Kinetics and mechanism of nucleophilic substitution of dichloro{1-methyl-2-(arylazo)imidazole}palladium(II) by pyridine bases

The reaction of dichloro{1-methyl-2-(arylazo)imidazole}palladium(II), Pd(RaaiMe)Cl₂ where RaaiMe = p-R–C₆H₄N=N–C₃H₂N₂-1-Me; R = H(1), Me(2), Cl(3), with pyridine bases [RPY: R = H (a), 4-Me (b), 4-Cl (c), 2-Me (d), 2,6-Me₂ (e), 2,4,6-Me₃ (f)] has been studied spectrophotometrically in MeCN at 451 nm. The products (4) have been isolated and characterised as trans-Pd(RPy)₂Cl₂. The kinetics of the nucleophilic substitution has been examined under pseudo-first-order conditions at 298 K. A single phase reaction step has been observed for bases such as Hpy (a), 4-MePy (b) and 4-ClPy (c) and follows the rate law: rate = (a + k[RPy]²[Pd(RaaiMe)Cl₂]). The bases 2-MePy (d), 2,6-Me₂Py (e) and 2,4,6-Me₃Py (f) exhibits a bi-phasic reaction and follows the rate laws: rate–1 = (a + k[RPy][Pd(RaaiMe)Cl₂]) and rate–2 = (a + k[RPy][Pd(RaaiMe)-Cl₂]), where k is the third-order rate constant; k is the second-order first phase rate constant, k is the second-order second phase rate constant and a/a/a correspond to the solvent dependent constant of the respective reaction path. The rate data supports a nucleophilic association path. External addition of Cl^– (LiCl) suppresses the rate, which follows the order: k/k/k (3) > k/k,k (1) > k/k,k (2). The k values are linearly related to the Hammett constants. The 2-substituted pyridines (d–f) remarkably reduce the rate and show a bi-phasic reaction behaviour as compared with 4-Rpy (a–c). This is attributed to the steric effect that destabilises the transition state. The rate decreases with increasing steric crowding at the ortho-position and follows the order: (d) > (f) > (e). The 4-substituted pyridines control the rate via an inductive effect and follow the order: (b) > (a) > (c).     

Photoelectron spectra and electronic structure of VF5 and VOF3

Using the gas-phase photoelectron spectroscopy and SCF-X-SW methods, we have studied the electronic structure of fluorides of five-valent vanadium VF₅ and VOF₃. The values of the ionization potential calculated in the transition state approximation agree well with the experimental values. The V-F covalent bonding is accomplished by the ie, 2e, 3a₁ electrons in the pentafluoride and the 3e, 2e, 3a₁ electrons in VOF₃. The short V=O bond is realized by the 3a₁ and 5a₁ electrons and the 5e electrons. We show that the shortening of the VO bond compared with the VF bond is also due to the higher effective charge on the oxygen.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol., No. 3, pp. 348–353, May–June 1988.The authors thank É. G. Rakov and A. S. Dudin for providing the vanadium compounds, and V. A. Andreev for helping with the calculation for VF₅.     

Kinetics and Mechanism of the oxygenation of triphenylphosphine by bis(ethyl-L-cysteinato)dioxomolybdenum(VI)

Summary The kinetics of oxygen-transfer from [MoO₂(Et-L-cys)₂] to PPh₃ and the reaction between [Mo₂O₃(Et-L-cys)₄] and O₂ in benzene solution have been investigated using spectrophotometric techniques between 25 and 40°. The rate laws-d[Mo⁶⁺]/dt = k₁[Mo⁶⁺][PPh₃] with k₁ (at 35°) = 2.95×10^–4dm³mol^–1s^–1 and -d[Mo⁵⁺]/dt = 2k₃[Mo⁵⁺][O₂] with k₃ (at 35°) = 6.3×10^–2 dm³mol^–1s^–1 account for the kinetic data obtained with activation parameters (at 35°) of H = 46 kJ mol^–1, S = –153 JK^–1mol^–1, and H = 50.8 kJ mol^–1, S = –95 JK^–1 mol^–1 respectively.     

Studies on the kinetics and mechanism of dissociation of copper(II) and nickel(II) complexes of the macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene in perchloric acid media

Summary Acid catalysed dissociation of the copper(II) and nickel(II) complexes (ML²⁺ of the quadridentate macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene (L) has been studied spectrophotometrically. Both complexes dissociate quite slowly with the observed pseudo-first order rate constants (k_obs) showing acid dependence; for the nickel(II) complex (k_obs)=k_O+k_H[H⁺], the k_o path is however absent with the copper(II) complex. At 60°C (I=0.1M) the k_H values areca 10^–4 M^–1 s^–1 for both complexes; k _H ^Cu /k _H ^Ni =ca. 3.9, comparable to some other square-planar complexes of these metal ions. The rate difference is primarily due to H values [copper(II) complex, 29.4±0.5 kJ mol^–1; nickel(II) complex, 35.6±1.5 kJ mol^–1] with highly negative S values [for copper(II), –215.5 ±6.1 JK^–1 mol^–1 and for nickel(II), –208.1 ±5.6 JK^–1 mol^–1] which are much higher than the entropy of solvation of Ni²⁺ (ca. –160 JK^–1 mol^–1) and Cu²⁺ (ca. –99 JK^–1 mol^–1) ions; significant solvation of the released metal ions and the ligand is indicated.     

Kinetics and mechanism of formation and dissociation of copper(II) and nickel(II) complexes of the Schiff base bis(acetylacetone)ethylenediimine in aqueous-organic solvent media

Summary In NH₄NO₃+NH₄OH buffered 10% (v/v) dioxan-water media (pH 7.0–8.5), thePseudo-first-order rate constant for the formation of the title complexes M(baen),i.e. ML, conforms to the equation 1/k_obs=1/k+1/(kK_o.s · T_L), where T_L stands for the total ligand concentration in the solution, K_o.s is the equilibrium constant for the formation of an intermediate outer sphere complex and k is the rate constant for the formation of the complex ML from the intermediate. Under the experimental conditions the free ligand (pK_a>14) exists virtually exclusively in the undissociated form (baenH₂ or LH₂) which is present mostly as a keto-amine in the internally hydrogen-bonded state. Although the observed formation-rate ratio k^Cu/k^Ni is of the order of 10⁵, as expected for systems having normal behaviour, the individual rate constants are very low (at 25°C, k^Cu=50 s^–1 and k^Ni=4.7×10^–4s^–1) due to the highly negative S values (–84.2±3.3 JK^–1M^–1 for CuL and –105.8±4.1 JK^–1M^–1 for NiL); the much slower rate of formation of the nickel(II) complex is due to higher H value (41.2±1.0 kJM^–1 for CuL and 78.2±1.2 kJM^–1 for NiL) and more negative S value compared to that of CuL. The K_o.s values are much higher than expected for simple outer-sphere association between [M(H₂O)₆] and LH₂ and may be due to hydrogen bonding interaction.In acid media ([H⁺], 0.01–0.04 M) these complexes M(baen) dissociate very rapidly into the [M(H₂O)₆]²⁺ species and baenH₂, followed by a much slower hydrolytic cleavage of the ligand into its components,viz. acetylacetone and ethylenediamine (protonated). For the dissociation of the complexes k_obs=k₁[H⁺]+k₂[H⁺]². The reactions have been studied in 10% (v/v) dioxan-water media and also ethanolwater media of varying ethanol content (10–25% v/v) and the results are in conformity with a solvent-assisted dissociativeinterchange mechanism involving the protonated complexes.     

Triterpene glycosides of Astragalus and their genins. XX. Cycloorbigenin from Astragalus orbiculatus

A new genin — cycloorbigenin (I), C₃₀H₄₈O₅, mp 217–219°C, [] _D ²⁰ +28.3° (c 1.19; ethanol) has been obtained from a glycoside isolated from the epigeal parts of the plantAstragalus orbiculatus (Leguminosae), and on the basis of chemical transformation and spectral characteristics its structure has been established as 16,23:16,24-diepoxy-23(R),24(S)cycloartane-3,7,25-triol. The acetylation of (I) with acetic anhydride in pyridine yielded its diacetate (II), C₃₄H₅₂O₇, mp 148–150°C, [] _D ²⁰ +32.6° (c 0.92; methanol) and its triacetate (III), C₃₆H₅₄O₈, mp 137–139°C, [] _D ²⁰ +75° (c 0.4; methanol). The Jones oxidation of (I) led to a diketone (IV), C₃₀H₄₄O₅, mp 155–158°C, [] _D ²⁰ -73° (c 0.63; methanol). Details of the PMR, IR, and mass spectra are given for all the compounds.Institute of The Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 455–460, July–August, 1986.     

Effect of ionic interactions on the oxidation of Fe(II) with H2O2 in aqueous solutions

The oxidation of Fe(II) with H₂O₂ has been measured in NaCl and NaClO₄ solutions as a function of pH, temperature T (K) and ionic strength (M, mol-L^–1). The rate constants, k (M^–1-sec^–1), d[Fe(II)]/DT=-k[Fe(II)][₂O₂]at pH=6.5 have been fitted to equations of the formlog k = log k₀+ AI ^1/2+BI+CI ^1/2/T Where log k₀=15.53-3425/T in water; A=–2.3, –1.35; B=0.334, 0.180; and C=391, 235, respectively, for NaCl (=0.09) and NaClO₄ ( =0.08). Measurements made in NaCl solutions with added anions yield rates in the order B(OH) ₄ ^– >HCO ₃ ^– >ClO ₄ ^– >Cl^–>NO ₃ ^– >SO ₄ ^2– and are attributed to the relative strength of the interactions of Fe²⁺ or FeOH⁺ with these anions. The FeB(OH) ₄ ⁺ species is more reactive while the FeCO ₃ ⁰ , FeCl⁺, FeNO ₃ ⁺ and FeSO ₄ ⁰ species are less reactive than the FeOH⁺ ion pair. The general trend is similar to our earlier studies of the oxidation of Fe(II) with O₂ except for B(OH) ₄ ^– . The effect of pH on the logk was found to be a quadratic function of the concentration of H⁺ or OH^– from pH=4 to 8. These results have been attributed to the different rate constants for Fe²⁺ (k₀) and FeOH⁺ (k₁) which are related to the measured k by, k=k_0Fe + k_1FeOH, where _i is the molar fraction of species i. The rates increase due to the greater reactivity of FeOH⁺ compared to Fe²⁺. k₀ is independent of composition and ionic strength but k₁ is a function of ionic strength and composition due to the interactions of FeOH⁺ with various anions.     

Determination of the nature of the diperiodatocuprate(III) species in aqueous alkaline medium through a kinetic and mechanistic study on the oxidation of iodide ion

The nature of the diperiodatocuprate(III) (DPC) species present in aqueous alkaline medium has been investigated by a kinetic and mechanistic study on the oxidation of iodide by DPC. The reaction kinetics were studied over the 1.0 × 10^–3–0.1 mol dm^–3 alkali range. The reaction order with respect to DPC, as well as iodide, was found to be unity when [DPC] [I^–]. In the 1.0 × 10^–3–1.0 × 10^–2 mol dm^–3 alkali region, the rate decreased with increase in the alkali concentration and a plot of the pseudo-first order rate constant, k versus 1/[OH^–] was linear. Above 5.0 × 10^–2 mol dm^–3, a plot of k versus [OH^–] was also linear with a non-zero intercept. An increase in ionic strength of the reaction mixtures showed no effect on k at low alkali concentrations, whereas at high concentrations an increase in ionic strength leads to an increase in k. A plot of 1/k versus [periodate] was linear with an intercept in both alkali ranges. Iodine was found to accelerate the reaction at the three different alkali concentrations employed. The observed results indicated the following equilibria for DPC.[Cu(H₂IO₆)₂]^3- [Cu(H₂IO₆)]^- + H₂IO₆ ^3- [Cu(H₂IO₆)] + OH^- [Cu(HIO₆)]^- + H₂OA suitable mechanism has been proposed on the basis of these equilibria to account for the kinetic results.     

Spectrophotometric determination of mercury with 5,10,15,20-tetrakis(3-chloro-4-sulfophenyl)porphine by flow injection analysis

5,10,15,20-tetrakis(3-chloro-4-sulfophenyl)porphine (m-Cl-TPPS₄) was synthesized and used for the Spectrophotometric determination of mercury by flow injection analysis. A pseudo-first-order reaction kinetic mechanism was proposed with a rate constant of 0.8 min^–1 for Hg(II) withm-Cl-TPPS₄ in the presence of 8-hydroxyquinoline in a medium of 1.0M acetic acid and sodium acetate buffer solution (pH 6.22). In the optimum conditions of reaction temperature (85 ° C), stopped-flow time (60 s) and sampling volume (100 l), the method's relative standard deviation was 0.82% (n = 12) at 5.0 g ml^–1 mercury, with a linear range of 0–12.0 g ml^–1 and an analytical frequency of 60h^–1. The detection limit (3) was 0.025 g ml^–1. Interference studies showed that most metal ions co-existing with Hg²⁺ could be tolerated at 100-fold excess levels, but Zn²⁺, Cu²⁺ and Mn²⁺ needed to be masked. The method has been applied to the analysis of water samples with satisfactory results.     

Synthetic and kinetic studies on copper(II), nickel(II) and cobalt(III) complexes of 1,4,7,10,13-pentaazacyclohexadecane ([16]aneN5)

Summary The pentadentate macrocycle 1,4,7,10,13-penta-azacyclo-hexadecane [16]aneN₅=(3)=L} has been prepared and a variety of copper(II), nickel(II) and cobalt(III) complexes of the ligand characterised. The copper complex [CuL](ClO₄)₂, on the basis of its d-d spectrum, appears to be square pyramidal, while [NiL(H₂O)](ClO₄)₂ is octahedral. The copper(II) and nickel(II) complexes dissociate readily in acidic solution and these reactions have been studied kinetically. For the copper(II) complex, rate=k_H[complex][H⁺]² with k_H =4.8 dm⁶ mol^–2s^–1 at 25 °C and I=1.0 mol dm^–3 (NaClO₄) with H=43 kJ mol^–1 and S ₂₉₈ =–89 JK^–1 mol^–1. Dissociation rates of the copper(II) complexes increase with ring size in the order: [15]aneN₅ < [16]aneN₅ < [17]aneN₅. For the dissociation of the nickel(II) complex, rate=k_H[Complex][H⁺] with k_H=9.4×10^–3 dm³mol^–1 s^–1 at 25 °C and I =1.0 mol dm^–3 (NaClO₄) with H=71 kJ mol^–1 and S ₂₉₈ =–47 JK^–1mol^–1.The cobalt(III) complexes, [CoLCl](ClO₄)₂, [CoL(H₂O)]-(ClO₄)₃, [CoL(NO₂)](ClO₄)₂, [CoL(DMF)](ClO₄)₃ (DMF=dimethylformamide) and [CoL(O₂CH)](ClO₄)₂ have been characterised. The chloropentamine [CoCl([16]aneN₅)]²⁺ undergoes rapid base hydrolysis with k_OH=1.1× 10⁵dm³ mol^–1s^–1 at 25°C and I=0.1 mol dm^–3 (H=73 kJ mol^–1 and S ₂₉₈ =98 JK^–1 mol^–1). Rapid base hydrolysis of [CoL(NO₂)]²⁺ is also observed and the origins of these effects are considered in detail.     

Transfer of F− in the reaction of SF 6 − with SOF4: Implications for SOF4 production in corona discharges

The temperature (T) and electric field-to-gas pressure (E/P) dependences of the rate coefficientk for the reaction SF ₆ ^– +SOF₄SOF ₅ ^– +SF₅ have been measured. ForT<270 K,k approaches a constant of 2.1×10^–9 cm³/s, and for 433>T>270 K,k decreases withT according tok (cm³/s)=0.124 exp [–3.3 lnT(K)]. ForE/Pk has a constant value of about 2.5×10^–10 cm³/s, and for 130 V/cm·torr>E/P>60 V/cm·torr, the rate is approximately given byk (cm³/s)7.0×10^–10 exp (–0.022E/P). The measured rate coefficient is used to estimate the influence of this reaction on SOF₄ production from negative, point-plane, glow-type corona discharges in gas mixtures containing SF₆ and at least trace amounts of O₂ and H₂O. A chemical kinetics model of the ion-drift region in the discharge gap is used to fit experimental data on SOF₄ yields assuming that the SF ₆ ^– +SOF₄ reaction is the predominant SOF₄ loss mechanism. It is found that the contribution of this reaction to SOF₄ destruction falls considerably below the estimated maximum effect assuming that SF ₆ ^– is the predominant charge carrier which reacts only with SOF₄. The results of this analysis suggest that SF ₆ ^– is efficiently deactivated by other reactions, and the influence of SF ₆ ^– +SOF₄ on SOF₄ production is not necessarily more significant than that of other slower secondary processes such as gas-phase hydrolysis.     

Synthesis and characterization of α-[P2W15O62M3]12- (M = Ti or Zr) heteropolyanions

Summary The trisubstituted Dawson anions -XaHb[P₂W₁₅O₆₂M₃]· nH₂O (X = K or Me₄N; a + b = 12; M = Ti or Zr) have been prepared from the lacunary precursor Na₁₂[-P₂ W₁₅O₅₆]·24H₂O and characterized by elemental analysis, i.r., u.v. and ¹⁸³W-n.m.r. spectroscopy, and by an electrochemical method. The ¹⁸³W-n.m.r. spectrum of -[P₂W₁₅Ti₃O₆₂]^12– exhibits three lines of 122 intensity at –148.32, –182.91 and –212.95 p.p.m., as expected for the C _3v structure of the trisubstituted -Dawson anion.Author to whom all correspondence should be directed.     

New path for the decomposition of bipolar adducts formed by the reaction of olefin oxides with diphenyl-β-hydroxyalkylphosphines

Conclusions A new decomposition path to olefin oxide and phosphine has been found for the R₃P+C-CO-bipolar ion that forms in the olefin oxide-diphenyl--hydroxyalkylphosphine system. It takes place along with the known degradation paths (formation of unsaturated and carbonyl compounds).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimcheskaya, No. 1, pp. 169–173, January, 1985.     

Spatial structures of natural sesquiterpenes of the humulene type

The spatial structures of the natural sesquiterpenes -,-, and -humulenes have been studied by the method of molecular mechanics (MM). The conformational states of the 11-membered macrocycle in the humulenes have been analyzed on the basis of calculated figures and x-ray structural results. It has been shown that four conformational states, differing by the mutual positions of the C14 and C15 methyl groups and the C7 and C8 atoms relative to the plane of the macrocycle are characteristic for all humulenes. These conformational states are denoted as₁₄U⁸ ₇,¹⁴U⁸ ₇,₁₄U⁷ ₈, and¹⁴U⁷ ₈. The conformational flexibility of the 11-membered ring in the humulenes has been evaluated.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 40 64 75. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 287–296, May–June, 1997.     

Complex formation between nickel(II) and 2-(2-aminoethyl) benzimidazole: A kinetic and equilibrium study

Summary The reversible complex formation between 2-(2-aminoethyl) benzimidazole (AEB) and nickel(II) was studied by stopped flow spectrophotometry at I = 0.30 mol dm^–3. Both the neutral and monoprotonated form of AEB reacted to give the NiAEB²⁺ chelate. At 25 °C, the rates and activation parameters for the reactions Ni_II + AEB NiAEB²⁺ and Ni^II + AEBH⁺ NiAEB²⁺ + H⁺ are k _f ^L(dm^–3 mol^–1 s^–1) = (2.17 ± 0.24) × 10³, H (kJ mol^–1) = 40.0 ± 0.8, S (JK^–1 mol^–1) = – 47 ± 3 and k _inff ^pHL (dm³ mol^–1 s^–1) = 33 ± 10, H (kJ mol^–1) = 42.0 ±2.7, S (JK^–1 mol^–1) = – 72 ± 9. The dissociation of NiAEB²⁺ was acid catalysed and k _obs for this process increased linearly with [H⁺] in the 0.01–0.15 mol dm^–3 (10–30 °C) range with k _H(dm³ mol^–1s^–1) (25 °C) = 329 ± 6, H (kJ mol^–1) = 40 ± 2 and S (JK^–1 mol^–1) = – 61 ± 8. The results also indicated that the formation of NiAEB²⁺ involves a chelation-controlled, rate-limiting process. Analysis of the S ° data for the acid ionisation of AEBH _inf2 ^p2+ and the formation of NiAEB²⁺ showed that the bulky AEBH⁺ ion has a solvent structure breaking effect as compared to AEB [s _aqS ° (AEBH⁺) – s _aq ° (AEB) = 69 JK^–1 mol^–1], while AEBH _inf2 ^p2+ is a solvent ordering ion relative to NiAEB²⁺ [s _aq° (NiAEB²⁺) – ovS _aq ° (AEBH _inf2 ^p2+ ) = 11 JK^–1 mol^–1].Author to whom all correspondence should be directed.     

Kinetics of oxidation of manganese(II) by peroxomonosulfuric acid in aqueous acidic solution

Summary Oxidation of Mn _aq ²⁺ by HSO ₅ ^– in acetate buffer to manganese(IV) is autocatalytic, and obeys a rate expression of the general form -d[Mn^II]/dt = k₀[Mn^II] + k₁[Mn^II][MnO_x]. The first-order (k₀) and heterogenetic (k₁) rate constants show first-order dependences on [HSO ₅ ^– ] and on 1/[H⁺]. The reaction is catalyzed by the addition of the chelating ligand glycine; k₁ shows a first-order dependence on [glycine] at a fixed pH. This catalysis is ascribed to complexation, whereby the redox potential for Mn(gly) _n ^(2–n)+ is lower than that for Mn _aq ²⁺ , facilitating oxidation. The stoichiometry of the reaction is Mn²⁺: HSO ₅ ^– = 11, and the manganese(IV) oxide formed is of battery-active grade. Purity of the recovered product is not affected by the presence of high concentrations of natural sugars in the initial solution.     

Raman spectra from partially deuterated water and ice VI to 10.1 kbar at 28°C

High-pressure argon-ion laser-Raman spectra (4880 Å excitation) have been obtained from partially deuterated water and ice VI (20 volume % D₂O) in the OD and OH stretching regions to pressures of 10.1 kbar at 28°C. The Raman spectra from ice VI are the first to be reported at room temperature, and they are similar to the liquid spectra obtained at 9.7 kbar. Raman shifts corresponding to contour intensity maxima were observed to change with pressure rise in the OD and OH stretching regions from v =2513–2490 cm^–1 and v = 3402–3380 cm^–1, respectively, for pressures from 1 bar to 10.1 kbar (ice VI). In addition, a shoulder observed at 1 bar on the OD contour near v = cm^–1 became less distinct and was visually absent for pressures from 6.4 to 10.1 kbar, although a shoulder on the OH contour at about v = cm^–1 intensified gradually for pressures to 9.7 kbar, and abruptly upon freezing at 10.1 kbar. The small effects of pressure on the OD component percentages obtained from computer analysis indicate that hydrogen-bond breakage is not a significant effect of pressure rise, and a downward change in the position of the OD stretching component having the largest Raman shift indicates that the nonhydrogen-bonded OD units or broken O-D...O bonds that exist at 1 bar are probably transformed by close packing due to compression into weak O-D...O bonds that are angularly deformed. In addition, intensification of the OH component at v = cm^–1 upon freezing or upon pressurizing the liquid to 9.7 kbar is indicated by the computer analyses, and an increase in intermolecular coupling is thus favored, as opposed to enhancement of Fermi resonance, because the positions of components at v = cm^–1 and v = cm^–1 are nearly independent of pressure. The computer results also strengthen previous evidence indicating that the OD component which occurs at about v = cm^–1 at 1 bar arises from broken O-D...O bonds, when it is understood that the severely deformed O-D...O bonds of ice VI give rise to intensity at a Raman shift of v = cm^–1, a difference of 37 cm^–1 in the direction of stronger hydrogen-bonding.This paper was presented at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.     

Kinetic studies on the formation of ternary complexes in the reaction of diaquonitrilotriacetatonickelate(II) with amino acids in aqueous solution

Summary Kinetics of formation of ternary complexes in the reaction of Ni(NTA)(H₂O) ₂ ^– with several amino acids, LH^± (glycine, -alanine, -alanine, L-valine and L-phenylalanine) have been studied by a pH indicator method using stopped-flow spectrophotometry. The results conform to 1/k_obs = 1/k + [H⁺]/ kKT_L where K is the equilibrium constant for formation of Ni(NTA)(–L)(H₂O)^2–, and k is the specific rate constant for the subsequent rate-determining ring-closure step leading to Ni(NTA)(=L)^2–. For the different amino acids studied, the k values decrease in the sequence: glycine > -alanine > L-phenylalanine > L-valine > -alanine. These k values areca. 1000 times lower than the values for complexation of Ni(NTA)(H₂O) ₂ ^– with NH₃ and imidazole and the spread in k values is much less than the pK_a values of the amino acids. The relative rates are enthalpy controlled and the S values are highly negative in conformity with ring closure as the rate determining step.     

Infrarot-spektralphotometrische Bestimmung der ?freien? Silanolgruppen in pyrogenen Kiesels?uren

Zusammenfassung Die technischen Eigenschaften pyrogener Kieselsäuren werden entscheidend vom Gehalt an »freien« Silanolgruppen mitbestimmt. Die IR-Absorption dieser Gruppen in CCl₄ gehört zu einer Bande bei 3700 cm^–1. Es wurde festgestellt, daß die maximale Extinktion in CCl₄ unter bestimmten Bedingungen unabhängig von der Anzahl ebenfalls vorhandener »gebundener« Silanolgruppen (max. Extinktion bei etwa 3350–3450 cm^–1) sowie auch weitgehend unabhängig vom Wassergehalt (Feuchtigkeit) ist. Diese Tatsache wurde zur Ausarbeitung eines Routineverfahrens benutzt.

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Infrared-spectrophotometric determination of free silanol groups in pyrogenic silicic acids

Summary The technical properties of pyrogenic silicic acids are decisively co-determined by their content of so-called feee silanol groups. Their infrared absorption in a CCl₄ suspension belongs to a band at about 3,700cm^–1. It has been found that under certain conditions their maximum absorbance in CCl₄ is independent of the number of so-called bonded silanol groups (maximum absorption at about 3,350–3,450cm^–1), additionally existing in all commercial pyrogenic silicic acids, and also in wide ranges of the water content (humidity). This fact has been used as a basis for a quantitative routine determination of free silanol groups.

Zum Gedenken an Heinz Dannenberg     

The Ionic Strength Dependence of Rare Earth and Yttrium Fluoride Complexation at 25°C

Formation constants for the complexation of yttrium and rare earth elements(YREE) by fluoride ions have been measured at 25°C. The ionic strength ()dependence of YREE formation constants in perchlorate solution for ionicstrengths between 0 and 6 molar can be expressed aslog_F1 (M, ) =log_F1 ^o (M) –3.066 ^0.5/(1 + 1.769 ^0.5)+ 0.1645 where log_F1 ^o(M) represents MF²⁺formation constants at zero ionic strength.The log_F1 ^o(M) results obtained inthis work are: Y(4.46), La(3.62), Ce(3.86),Pr(3.84), Nd(3.82), Sm(4.15), Eu(4.27), Gd(4.24), Tb(4.37), Dy(4.39), Ho(4.28),Er(4.27), Tm(4.29), Yb(4.39), and Lu(4.25). The relative magnitudes of YREEformation constants are independent of ionic strength. The pattern oflog_F1(M,),formation constants obtained in this work [relative magnitudes oflog_F1 ^o (M)],exhibits a shallow minimum between Dy and Yb. In contrast to the smoothpattern of stability constants expected if fluoride were to interact with bare ions(with monotonically decreasing crystal radii between La and Lu), theinteractionof F^– with YREEs, which have extensive hydration spheres[M(H₂O)_8–9 ³⁺] resultsin a relatively complex pattern of lanthanide stability constants. The fluoridecomplexation behavior of yttrium differs distinctly from the behavior of any rareearth. Although the crystal radius of Y3;pl is approximately equalto that of Ho³⁺,differences in the covalence/ionicity of Y³⁺ relative to therare earths leads to aYF²⁺ stability constant that exceeds that of any rare earthelement (REE).