Large isotope effect in the quenching of I(52P12) by benzene and benzene-d6

Spin—orbit relaxation of I(5²P_{$\text{1}\text{2}$})(ΔE = 0.94 eV) by benzene-d₆, has been studied at 297 K, using time-resolved atomic resonance fluorescence. A large isotope effect is observed, k_C6H6 = (4.6 ± 0.7) × 10^?13 cm³ molecule^?1 s^?1, and k_C6D6 = (9.9 ± 1.0) × 10^?15 cm³ molecule^?1 s^?1, despite evidence that formation of a bound collision complex may contribute to the quenching mechanism. The roles of resonant energy transfer channels, Franck—Condon factors and the density of final states, in the quenching process, are discussed.     

Ozonolytic determination of double-bond positions in olefins by gas chromatography

Summary Olefinic compounds are reacted with ozone in CCl₄/CH₃OH solution, yielding specific cleavage products, from which the original position of the double bond can be deduced. By reduction with dimethylsulfide the resulting methoxyhydroperoxides are converted into aldehydes and ketones, which are determined by temperature-programmed gas chromatography on silicone columns. The lower compounds (C₁-C₁₀) are analyzed as dinitrophenylhydrazones, the less volatile higher ones as such. 100% yields were obtained for monoolefins of different structures and chain lengths (up to C₃₀).

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Ozonolytische Bestimmung von Doppelbindungen in Olefinen durch Gas-Chromatographie

Zusammenfassung Die Olefine werden in CCl₄/CH₃OH-Lösung mit Ozon behandelt, wodurch spezifische Spaltprodukte entstehen, aus denen man die ursprüngliche Lage der Doppelbindung ableiten kann. Die entstandenen Methoxyhydroperoxide werden durch Reduktion mit Dimethylsulfid zu Aldehyden und Ketonen umgesetzt, die durch temperaturprogrammierte Gas-Chromatographie an Siliconsäulen analysiert werden. Die niedrigeren (C₁-C₁₀) Verbindungen werden als Dinitrophenylhydrazone bestimmt, die weniger flüchtigen höheren als solche. Für aliphatische Monoolefine verschiedener Struktur und Kettenlänge (bis C₃₀) wurden 100%ige Ausbeuten erhalten.

Lecture presented at Euroanalysis I Conference, 28. 8.–1. 9. 1972 in Heidelberg, Germany.     

Hydrogen bond-assembled fullerene molecular shuttle

A novel [2]rotaxane has been prepared in which fullerene C(60) behaves as both a stopper and a photoactive unit. The amphiphilic nature of the rotaxane thread can be used to shuttle the macrocycle from close to the fullerene spheroid (in nonpolar solvents) to far away (in polar solvents). The differing location of the macrocycle in dichloromethane and dimethyl sulfoxide gives rise to effects detectable by (1)H NMR and time-resolved spectroscopy.     

An industrially viable catalyst system for palladium-catalyzed telomerizations of 1,3-butadiene with alcohols

The telomerization reaction of 1,3-butadiene with alcohols to give alkyl octadienyl ethers in the presence of palladium-carbene catalysts has been studied in detail. Unprecedented catalyst efficiency with turnover numbers (TON) up to 1,500,000 and turnover frequencies (TOF) up to 100,000 h(-1) have been obtained after optimization for the reaction of methanol in the presence of an excess of in situ generated carbene ligands. High yields (75-97 %) and catalyst productivities (TON 15,000-100,000) are observed for other aliphatic alcohols and phenols. For comparison five carbene-palladium(0) complexes have been synthesized and characterized by X-ray crystallography. Both electronic and steric effects on the stability and reactivity of the catalysts have been discussed on the basis of density functional theory calculations.     

Biomimetic synthesis of (+/-)-pinnatal and (+/-)-sterekunthal A

Concise biomimetic syntheses of the antimalarial naphthoquinones (+/-)-pinnatal and (+/-)-sterekunthal A are described.     

Liquid chromatographic-mass spectrometric studies on the in vitro degradation of a poly(ether ester) block copolymer

A detailed study on the in vitro degradation of a poly(ethylene glycol) and poly(butylene terephthalate) (PEOT/PBT) segmented block copolymer was carried out using liquid chromatography/electrospray-mass spectrometry. Accelerated hydrolysis of PEOT/PBT was achieved by placing the material for 14 days in a refluxing phosphate buffered saline (pH 7.4) solution. All major degradation products and several side-products were identified using both the positive and the negative ion mode. The data indicate that degradation does not only occur in the "soft", but also in the "hard" segments of the polymer. Liquid chromatographic separation is required to distinguish between degradation products with different sequences but identical molecular mass. The addition of ammonium and sodium ions provided important complementary information on the number of monomer units present in the degradation products.     

Trichloropalladate(II) Complexes with Pyridine Ligands – Molecular Structure and Conformational Analysis of [K(18C6)][PdCl3(py)]

[K(18C6)]₂[Pd₂Cl₆] ( 1 ) (18C6 = 18‐crown‐6) was found to react with pyridines in a strictly stoichiometric ratio 1 : 2 in methylene chloride or nitromethane to yield trichloropalladate(II) complexes [K(18C6)][PdCl₃(py*)] (py* = py, 2a ; 4‐Bnpy, 2b ; 4‐tBupy, 2c ; Bn = benzyl; tBu = tert‐butyl). The reaction of 1 with pyrimidine (pyrm) in a 1 : 1 ratio led to the formation of the pyrimidine‐bridged bis(trichloropalladate) complex [K(18C6)]₂[(PdCl₃)₂(μ‐pyrm)] ( 3 ). The identities of the complexes were confirmed by means of NMR spectroscopy (¹H, ¹³C) and microanalysis. The X‐ray structure analysis of 2a reveals square‐planar coordination of the Pd atom in the [PdCl₃(py)]^? anion. The pyridine plane forms with the complex plane an angle of 55.8(2)°. In the [K(18C6)]⁺ cation the K⁺ lies outside the mean plane of the crown ether (defined by the 6 O atoms) by 0.816(1) Å. There are tight K···Cl contacts between the cation and the anion (K···Cl1 3.340(2) Å, K···Cl2 3.166(2) Å). To gain an insight into the conformation of the [PdCl₃(py)]^? anion, DFT calculations were performed showing that the equilibrium structure ( 6eq ) has an angle between the pyridine ligand and the complex plane of 35.3°. Rotation of the pyridine ligand around the Pd–N vector exhibited two transition states where the pyridine ligand lies either in the complex plane ( 6TS _pla, 0.87 kcal/mol above 6eq ) or is perpendicular to it ( 6TS _per, 3.76 kcal/mol above 6eq ). Based on an energy decomposition analysis the conformation of the anion is discussed in terms of repulsive steric interactions and of stabilizing σ and π orbital interactions between the PdCl₃^? moiety and the pyridine ligand.     

Atropo-enantioselective total synthesis of knipholone and related antiplasmodial phenylanthraquinones

The "lactone concept" has been efficiently employed for the first atropo-enantioselective synthesis of knipholone and related natural phenylanthraquinones. Besides the regio- and stereoselective construction of the biaryl axis, another important step was the "synthetically late" introduction of the C-acetyl group, either by a Friedel-Crafts type acetylation or by an ortho-selective Fries rearrangement first tested on simplified model systems and subsequently applied to the highly atroposelective preparation of the natural products and of simplified analogs thereof for biotesting. The synthetic availability of these natural biaryls, their precursors, and their unnatural analogs permitted a broader investigation of the antiplasmodial activities of these interesting biaryls.     

13C and (15)N chemical shift tensors in adenosine,guanosine dihydrate, 2'-deoxythymidine,and cytidine

The (13)C and (15)N chemical shift tensor principal values for adenosine, guanosine dihydrate, 2'-deoxythymidine, and cytidine are measured on natural abundance samples. Additionally, the (13)C and (15)N chemical shielding tensor principal values in these four nucleosides are calculated utilizing various theoretical approaches. Embedded ion method (EIM) calculations improve significantly the precision with which the experimental principal values are reproduced over calculations on the corresponding isolated molecules with proton-optimized geometries. The (13)C and (15)N chemical shift tensor orientations are reliably assigned in the molecular frames of the nucleosides based upon chemical shielding tensor calculations employing the EIM. The differences between principal values obtained in EIM calculations and in calculations on isolated molecules with proton positions optimized inside a point charge array are used to estimate the contributions to chemical shielding arising from intermolecular interactions. Moreover, the (13)C and (15)N chemical shift tensor orientations and principal values correlate with the molecular structure and the crystallographic environment for the nucleosides and agree with data obtained previously for related compounds. The effects of variations in certain EIM parameters on the accuracy of the shielding tensor calculations are investigated.