63Cu-NMR.-Untersuchungen von Kupfer (I)-Tetrapyridin-und Kupfer (I)-Tetraacetonitrilsolvaten

⁶³Cu-NMR.-Spectra of Cu(CH₃CN)₄X (X = ClO, BF, PF) and Cu(C₅H₅N)₄X (X = ClO, BF) in solution are reported at different temperatures and concentrations. The influence of temperature on the linewidth and chemical shift indicates an equilibrium of Cu(CH₃CN) and Cu(C₅H₅N) with another complex of lower symmetry. The preferential solvation of Cu (I) by pyridin in a mixture acetonitrile/pyridine is clearly shown.     

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.     

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.     

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.     

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.     

Contribution à l'étude du système quaternaire K+?NH4+?CrO?SO?H2O: II. L'isotherme de 25°C

The study at 25°C of the system K⁺? NH? CrO? SO? H₂O has shown experimentally the existence of a new type of quaternary system of solubility with two cations and two anions. The solubility diagramm is caracterized by the presence of two adjacent ternary limiting systems with a miscibility gap, three univariant lines (one of them being evanescent), one invariant point, three binary and one ternary miscibility gaps.     

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.     

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].     

Chemistry of Superoxide Ion as Revealed by the Differential Oxidation of Arylpyruvates

Superoxide ion apparently reacts with acidic substrates via species such as O₂, HO₂, O, HO and H₂O₂. Arylpyruvates give arylacetates and arylaldehydes indicating competing nucleophilic and free radical oxidation. Benzaldehyde is further oxidized by free radical and nucleophilic dioxygen species giving benzoic acid. p-Hydroxybenzaldehyde gives the corresponding benzoic acid which is best accounted for by HO₂, since O and O₂ are without effect. Hydroquinone is also produced presumably by nucleophilic attack of HO. Replacement of the acidic hydrogen atoms by sodium changes the product distribution in accord with these findings.     

Structures and Electrical Properties of Some New Organic Conductors Derived from the Donor Molecule TMET (S,S,S,S,-Bis(dimethylethylenedithio) tetrathiafulvalene)

Crystal structures and electrical properties of radical-cation salts of the chiral organic donor TMET (S,S,S,S,-bis-(dimethylethylenedithio)tetrathiafulvalene) are described. Two structural types, 2:1 with octahedral anions Pf, AsF, SbF, I (incommensurate), and 3:2 with tetrahedral anions BF^?₄, CIO^?₄, ReO^?₄ are observed. Resistivity measurements between 2 and 298 K indicate that the 3:2 types are organic metals, while the other compounds are semiconductors. (TMET)₃(CIO₄)₂ is metallic down to about 120 K at ambient pressure and remains metallic down to 2 K at 8 kbar.     

Endohedral fullerene-noble gas clusters formed with high-energy bimolecular reactions of C (x = 60, 70; n = 1, 2, 3)

Results are reported for high-energy beam experiments which establish the formation of endohedral carbon cluster-noble gas compounds by bimolecular reactions of C (x = 60, 70; n = 1, 2, 3) with He and C with Ne. The ions were accelerated up to 8 ke V in a four-sector mass spectrometer and allowed to collide with the noble gas in a collision chamber at room temperatur. Product ions were monitored with a B/E = constant linked scan. Within the sensivity of the experiments, no carbon cluster-gas compounds were observed in the reactions of C with H₂, D₂, O₂, Ar, and SF₆, or of C with O₂. The observed fall in the cross-section for carbon cluster-noble gas compounds with increasing size of the noble gas, the observation of unimolecular loss of C₂ from mass-selected C_xHe⁺ ions, and the elimination of carbon fragments instead of He observed in the formation of the collision-induced C_xHeⁿ⁺ product ions are taken as evidence for endohedral compound formation. Results of ab initio molecular-orbital calculations for the perpendicular penetration of the plane of ionized benzene with He, Ne, and Ar indicate that sufficient kinetic energy should be available in the collisions with C to penetrate the C cage at the collision energies of the experiments.     

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.     

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.     

Hydrogen Generation by Visible Light Irradiation of Aqueous Solutions of Metal Complexes. An approach to the photochemical conversion and storage of solar energy

We describe a photochemical system for the generation of hydrogen by water reduction under visible light or sunlight irradiation of aqueous solutions containing the following components: a photosensitizer, the Ru (bipy) complex, for visible light absorption; a relay species, the Rh (bipy) complex, which mediates water reduction by intermediate storage of electrons via a reduced state; an electron donor, triethanolamine (TEOA) which provides the electrons for the reduction process and a redox catalyst, colloïdal platinum, which facilitates hydrogen formation. The conditions for efficient hydrogen production and the influence of the concentration of the components have been investigated; the metal complexes act as catalysts with high turnover numbers; excess bipyridine facilitates the reaction. The process contains two catalytic cycles: a ruthenium cycle and a rhodium cycle. The Ru cycle involves oxidative quenching of the *Ru(bipy) excited state by Rh(bipy) forming Ru(bipy) which is converted back to Ru(bipy) by oxidation of the electron donor TEOA, which is thus consumed. The Rh cycle comprises a complicated set of transformations of the initial Rh(bipy) complex. The reduced rhodium complex formed in the quenching process undergoes a series of transformations involving the Rh(bipy) complex and hydridorhodium-bipyridine species, from which hydrogen is generated by reaction with the protons of water. In view of the storage of two electrons in the reduced rhodium species, the process is formally a dielectronic water reduction. The properties and eventual participation of [Rh(III)(bipy)₂LL′]ⁿ⁺(L,L′ = H₂O, OH^?) species are investigated. It is concluded that at neutral pH in presence of excess bipyridine, the cycle involving regeneration of the Rh(bipy) complex is predominant. A number of experiments have been performed with modified systems. Hydrogen evolution is observed with other photosensitizers (like proflavin), other relay species (like Rh(dimethylbipy) or Co(II)-bipyridine complexes), other donor species, or in absence of the platinum catalyst. It also occurs in absence of photosensitizer by sunlight of UV. irradiation of Rh(bipy) or by visible light irradiation of iridium (III)-bibyridine complexes. These systems deserve further investigations. The present photochemical hydrogen generating system represents the reductive component of a complete water splitting process. Its role in solar energy conversion and in photochemical fuel production is discussed.     

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.     

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.     

Remarkable Enhancement of the Rate of Cathodic Reduction of Hydrocarbonate Anions at Palladium in the Presence of Caesium Cations

Steady-state polarization curves, representative of the electrochemical reduction of HCO ions at a smooth palladium cathode, obtained in CsHCO₃ and NaHCO₃ solutions, are compared. The net current densities, associated with the electroreduction of HCO anions, measured in 1M CsHCO₃ solution are shown to be up to 9 times larger than those observed in 1M NaHCO₃ solution. This result is interpreted in terms of a direct involvement of the Cs⁺ cations in the reaction at the cathode through the formation of ion pairs with the HCO anions.     

Molecular Ions of Transient Species: Vinyl-Alcohol Cation

Vinyl alcohol 1 was prepared by thermolysis of cyclobutanol and its photoelectron spectrum was determined. I = 9.18 eV and I = 9.52 eV were found, the vibrations progression (? = 1400 cm^?1) for this lowest energy transition 1(X)→1⁺(X?) indicating significant skeletal changes in the ion. The question of the relative stability of the syn ( 1 )- vs. anti-ions ( 1 ) is discussed in the light of theoretical calculations. The energy of the second π-state of 1 ⁺ is estimated at 13.6–14.1 eV above the ground state of 1 .     

Zur Stabilität der Metallkomplexe von Methantriessigsäure und cis,cis-1,3,5-Cyclohexantricarbonsäure

The stabilities of the Mn²⁺-, Co²⁺-, Ni²⁺-, Cu²⁺- and Zn²⁺-complexes with 2-(carboxymethyl)glutaric acid ( 2 ) and cis,cis-1,3,5-cyclohexanetricarboxylic acid ( 3 ) were measured potentiometrically at 25° and I = 0.5 (KNO₃). Beside the complexes ML^? protonated species MLH and MLH are also formed. Their stability constants are given in Table 1. A comparison between the stabilities of 2 or 3 and those of acetate, as a model for a monocarboxylate, or succinate and glutarate, as examples for dicarboxylates, indicates that in all species only one carboxylate is strongly bound whereas the second and third ones are probably not. The observation that Δlog K₁ = log K ? log K as well as Δlog K₂ = log K ? log K are practically constants with values of 0.34 ± 0.05 and 0.49 ± 0.07, respectively, for both ligands and the five metal ions studied is also in line with the proposed monodentate structures of the complexes ML^?, MLH and MLH.