 times (TMC) and 47 times (TEC) faster than in the absence of chloride. The mechanism of complex formation is discussed.     

Intra- and intermolecular complexation in C6 monoazacoronand substituted cyclodextrins

The preparation of 6(A)-deoxy-6(A)-(6-(2-(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)acetamido)hexylamino)-alpha-cyclodextrin, 3, 6(A)-deoxy-6(A)-(6-(2-(1,4,7,10,13-pentaoxa-16-azacyclooctadecan-16-yl)acetamido)hexylamino)-alpha-cyclodextrin, 4, and their beta-cyclodextrin analogues, 5 and 6, are described. (1)H (600 MHz) ROESY NMR spectra of the C(6) substituted beta-cyclodextrins, 5 and 6, are consistent with the intramolecular complexation of their azacyclopentadecanyl- and azacyclooctadecanyl(acetamido)hexylamino substituents in the beta-cyclodextrin annulus in D(2)O at pD = 8.5 whereas those of their alpha-cyclodextrin analogues, 3 and 4 are not complexed in the alpha-cyclodextrin annulus. This is attributed to the monoazacoronand components of the substituents being able to pass through the beta-cyclodextrin annulus whereas they are too large to pass through the alpha-cyclodextrin annulus. However, the substituents of 3 and 4 are intermolecularly complexed by beta-cyclodextrin to form pseudo [2]-rotaxanes. Metallocyclodextrins are formed by 5 through complexation by the monoazacoronand substituent component for which log (K/dm(3) mol(-1))= <2, 6.34 and 5.38 for Ca(2+), Zn(2+) and La(3+), respectively, in aqueous solution at 298.2 K and I= 0.10 mol dm(-3)(NEt(4)ClO(4)).