Kinetik und stereochemischer Verlauf der thermischen Umlagerung der vier stereoisomeren Propenyl-(but-2′-enyl)-äther

The gas-phase rearrangement of (1Z, 2′E)-, (1Z, 2′Z)-, (1E, 2′E)-, and (1E, 2′Z)- propenyl but-2′-enyl ether (Z, E)-, (Z, Z)-, (E, E)-, and (E, Z)-1) into erythro- and threo-2, 3-dimethyl-pent-4-en-al (erythro- and threo-2) was investigated over a temperature range from 142,5° to 190,0° at 20–35 Torr (for kinetic data and activation parameters see table 2). All four stereoisomeric ethers 1 rearrange preferentially via a chair-like transition state C into the aldehydes 2 (ΔΔG^≠ (160°) = 2,5–2,7 kcal/mol for B – C (B = boat-like transition state). The relative rates (k_rel) for (Z, Z)-1, (Z, E)-1, (E,Z)-1, and (E,E)-1 at 160° are 1,0, 2,9, 4,3 and 9,0 respectively (see table 5). Taking into account the relative enthalpies of the ethers 1 and the steric interaction in the C transition state of the ethers 1 (see table 6), k_rel values can be estimated. They are in good agreement with those observed (see table 5).     

Photoisomerization of Arylazopyrazole Photoswitches: Stereospecific Excited‐State Relaxation

Electronic structure calculations and nonadiabatic dynamics simulations (more than 2000 trajectories) are used to explore the Z–E photoisomerization mechanism and excited‐state decay dynamics of two arylazopyrazole photoswitches. Two chiral S₁/S₀ conical intersections with associated enantiomeric S₁ relaxation paths that are barrierless and efficient (timescale of ca. 50 fs) were found. For the parent arylazopyrazole (Z8) both paths contribute evenly to the S₁ excited‐state decay, whereas for the dimethyl derivative (Z11) each of the two chiral cis minima decays almost exclusively through one specific enantiomeric S₁ relaxation path. To our knowledge, the Z11 arylazopyrazole is thus the first example for nearly stereospecific unidirectional excited‐state relaxation.     

Oligonucleotide charge states in negative ionization electrospray-mass spectrometry are a function of solution ammonium ion concentration

The charge state distribution for oligonucleotides detected using negative ionization electro-spray-mass spectrometry has been studied as a function of solution counterion concentration. In the absence of added buffer, an average charge state (Z) of ?7.2 is observed for a 10 µM aqueous solution of a 14mer DNA at pH 7.0, with [M ? 8H]^8? the most abundant ion. As the solution concentration of ammonium acetate increases from 0.1 to 33 mM, Z shifts to ?3.8 with [M ? 4H]^4? the most abundant charge state. The shift in most abundant charge state from [M ? 7H]^7? to [M ? 4H]^4? occurs abruptly between 1.0 and 10 mM NH₄OAc. Above 100 mM NH₄OAc, the value of Z plateaus at ?3.1, with [M ? 3H]^3? the most abundant charge state. The addition of 1–50 mM glycine to the analyte solution does not alter Z, suggesting that the changes in charge state observed by using ammonium acetate result from a solution equilibration of cations around the DNA strand, rather than nonspecific gas-phase proton transfers during the electrospray process. The fraction of neutralized phosphate groups reaches a maximum of 0.79 ± 0.03 independent of length and sequence.     

Z/E PHOTOISOMERIZATION OF UROCANIC ACID*

The E ? Z photoisomerization of the title compound (UA) (a naturally occurring sunscreen) has been studied in aqueous solution. At a UA concentration of 6mM and using 313nm excitation, φ_E→z= 0.52, φ_Z→E= 0.47 and the photostationary state is 34% E. Under these conditions, loss of UA is minimal. Low energy triplet quenchers fail to impede the isomerization, but the reaction can be induced by several triplet sensitizers. The E_T for UA is estimated to be approximately 55 kcal/mol.     

ChemInform Abstract: The Crystal Structures of Ni3+xSn4Zn and Ni6+xSn8Zn and Their Structural Relations to Ni3+xSn4, NiSn and Ni5‐δZnSn4.

Two new compounds, Ni_3+xSn₄Zn (x≈1.35, monoclinic, space group I2/m, Z = 2, single crystal XRD) and Ni_6+xSn₈Zn (x≈1.35, monoclinic, space group C2/m, Z = 2) are prepared by solid state reaction of the elements (Al₂O₃ crucible in evacuated quartz tubes, 1453 K).     

Standard potentials of the silver—silver tungstate,silver—silver phosphate,and silver—silver arsenate electrodes in water—dioxane and water—urea mixtures and related thermodynamic functions

The standard potentials of the silver—silver tungstate, silver—silver phosphate and silver—silver arsenate electrodes in four different compositions of water—dioxane and water—urea mixtures at seven different temperatures from 5 to 35°C have been determined from EMF measurements of cells of the type Ag(s), AgCl(s), NaCl(c)//Na_xZ(c/x), Ag_xZ(s), Ag(s), where x is 2 or 3, and Z is WO₄, PO₄ or AsO₄. These values have been used to evaluate the transfer thermodynamic quantities accompanying the transfer of 1 g ion of WO^2?₄. PO^3?₄ or AsO^3?₄ ion from the standard state in water to the standard state in water—dioxane or water—urea mixtures.     

σ‐Hole Bond Versus Hydrogen Bond: From Tetravalent to Pentavalent N,P, and As Atoms

Ab initio calculations were performed on complexes of ZH₄⁺ (Z=N, P, As) and their fluoro derivatives, ZFH₃⁺ and ZF₄⁺, with a HCN (or LiCN) molecule acting as the Lewis base through the nitrogen electronegative center. It was found that the complexes are linked by the Z? H???N hydrogen bond or another type of noncovalent interaction in which the tetravalent heavy atom of the cation acts as the Lewis acid center, that is, when the Z???N link exists, which may be classified as the σ‐hole bond. The formation of the latter interaction is usually preferable to the formation of the corresponding hydrogen bond. The Z???N interaction may be also considered as the preliminary stage of the S_N2 reaction. This is supported by the observation that for a short Z???N contact, the corresponding complex geometry coincides with the trigonal‐bipyramidal geometry typical for the transition state of the S_N2 reaction. The Z???N interaction for some of complexes analyzed here possesses characteristics typical for covalent bonds. Numerous interrelations between geometrical, topological and energetic parameters are discussed. The natural bond orbital method as well as the Quantum Theory of “Atoms in Molecules” is applied to characterize interactions in the analyzed complexes. The experimental evidences of the existence of these interactions, based on the Cambridge Structure Database search, are also presented. In addition, it is justified that mechanisms of the formation of the Z???N interactions are similar to the processes occurring for the other noncovalent links. The formation of Z???N interaction as well as of other interactions may be explained with the use of the σ‐hole concept.     

Polymeric Carbon Nitride/Reduced Graphene Oxide/Fe2O3: All‐Solid‐State Z‐Scheme System for Photocatalytic Overall Water Splitting

The charge transfer between hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatalysts (OEPs) is the rate‐determining step that controls the overall performance of a Z‐scheme water‐splitting system. Here, we carefully design reduced graphene oxide (RGO) nanosheets for use as solid‐state mediators to accelerate the charge carrier transfer between HEPs (e.g., polymeric carbon nitride (PCN)) and OEPs (e.g., Fe₂O₃), thus achieving efficient overall water splitting. The important role of RGO could also be further proven in other PCN‐based Z‐systems (BiVO₄/RGO/PCN and WO₃/RGO/PCN), illustrating the universality of this strategy.     

Synthese und Untersuchung von kohlenstoff‐ und stickstoffzentrierten (Z) trigonal‐prismatischen Wolframclustern des Formeltyps A[W6ZCl18]

Synthesis and Characterization of Carbon‐ and Nitrogen‐Centered (Z) Trigonal Prismatic Tungsten Clusters of the Formula Type A[W₆ZCl₁₈] Solid state reactions of tungsten(IV) chloride and carbodiimides (cyanamides) of lithium, sodium, calcium and silver yielded black crystalline powders of centered (Z) trigonal‐prismatic tungsten cluster compounds having the general formula A[W₆ZCl₁₈] with A = Li, Na, Ca, Ag and Z = C, N. Crystal structures of the corresponding compounds were investigated by powder and single‐crystal X‐ray diffraction. The nature of the central atom was studied by mass spectrometry and nuclear magnetic resonance spectrometry. It turned out that the central atom can be C (in the case of the lithium and sodium compound) or N (in the case of the sodium and calcium compound).     

Untersuchungen an elektronenleitenden Oxidsystemen. XXIII. Struktur und Eigenschaften stabiler Spinelle in den Reihen MzNiMn2−ZO4 (M=Li,Fe)

Investigations on Electronically Conducting Oxide Systems. XXIII. Structure and Properties of Stable Spinells in the Series M_ZNiMn_2?ZO₄ (M=Li, Fe) . Preparation and properties of the spinel compounds Li_ZNiMn_2?ZO₄ (0 ? Z ? 2/3) and Fe_ZNiMn_2?ZO₄ (0 ? Z ? 1) are reported. Compounds of the series Zn_ZNiMn_2?ZO₄ are involved. Different from the initial compound NiMn₂O₄ ( Z=0), the spinels with Z ≥ 1/6 submitted to annealing at 400?500°C appear to be in the thermodynamically stable state. For M=Li and Zn the upper limit of thermal stability of the compounds becomes reduced which, however, is not valid for the series with M=Fe. The alterations are interpreted in view of structural chemistry. The electrical properties are discussed in the frame of the polaron hopping model in close relation with the cationic distribution.     

Photochemistry of dienones-VIII: The low temperature photochemistry of (z)-β-ionone and its photo-isomers

The isomerizations of (E)-β-ionone 1, and of mixtures of the isomeric pyran 2 and (Z)-β-ionone 3 in CD₃OD as solvent on direct irradiation with λ 254 nm and on triplet photo-sensitization have been studied at temperatures ? - 50°, where the thermal isomerization between 2 and 3 is fully inhibited. The direct irradiation of 1 at -60° leads to 3 and (Z)-retro-γ-ionone 4 as primary products; 3 is subsequently rapidly photo-converted into mainly 2. Evidence is presented that 4 is also a primary photoproduct from both 2 and 3. The quantum yield ratio φ_2→4:φ_3→4 ?0.50. On starting with either 1 or mixtures of 2 and 3 the same photo-stationary equilibrium ratio of 1-3 is eventually obtained, viz 1:2:3 ?17:72:11. 4 is photostable relative to 1–3.The perdeuterobenzophenone triplet photo-sensitization with λ 366 nm at -50° of 1 leads to 3 as the sole primary product, which isomer on triplet sensitization yeilds both 1 and 2. The triplet sensitized conversion is much faster for (Z)- than (E)-β-ionone. On starting with either 1 or mixtures of 2 and 3, eventually the same photo-stationary state is obtained, viz 1:2:3?39:46:15. (Z)-retro-λ-ionone 4 is not formed in the triplet sensitized irradiations of 1,2 and 3 and in the direct irradiation it apparently results from the singlet excited state of the three substrates.The UV spectrum of the (unstable) (Z)-β-ionone 3 has been indirectly determined; its absorption occurs at lower wavelength and is of lower intensity than that of the (E)-isomer 1.     

Conformations of the Ten-membered Ring in 5, 10-Secosteroids. III: (Z)-3β- and (Z)-3α-Hydroxy-5, 10-seco-1 (10)-cholesten-5-one Esters and (Z)-5, 10-seco-1 (10)-Cholestene-3, 5-dione

(Z)-3β-Acetoxy- and (Z)-3 α-acetoxy-5, 10-seco-1 (10)-cholesten-5-one ( 6a ) and ( 7a ) were synthesized by fragmentation of 3β-acetoxy-5α-cholestan-5-ol ( 1 ) and 3α-acetoxy-5β-cholestan-5-ol ( 2 ), respectively, using in both cases the hypoiodite reaction (the lead tetraacetate/iodine version). The 3β-acetate 6a was further transformed, via the 3β-alcohol 6d to the corresponding (Z)-3β-p-bromobenzoate ester 6b and to (Z)-5, 10-seco-1 (10)-cholestene-3, 5-dione ( 8 ) (also obtainable from the 3α-acetate 7a ). The ¹H-and ¹³C-NMR. spectra showed that the (Z)-unsaturated 10-membered ring in all three compounds ( 6a , 7a and 8 ) exists in toluene, in only one conformation of type C ₁, the same as that of the (Z)-3β-p-bromobenzoate 6b in the solid state found by X-ray analysis. The unfavourable relative spatial factors (interdistance and mutual orientation) of the active centres in conformations of type C ₁ are responsible for the absence of intramolecular cyclizations in the (Z)-ketoesters 6 and 7 ( a and c ).     

Synthesis,photooxidation and Z ⇌ E photoisomerization of benzalpyrrolinones

In contrast to oxodipyrromethenes and bilirubin, benzalpyrrolinones (H, P-OCH₃, p-Cl, p-N(CH₃)₂ and o-CH₃) and α-pyridalpyrrolinones appear not to undergo dye-sensitized photo-oxygenation. They do, however, undergo an unsensitized E ? Z photoisomerization reminiscent of stilbene photoisomerization, and the photostationary state varies with substituent. Intramolecular H-bonding is implicated in the α-pyridalpyrrolinone isomerization. In each case, the Z isomers are the therrnodynamically more stable ones, but the corresponding E isomers have been isolated and characterized following photoirradialion.     

Structure-Reactivity correlations in the addition of water to aromatic substrates

The addition of water to 1-(4-Z-2,6-dinitrophenyl)-3-methylimidazolium chloride (Z = CN, NO₂, and CF₃) is catalyzed by carboxylate bases. There is a decrease in ρ with increasing basicity of the catalyst with ?ρ/?pK_BH = ?0.090. The results indicate that the mechanism of the reaction involves the addition of water to the aromatic substrates catalyzed by general bases. The changes in the structure-reactivity parameters with changing pK of the catalysts or Z in the substrate are rationalized in terms of changing bond lengths in the transition state. The activation parameters and the kinetic solvent isotopic effect were determined for the water, acetate, and OH^? catalyzed reactions and are consistent with the proposed mechanism.     

Reconstructing biochemical cluster networks

Motivated by fundamental problems in chemistry and biology we study cluster graphs arising from a set of initial states S í \mathbbZⁿ₊{S\subseteq\mathbb{Z}^n_+} and a set of transitions/reactions M í \mathbbZⁿ₊×\mathbbZⁿ₊{M\subseteq\mathbb{Z}^n_+\times\mathbb{Z}^n_+}. The clusters are formed out of states that can be mutually transformed into each other by a sequence of reversible transitions. We provide a solution method from computational commutative algebra that allows for deciding whether two given states belong to the same cluster as well as for the reconstruction of the full cluster graph. Using the cluster graph approach we provide solutions to two fundamental questions: (1) Deciding whether two states are connected, e.g., if the initial state can be turned into the final state by a sequence of transition and (2) listing concisely all reactions processes that can accomplish that. As a computational example, we apply the framework to the permanganate/oxalic acid reaction.     

Reactivite des dienes fonctionnels—VII : Cycloaddition du diphenylcetene avec les dienes conjugues

Functional conjugated dienes undergo 1,2 and 1,4 cycloaddition to diphenylketene to give cyclobutanones and dihydropyrans by a two-step ionic process. The electron delocalization of ketenophiles plays a prominent part in the mechanism but it is difficult to show a quantitative relation between second order rate constants and electronic distribution computed by the CNDO/2 method. The Z and E isomers of dienes give different adducts and rate constant ratio k_E/k_Z is related to transition state stabilization.     

Zeeman experiments on the 3Eu α 1A1g transition of platinum porphein in an n-alkane single crystal at 4.2

We report absorption spectra from the ground state to the photoexcited triplet state of platinum porphin (PtP) in single crystals of n-octane (C₈) and n-decane (C₁₀) at 4.2 K, with and without a magnetic field. For PtP in C₁₀ the same transition was studied in emission. From the experiments, values are derived of the spin-orbit coupling parameter Z, the crystal field splitting δ and the orbital angular momentum A for PtP in the two hosts: Z = 76 ± 2 cm^?1 (C₈, C₁₀), δ = 71 ± 1 cm^?1 (C₈), 55 ± 1 cm^?1 (C₁₀) and A = 1.6 ± 0.1 (C₈, C₁₀). For the ratio of the in-plane and the z-polarized electric dipole transition moments we obtain ¦Mx,y¦/¦Mz¦=76± 0.3 (C₈).     

Notiz über einen Perowskit der Zusammensetzung BaInO2,5

A Non-stoichiometric Perovskite: BaInO_2.5 Single crystals of BaInO_2.5 were prepared by solid state reactions. Out of X-ray diffraction measurements we found a perovskite structure (a = 4.219(2) Å space group O_h¹–Pm3m, Z = 1). BaInO_2.5 is a rare example of a defect perovskite compound with a pure M³⁺ metal ion and an alkaline earth element.     

Heteroleptische Diorganylzink-Verbindungen mit einem Bis(trimethylsilyl)phosphanido-Substituenten

Heteroleptic Diorganylzinc Compounds with a Bis(trimethylsilyl)phosphido Substituent Dialkylzinc ZnR₂ (Me, Et, iso-Pr, nBu, tBu, CH₂SiMe₃) reacts with one equivalent of bis(trimethylsilyl)-phosphine in carbohydrates to the heteroleptic compounds RZnP(SiMe₃)₂; dependent from the steric demand of the alkyl group R the derivatives are dimeric or trimeric in solution as well as in the solid state. Monomeric bis(trimethylsilyl)phosphido-tris(trimethylsilyl)methylzinc yields from the reaction of lithium tris(trimethylsilyl)methanide and lithium bis(trimethylsilyl)phosphide with zinc(II) chloride. Bis(trimethylsilyl)phosphido-methylzinc crystallizes in the orthorhombic space group P2₁2₁2₁ with {a = 1 007.6(1); b = 1 872.3(3); c = 2 231.0(4) pm; Z = 4} as a trimeric molecule with a central cyclic Zn₃P₃ moiety in the twist-boat conformation. Bis(trimethylsilyl)phosphido-n-butylzinc, that crystallizes in the orthorombic space group Pben with {a = 1 261.7(2); b = 2 253.0(4); c = 1 798.9(2) pm; Z = 4}, shows a simular central Zn₃P₃ fragment. The sterically more demanding trimethylsilylmethyl substituent leads to the formation of a dimeric molecule of bis(trimethylsilyl)phosphido-trimethylsilylmethylzinc {monoklin, P2₁/c; a = 907.2(4); b = 2 079.8(8), c = 1 070,2(3) pm; β = 103,48(1)°; Z = 2}. Bis(trimethylsilyl)phosphido-iso-propylzinc shows in solution a temperature-dependent equilibrium of the dimeric and trimeric species; the crystalline state contains a 1:1 mixture of these two oligomers {orthorhombisch; Pbca; a = 1 859.0(3); b = 2 470.9(2); c = 3 450.7(3) pm; Z = 8}. The Zn? P bond lengths vary in a narrow range around 239 pm, the Zn? C distances were found between 196 and 203 pm.     

Macrocycles with 1,2-dicyano-1,2-dithioethene units

Abstract

The new unsaturated macrocyclic tetrathioethers (Z,Z)-4 (n = 0), (Z,Z)-5 (n = 1), (Z,Z)-6 (n = 2) and (Z,Z)-7 (n = 3) were synthesized by the cyclization of (Z)-disodium-1,2-dicyanoethene-1,2-dithiolate (Z)-3 with ω,ω'-dibromoalkanes BrCH₂CH₂(CH₂)nCH₂Br (n = 0;1;2;3) on refluxing in dioxane in yields up to 15%. By reaction of the dithiolate (Z)-3 with 1,3-dibromopropane the unsaturated hexathioether (Z,Z,Z)-6 was also obtained. By the cyclization of dithiolate (Z)-3 with 1,5-dibromopentane and 1,6-dibromohexane the (Z,E)- and (E,E)-isomers, respectively, were formed in addition to the (Z,Z)-isomers. The (E,E)- and (Z,E)-isomers are photochemically convertable to the corresponding themodynamically more stable (Z,Z)-isomers by irradiation with UV-light. The (E,E)-isomers can be synthesized in a straightforward manner using the (E)-disodium-1,2-dicyanoethene-1,2-dithiolate (E)-3. Crystal structures of (Z,Z)-5, (Z,Z)-6, (E,E)-6, (Z,E)-7 and (E,E)-7 are reported.