über die Reaktionen von CH3OCl,CF3OCl,CF3OF und CF3OH mit dem supersauren System HF/MF5 (M = As,Sb) Darstellung und Charakterisierung von CH3OCl(H)+MF6− und CF3OCl(H)+MF6−

On the Reactions of CH₃OCl, CF₃OCl, CF₃OF, and CF₃OH with the Superacid System HF/MF₅ (M = As, Sb). Preparation and Characterization of CH₃OCl(H)⁺MF₆^? and CF₃OCl(H)⁺MF₆^? The preparation of the chlorine oxoniumsalts CH₃OCl(H)⁺MF₆^? and CF₃OCl(H)⁺MF₆^? by protonation of CH₃OCl and CF₃OCl in the superacid solution of HF/MF₅ (M = As, Sb) is described. However CF₃OF and CF₃OH have not been protonated under the same conditions. In the case of CF₃OH the formation of F₂CO · MF₅ is observed. The novel compounds are characterized by nmr- and vibrational spectroscopy.     

On Excited States of Alkyl Bridged [14] Annulenes

The linear dichroic absorption spectrum of 1,3,6,8-trans-15, 16-hexamethyl-dihydropyrene has been measured in stretched polyethylene at 77K, and CNDO-CI calculations with inclusion of singly and doubly excited configurations have been carried out on a series of alkyl bridged [14] annulenes with pyrene- and anthracene-shaped perimeters. Transitions to e_3g → e_4u type ¹B states are well described, and the results indicate that additional low-energy excited states originate from e_3g→ e_5g and e_2u → e_4u type configurations interacting strongly with doubly excited configurations of the e_3g, e_3g → e_4u, e_4u type. The second excited singlet state responsible for the weak transition observed between the ¹L bands may be assigned to one of these additional states, but it is probably of complex nature, similar to the ‘phantom’ state in linear polyenes.     

On the exceptional reactivity of the norbornene double bond

The anomalous reactivity of the bicyclo [2.2.1] heptene double bond is interpreted in terms of hyperconjugative effects, leading to an exceptionally low endo out-of-plane deformation potential.     

Cooperative effects in binuclear zirconocenes: Their synthesis and use as catalyst in propene polymerization

The mono- and bis-cyclopentadienyl compounds 1-(Cp″)-4-(CH₃)C₆H₄ (1) and 1, 4-(Cp″)₂C₆H₄ (2) (Cp″ = 3,4-dimethylcyclopenta-1,3-diene-1-yl) have been synthesized. The reactions of the lithium salts of 1 and 2 with CpZrCl₃ · dme (dme = dimethoxyethane) and Cp*ZrCl₃(CP* = C₅(CH₃)₅) yielded the mono- and bi-nuclear bridged zirconocenes 1-(Cp″ZrCpCl₂)-4-(CH₃)C₆H₄ (3), 1,4-(Cp″ZrCpCl₂)₂C₆H₄ (4) and 1,4-(Cp″ZrCp*Cl₂)₂C₆H₄ (5). When activated with methylaluminoxane (MAO), the mono- and bi-nuclear zirconocenes 3 and 4 catalyse the polymerization of propene. The influence of the catalyst composition on the polymerization kinetics and molecular weight is discussed.     

Improved method for the determination of sulfur components in natural gas

Determination of trace concentrations of sulfur components in natural gas is a true analytical challenge. Only analytical procedures based on gas chromatography can meet the sensitivity and accuracy requirements dictated by environmental regulation institutions and modern chemical industry. In the present contribution the sample pretreatment and chromatographic separation steps have been evaluated and optimized based on the use of a flamebased sulfur chemiluminescence detector (SCD) for target compound detection. The proposed instrument consists of a programmed temperature vaporizing (PTV) injector employing a liner packed with Chromosorb 104, a 4 μm thick film apolar column and a flame-based SCD. Using a 13 mL sample loop the detection limit achievable with the new method is 3 μg S/m³. The precision of replicate measure. ments is generally in the range of 5–15% relative standard deviation. Lower detection limits can be achieved by preconcentrating larger sample volumes, e.g. 100 mL.     

A Short Synthesis of (−)-Carbovir

(–)-Carbovir ((–)- 1 ) was synthesized via the cyclic carbonate 2 in four steps starting from enantiomerically enriched (–)-(S)-(cyclopent-2-enyl)methanol ((–)- 3 ).     

Magnetic circular dichroism of heterocycles: thiophene

The existence of two transitions with opposite MCD in the 38000–45000 cm^?1 absorption of thiophene is demonstrated. The B-term for the lowest transition is negative, while it is positive for the higher one. Quantum mechanical calculations in the π-electron approximation resulted in energies, oscillator strengths and MCD terms in good agreement with the observed spectra.     

Simultaneous determination of ondansetron and tropisetron in human plasma using HPLC with UV detection

A rapid and sensitive HPLC method for the simultaneous quantitation of ondansetron and tropisetron, two serotonin (5-HT) receptor antagonists frequently used in treatment and prevention of nausea and emesis, is described. The procedure involves liquid-liquid extraction of human plasma with dichloromethane coupled with reversed-phase HPLC and UV detection. The lower limits of quantification (LOQ) were 0.62 ng/mL for ondansetron and 1.25 ng/mL or tropisetron. Intra- and inter-assay coefficients of variation ranged from 1.5 to 7.5% and 5.3 to 13.7%, respectively. The sensitivity and precision were sufficient for determination of plasma concentrations after therapeutic administration of both drugs and the method can be used for the estimation of pharmacokinetic parameters.     

Ring expansion and hydrogen scrambling in the molecular ion of 3-methylthiophene

The ratio [M ? D]/{[M-D] + [M ? H]} in the 70 eV mass spectra of six deuterated 3-methylthiophenes has been determined. From these values the mole fractions of the molecular ions that lose hydrogen atoms specifically from the various positions of the molecule were calculated, as well as the mole fraction in which the hydrogen atoms are fully scrambled before hydrogen elimination. It appears that hydrogen atoms are mainly lost from a fully scrambled [C₅H₆S]⁺· ion and from the α-position of the original molecular ion. A deuterium isotope effect of 1·60 to 1·72 was calculated for the hydrogen elimination. The reaction was also studied at low electron energies. In order to determine the degree of scrambling in the [C₅H₅S]⁺ ions, some decomposition reactions of this ion were investigated.     

Propene polymerization using homogeneous MAO-activated metallocene catalysts: Me2Si(Benz[e]Indenyl)2ZrCl2/MAO vs. Me2Si(2-Me-Benz[e]Indenyl)2ZrCl2/MAO

Propene was polymerized at 40°C and 2-bar propene in toluene using methylalumoxane (MAO) activated rac-Me₂Si(Benz[e]Indenyl)₂ZrCl₂ ( BI ) and rac-Me₂Si(2-Me-Benz[e]Indenyl)₂ZrCl₂ ( MBI ). Catalyst BI /MAO polymerizes propene with high activity to afford low molecular weight polypropylene, whereas MBI /MAO is less active and produces high molecular weight polypropylene. Variation of reaction conditions such as propene concentration, temperature, concentration of catalyst components, and addition of hydrogen reveals that the lower molecular weight polypropylene produced with BI /MAO results from chain transfer to propene monomer following a 2,1-insertion. A large fraction of both metallocene catalyst systems is deactivated upon 2,1-insertion. Such dormant sites can be reactivated by H₂-addition, which affords active metallocene hydrides. This effect of H₂-addition is reflected by a decreasing content of head-to-head enchainment and the formation of polypropylene with n-butyl end groups. Both catalysts show a strong dependence of activity on propene concentration that indicates a formal reaction order of 1.7 with respect to propene. MBI /MAO shows a much higher dependence of the activity on temperature than BI /MAO. At elevated temperatures, MBI /MAO polymerizes propene faster than BI /MAO. © 1995 John Wiley & Sons, Inc.