Refine
Year of publication
Document Type
- Article (354) (remove)
Language
- German (354) (remove)
Has Fulltext
- yes (354)
Is part of the Bibliography
- no (354) (remove)
Keywords
- Crystal Structure (23)
- Synthesis (14)
- ESR Spectra (12)
- IR Spectra (10)
- PE Spectra (7)
- Phase Diagrams (7)
- X-Ray (7)
- Addition Compounds (6)
- X-Ray Structure Analysis (6)
- Cyclic Voltammetry (5)
Institute
- Biochemie und Chemie (354) (remove)
From theoretical considerations a dynamically distorted octahedron as a result of vibronic coupling between the ground state and the first excited state should exist for 14 electron AX6E systems like TeX62- . A high symmetry crystal field yielding at least a center of symmetry for the Te position stabilizes this fluctuating structure, otherwise statical distortion will be observed. From X-ray diffraction experiments on antifluorite type compounds A2TeX6 (A = Rb. Cs: X = Cl, Br) the averaged structure (m3̅m symmetry) of the anions was found even at very low temperatures. The thermal parameters are not significantly different from those of similar SnX62 compounds. Distortions therefore are very small and are evident from FTIR spectroscopic measurements only. Here very broad T1u-deformation vibration bands are observed down to temperatures <10 K without splitting: Astatically distorted species could not be frozen out. In contrast to XeF6 for TeX62- the energy gap between the threefold, fourfold or sixfold minima of the potential surface (according to the symmetry of one component of the T1u-vibration) is very small and shifted to temperatures lower than reached with the devices used for these experiments.
The synthesis of [Ph4As+]2[Cl4Re(NS)(NSCl)2-] · CH2Cl2 (4) from the reaction of S4N4, Cl4ReN, and Ph4AsCl is reported. CH2Cl2 is used as solvent. The reaction of S4N4 with Re2Cl10 similarly leads to the salt [Ph4As+][Cl2ReNS-] (5) in a smaller yield. 4 crystallizes in the triclinic space group P1̅ with Z = 2, a - 10.434(2), b = 12.1454(6), c = 21.125(2) Å, a = 81.210(6), β = 86.70(1), γ = 76.624(8)°.
Die Sonne strahlt weltweit pro Tag genügend Licht ein, um den Weltenergiebedarf für ein ganzes Jahr abzudecken. Somit ist sie die Quelle aller erneuerbarer Energien, denn neben der Erzeugung von Elektrizität aus Licht (Photovoltaik) regelt sie die Gezeiten und damit auch Wind und Wellen, die bei der Windkraft und in Gezeitenkraftwerken genutzt werden. Außerdem liefert sie die Energie für die Photosynthese in nachwachsenden Rohstoffen. Es gibt diesbezüglich nur ein grundlegendes Problem: Erneuerbare Energien fi nden wir in ausreichender Menge vor allem an Stellen mit mangelnder Infrastruktur. Sonnenenergie gibt es am meisten in der Wüste, Wind auf dem Meer und Biomasse im Dschungel. An Orten hoher Industrialisierung und damit auch hoher Bevölkerungsdichte ist für die »Erneuerbaren « so gut wie kein Platz. Es gibt demnach kein Energieproblem, aber ein Problem der Energiespeicherung und des Energietransportes.
Seit der TIMSS- und PISA-Studie sind sie wieder einmal Thema der bildungspolitischen Diskussion: die naturwissenschaftlichen Fächer in unseren Schulen. Deutschen Schülerinnen und Schülern wird bescheinigt: Sie haben nur mangelnde Kenntnisse, verstehen zu wenig, sind nicht recht in der Lage, Fragestellungen methodisch anzugehen, und denken zu wenig darüber nach, wie sie naturwissenschaftliche Probleme lösen könnten. Aber auch viele Erwachsene geben offen zu, besonders von den »harten« Naturwissenschaften wie Chemie wenig zu verstehen und sich nie besonders dafür interessiert zu haben. Wie kommt es, dass eine Wissenschaft, die wesentlich zum Verständnis unserer stofflichen Umwelt beiträgt und deren praktische Anwendung unser tägliches Leben in hohem Maße beeinflusst, auf ein so geringes Interesse stößt?
As we have shown in a recent paper, the principle of competition between "statistical" and "chemical" mixing represents a molecular thermodynamic approach to all known types of phase separation. This principle is effective if the contributions of two independent spontaneous processes enter into the thermodynamic potential by which the resulting equilibrium state of the system is determined. This is equivalent with the statement that two different forms of entropy exist which are not interchangeable, and for which the law of increasing entropy independently must be valid. As "cooperativity" is introduced by this principle, critical phenomena may be described by simple equilibrium models in which only nearest-neighbour interactions are considered.
Starting from the molar Gibbs free energy GM of the most simple binary equilibrium model z = 1 with nearest-neighbour pairs, nonclassical critical-point exponents α = 0.33 of the molar heat capacity, β = 0.33 of the coexistence curve, γ = 1.33 of the isothermal compressibility, and δ = 4.33 of the critical isotherm, are derived, which are consistent with the well-known exponent in equalities. These non-classical critical-point exponents are independent of the chemical nature of the particles because they are obtained by applying thermodynamic arguments on the coupling constant τ, by which the contribution of "statistical mixing" to GM is weighted.
The enthalpies of mixing at 25° of diethyl ether, di-n-propyl ether, di-n-butyl ether, di-isopropyl ether, propylene oxide, tetrahydrofuran, and tetrahydropyran with chloroform are determined by an isothermal titration method. As a result, the functions HM-f(N CHCl3) are obtained with a step width of 0.025 of the mole fraction and a relative accuracy of 1 per cent or better. Evaluation of the heat of mixing data by means of equilibrium models ("ideal associated mixture") shows that the systems of aliphatic ethers with chloroform behave rather precisely as one-step equilibria of the type A + B = AB (A = ether; B = chloroform). In the systems of cyclic ethers with chloroform, a second equilibrium step, AB + B = AB2 , must be considered, the importance of which decreases with increasing ring size of the ether. The equilibrium data calculated for the seven ether-diloroform systems are discussed.
This paper contains further applications on symmetrical liquid mixtures of the molecular thermodynamic theory which has been developped in part I of this series. The essential feature of this theory is the superposition of "chemical" and “random” exchange equilibria between “complexes” formed by a given molecule and its z nearest neighbours, thus allowing a unified treatment of the thermodynamic phenomena in binary liquid mixtures using the equilibrium constant K of the ideal law of mass action and the energy w of pair interactions as parameters.
The temperature and pressure dependences of K and the evaluation of experimental excess enthalpy and excess volume data are treated. Formulas and examples for the calculation of K and w from isothermal and non-isothermal vapour-liquid equilibrium data are given. The conditions for azeotropy with minimum or maximum vapour pressure, resp., are derived. Melting curves for a symmetric eutectic system with superposed miscibility gap are discussed. Further models for partially miscible liquids with competing self-association and complex-formation are treated showing the phenomenon of two separated miscibility gaps.
A thermodynamic theory of liquid mixtures based on a simple molecular model is developed which describes the equilibrium state as the result of a coupling between a "chemical" and a "statistical" equilibrium. The intermolecular interactions are taken into account by considering "complexes" formed between a given molecule and its z nearest neighbours. The equilibrium mole fractions of these complexes are calculated by application of the ideal law of mass action to an appropriate set of "exchange equilibria". Formulae for the excess functions GE and HE and for the activities of the components are derived for the cases z=1 and z=4. GE depends on an equilibrium constant K describing the deviation from random distribution of the equilibrium mole fractions of the complexes. HE depends on K and on an energy parameter w which is related to differences of pair interactions. K and w are independent parameters, and there is no limitation in respect to amount and sign of the excess functions. The conditions for the existence of a critical solution point are formulated; at this point GE has a value of about 0.56 R T. If a model with two equilibrium constants is used allowing for instance competition between "self-association" and "complex-formation", the existence of closed miscibility gaps becomes possible. Closed miscibility curves are calculated and the conditions for their appearance are discussed. The relations between this theory and Guggenheim's statistical lattice theory of symmetrical mixtures are pointed out.
Phasentrennung als Folge der Konkurrenz zwischen "statistischer" und "chemischer" Vermischung
(1977)
The fact that common thermodynamic conditions are valid for all known types of critical phases (liquid-liquid, liquid-gas, and "gas-gas") suggests that a common principle for the interpretation of material phase instability from a molecular point of view must exist. In this paper we show that the principle of competition between "statistical mixing" (i. e. random mixing) and "chemical mixing" (i. e. mixing effected under the influence of chemical interactions) can give this common inter pretation. If the equilibrium states resulting from both types of mixing are sufficiently different, phase separation occurs. We refer to our earlier papers (since 1972) in which we have applied this principle to describe liquid-liquid phase equilibria by "chemical" models, using the equilibrium constants of exchange equilibria between nearest-neighbour complexes as a measure of "chemical" mixing. In this paper we show that the well-known reduced gas-liquid coexistence curve, T/Tc =f(q/qc), can accurately be fitted by a very simple "mixture" model of molecules A with "vacan cies", provided that the contributions of both statistical and chemical mixing are incorporated into the formula for GE. From a discussion of the application to "gas-gas" phase equilibria in the hyper critical region it results that the weight factor r, by which the contribution of statistical mixing enters into GE, must depend on the density of the gas mixture. Phase separation can only occur if, by increasing pressure, the contributions to GE of statistical and chemical mixing have reached the same order of magnitude. From an attempt to apply the same principle to solid-liquid equilibria it is shown under which external conditions a critical point for this type of phase transition can be expected.
The theoretical IR-frequencies of sulphurdifluoride are computed from force constants, which are evaluated by means of molecules with S-F-bonds: ν1 = 795 ± 10cm-1, ν2 = 430± 5cm-1, ν3= 830 ± 10cm-1.
The two-electron reduction of tetraphenyl-p-quinodimethane M via its radical anion M⊖ to its dianion M⊖⊖ is explored both by cyclovoltammetry and ESR/ENDOR spectroscopy. Contact of the diglyme solution with added 15-crown-5 under aprotic conditions with a sodium metal mirror yields black crystals of a solvent-separated contact ion triple [M⊖⊖][Na⊕(OCH2CH2)5(H3CO(CH2CH2O)2CH3)]2. The two-electron-insertion into the pquinodimethane derivative R2C⊖=C(HC=CH)2C=CR2 changes its structure drastically to that of a twofold carbanion substituted benzene, R2C⊖ -(C6H4)- ⊖CR2. MNDO calculations provide a rationale for both the tremendous solvation of a Na⊕ center coordinated to seven oxygen centers of 15-crown-5 and of one diglyme molecule and the structural changes as well as the charge distribution in the unique Tetraphenyl-p-quinodimethane dianion (H5C6)2C⊖-(C6H4)- ⊖C(C6H5)2, in which the two negative charges are largely localized at the carbanion center of the benzene -substituents.
Tetraphenylbutatriene is reduced under aprotic conditions to its ESR/ENDOR-spectroscopically characterized radical anion and to its dianion, with both electron transfers quasireversible according to cyclovoltammetric measurements. The alkali cation salts, the red contact ion pair [(H5C6)4C4·⊖][Na⊕ (H3COCH2CH2OCH3)3] and the dark violet contact ion triple [(H5C6)4C4⊖⊖][Li⊕(H3COCH2CH2OCH3)3]2 can be prepared by single electron reduction at a sodium metal mirror or by twofold de-protonation of 1,1,4,4-tetraphenylbutyne-2 using lithium-n-butyl. Their single crystal structures as well as that of the parent acetylene have been determined at low temperatures. The essential structural changes observed are the twisting of both molecular halves (H5C6)2CC relative to each other with increasing negative charge. The simultaneously resulting bond alternancy >C = C = C = C< → >C⊖ - C ≡ C⊖ - C < within the cumulene chain is discussed based on MNDO calculations for the structures determined.
In the pyrolysis of 1,2,3-benzoselenodiazole using a short-distance furnace, a short-lived intermediate is detected photoelectron spectroscopically. Mass spectra recorded under similar conditions suggest an isomer C6H4Se rearranging to the more stable final product 6-fulveneselone. The ionization pattern obtained by computerized spectra stripping is assigned to benzselenirene by molecular radical cation state comparison based on MNDO calculations.
Thermal decompositions of azo compounds in the gas phase under reduced pressure are further investigated using photoelectron spectroscopic gas analysis. Passing diallyl, diphenyl and phenylmethyl derivatives either through a short-pathway pyrolysis (SPP) apparatus or through an external thermal reactor (ETR) results in the following fragmentations: Under nearly unimolecular conditions (SPP, 10-4 mbar pressure), diallyldiazene decomposes above 600 K to N2 and hexadiene-1,5 with the allyl radical as a detectable intermediate. The PE spectra recorded for diphenyldiazene above 1000 K (ETR, 1-2 mbar pressure) show N2, benzene, as well as traces of diphenyl. Phenylmethyldiazene yields above 800 K (SPP) predominantly N2, toluene, diphenyl and ethane with the methyl radical as the only detectable intermediate. Insertion of quartz wool into the pyrolysis tube (ETR) lowers the fragmentation temperatures, and in addition, above 850 K, HCN and aniline are PE spectroscopically identified. Surprisingly, this second reaction channel can be heterogeneously catalyzed: phenylmethyldiazene decomposes under 10-2 mbar pressure at a [Ni/SiO2] catalyst surface selectively to HCN and aniline.
Photoelektronen-Spektren und Moleküleigenschaften, 110 [1,2]. Tricyanmethan-Derivate X—C(CN)3
(1987)
The photoelectron spectra of tricyanomethane derivatives X-C(CN)3 with substituents X = H, CH3, Br and C6H5 have been recorded and are assigned based on MNDO calculations as well as on radical cation state comparison with the iso(valence)electronic P(CN)3, within the series of cyanomethanes H4-nC(CN)n, and with each other. For HC(CN)3, no traces of the isomeric dicyano, ketimine HN = C=C(CN)2 are detected in the gas phase. Tricyanomethylbenzene, H5C6-C(CN)3, exhibiting the highest first ionization energy of any known singly acceptor substituted phenyl derivative, demonstrates the tremendous electron withdrawing effect of the -C(CN)3 group.
The HCl elimination from β-chloroethyl azide (1-azido-2-chloroethane) over potassium tert. butanolate at 350 K in a low pressure flow system is optimized using PE spectroscopic real-time gas analysis. The highly explosive vinyl azide formed can be purified by cool-trapping the by-products. Its subsequent and virtually hazard-free pyrolysis yields 2H-azirine, which can be isolated at temperatures below 240 K.
In contrast, the direct pyrolysis of β-chloroethyl azide requires temperatures above 710 K and results in a simultaneous split-off of both HCl and N2, yielding acetonitrile as the main thermolysis product. No intermediates such as β-chloroethanimine or ketenimine are observed, a result which is interpreted in terms of chemical activation.
Trifluoromethyl azide decomposes in a low-pressure flow system at rather high temperatures by splitting off N2. The nature of the resulting products depends largely on the wall material of the pyrolysis tube: using molybdenum above 1120 K, FCN is observed exclusively. Neither F2C=NF nor F3C-N=N-CF3 can be detected as intermediates by comparing their PE spectra with those continuously recorded while increasing the temperature. F3C-N = N - CF3 fragments already at 870 K to give N2 and F3C-CF3. The PE spectra of F3CN3 and F2C=NF are assigned based on MNDO calculations.
The PE spectra of the nitrogen-rich title compounds cyanogen azide NC-N3, azodicarbonitrile NC - N = N - CN, azidoacetonitrile NC - H2C - N3, tetrazolo[1,5-a]pyridine (H4C5N)(N )3 and trimethylenetetrazole (H2C)3(CN4) are presented and assigned by radical cation state comparison with related compounds or by Koopmans’ correlation with MNDO eigenvalues. In a low pressure flow system the compounds decompose at higher temperatures, with elimination of the thermodynamically favorable N2 molecule. PE-spectroscopic real-time analysis reveals as further products: NC - N3 → C∞, NC - N = N - CN → NC - CN , NC - H2C - N3 → 2HCN (+ traces NC - HC = NH?) and (H2C)3(CN4) → H2C = N - CN + H2C = CH2. For tetrazolo[1,5-a]pyridine, a preceding ring opening to the corresponding 2-azidopyridine is observed.
The reversible one-electron insertion into mono- and 1,4-di-substituted benzene derivatives is favored by dialkoxyboron and especially by dialkylboron groups. The assumption that it should be the symmetric e2u benzene molecular orbital which is occupied in the resulting radical anions can be supported by comparison of ESR coupling constants.
The sodium salt of the most simple polynitro-substituted hydrocarbon anion. Na⊕⊖C(NO2)3, (for a hazard warning cf. [***]) crystallizes from ether solutions without and with addition of 18-crown-6 either in a polymer band. [(Na⊕⊖C(NO2)3)dioxane]∞, or as a solvent- separated ion pair, [(Na⊕/18-crown-6)(THF2]⊕[(Na⊕/18-crown-6)(O2N-C⊖(NO2)2)2]⊖. The Na⊕ cations are each 8-fold coordinated in hexagonal bipyramidal arrangement. According to extensive quantum-chemical calculations based on the structure coordinates, the formation of these novel salts can be traced back to the charge distribution in the anions ⊖C(NO2)3. which due to negatively charged oxygen centers are favorable complex ligands. The structure determining effects of solvation are discussed.
For the first time, 107,109Ag ENDOR measurements in solution are reported. In addition, the formation of the known paramagnetic contact ion pair [Ag⊕(PR3)2(R2H2C6O2·⊖] on reduction of 3,5-di(tert-butyl)-o-benzoquinone in THF solution containing soluble silver salts and triphenylphosphine is studied by cyclic voltammetry.
Conditions for ENDOR measurem ents of organosulfur radical cations are discussed and tested. The one electron oxidation of a variety of aromatic sulfur com pounds comprising benzene-1,2-dithiole, 1,4-dithiine, thianthrene and diphenylsulfide derivatives as well as 33S isotope-marked bis(2,5-dimethoxyphenyl)disulfide is accomplished using the oxygen-free, powerful and selective AlCl3/H2CCl2 reagent. Partly with substantial structural changes, paramagnetic M⊕ species of 1,2-benzodithiete, 1,4-dithiine, thianthrene and diphenyl sulfide result. Their temperature-dependent ENDOR signal patterns provide numerous information e.g. on radical cation structure and dynamics, on the rather high sulfur spin populations or on the spin rotation interaction dominated relaxation behaviour. Accordingly, to obtain optimum ENDOR effects in organosulfur radical cations low temperature measurements are required, and especially for still undiscovered 33S ENDOR couplings, small g factor anisotropies and 33S spin densities appear to be necessary.
Ion pairs of 1,10-phenanthrolin-5,6-dione radical anion [M · ⊖Me⊕n] ·⊕(n−1) with Me⊕n = Mg⊕⊕, Ca⊕⊕, Sr⊕⊕, Zn⊕⊕, Cd⊕⊕, Pb⊕⊕ and La⊕⊕⊕ are advantageously prepared in aprotic DMF solution containing appropriate metal salts Me⊕nX⊖ by using the ‘mild’ single-electron reducing agent tetra(n-butyl)ammonium-boranate R4N⊕BH4⊖ . For comparison, the ‘naked’ radical anion with the largely interaction-free [K⊕(2.2.2)-cryptand]⊕ counter cation is chosen, which is formed on reduction with potassium in THF solution of (2.2.2)-cryptand. Addition of excess Na⊕[B(C6H5)4]⊖ to the reduction solution only yields a solvent-separated ion pair (M · ⊖)DMF ··· (Na⊕)DMF, whereas in the presence of multiply charged counter cations Me⊕n the respective contact ion pair radical cations [M · ⊖Me⊕n] · ⊕(n−1) are formed. Their g values decrease with increasing nuclear charge of Me⊕n and their metal-s-spin densities increase with the effective counter cation charge n⊕/rMe⊕n. The ESR /ENDOR data recorded suggest Me⊕n complexation by the δ⊖OC -COδ⊖ chelate tongs and the ion pair stability, which is modified by the dielectric properties of the solvent used, may be rationalized by the Coulombic attraction between the radical anion M · ⊖ and the counter cations Me⊕n.
Reduction of naturally occurring para-and ortho-benzoquinone derivatives M to their respective radical anions M·⊖ can be accomplished under largely aprotic conditions either by cautious low-temperature reaction in THF containing an excess of (2.2.2) cryptand at a potassium mirror or by using the "mild" single electron transfer reagent tetrabutylammonium boranate R4N⊕BH4⊖ in DMF. On addition of soluble alkali tetraphenylborates Me⊕[B(C6H5)4]⊖ , their hitherto unknown radical ion pairs [M·⊖ Me⊕]· and/or triple ion radical cations [Me⊕M·⊖Me⊕]·⊕ form, which might be of biological relevance in molecular carrier and "turn off -turn on" switch processes. On addition of metal perchlorates Me⊕n(ClO4⊖)n with multiply charged counter cations Me⊕n the respective paramagnetic species [M·⊖Me⊕n]·(n-1)⊕ result. Assuming exclusive one-electron transfer reductions without any redox fragmentation reactions, ESR, ENDOR and GENERAL TRIPLE spectra are presented and discussed for the following radical anions and radical ion pairs: mitomycin C (M·⊖ and [M·⊖Mex⊕]·(x-1)⊕ with Me⊕ = Li⊕, Na⊕), streptonigrine (M·⊖ and [M·⊖Lix⊕]·(x-1)⊕), Entobex® (M·⊖ and [M·⊖Me⊕n]·(n-1)⊕ with Me⊕n = Li⊕, Na⊕, Cd⊕⊕, (H5C6)2Tl⊕) as well as brucinequinone ([M·⊖ Me⊕n]·(n-1)⊕ with Me⊕n = Li⊕, Cd⊕⊕, Pb⊕⊕, La⊕⊕⊕).
The radical anion of dimesityltetraketone (ERed, I = -0.40 V) is easily generated in THF by potassium mirror/[2.2.2]-cryptand reduction. Its contact ion pairs with Na⊕, Cs⊕ and Ba⊕⊕ counter cations, prepared in THF solution by single electron transfer from the respective metals, are characterized by their ESR/ENDOR spectra, which exhibit temperature-dependent metal couplings of aNa⊕ = 0.061 mT (190 K), aCs⊕ = 0.021 mT (190 K), and aBa⊕⊕ = 0.145 mT (295 K).
The structurally different radical anions M⊖ of peralkylated 1-sila-2,5-diazacyclopentane-3,4-dithione and of tetrakis(isopropylthio)-p-benzoquinone are generated by reduction with potassium/2.2.2-cryptand under aprotic conditions in THF solution. On addition of Li⊕B(C6H5)4⊖, both form hitherto elusive sulfur-containing contact ion pairs, which are characterized by their ESR/ENDOR spectra.
Cyclovoltammetric measurements of solutions containing the rather basic tetra-(2′-pyridyl)pyrazine allow to detect even traces of water and thus can be used as a touchstone for aprotic (cH⊕ < 1 ppm) conditions. On exchange of the “innocent” tetrabutylammonium R4N⊕ as supporting electrolyte cation by “interactive” ones such as Li⊕) or Na⊕, considerable changes in the reduction potentials are observed due to ion pair formation.
Tetraphenyl-p-benzoquinone, according to its single crystal structure, shows some steric congestion: its quinone ring is distorted by 7° to a chair conformation, and its phenyl substituents are twisted around their CC axes between 46° and 72°. The half-wave reduction potentials of -0.57 and -1.25 V in acetonitrile confirm negligible π interaction of the phenyl substituents. Addition of alkalimetal tetraphenylborate salts lowers the second reduction potential due to contact ion formation, which can be confirmed by UV/VIS spectra recorded under aprotic conditions. Extensive ESR/ENDOR investigations prove the formation of the following species in THF solution: Tetraphenyl-p-benzosemiquinone radical anion contact ion pairs [M·⊖ Me⊕solv]' (Me⊕: Li⊕, Na⊕, Rb⊕, Cs⊕) and contact triple ion radical cations both with identical cations [M·⊖ (Me⊕solv)2]·⊕ (Me⊕: Li⊕, Na⊕, Cs⊕) and different cations [M·⊖ (Li⊕solv)(Me⊕solv)]·⊕ (Me⊕: Na⊕, Cs⊕). Addition of crown ethers can lead to external solvation of the Me⊕ counter cations, whereas cryptands form internal solvation complexes. The radical anion of 2,6-diphenyl-p-benzosemiquinone adds cations at its phenyl-free molecular half. The radical anion salt [tetraphenyl-p-benzosemiquinone·⊖ (Na⊕(tetrahydropyrane) 2)] could be crystallized and its structure determined at 200 K. In agreement with the Hirota sign rules for contact radicals in solution, the Na⊕ ion is found 62 pm above the π plane and 29° outside the axis of the CO bound, which is elongated due to one-electron reduction by 5 pm to 127 pm.
The one-electron transfer to large π-delocalized hydrocarbons provides an interesting possibility to crystallize solvent-separated ion-pair salts containing optimally solvated cations. Accordingly, the reduction of 9.9′-bianthryl in aprotic 1.2-dimethoxyethane (DME) solution at a sodium metal mirror allows to grow dark blue, brick-like crystals of its radical anion and threefold DME-solvated sodium cation. The structure of the radical anion is very similar to that recently published for the neutral molecule. According to AM 1 enthalpy hypersurface calculations based on the structural data, the torsion angle between 60° and 120° is determined by the lattice packing and the negative charge is -π-delocalized predominantly within only one anthracene subunit. The counter cation [Na⊕(DME)3], reported only three times so far, shows a sixfold propeller-like coordination of approximate D3 skeletal symmetry with contact distances Na⊕···O between 232 and 243 pm and angles ≮ONa⊕O varying between 69° and 159°. Due to the small repulsion between the chelating DME molecules, the isodesmically calculated Na⊕ solvation enthalpy is more negative than that of the analogous tetrahydrofuran complex [Na⊕(THF)6] - as confirmed by the laboratory experience that salts of less stable anions are preferentially crystallized from a strongly cation solvating DME solution.
The photoelectron (PE) spectra of bis(dialkylamino) acetylenes R2N-C≡C-NR2 and of tetrakis(dialkylamino) allenes (R2N)2C=C=C(NR2)2 with R = CH3, C2H5 exhibit characteristic ionization patterns which are assigned to π radical cation states of the two molecular halves twisted against each other. The low first ionization potentials between 7.0 eV and 7.7 eV stimulated attempts to oxidize using AlCl3 in H2CCl2 or D2CCl2. The hyperfine structured ESR spectra observed can be unequivocally assigned to the ethylene radical cations R2N-HC=CH -NR2˙⊕ which are formed from the obviously non-persistent species R2N-C≡C-NR2˙⊕ via a hydrogen transfer. During the oxidation of the dialkylamino-substituted allenes no paramagnetic intermediates could be detected, presumably due to a rapid dimerisation of the allene radical cation (R2N)2C=C=C(NR2)2˙⊕.
Novel radical anions of trimethylstannyl substituted naphthalenes and their ESR spectra are reported. Both 119 Sn and 117 Sn coupling can be assigned unequivocally. The perturbation of π systems by R3X substituents of group IV b elements X = C, Si, Ge, Sn and Pb is discussed with respect to photoelectron ionization potentials, charge transfer excitations, half-wave reduction potentials and ESR spin distribution.
UV/VIS and ESR spectra of electron transfer reaction products in aprotic (cH⊕ < 0,1 ppm) solution can be measured in an especially designed and sealed glass apparatus and provide information on unknown facets of the microscopic pathway through the network of interdependent equilibria. For tetraphenyl-p-benzoquinone in tetrahydrofuran, single-electron reduction by a sodium metal mirror produces a red solution and, unexpectedly, after addition of 2.2.2. cryptand, contact with a potassium metal mirror generates a green (!) one. For both, ESR/ENDOR spectra prove the presence of tetraphenyl-p-benzoquinone radical anion. UV/VIS measurements provide the clue: In the equilibrium revealed by repetetive spectra recording, M·⊖solv + Me⊕solv ⇄ [M·⊖···Me⊖]solv, the radical anion is green (vm = 16900 cm-1) and the contact ion pair red (vm=18900 cm-1 ). On ion pair formation, therefore, the excitation energy of the radical anion increases by 0.25 eV.
In an especially designed and sealed glass apparatus, a combination of UV/VIS and ESR spectroscopy measurements are performed to follow electron transfer reactions in aprotic (cH⊕ < 0,1 ppm) solution. For the sodium metal reductions of the tetracyano-substituted title compounds, the novel technique provides the following detailed information: 1,2,4,5- tetracyanobenzene is uniformly reduced to its radical anion, for which additional geometryoptimized MNDO calculations predict an already significant cyanine disortion. For 7,7,8,8- tetracyano-p-quinodimethane, UV/VIS band shape analysis allows to detect in the saturated THF reduction solution the 16300 cm-1 absorption of the donor/acceptor complex formed in the equilibrium TCNQ·⊖ + TCNQ ⇆ {TCNQ·⊖···TCNQ}, which according to a literature search has been crystallized and structurally characterized in paramagnetic salts such as [Me2⊕ (TCNQ·⊖)2(TCNQ)].
The reduction potentials of 40 aromatic nitro compounds Rπ(NO2)n with Rπ = benzene, naphthalene, anthracene, fluorene and carbazole and n = 1 to 4 nitro groups are determined by cyclic voltammetry in DMF under aprotic conditions. The perturbation by the strongly electron accepting substituents can be rationalized via correlation with HMO eigenvalues. Based on reversibility criteria, the electrochemical behaviour is discussed and the compounds are classified with respect to reversible or irreversible one-electron transfer as well as up to 4 (quasi)-reversible reduction steps. The CV data measured can be used to predict redox reactions of aromatic nitro compounds in inert solvents.
1,4-Bis(trimethylsiloxy)benzene has been crystallized both by vacuum sublimation and from «-heptane solution, which each yielded colourless plates with identical monoclinic unit cell dimensions (P2/n, Z = 4). The conformation of C[ symmetry shows the two (H3C)3SiO-substituents to be conrotationally twisted around the O-( C6H4)-O axis by dihedral angles o f ± 60°. According to the photoelectron spectroscopic ionisation pattern and its Koopmans’ assignment, IEVn = -εJAM 1, by AM 1 eigenvalues, the gas phase structure should also be of C, symmetry. The results of geometry-optimized MNDO , AM 1 or PM 3 calculations for the monosubstituted derivative H5C6-OS i(CH3)3 are compared with respect to the quality of their fit to the measured data.
Crystals of lemon yellow dipotassium nitranilate and of yellow disodium nitranilate dihydrate have been grown and their structures determined at 290 and 200 K. The six-member- ed, O2N-disubstituted rings show a pronounced cyanine distortion with all four CO bonds identical and the two (OCC(NO2)CO)⊖ chains connected by single CC bonds of each 156 pm length. In the anhydrous K⊕ salt, the ring is planar, but in the Na⊕ hydrate salt it exhibits a twist conformation. Quantum chemical calculations allow to reproduce the structure in every detail, demonstrate strong charge alternation along the cyanine chains with considerable delocalization into the O2N acceptor substituents, and suggest that the rather long connecting CC bonds contain positively charged carbon centers on both ends. In addition, metal ion coordination effects as well as the rather high pKa value of nitranilic acid are rationalized.
Chelate complexes of 1,2-dimesitoylbenzene radical anion with alkali metal cations exhibit in aprotic solution extremely large ESR /ENDOR metal coupling constants. For rationalization, structures of both the neutral molecule (H3C)3H2C6 - CO - C6H4 - CO - C6H2(CH3)3, in which the two carbonyl groups are twisted out of the benzene ring plane by dihedral angles of ± 3̄7̄°, and a sodium contact ion quadruple have been determined. One of the dimers [dimesitoylbenzeneH⊖ (Na⊕H2N H2C - CH2NH2)]2, although generated by Na metal mirror reduction of 1,2-dimesitoylbenzene in aprotic DME solution with added ethylendiamine for better electron transfer, surprisingly contains two 245 pm short (!) hydrogen bridges ⊖O ··· (H)O and in addition two solvation bridges e ⊖O ··· Na⊕(H2NH2C - CH2NH2) ··· O⊖. Results of MNDO calculations based on the experimental coordinates support the proposed concept.
From the electron and proton transfer equilibria network of quinones in solution a novel intermediate can be prepared by deprotonation of 2,5-bis(trimethylsilyl)hydroquinone to its monoanion using sodium metal. The sodium salt crystallizes in polymer strings connected via O⊖···(H)O hydrogen bridges, which are capped additionally by twofold dimethoxy-ethanesolvated Na⊕ countercations. The single crystal structure determination reveals one of the shortest O⊕ ··· HO distances observed so far of only 246 pm. MNDO calculations further confirm the assignment of hydroquinone monoanion building blocks in the polymer chain. For structural comparison as well as for attempts of its sodium reduction, 2,5-bis(trimethylsilyl)-p- benzoquinone has been synthesized. Its single crystal structure is reported, which does not show any cyanine distortion.
The structures of seven di- or tetrasubstituted p-benzoquinone derivatives O=C(XC=CH )2C=O and O=C(XC=CX)2C=O with substituents X = -OCH3, -N(CH2)5, - N(CH2CH2)2O, -Cl, -CN and -⊕N(HC=CH)2C-N(CH3)2 are presented and discussed in comparison with published ones substituted by X = -Si(CH3)3, -C6H5, -N(CH3)2, -⊕N(HC=CH)2CN(CH3)2, -O⊖ , and - NO2. Based on the introduction, in which halfwave-reduction potentials, geometry-optimized quantum-chemical calculations on substituent perturbation and known structural data of p-benzoquinone derivatives are used to characterize their molecular ground states. The structural changes indicate how substituent perturbations might be rationalized. Of the categories defined - imperturbed, donor, donor/acceptor and acceptor perturbed - the donorsubstituted p-benzoquinones do exhibit the largest differences, often called cyanine distorsion. In very satisfactory agreement with extensive semiempirical calculations, all effects determined experimentally are discussed in terms of varying charge distribution. With respect to the biochemical importance of p-benzoquinone derivatives, this first structural summary points out important facets.
Di(methylthio)acetylene H3CS-C≡C-SCH3 reacts with S2C12 in AlCl3/H2CCl2 solution to the tetra(thiomethyl)thiophene radical cation (H3CS)4C4S·⊕ and with H3CSCl to the tetra(thiomethyl)ethene radical cation (H3CS)2C·=⊕C(SCH3)3. Their ESR spectra are assigned by comparison with literature data or those of analogous products obtained from other acetylene derivatives R-C≡C-R with R = SCH2CH3, CH3, C6H5 as well as based on HMO arguments. The possible course of the oxidative sulfuration is discussed.
The neutral title compound, 8,8-bis(dimethylamino)dibenzo-[a,d]-heptafulvene, exhibits a first vertical ionization potential of only 6.98 eV and, therefore, can also be oxidized by AlCl3 in H2CCl2 solution. The radical cation generated shows a complex multiplet signal pattern, which is assigned based on additional ENDOR measurements. The photoelectron (PE) and ESR spectra of the 112 valence electron molecule are interpreted by “pararneter-optimized” HMO and by geometry-optimized MNDO calculations, which both suggest a non-planar π-type ground state with most of the charge and the spin distributed over the dibenzoheptatriene part of the radical cation.
The following mixed-stack donor/acceptor complexes {D · · · A }∞ have been crystallized and their structures determined: { 1 ,2,4,5-tetramethylbenzene · · · tetrabromo-p -benzoquinone}∞ , {hexamethylbenzene · · · tetrabromo-p-benzoquinone}∞ , { ( 1 ,2 ,4,5-tetramethyl-benzene)2 · · · tetrachloro -p -benzoquinone}∞ , {pyrene · · · tetrafluoro-p-benzoquinone}∞ , {pyrene · · · tetrabromo-p-benzoquinone}∞ and {perylene · · · tetrabromo-p-benzoquinone}∞ . They exhibit an interesting lattice packing, especially the 2:1 tripeldecker sandwich of tetrachloro-p-benzoquinone, which crystallizes in a herringbone pattern. Their interplanar distances are around 340 pm, i. e. two van der Waals π radii. None of them , however, exhibits in neither the donor nor the acceptor components significant structural changes due to complex formation. Their colours range from orange-red to black in the crystal and to green in H2CCl2 solution. Their long-wavelengths charge transfer absorption maxim a correspond to a lowering in excitation energy of up to 2 eV relative to that of the components. The different charge transfer in the ground and excited states of the donor/acceptor complexes investigated is further discussed referring to data such as cyclovoltammetric reduction potentials as w ell as to results from semiempirical calculations based on the crystal structure data determined and including configuration interaction.
Raney nickel, a highly reactive and air-sensitive solid, if prepared and investigated under oxygen-free conditions, exhibits interesting catalytic properties. Using photoelectron spectroscopy for real-time gas analysis in a flow reactor, the following results are obtained with alkyl and acylhalides: Dehydrohalogenation temperatures are lowered relative to thermal HHal elimination up to 350 K. Monochloro and bromo propanes and butenes yield propene and butadiene, respectively. 1,1-Dichloro ethane or 1,1-dibromo propane only split off one HHal and form chloroethene or 1-bromopropene-2. HCl elimination from 2-methyl propionic acid chloride, expectedly, produces dimethyl ketene. Most interesting, however, is the ring opening of monobromo cyclobutane to 1-bromo-butene-3, observed already at room temperature, which strongly suggests the intermediate formation of a chemisorbed surface carbene at Raney nickel. The formation of hexadiene-1,5 as a by-product in the HCl elimination of 1-chloropropane, i. e. a surface carbene dimer, indicates their presence also in other dehydrohalogenations heterogeneously catalyzed by Raney nickel.
Raney nickel, a highly reactive and air-sensitive solid, if prepared and investigated under oxygen-free conditions, exhibits interesting catalytic properties. Using photoelectron spectroscopy for real-time gas analysis in a flow reactor, the following results are obtained with alkyl and acylhalides: Dehydrohalogenation temperatures are lowered relative to thermal HHal elimination up to 350 K. Monochloro and bromo propanes and butenes yield propene and butadiene, respectively. 1,1-Dichloro ethane or 1,1-dibromo propane only split off one HHal and form chloroethene or l-brom opropene-2. HCl elim ination from 2-methyl propionic acid chloride, expectedly, produces dimethyl ketene. Most interesting, how ever, is the ring opening of monobromo cyclobutane to 1-brom o-butene-3, observed already at room temperature, which strongly suggests the intermediate formation of a chem isorbed surface carbene at Raney nickel. The formation of hexadiene-1,5 as a by-product in the HCl elim ination of 1-chloropropane, i.e. a surface carbene dimer, indicates their presence also in other dehydrohalogenations heterogeneously catalyzed by Raney nickel.
The following mixed-stack donor/acceptor complexes {D···A}∞ have been crystallized and their structures determined: {hexamethylbenzene···3,5-dicyano-1-nitrobenzene hexamethylbenzene···3,5-dinitro-1-cyanobenzene}∞, {pyrene···3,5-dinitro-1-cyanobenzene}∞, {anthracene···(3,5-dinitro-1-cyanobenzene)2}∞, {N,N-dimethylanilin···3,5-dinitro- 1-cyanobenzene}∞ and { 1-3-phenylenediamine···3,5-dinitro-1-cyanobenzene}∞. Their lattice packing consists of parallel layers, which contain either donors and acceptors as for hexamethylbenzene and pyrene or composite ones as in the 1:2 complex of anthracene with each one of the acceptors above and below its peripheral rings. The isostructural hexamethylbenzene complexes exhibit almost identical packing coefficients as well as a hexagonal coplanar arrangement of the C6(CH3)6 donors. Weak intermolecular van der Waals interactions are also observed between antiparallel cyano substituents. The interplanar n distances range between 334 and 353 pm, i. e. around 340 pm of two van der Waals n radii. In none of the complexes, however, significant structural changes in either the donor or the acceptor components due to the complex formation are observed. In both the crystals as well as in solution, the donor/acceptor complexes exhibit colours between yellow and red; their long-wavelength charge transfer absorption maxima, therefore, correspond to a lowering in excitation energy of only up to 1 eV relative to that of the components. The different charge transfer in the ground and the CT excited states is also discussed referring to other data such as vertical first ionization energies or interplanar distances {D···A}, as well as to results from semiempirical calculations based on the crystal structure data determined and including approximate configuration interaction.
The isolation of transcribed DNA sequences of P815 cells and the partial characterization with respect to their sequence composition and relative rates of enzymatic DNA methylation are reported in this paper. Transcribed regions were purified by affinity chromatography using immobilized heterogenous nuclear RNA of P815 cells. About 10% of total genome was found in this fraction. Reassociation analyses showed differences in sequence composition of transcribed versus non-transcribed DNA fractions. The relative proportion of inverted repeats was doubled in the transcribed fraction whereas ordinary highly repetitive sequences comprising mainly of satellite DNA were found almost exclusively in the non-transcribed regions of the P815 genome. About 70% of transcribed portions corresponds to unique and intermediary DNA sequences. After labelling of cells with L-[Methyl-3H]methionine and [14C]deoxycytidine relative rates of enzymatic DNA methylation were computed for different kinetic components of transcribed and non- transcribed portions of P815 genome. No difference was found except in inverted repeats. In transcribed DNA the relative rate of enzymatic DNA methylation was only about 40% of that of the non-transcribed ones. We have quantitated this hypomethylation and found that there is, in average, about one 5-methylcytosine residue in 100 nucleotides of transcribed inverted repeats, compared to about 2.5 5-methylcytosines in non-transcribed fractions. In view of these data we propose that the enzymatic methylation of inverted DNA repeats negatively controls the transcriptional process in a given genomic region.
A specific class of DNA sequences, the inverted repetitive sequences, forms a double-stranded structure within a single linear polynucleotide chain in denatured DNA. The reassociation process is unimolecular and occurs very fast. Quantitative analyses have shown that these sequences com-E rise about 4-5% of the nuclear DNA of various mammalian cells (P815 mouse mastocytoma, Hela, L cells, Raji and Chang cells, and human embryonic hepatocytes) and are interspersed within sequences of other degrees of repetitiveness.
After labeling the cells with L-[Metnyl-3H]methionine and [14C]deoxycytidine, relative rates of enzymic DNA methylation were computed on the basis of 3H and 14C radioactivities found in py rimidine residues of the nuclear DNA. The results indicate that DNA of inverted repetitive sequences is methylated to a level about 50% higher than the ordinary repetitive sequences and to about 300% higher than the unique and intermediary sequences.
The biological function of the inverted repeats as well as the role of their enzymic hypermethyl ation is unknown.
The sequence complexity of nuclear RNA from mouse liver, mouse spleen and highly malignant P815 mastocytoma was measured by nRNA driven hybridization to unique DNA sequences of P815 cells. The unique DNA sequences represent 63% of the total nuclear DNA of P815 cells and their availibility in hybridization experiments was found to be 76%. Of these sequences 7.8% formed hybrids with nuclear RNA of this cell, about 11.5% with mouse spleen and about 14.5% with mouse liver nuclear RNA. Assuming an asymmetrical transcription, the complexities of these transcripts are 2.8 × 108 nucleotides for mouse P815 mastocytoma, 4.3 × 108 for mouse spleen and about 5.3 × 108 nucleotides for mouse liver.
Cellular specifity of the transcribed information was analyzed in additivity experiments, in which unique DNA sequences, not complementary to the nuclear RNA of one cell were annealed to the nuclear RNAs of the two other tissues/cells. In these experiments most of the nuclear RNA se quences of P815 cells were found to be also present in the nucleus of mouse liver and spleen. Only a small portion of the unique DNA sequences of P815 mastocytoma (about 1.2% corresponding to 4.4 ×107 nucleotides) was found to be complementary only to P815 mastocytoma nuclear RNA.
Der wissenschaftliche Fortschritt in Chemie, Biowissenschaften und Medizin basiert auf den immer detaillierteren Erkenntnissen über die molekularen Prozesse des Lebens. Eine Voraussetzung dafür sind Fortschritte bei den analytischen Methoden, Techniken und Instrumenten. In dem heute zur Verfügung stehendem Instrumentarium spielt die Massenspektrometrie eine zunehmend wichtige Rolle. Wenn aktuell ein neuer Doping-Skandal durch die Presse geht, sind immer massenspektrometrische Techniken im Spiel: Sie ermöglichen den Nachweis von erlaubten und verbotenen Substanzen aller Art – auch Dopingmitteln.
The compound [(PyH)3Br][AlBr4]2 is formed by melting stoichiometric amounts of AlBr/PyHBr in a ratio of 2:3. It crystallizes in the orthorhombic space group Pbca with lattice constants a = 1365.5(2), b = 1616.0(2), c = 2783.7(3) pm, Z = 8, Dc = 2.21 g/cm3. The structure was solved from 2810 diffractometer measured intensities (Cu -Kα radiation) and refined to Rw (F) = 0.071. The cation shows three pyridinium ions attached via N - H - Br hydrogen bonds to a central bromide ion. The N - Br distances are 321(1), 321(2) and 332(2) pm.
[Ph3PN(H)Ph][AuI2] (2) is formed by the reaction of AuI with N-Phenyl-iminotriphenylphosphorane, Ph3PNPh in a toluene suspension. 2,3-Bis(triphenylphosphinimino)maleic acid-N-methylimide (3) has been prepared by the Staudinger reaction of 2,3-bis(azido)maleic acid-N-methylimide with PPh3 in THF solution in the form of red crystals. Crystal structure determinations of three iminophosphoranes were carried out by X-ray methods.
Ph3PNPh (1): space group P21/c, Z = 4, 2176 independent observed reflexions, R = 0.057. Lattice dimensions (-30 °C): a = 1126.4, b = 1148.6, c = 1476.0 pm; β = 97.21°. The compound forms monomeric molecules with P=N = 160.2 pm and an PNC angle of 130.4°.
[Ph3PN(H)Ph][AuI2] (2): space group P1̄, Z = 2, 1780 independent observed reflexions, R = 0.057. Lattice dimensions (18 °C); a = 824.9, b = 1022, c = 1476.2 pm; α = 89.23°, β = 87.41°, γ = 85.65°. The compound consists of ions [Ph3PN(H)Ph]⊕ with P=N = 162.4 pm and PNC = 129.3°, and anions [AuI2]⊖ with Au-I = 261.9 and 259.3 pm, IAuI = 176.8°.
(Ph3P)2N2C4O2 (NMe) (3): space group P1̄, Z = 2, 4972 independent observed reflexions, R = 0.050. Lattice dimensions (-90 °C): a = 904.7, b = 993.8, c = 2017.4 pm; α = 101.55°, β = 96.39°, γ = 105.81°. The compound forms monomeric molecules with syn-conformation of the two NPPh3 groups. Bond lengths: P=N = 157.1; 155.3 pm, bond angles: PNC = 133°; 136°.
The effect of NNMG on the template activities of different polynucleotides (polyuridylic acid, polycytidylic acid, polyadenylic acid and copolymer of adenylic and guanylic acid 5,5:1) and t-RNS was studied. The maximum inhibition of the messenger activity was found for poly-C, followed by poly-Α and poly-U. The acceptor activity of t-RNA was found to be inhibited by NNMG: maximum for proline, followed by serine, leucine, phenylalanine and lysine. The mechanism of these inhibitions was studied using NNMG radioactively labelled on the methyl group. Different amounts of radioactivity were found in the various polynucleotides and t-RNS.
This paper reports about the fine structure in the O—K-spectra of the oxides BeO, MgO, CaO, SrO, BaO, Sc2O5, Y2O3, La2O3, Sm2O3, Yb2O3, NiO and ZnO. The spectra show the satellite lines α3, α4, α5, α6 on the short wavelength side of the main line α1,2 and a shoulder β′ on its long wavelength side. The wavelengths of all lines depend on the nature of the oxide. For the positions of the lines Kα1.2 in the spectra no systematic relation to other data of the oxides is observed. On the other hand the distance of the a4-satellite from the α1,2-line decreases with increasing electronegativity of the metal atom in the oxide. This distance can be used as a measure for the ionic character of the metal-oxygen bond in these compounds.
The O-K-spectra of α- and γ-Al2O3, β-Ga2O3 and In2O3 were measured with a high-resolution grating spectrograph using the method of electron excitation. The spectra were corrected for the nonlinear response of the photographic emulsions. A shift of the O - K-emission band edge of γ-Al2O3 to lower energy with respect to that of α-Al2O3 and a fine structure within all O -K-bands were observed for the first time. The details of the spectra are discussed in relation to the energy level diagrams, crystal structure, and chemical bond of these oxides.
Das OH-Radikal, bestehend aus einem Sauerstoff- und einem Wasserstoffatom, ist verantwortlich für den Selbstreinigungsmechanismus der Atmosphäre. Als Oxidationsmittel reagiert es mit praktisch allen Spurengasen, wie z. B. dem giftigen Kohlenmonoxid, dem Treibhausgas Methan und dem Schwefeldioxid, und macht sie wasserlöslich, so daß sie im Regen gelöst ausgewaschen werden können (Waschmitteleffekt). Welche Schlüsselstellung das OH-Radikal in der Atmosphärenchemie hat, beschreibt der Chemiker Franz Josef Comes. Gleichzeitig stellt er das weltweit empfindlichste Absolutverfahren zur Bestimmung von troposphärischen OH-Konzentrationen vor, das in Frankfurt entwikkelt und zur Zeit getestet wird.
The reactive intermediate methyleneaminoacetonitrile H2C = N - C H2 - CN has been generated via thermal retrotrimerization of N ,N',N"-tris(cyanomethyl)hexahydro-s-triazine and characterized by its photoelectron, mass and low-temperature NMR spectra. A fully geometryoptimized MNDO calculation allows to assign the observed ionization energies and yields estimates for other molecular properties, e.g. a rather high dipole moment.
Nach Kultivierung von Enterococcus Stei mit 14C-markiertem 5-Chlor-, 5-Brom- oder 5-Jod-Uracil wurde aus den Zellen die DNS isoliert und hoch gereinigt. Durch UV-Bestrahlung dieser DNS in wäßriger Lösung werden die eingebauten 5-Halogen-Uracile photochemisch verändert. Beim Abbau dieser bestrahlten DNS findet man neben geringen Mengen nicht-identifizierter Photoprodukte als überwiegendes Strahlenprodukt nach Hydrolyse mit Perchlorsäure Uracil und nach fermentativem Abbau Uracildesoxyribosid. Die Dehalogenierung von BU und JU in der DNS verläuft in Abhängigkeit von der Bestrahlungsstärke etwa gleich schnell, während CU sehr viel langsamer dehalogeniert wird.
Die photochemische Dehalogenierung des BU erfolgt in der nativen DNS am leichtesten, weniger gut in der Hitze-denaturierten DNS und nur in geringem Maße in der Apurinsäure.
Glycol causes a denaturation of the DNA double helix structure in solution. As could be shown earlier, heat denaturation of DNA leads to an increased dimerization of thymine following uvirradiation. In contrast to this, thymine dimer is not increased - but is even slightly decreased - when DNA is uv-irradiated in the presence of glycol. These results are discussed with regard to the distortion of the hydration layer and the hydrophobic stacking of bases, as influenced by glycol.
Gel layers of crosslinked polyacrylamide with different porosities were synthesized and their properties compared. At high concentrations of bifunctional monomers macroporous gels were formed, one of which (total monomer concentration 14%, methylene-bis-acrylamide 40%) permitted even high molecular weight proteins (e. g. γ-globulins) to diffuse more readily than in 2 per cent agar or agarose gels. Different immunological techniques were modified for application with this polymer. It offers advantages especially with respect to porosity, storability and sample application for quantitative diffusion techniques.
The NAD analogue [3-(3-acetylpyridinio)-propyl] adenosine pyrophosphate forms enzymically inactive complexes with glyceraldehyde-3-phosphate dehydrogenase from yeast and rabbit skeletal muscle. In the latter enzyme four mol of the analogue are bound with equal affinity inhibiting the enzyme in a competitive way: KI = 0.3 mM as compared to the dissociation constant KD=O.6 mм.
The brominated derivative [3- (3-bromoacetylpyridinio) -propyl] adenosine pyrophosphate is covalently bound to both enzymes causing irreversible loss of enzymic activity. Complete inactivation of the enzyme from muscle requires two moles of the analogue per mol of tetramer. The remaining two sites are still able to bind two mol of NAD+ without regain of enzymic activity. In the case of the yeast enzyme four mol of the analogue are bound. Inactivation of the rabbit muscle enzyme is accompanied by the disappearance of two out of four highly reactive sulfhydryl groups; in the yeast enzyme the four active site cysteine residues are still able to react with DTNB1 the reactivity being diminished significantly.
Hybrid formation between the native enzymes from yeast and skeletal muscle is not affected by the modification of the enzyme. Similarly the sedimentation properties of the covalently modified enzyme are indistinguishable from those of the native molecule. This indicates that both the native and the irreversibly inhibited enzyme are identical regarding their quaternary structure.
Hemoproteinoids related to contemporary porphyrin-dependent peroxidases were synthesized under simple conditions. The peroxidative activity of hematin increased by a factor of 50 if the hematin was bound to proteinoids whereas the catalatic activity of hematin decreased rather under the same conditions. The peroxidative activity of hemoproteinoids particularly increased with their lysine content whereas the catalatic activity especially decreased in proteinoids with high phenylalanine content. The isoelectric points of the lysine-rich peroxidic hemoproteinoids were about 8. Their relatively broad pH-activity optimum was about pH 7.0. The molecular weights were a little below 20 000. Hematin content and amino acid composition of the synthetic materials were varied greatly. The substrate specificity appeared as broad as that of biogenous peroxidases, e. g., horseradish peroxidase. Among the many substrates was NADH. The possible importance of the peroxidative oxidation of NADH-type coenzymes by primitive heterotrophic organisms or prebiological systems in an anaerobic environment is discussed.
"Ästhetisch ist, was hilft"
(2017)
Chemiker stehen in ihrer Mentalität Architekten nahe: Sie planen und bauen Moleküle. Schon lange, bevor der atomare Aufbau der Materie experimentell bewiesen war, entwickelten sie genaue Vorstellungen über die Raumstruktur von Molekülen. Erst zu Beginn des 20.Jahrhunderts wurden diese "Arbeitshypothesen" – zum Beispiel das von Jacobus van’t Hoff postulierte Tetraedermodell für den vierbindigen Kohlenstoff – von den Physikern glänzend bestätigt. Zwar ist es mittlerweile möglich, die Struktur von unbekannten Molekülen zuverlässig vorherzusagen; doch nach wie vor sind genaue experimentelle Strukturbestimmungen ein unverzichtbarer Bestandteil vieler Forschungsprojekte.
Alkylating NAD-Analogs, Glyceraldehyde-3 Phosphate Dehydrogenase, Half-of-the-Sites Reactivity co-(3-Bromoacetylpyridinio)alkyldiphosphoadenosines with alkyl chain lengths of 2 -6 me thylene groups inactivate glyceraldehyde-3 phosphate dehydrogenase from rabbit muscle. Half-of-the-Sites reactivity is observed in each case: The analogs are covalently bound to highly reactive cysteine residues in two of the four subunits. The remaining two subunits still bind N AD and the reactive SH-groups, although modified by SH-reagents of low molecular weight are not labeled by any of the brominated coenzyme models. This behaviour may be explained by the assumption, that the modification of 2 subunits induces structural changes in the neighboured unoccupied subunits which prevent any attack on reactive cysteine residues caused by fixation and orientation of the bromoketo-coenzyme analog when bound to the active center. Structural similarities of the covalently bound coenzyme analogs in the active center and the native ternary GAPDH-NAD-substrate complex suggest that half-of-the-sites reactivity is a natural characteristic of the enzymes catalytic mechanism.
Triphenylmethylphosphonium nitrite and formate have been prepared by the reaction of [PPh3Me]I with silver nitrite, and lead formate, respectively, in aqueous solutions. [PPh3Me]NO2 (1) forms pale yellow crystals, and [PPh3Me]HCO2·H2O (2) forms white crystals. Both compounds are soluble in water, ethanol, and dichloromethane. In moist air 2 is hydrated to yield [PPh3Me]HCO2·2H2O (3). The compounds were characterized by their IR spectra, 1 and 2 also by X-ray crystal structure determinations.
[PPh3Me]NO2 (1): space group P21/n, Z = 4, 2088 independent observed reflexions, R = 0.062. Lattice dimensions (20 °C): a = 914.7(3), b = 1887.5(9), c = 1080.0(4) pm, β = 110.29(3)°. The compound consists of PPh3Me+ ions and NO2- anions with bond lengths of 114.2(6) pm and a bond angle of 124.1(7)°.
[PPh3Me]HCO2·H2O (2): space group P21/n, Z = 4, 2973 independent observed reflexions, R = 0.069. Lattice dimensions (-20 °C): a = 931(2), b = 1558(3), c = 1281(2) pm, β = 105.9(1)°. The compound consists of PPh3Me+ ions and formate anions which form centrosymmetric dimeric units [HCO2·H2O]22- through hydrogen bridges of the water molecules. Bond lengths CO 122.4(4) and 120.9(4) pm. bond angle OCO 129.9(4)°.
The He I photoelectron spectra of certain MeEHal2 and Me2EHal compounds (E = (N), P, As, Sb; Hal = (F), Cl, Br, J; Me = CH3) are interpreted in terms of a “composite molecule” approach derived for C3vCs systems. Although an “internal standard” is missing here, substituent group-orbitals (nHal, C—H) may be classified with respect to their orientations in space (R, V, T). Ionisation energies are assigned according to this assumption.
PE data of the isoelectronic EMe3/EHal3 compounds and of related molecules (Me2EH, MePH2, CF3PBr2) as well as EHMO calculations with partial inclusion of spin orbit coupling are used to confirm the assignments given for Me2EHal/MeEHal2 series.
Correlations between PE ionisation energies (e.g. nE (IE)) and molecular or atomic properties are critically revised and discussed.
The He I photoelectron spectra of the series RnP(CN)3-n (R = Me, CF3; n = 0, 1, 2, 3) and of Me2ECN (E = N, P, As) are interpreted. The PE assignments are based on the comparison with the PE data of analogous halogeno and hydrogen derivatives as well as related cyano compounds and on simple MO considerations (composite molecule-approach). Hyperconjugative and inductive effects of the substituents CN, Me, F, Cl, and especially CF3 are assessed. The various effects of changing substituents or central atoms within the series are used to confirm the interpretation. The differing electronic structures of halogeno and cyano (pseudohalogeno) compounds are discussed on the basis of their PE spectra.
Coordination of substitutionally inert [Ru(bpy)2]2+ fragments (bpy: 2,2′-bipyridine) to the a-iminoketone chelate ligands pyrazine-2-dimethylcarboxamide (4) and 4,7-phenanthroline-5,6-dione (5) yields the complexes [(N,O-4)Ru(bpy)2]2⊕, [(O,O′-5⊖)Ru(bpy)2]⊕ and {(N,O; N′,O′-5)[Ru(bpy)2]2}4⊕ which exhibit a rich electrochemistry. The distinctly different electronic structures of the complexes are evident from the ESR behaviour of paramagnetic intermediates: N.O-coordinated complexes have the unpaired electron residing in the ligand n system upon reduction, albeit with g<2 for the binuclear complex of 5. The paramagnetic O,O′-coordinated mononuclear complex with 5 has its redox potentials shifted positively relative to that of the binuclear system. These results are particularly noteworthy because 4 and 5 can be regarded as model compounds for the flavin and methoxatin dehydrogenase cofactors.
Lumineszenz von Hefe
(1968)
Cp2TiSe5 has been prepared by the reaction of trim ethyltetradecylammonium-polyselenide with Cp2TiCl2 in ethanol solution and subsequent extraction of the dry residue with dichloromethane. Cp2TiSe5 crystallizes in the space group P1 with two formula units in the unit cell (2559 observed, independent reflexions, R = 0.074). The cell dimensions are a = 808.6, b = 822.6, c = 1190.7 pm, α - 96.28°, β - 106.06°, γ = 108.78°. The structure consists of discrete Cp2TiSe5 molecules with the TiSe5, ring in the chair conformation.
The title compound has been prepared by the reaction of N-trimethylsilyl-iminotriphenylphos-phorane with copper(II) chloride in boiling CCl4 /C2H5OH, and forms moisture sensitive crystals, which are green in transmittance and black in reflexion. [Me3SiNPPh3 · CuCl2 ] 2 was characterized by its IR spectrum as well as by a crystal structure determination (4197 observed, independent reflexions, R = 0.049). The lattice dimensions are at 20 °C: a = 1102.7. b = 1407.3. c = 1560.2 pm; β = 94.27°; space group P21/n with two formula units in the unit cell. The complex consists of centrosymmetric, dimeric molecules with a planar Cu2 Cl2 ring (Cu-CI bond lengths 229 and 231 pm). A terminally bonded CI atom (Cu-CI = 221 pm) and the N atom of the Me3SiNPPh3 ligand (Cu-N = 198.5 pm) complete the coordination number four of the nearly planar surroundings of the Cu atoms.
Kristallstruktur von 1,1-Dichlor-3,5-diphenyl-4-H-1,2,4,6-λ4-selenatriazin, SeCl2C2N3H(C6H5)2
(1989)
The title compound has been prepared as a byproduct of the reaction of Se2Cl2 with Ν,Ν,N′-Tris(trimethylsilyl)benzamidine in CH2Cl2 solution. [SeCl2(HNC2N2Ph2)]2 was characterized by an X-ray structure determination. Space group P21/n, Z = 2, 2979 observed independent reflexions, R = 0.032. Lattice dimensions (-65 °C): a = 1050.1(4), b = 1018.9(4), c = 1402.1(6) pm; β = 99.78(3)°. The compound forms centrosymmetric dimeric molecules with SeCl2Se bridges (bond lengths 241.6(1) and 339.3(1) pm), the selenium atoms being members of nearly planar [xxx] selenatriazine rings with Se—N bond lengths of 182.2(2) and 181.5(2) pm.
W2NCl7 has been prepared by the reaction of tungsten pentachloride with the bromide of Millon's base, [Hg2N]Br, in boiling CCl4. The product forms a dark brown, moisture sensitive crystal powder (μeff = 0.7 B.M. at 21 °C). With phosphoryl chloride, the complex W2NCl7·2 POCl3 is formed. The reaction with chlorine leads to the mixed-valenced W(V)/W(VI) complex W2NCl8 (μeff = 0.5 B.M. at 22 °C), which reacts with tetraphenylphosphonium chloride in CH2Cl2 to form (PPh4)2[W2NCl10] ·2CH2Cl2. The reactions of W2NCl7 with PPh4Cl in molar ratios in CH2Cl2 solution lead to several complexes; one of them was identified bv X-ray diffraction methods to be (PPh4)2[W3Cl9(μ3-N)(0)(μ2-NCl)]2 ·1,5 CH2Cl2, which forms black crystals. The compound crystallizes monoclinically in the space group P21/n with two formula units per unit cell (7318 observed, independent reflexions, R = 0.083). The lattice dimensions are (20 °C): a = 994.4; b = 2673; c = 1518.2 pm; β = 101.00°. The compound consists of PPh4⊕ cations and centrosymmetric anions [W3Cl9(μ3-N)(O)(μ2-NCl)]22⊕. The tungsten atoms form a scalene triangle with WW bond lengths of 282 and 278 pm, respectively. The hypothenuse of this triangle is a nearly linear W - N -W bridge with WN distances of 199 and 182 pm. One of the WW edges is bridged by a μ-NCI group with WN bond lengths of 196 und 189 pm. respectively.
The N,N'-bis(trimethylsilyl)benzamidinato complexes [C6H5 -C(NSiMe3)2MCl3]2(M = Ti. Zr) have been prepared by the reactions of N,N,N'-tris(trimethylsilyl)benzamidine [C6H5-C(NSiMe3)N(SiMe3)2] with titanium tetrachloride, and zirconium tetrachloride, respec-tively. The compounds form moisture sensitive, dark red (Ti) and white (Zr) crystals, which were characterized by crystal structure determinations. [C6H5-C(NSiMe3)2TiCl3]2 : space group P21/rc. Z = 2, 4373 observed independent reflexions, R = 0.034. Lattice dimensions (-90 °C): a - 959.0(8); b = 1196.5(8); c = 1770.9(11) pm; β = 93.79(4)°. [C6H5-C(NSiMe3)2ZrCl3]2 : space group P21/n. Z = 2, 3160 observed independent reflexions, R = 0.031. Lattice dimensions (-90 °C): a = 971.6(7); b = 1222.2(9); c = 1792.9(10) pm; β = 93.51(5)°.
Both complexes crystallize isotypically, forming centrosymmetric dimeric molecules via chloro bridges with bond lengths of 242.0 and 253.8 pm (Ti), and of 253.7 and 264.9 pm (Zr). The metal atoms complete their distorted octahedral surroundings with two chlorine ligands and the nitrogen atoms of the chelating amidinato ligand. The N atoms of the amidinato group are in equatorial and axial positions. This accounts for the different metal-nitrogen bond lengths of 207 pm (ax) and 199 pm (eq) in the titanium compound and 219 pm (ax) and 214 pm (eq) in the zirconium complex.
Zur Reaktion von [(η3-C4H7)PdCl]2 mit Se(SiMe3)2. Die Kristallstruktur von [(η3-C4H7)6Pd6Se3]
(1988)
[(η3-C4H7)PdCl]2 reacts with Se(SiMe3)2 to form [(η3-C4H7)6Pd6Se3] (1). 1 has been characterized by X-ray crystal structure analysis. It contains a distorted trigonal prismatic Pd6-cluster. Three faces of the Pd-prism are occupied by μ4-Se ligands. 1 crystallizes in the space group Pnma with 4 formula units per unit cell. The lattice constants at 200 K are: a = 1175.1(8), b = 1611.4(12), c = 1720.3(12) pm.
[η5-CpMCl4] (M = Nb, Ta) reacts with E(SiMe3)2 (E = S, Se) to form different multinuclear clusters. The cation [Cp8Ta6S10]2+ (1) consists of a planar Ta2S2-ring of which each Ta is coordi-nated to two Cp2TaS2-fragments. [Cp4Ta4S13] (3) can be derived from [Cp3Ta3S7Cl2] (2) by addition of a CpTaS6-unit to a triangle of Ta-atoms bridged by S- and S2-ligands. The niobium atoms in [Cp3Nb3Se5Cl2] (4) arrange in a chain structure with Nb coordination numbers varying from 4-6.
(η5-C5H5)Fe(CO)2Br reacts with Se(SiMe3)2 to form the title compound 1, which has been characterized by X-ray crystal structural analysis. 1 crystallizes in the space group P212121 with 4 formula units per unit cell. 1 consists of [Se{Fe(CO)2(C5H 5)}3]+- cations and [Fe4Se4Br4]2--anions, the latter with a heterocubane structure.
(NBu4)2[Co4(SPh)6Cl4] (1) and [Co4(SPh)6CU(PPh3)2] (2) are prepared by the reaction of (NBu4)[CoCl3(PPh3)] and [CoCl2(PPh3)2] with PhSSiMe3 in toluene solution. The overall description of the Co4S6Cl4 core is that o f a nearly regular Co4-tetrahedron inscribed in an irregular octahedron of the bridging S atoms resulting in an “adamantane” type cage. 1 reacts with Na[Mn(CO)5] to form the dinuclear complex (NBu4)[Mn2(SPh)3(CO)6] (4). Each Mn atom is bound to three CO and three μ2-SPh ligands. The Mn-Mn contact (318.5 pm) does not indicate metal-metal bonding.
(NBu4)[CoCl3(PPh3)] reacts with Se(SiMe3)2 to form the new clusters [Co8Se8(PPh3)6][CoCl3(PPh3)] (6) and [Co8Se8(PPh3)6][Co6Se8(PPh3)6] (7). The structures of 6 and 7 have been determ ined by X-ray diffraction. 6 and 7 crystallize in the space group P1̄ with two formula units per unit cell and with the following lattice constants at 180 K: 6: a = 1413.8(10), b - 2224.2(23), c = 2348.4(17) pm, α = 86.06(5), β = 86.58(5), γ = 76.11(5)°; 7: a = 1465.9(4), b = 1627.6(6), c = 2505.7(6) pm, α - 98.69(2), β = 96.23(2), γ = 113.06(2)°. The cluster structures of the [Co8Se8(PPh3)6]n (n = 0, 1 +) depend on the total number of electrons in the cluster units.
MoF4(NCl) has been prepared as a yellow crystal powder by the reaction of diluted fluorine with MoCl3(N3S2) at room temperature. The compound is associated via fluorine bridges, according to the IR spectrum. With acetonitrile, the monomeric complex [CH3CN -MoF4(NCl)] is obtained, which was characterized by its IR and 19F NMR spectra as well as by an X-ray structure determination. Crystal data: space group Pm, Z = 2 (1068 observed, independent reflexions, R = 0.03). Lattice dimensions at -90 °C: a = 507.1. b = 704.8, c = 995.8 pm, β = 102.02°. The unit cell contains two crystallographically independent molecules [CH3CN -MoF4(NCl)], the Mo≡N-Cl groups being linear (bond angles 176°, 178°) with bond lengths MoN = 172 and NCI = 159, 162 pm. In the trans position to the MoNCl group, the nitrogen atom of the acetonitrile molecule is coordinated.
[Na-15-Crown-5][MoF5(NCl)] has been prepared as yellow crystals by the reaction of NaF with MoF4(NCl) in the presence of 15-crown-5 in acetonitrile solution. The compound was characterized by its IR and 19F NMR spectra as well as by an X-ray structure determination. Crystal data: space group P21/n, Z = 4 (3736 observed, independent reflexions, R = 0.034). Lattice dimensions at -70 °C: a = 823.5(4). b = 1612.2(9), c = 1383.4(8), β = 99.35(3)°. The compound forms ion pairs, in which the sodium ion is seven-coordinated by the oxygen atoms of the crown ether molecule and by two fluorine ligands of the [MoF5(NCl)]- unit with Na-F distances of 228.3 and 249.6 pm. The Mo=N-Cl group of the anion is nearly linear (bond angle 175.8°) with bond lengths MoN = 172.9 and NCl = 161.8 pm.
The stereochemistry of the bisaboloids in chamomile-with the exception of bisabolol-oxide C-has been elucidated. The in-vitro-examination of the mutual convertibilities of some bisaboloids gave evidence for the stereochemical accordance of the common chiral centres of all the bisaboloids. The absolute configurations of the remaining third asymmetric carbon atoms in bisabololoxide A and B have been determined by NMR spectrometric studies in comparison with their unnatural semisynthetic epimers. All the stereogenic centres of the bisabololoxides A and B, of (-)-α-bisabolol and of bisabolonoxide A turn out to be S-configurated.
Untersuchungen zur Charakterisierung des Prochamazulens Matrizin aus Matricaria chamomilla L.
(1982)
The relative configuration of the thermolabile chamazulene precursor matricine has been established by NMR spectrometric studies.
The NMR spectral data prove to be consistent with the well-known structure of the chamomile component. On the basis of our results the levorotatory natural substance moreover can be specified stereochemically as (−)-(3S*, 3aR*, 4S*, 9R*, 9aS*, 9bS*)-4-acetoxy-2,3,3a,4,5,9,9a,9b-octahydro-9-hydroxy-3,6,9-trimethylazuleno[4,5-b]furan-2-one.
The near and far UV spectra of the aminoboranes (Me2N)n B X3-n, n = 1, 2, 3, Me=CH3, X= H, Me, F, Cl, Br are presented. In most of the monoamino boranes the π→π * transitions dominate. In the di- and triamino boranes there were additionally found a Rydberg series and some single Rydberg transitions, partly preceding the π→π * band. The Rydberg assignments were settled particularly by comparing the band positions of the compounds relatively to their ionization energies.
Die Bedeutung der Toleranztests für die Diagnostik der diabetischen Vorstadien wird aus den pathophysiologischen Voraussetzungen abgeleitet. Die Methode der Wahl ist der einzeitige orale Glucosetoleranztest mit 100g Glucose (oder Glucoseoligosacchariden). Dieses Verfahren erfaßt die wesentlichen physiologisch wichtigen Funktionen, was für den intravenösen Glucosetoleranztest und für den Tolbutamidtest nicht gilt. Gleichzeitig ist der orale Toleranztest am einfachsten durchzuführen. Lediglich bei Störungen im Gastrointestinalbereich kann der intravenöse Glucosetoleranztest angezeigt sein. Es gibt keine wichtige Indikation für den Tolbutamidtest.
Die verschiedenen Störmöglichkeiten sowie Fehlermöglichkeiten bei der Durchführung des Glucosetoleranztests werden besprochen.
The enzyme 20-β-Hydroxy-steroid-dehydrogenase obtained from the culture of Streptomyces hydrogenans and dissolved in 0.05 M Tris puffer, pH 7.3, has been investigated by means of a ultracentrifuge at 20 °C. The sedimentation- as well as the diffusion-coefficients obtained from various solutions at different concentrations were extrapolated to the concentration c = 0. The resulting zero-value for the sedimentation coefficient is s0 = 6.64 s and for the diffusion coefficient is D0 = 5.51 × 10-7 cm2/sec. Supposing the partial specific volume of the enzyme under consideration analogously to other similar proteins is V+=0.749 ml/g, the molecular weight has been estimated as M = 118 400.
A single procedure for the preparation of lactate dehydrogenase (EC 1.1.1.27), the mitochondrial and cytoplasmic forms of malate dehydrogenase (EC 1.1.1.37), adenylate kinase (EC 2.7.4.3) and pyruvate kinase (EC 2.7.1.40) from pig heart is described. The five enzymes are obtained in preparative amounts in homogenous form with specific activities equal to or higher than those pre viously reported. Some molecular properties of pig heart pyruvate kinase are determined.
The behaviour of molecules and particles of 2 - 400 nm radius in gel chromatography was investigated using sephadex, agarose, and pearl shaped cellulose gel of different porosity. Correlations between elution volumes and particle sizes are given in Fig. 1. We found that particles from ca. 400 nm diameter upwards were more or less irreversibly adsorbed depending on the particle size. So only 15% of rat liver mitochondria and 5% of E. coli bacteria could be eluted from the loose cellulose Cu3. It is assumed that this adsorption is due to the absence of Brownian motion of large particles, which therefore are more or less subject to the gravity and to adsorption forces of the gel.
Stereoisomere Aromastoffe XIX: Asymmetrische Reduktion von 4(5)-Oxocarbonsäuren mit Bäckerhefe
(1987)
Asymmetric reduction of 4(5)-oxocarboxylic acids (esters)by baker’s yeast and cyclizationin acidic media yield soptically active γ(δ)lactones. The evaluation of their chirality and optical purity was carried out by HPLC (HRGC)analysis of the corresponding 1,4(1.5)-diols via diastereomeric esters with(R)-Mosher acid(MTPA) and (S)-O-acyllactic acids respectively. By increasing the 4(5)alkyl side chain 4R(5R) configurated γ(δ)-lactones with high ee-values are generated.
Lieblingsbild
(2017)
Wir untersuchen eine neuartige Gruppe von Polarisationsmitteln – gemischtvalente Verbindungen – mittels theoretischer und experimenteller Methoden und demonstrieren ihre Leistungsfähigkeit in NMR-Experimenten mit Hochfeld-DNP (DNP=Dynamic Nuclear Polarization, dynamische Kernpolarisation) im festen Zustand. Diese gemischtvalenten Verbindungen stellen eine Gruppe von Molekülen dar, bei denen die molekulare Mobilität auch in Festkörpern erhalten bleibt. Folglich können solche Polarisationsmittel unter günstigen Bedingungen für die dynamische Kernpolarisationsbildung bei ultrahohen Magnetfeldern verwendet werden, um Overhauser-DNP-Experimente im Festkörper durchzuführen.
Zeit ist einer jener Begriffe, für die man die Augustinische Charakterisierung gelten lassen wollte, es sei klar, was sie bedeuten, solange nicht danach gefragt werde (Augustinus Confessiones Lib. XI, 17). Die Frage aber nach dem, was "Zeit" eigentlich ist, erscheint umso berechtigter, als es insbesondere die Naturwissenschaften sind, die für sich in Anspruch nehmen, hier Antworten geben zu können. Die zu erwartenden Antworten wären danach wesentlich empirischer Natur – also direkt oder indirekt experimentell gestützt und mithin Ergebnis dieser Forschung. ...
WCl4(NCl) has been prepared as a red-brown crystal powder by the reaction of tungsten hexacarbonyl with excess nitrogen trichloride in boiling CCl4. The complex is associated via chloro bridges, forming dimeric units, according to the IR spectrum. Thermal decomposition at 200 °C leads to tungsten nitride trichloride, WNCl3,. With acetonitrile, WCl4(NCl) reacts with formation of the monomeric complex [CH,CN-WCl4(NCl)], which was characterized by its IR spectrum as well as by an X-ray structure determination. Crystal data: space group P21/m, Z = 2 (1387 independent observed reflexions, R = 0.07). Lattice dimensions at 20 °C: a = 590.4(3), b = 729.0(3), c = 1124.6(4) pm, β = 100.63(2)°. The complex forms monomeric molecules, in which the tungsten atom has a distorted octahedral environment of four chlorine atoms in equatorial positions, and the acetonitrile molecule in trans-position to the group. Bond lengths WN = 172 and NCI = 161 pm; bond angle WNCl = 175.5°.