Ternäre Komplexe in Lösung IV. Einfluss von 2,2′-Bipyridyl auf Stabilität und Acidität des Cu2+-Adenosin-5′-monophosphat-N(1)-oxid-1:1-Komplexes

The ternary Cu²⁺?2,2′-bipyridyl-adenosine-5′-monophosphate-N(1)-oxide complex was investigated and compared with the binary Cu²⁺-adenosine-5′-monophosphate-N(1)-oxide complex (I) (cf. [2]). In both complexes Cu²⁺ is bound to the o-amino-N-oxide group of adenosine-5′-monophosphate-N(1)-oxide (HL). The stabilities of the complexes monoprotonated at the phosphate group are of the same order: log K = 11,20, and log K = 11,19. The acidity constants for the deprolonation of the phosphate group in these complexes are slightly different (pK = 5,55, and pK = 5,88), but as expected both values are lower than the corresponding value pK = 6,12 of the ligand.     

Structural Comparison of Oligoribonucleotides and Their 2′-Deoxy-2′-fluoro Analogs by heteronuclear NMR spectroscopy

1-(2′-Deoxy-2′-fluororibofuranosyl)pyrimidines were synthesized and incorporated into an RNA oligonucleotide to give 5′-r[C_fGC_f(U_fU_fC_fG)GC_fG]-3′ (C_f: short form of C = 2′-deoxy-2′-fluorocytidine; U_f: short form of U = 2′-deoxy-2′-fluorouridine). The oligomer was investigated by means of UV, CD, and NMR spectroscopy to address the question of how F-labels can substitute ¹³C-labels in the ribose ring. Through-space (NOE) and through-bond (scalar couplings) experiments were performed that make use of the ameliorated chemical-shift dispersion induced by ¹⁹F as an alternative heteronucleus. A comparison of the structures of fluorinated vs. unmodified oligomer is given. It turns out that the fluorinated oligonucleotide exists in a 14:3 equilibrium between a hairpin and a duplex conformation, in contrast to the unmodified oligonucleotide which predominantly adopts the hairpin conformation. Furthermore, the fluorinated hairpin structure adopts two distinct conformations that differ in the sugar conformation of the U and C nucleoside units, as detected by the ¹⁹F-NMR chemical shifts. The role of the 2′-OH group as stabilizing element in RNA secondary structure is discussed.     

Photoinduced electron transfer from polystyrene pendant tris(2,2′-bipyridyl)ruthenium (II) complex to methylviologen

The characteristics of the photoinduced electron transfer reaction from polystyrene pendant tris(2,2′-bipyridyl)ruthenium (II) complex [Ru(bpy)] to methylviologen (MV²⁺) were studied. The rate constant k₁ from the excited state of the complex, Ru(bpy), to MV²⁺ were determined for both the polymeric and monomeric complexes from the lifetime τ of Ru(bpy) and the quenching rate of Ru(bpy) by MV²⁺. The polymer pendant Ru(bpy) showed three kinds of τ components ranging from 7 to 474 ns, in contrast to the monomeric complex, which showed one component of 350 ns. The k₁ values for both complexes were almost the same, on the order of 10⁸ L/mol s. The photoinduced electron transfer from solid-phase Ru(bpy) to liquid-phase MV²⁺ was realized by utilizing the polymer complex, and the solid–liquid interphase reaction system is discussed.     

L'action de l'ammoniac sur l'hydroxyde de cadmium et la stabilité des complexes en milieu aqueux

The solubility of precipitated Cd(OH)₂ was determined at 25°C in 1 M NaClO₄, as a function of pH and of the ammonia content of the solutions. Formation constants were obtained for the following hydroxo, ammine and hydroxo-ammine complexes: CdOH⁺, Cd(OH)₂, Cd(OH), CdNH, Cd(NH₃), Cd(NH₃), Cd(NH₃) and Cd(OH)₂NH₃. The solubility product of the hydroxide was also calculated. The presence of polynuclear species was investigated by titrimetric determinations of the hydrogen ion concentration at constant metal concentration.     

Molecular Recognition in Anion Coordination Chemistry. Structure,Binding Constants and Receptor-Substrate Complementarity of a Series of Anion Cryptates of a Macrobicyclic Receptor Molecule

The crystal structures of four anion cryptates [X^? ? BT -6H⁺] formed by the protonated macrobicyclic receptor BT -6H⁺ with F^?, Cl^?, Br^? and N have been determined. They provide a homogeneous series of anion coordination patterns with the same ligand. The small F^?-ion is tetracoordinated, while Cl^? and Br^? are bound in an octahedron of H-bonds. The non-complementarity between these spherical anions and the ellipsoïdal cavity of BT -6H⁺ is reflected in ligand distortions. Structural complementarity is achieved for the linear triatomic substrate N, which is bound by two pyramidal arrays of three H-bonds, each interacting with a terminal N-atom of N. The formation constants of the complexes formed by BT -6H⁺ with a variety of anions (halides, N, NO, carboxylates, SO, HPO, AMP^2?, ADP^3?, ATP^4?, P₂O) have been determined. Very strong complexations are found, as well as marked electrostatic and structural effects on stability and selectivity; in particular the binding of F^?, Cl^?, Br^?, and N may be analyzed in terms of the crystal structure data. The cryptand BT -6H⁺ is a molecular receptor containing an ellipsoïdal recognition site for linear triatomic substrates of size compatible with the size of the molecular cacity. Further developments of various aspects of anion coordination chemistry are considered.     

A Unified Interpretation of Kinetic Data on the Acid-Induced Cleavage and of Product-Analysis Data on Spontaneous Cleavage of the Mono-ol Cation μ-Hydroxo-bis[pentaamminecobalt(III)] ([(NH3)5CoOHCo(NH3)5]5+)

Recent work on the spontaneous (= acid-independent) cleavage of the mono-ol cation, i.e. in Cl^?/ClO and NO/ClO mixed-electrolyte media has established (by analysis of anion-competition experiments) the existence of reactive ion pairs of the mono-ol cation with Cl^? and NO. Their existence must be allowed for in the analysis of the rate data for the acid-induced cleavage (pH 0–1) of the mono-ol cation in these mixed-electrolyte media. Thus, previous data for acidic Cl^?/ClO media have been re-interpreted in this work, and new data for NO/ClO media have been analyzed in the same sense. This analysis removes an apparent discrepancy in the orders of magnitude of ion aggregate stability constants between the mono-ol and similar binuclear cations.     

Conformations of the 10-membered Ring in 5, 10-Secosteroids. II. (E)-3α-Acetoxy-5, 10-seco-1 (10)-cholesten-5-one and (E)-5, 10-seco-1 (10)-cholestene-3, 5-dione

(E)-3α-Acetoxy-5, 10-seco-1(10)-cholesten-5-one ( 3 ) was synthesized by fragmentation of 3α-acetoxy-5α-cholestan-5-ol ( 1 ) using the photochemical version [3] of the lead tetraacetate reaction [4], and transformed into the corresponding 3-oxo-compound ( 5 ). Two conformations ( A and B ) were deduced for the 10-membered ring of 3 by analysis of the ¹H- and ¹³C-NMR. spectra in toluene. The major conformation ( A ) corresponds to that found in the solid state by X-ray analysis. According to its NMR. spectra in toluene, the medium-sized ring of the diketone 5 exists also predominantly in two conformations, the major one being analogous to A (the solid-state conformation of the 3β-acetoxy isomer ( 9 ) [1]) and the minor one to A (see above). The stereochemistry of the acidcatalyzed and thermal cyclisations of 3 as well as of the corresponding 5-oxime is discussed in terms of conformational factors.     

Charge Dispersal in the Quinuclidine Radical Cation and in the Quinuclidinium Ion,as Revealed by PE and ICR Spectroscopy

The n ionization energies I and the gas-phase basicities GB of CH₃-, Cl-, or CN-substituted quinuclidines have been measured by PE and ICR spectroscopy. The dependence of the shifts ΔI and ΔGB (relative to the values of the parent molecule) allow conclusions about the charge dispersal accompanying the n ionization or the protonation of quinuclidine in the gas phase. The agreement with the results of a minimal basis set ab initio calculation is excellent. Comparison of the solution pK_a values with either I or GB reveals that 2-substituted quinuclidines exhibit sizeable solvent-induced proximity effects, i.e. that the corresponding quinuclidinium ions are more acidic in solution than expected on the basis of the gas-phase basicities. This agrees with earlier results concerning 2-substituted pyridines.     

Stability,Formation and Dissociation Kinetics of the Pentacoordinate Ni2+ and CO2+ Complexes with 1,5-Diazacyclooctane-N,N′-diacetic Acid

The stability constants of the Ni²⁺ and Co²⁺ complexes with 1,5-diazacyclooctane-N,N′-diacetic acid (H₂DACODA) have been determined potentiometrically in 0.5M KNO₃ at 25°. Only M(DACODA) and M(DACODA)OH^? were observed. In addition the formation and dissociation kinetics of the pentacoordinate complexes M(DACODA) has been studied in aqueous solution using a stopped-flow technique. Formation follows the rate law v_f = k_f [M²⁺] [HDACODA^?]/[H⁺], which can be interpreted as a bimolecular process either between M²⁺ and DACODA^2? (k) or between MOH⁺ and HDACODA^? (k). The second order rate constants k are much higher than those expected from water exchange and can only be explained by a strong internal conjugate base effect. In the limiting case, however, this is equivalent to the second possible explanation, which assumes MOH⁺ and HDACODA^? as reactive species. The dissociation rate is given by v_d = (k^ML + k [H⁺]) · [M(DACODA)].     

Photoelectrochemistry with Colloidal Semiconductors; Laser Studies of Halide Oxidation in Colloidal Dispersions of TiO2 and α-Fe2O3

Aqueous sols of TiO₂ (anatase, particle radius 25 Å) were excited with (347.1 nm)-laser light and the reaction of valence-band holes with halide ions (X = I^?, Br^?, Cl^?) was investigated. Hole transfer takes place within the duration of the (10 ns)-laser pulse and results in the formation of anion radicals according to the sequence: The quantum yield of X increases in the order Cl < Br < I, attaining 0.8 for I at pH 1. It is affected by pH, halide concentration and the presence of a protective agent for the sol. RuO₂ deposited onto TiO₂ enhances markedly Cl and Br -formation, but has no effect on the yield of I. Laser-photolysis investigation of halide oxidation were also carried out with colloidal Fe₂O₃ (particle radius 600 Å). For I_2?formation, the quantum yield exceeds 0.9 indicating almost quantitative hole scavenging by iodide.     

Réactions chimiques oscillantes III. Effets de la température et de la composition chimique sur la «période d'induction» des systèmes BrO/Ce4+/Cyclohexanone et BrO/Ce4+/Cyclopentanone

Oscillating Chemical Reactions. III. Effects of the Temperature and Chemical Composition on the ‘Induction Period’ of the BrO /Ce⁴⁺/Cyclohexanon and BrO /Ce⁴⁺/Cyclopentanon Systems A study of the influence of the temperature and composition of the BrO/Ce⁴⁺/cyclohexanon (S1) and BrO/Ce⁴⁺/cyclopentanon (S2) systems has shown a very particular behaviour for τ_ind. for a given ratio α of concentrations: Moreover, for 0.27 ? α ? 0.32, log¹/τ_ind. is no longer a linear function of the inverse of the temperature: a break in the line log¹/τ_ind. = f(1/T) occurs.     

Variable-Temperature and Variable-Pressure Kinetic and Equilibrium studies of formation and dissociation of [V(H2O)5NCS]2+

The kinetics of formation and dissociation of [V(H₂O)₅NCS]²⁺ have been studied, as a function of excess metal-ion concentration, temperature, and pressure, by the stopped-flow technique. The thermodynamic stability of the complex was also determined spectrophotometrically. The kinetic and equilibrium data were submitted to a combined analysis. The rate constants and activation parameters for the formation (f) and dissociation (r) of the complex are: k/M ^?1 · S^?1 = 126.4, k/s^?1 = 0.82; ΔH /kJ · mol^?1 = 49.1, ΔH/kJ · mol^?1 = 60.6; ΔS/ J·K^?1·mol^?1= ?39.8, ΔSJ·K^?1·mol^?1 = ?43.4; ΔV/cm³·mol^?1 = ?9.4, and ΔV/cm³ · mol^?1 =?17.9. The equilibrium constant for the formation of the monoisothiocynato complex is K²⁹⁸/M ^?1 = 152.9, and the enthalpy and entropy of reaction are ΔH⁰/kJ · mol^?1 = ? 11.4 and ΔS⁰/J. K^?1mol^?1 = +3.6. The reaction volume is ΔV⁰/cm³· mol^?1 = +8.5. The activation parameters for the complex-formation step are similar to those for the water exchange on [V(H₂O)₆]³⁺ obtained previously by NMR techniques. The activation volumes for the two processes are consistent with an associative interchange, I_a, mechanism.     

L'action de l'ammoniac sur l'oxyde cuivrique. et les hydroxo-complexes de cuivre (II)

Using a new mathematical treatment, the nature and stability constants of the simple and mixed complex-species of copper(II) with hydroxyde and ammonia as ligands have been determined. The solubility curves of CuO in heterogeneous equilibrium have been identified in function of pH only and in function of pH and pNH_3tot at 25° and unit ionic strength (NaClO₄). The predominent species in the relatively dilute system limited by the ionic strength are [Cu²⁺], [Cu(OH)₂], [Cu(OH)], [Cu(OH)], [Cu(NH₃)], [Cu(NH₃)], [Cu(NH₃)], [Cu(NH₃) (OH)⁺], [Cu(NH₃)₃(OH)⁺] and [Cu(NH₃)₂(OH)₂].     

Towards the Construction of a Complete Cyclic Water Decomposition System,Design and Operation of an Oxygen Producing Half Cell

Oxygen is generated when aqueous solution of iron (III) tris(2,2′-bipyridyl), Fe(bipy), are brought in contact with catalytic amounts of powdered or colloidal RuO₂. The oxygen yield depends strongly on the pH, reaching a maximum between pH 7 and 8 where it corresponds to the stoichiometry of the reaction: The rate of the reaction is so fast that it occurs practically upon dissolution of Fe(bipy) in the aqueous phase. In acidic media (pH 4), no O₂ evolution is observed. Instead, Fe(bipy) is converted to an intermediate which in the presence of RuO₂ yields O₂ upon neutralization. The pH profile of the O₂ evolution occuring upon illumination of Ru(bipy) in the presence of the cobalt complex [Co(NH₃)₅Cl]²⁺ was also investigated. The surprisingly low energy losses (160 mV) in reaction (1) makes the construction of four quanta water splitting systems feasible.     

The Gas-Phase Ion/Molecule Reactions of C3H5X (X = C1, Br,I)

The ion/molecule reactions of the molecular ion, the C₃H ion, and the C₃H ion obtained from 3-chloropropene. 1-bromopropene, 2-bromopropene, 3-bromopropene, bromocyclopropane. and 3-iodopropene have been studied with their neutral precursor in a Fourier-transform mass spectrometer (FT/ICR). The molecular ions react to yield primarily C₆H except for the ion derived from 1-bromopropene that is unreactive. The kinetics of the 3-bromopropene molecular ion reveals that 18% of these ions must possess a different structure which is unreactive. The fact that C₃H ions obtained from 2-bromopropene are the only ones to undergo proton transfer is taken as evidence that only this parent compound gives rise to 2-propenyl cations by low-energy electron impact. The C₃H ions generated in these systems are shown to be roughly an equal mixture of propargylium ions that react to yield C₆H and unreactive cyclopropenium ions.     

Fundamental Aspects of Ionic Dissociations: The Fragmentation Pathways of Excited Bicyclobutane Cations

A most recently developed method to quantify the fragmentation pathways of excited radical cations is presented. Using bicyclobutane cation as an illustrative example, the RRKM analysis of the breakdown diagram determined by He-Iα photoelectron-photoion coincidence spectroscopy is outlined. The results imply complete isomerization to 1,3-butadiene cation preceding the dissociative processes. The rate-energy functions of four competitive primary fragmentation reactions, leading to C₃H, C₄H, C₄H and C₂H are established. There is compelling evidence that the production of C₂H fragment ions does not compete effectively with these four reactions. The extent of kinetic and competitive shift effects is determined. The derived enthalpies of formation are in excellent accord with the available high quality reference data. The relative importance of different fragmentation pathways which ultimately lead to fragment ions of identical mass to charge ratio is assessed.     

Synthesis and Crystal Structure of Tricaesium Heptaphosphide–Ammonia(1/3) Cs3P7·3 NH3

Recrystallization of Cs₃P₇ from liquid NH₃ yields the triammoniate Cs₃P₇·3 NH₃, which loses the weakly bound NH₃ of crystallization below 253 K. A low-temperature crystal-structure analysis shows that Cs₃P₇· NH₃ consists of a framework of heptaphosphanortricyclane anions P and Cs⁺ cations with NH₃ molecules completing the coordination of the cations. The framework is built from Cs₃P₇ layers connected by only few Cs…?P interactions, the interlayer gap being filled by a two-dimensional network of NH₃. The Cs₇P₇ part of the structure completes a family of alkali-metal-polyphosphide substructures which range from [RbP₇]^2? or [CsP₁₁]^2? chains over [Cs₂P_n]^? layers (n = 7, 11) to now [Cs₃P₇] frameworks.     

Ionization Energies of 1-R-2,2-dimesitylethenols, 1,2-Dimesityl-2-phenylethenol,and Some of their Keto Tautomers

The He(Iα) PE spectra of the two pairs of 1 (Ph)- 3 and 2–4 have been recorded and assigned by comparison with spectra of related molecules, or by using simple MO arguments. This analysis shows that the PE spectra are in agreement with the assumption that the twist angles observed in the crystalline state are more or less conserved in the gas phase. The observed first adiabatic ionization energies I allow the calculation of the molar reaction enthalpy Δ_rH which accompanies the enolization of the radical cations of the title compounds. It is found that Δ_rH ≈? –?70 kJ mol^?1 is roughly the same for both keto/enol pairs, in qualitative and quantitative agreement with expectation. These results complement and correct ionization energies determined previously by mass spectrometric methods and conclusions based on them.     

Conformational Analysis and Chromic Acid Oxidation. Oxidation Rates and Equilibrium Constants of Epimeric Alcohols

The linear free energy relationship of Sicher for relative reactivity towards chromic acid oxidation (ΔΔG) as a function of thermodynamic stability (ΔG) has been reexamined with 23 pairs of epimeric alcohols. The plot of ΔG vs. ΔG has a slope of 0.8, a correlation coefficient of 0.97 and a standard deviation of 0.23 kcal/mol on ΔΔG_Ox. The limitations of the relationship and the exceptions are discussed.     

Réactions de substitution dans l'anion tétrachloropalladate(II) avec des diamines. I. Ethylènediamine

The replacement of Cl^? by ethylenediamine (en) in PdCl has been followed spectrophotometrically at 25°C and μ = 1 (NaClO₄); it proceeds in two steps leading to Pd(en)Cl₂ and Pd(en), respectively. The observed rate constants are discussed in terms of the mechanism proposed by Reinhardt [1] for the successive ammination reactions of PdCl.