Effects of pi-bonding on stability and reactivity in

Recent synthetic and theoretical investigations of organometallic compounds containing the CpCr(NO) fragment have shown an interdependence of the electronic nature of ancillary ligands and the stability of a given Cr oxidation state. Understanding the correlation between ligand pi-bonding properties and the nature of the metal-based frontier orbitals permits the rationalization of observed reactivity patterns, and the identification and preparation of new classes of target molecules.     

Thermal degradation of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl acrylate copolymers

Vinylidene chloride polymers containing comonomer units capable of consuming evolved hydrogen chloride to expose good radical-scavenging sites might be expected to display greater thermal stability than similar polymers containing simple alkyl acrylates as comonomer. Incorporation of a comonomer containing the phenyl t-butyl carbonate moiety into a vinylidene chloride polymer has the potential to afford a polymer with pendant groups which might interact with hydrogen chloride to expose phenolic groups. Copolymers of vinylidene chloride with [4-(t-butoxycarbonyloxy)phenyl]methyl acrylate have been prepared, characterized, and subjected to thermal degradation. The degradation has been characterized by thermal and spectroscopic techniques. The degradation of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl acrylate copolymers is much more facile than the same process for similar copolymers containing either [4-(isobutoxycarbonyloxy)phenyl]methyl acrylate or methyl acrylate, a simple alkyl acrylate, as comonomer. During copolymer degradation, [4-(t-butoxycarbonyloxy) phenylmethyl acrylate units are apparently converted to acrylic acid units by extensive fragmentation of the sidechain. Thus, the phenyl t-butyl carbonate moiety does function as a labile acid-sensitive pendant group but its decomposition in this instance leads to the generation of a phenoxybenzyl carboxylate capable of further fragmentation.     

Laser-excited ionic fluorescence spectrometry of rare-earth elements in the inductively-coupled plasma

A pulsed tunable dye laser pumped with an excimer laser is used to excite ionic fluorescence of the rare earth elements in the inductively-coupled plasma. Because several fluorescence lines were observed after laser excitation, it was possible to draw partial energy-level diagrams for most of the rare earths. Non-resonance fluorescence lines were used for all measurements in order to minimize spectral interferences. Detection limits at given excitation wavelengths are reported for each element. Laser-excited ionic fluorescence eliminates the problem of spectral interferences which has been associated with the determination of the rare earths by atomic emission spectrometry in the inductively-coupled plasma.     

Structural isomerism of mono- and sesquiterpenoid skeletons

A generator of chemical structures (CONGEN) has been utilized to investigate two aspects of the structural isomerism of mono- and sesquiterpenoid skeletons: (1) the scope of possible isomers under various structural constraints; and (2) the scope of possible isomers based on a mechanistic model which allows interactive exploration of reactions of formation and interconversion. The possibilities, even under severe constraints, are many more than the structural types commonly encountered in natural. These results indicate the potential danger of structural assignment based in part on biogenic grounds.     

Two-component dendritic gel: effect of stereochemistry on the supramolecular chiral assembly

The self-assembly of diaminododecane solubilised by four different stereoisomeric dendritic peptides to form gel-phase materials in toluene was investigated. The second generation dendritic peptides were based on D- and L-lysine building blocks, and each contained three chiral centres. By designing dendritic peptides in which the configurations of the chiral centres were modified, and applying them as gelator units, the assembly of stereoisomers could be investigated. In all cases, the self-assembly of gelator units resulted in macroscopic gelation. However, the degree of structuring was modulated by the stereoisomers employed, an effect which changed the morphology and macroscopic behavior of the self-assembled state. Enantiomeric (L,L,L or D,D,D) gelator units formed fibrous molecular assemblies, whilst the racemic gel (50 % L,L,L : 50 % D,D,D) formed a flat structure with a "woven" appearance. Gelator units based on L,D,D or D,L,L dendritic peptides also formed fibrous assemblies, but small-angle X-ray scattering indicated significant morphological differences were caused by the switch in chirality. Furthermore, the macroscopic stability of the gel was diminished when these peptides were compared with their L,L,L or D,D,D analogues. In this paper it is clearly shown that individual stereocentres, on the molecular level, are directly related to the helicity within the fibre. It is argued that the chirality controls the pattern of hydrogen bonding within the assembly, and hence determines the extent of fibre formation and the macroscopic gel strength.     

Comparing the Atomic Structures of Binary MO2-SiO2 (M = Ti, Zr or Hf) Xerogels

The incorporation of transition-metal oxides into silica can give materials with useful optical, electronic or catalytic properties. For example, ZrO₂-SiO₂ and HfO₂-SiO₂ materials are of interest due to their high dielectric constants. Here we present a comparison of extended X-ray absorption fine structure and small-angle X-ray scattering results for acid-catalysed binary (MO₂) _x (SiO₂)_{1 – x} (M = Ti, Zr or Hf) xerogels, with x up to 0.4 and heat treatments up to 750°C. Detailed observations for TiO₂-SiO₂ and ZrO₂-SiO₂ xerogels provide a basis for interpretation of new results for HfO₂-SiO₂ xerogels. At low concentrations metal atoms are homogeneously incorporated into the silica network. Ti adopts coordinations of 4 or 6, and Zr and Hf both adopt higher coordination of 6 or 7 (the larger coordinations being due to ambient moisture). At higher concentrations, phase separation of metal oxide occurs. Such regions become clearly separated from the silica network for TiO₂, but remain very finely mixed with silica network for ZrO₂ and HfO₂.     

A simple and versatile method for the synthesis of acetals from aldehydes and ketones using bismuth triflate

Acetals are obtained in good yields by treatment of aldehydes and ketones with trialkyl orthoformate and the corresponding alcohol in the presence of 0.1 mol % Bi(OTf)3.4H2O. A simple procedure for the formation of acetals of diaryl ketones has also been developed. The conversion of carbonyl compounds to the corresponding 1,3-dioxolane using ethylene glycol is also catalyzed by Bi(OTf)3.4H2O (1 mol %). Two methods, both of which avoid the use of benzene, have been developed.     

Measurements by microdialysis of free tissue concentrations of propranolol.

To determine the free concentration of a drug (propranolol) in the interstitial space in humans in vivo, seven male students were investigated by microdialysis of the periumbilical subcutaneous tissue. The microdialysis catheters were calibrated in vivo and the propranolol concentration was determined by high-performance liquid chromatography. Ten hours after intake of 80 mg of propranolol, the total plasma and free interstitial propranolol concentrations were 80 +/- 43 and 7 +/- 2 nM, respectively. After a second dose, maximum concentration was reached after 80 +/- 10 min and 98 +/- 12 min, in plasma, and the concentrations in the interstitial water were 594 +/- 138 and 27 +/- 7 nM, respectively. In a second study, microdialysis was performed on the left ventricular wall in six pigs receiving an intravenous injection of 5 mg of propranolol followed by a constant propranolol infusion for 40 min (5 mg propranolol per h). The maximum concentrations of propranolol were 97 +/- 29 and 6 +/- 2 nM in plasma and in interstitial water, respectively. The data suggest that microdialysis is a useful tool for recording the free concentrations of a drug in the interstitial space.     

Electron impact and chemical ionization mass spectra of 2,2-diphenyl-3-arylcyclobutanone oximes. Formation of ion m/z 167

The electron impact (EI) and chemical ionization (CI) spectra of 2,2-diphenyl-3-aryl cyclobutanone oximes (1–5) are reported. Formation of diphenylmethyl cation at m/z 167 is a major fragmentation process in both EI and CI spectra. Labelling studies established that the hydrogen involved in this rearrangement transfers from the NOH group and not from cyclobutane ring positions. The [M + 3]⁺ ions are formed under CI conditions as a result of C?N double bond reduction. An interesting secondary kinetic isotope effect is observed in the formation of ion e at m/z 183 in both EI and CI spectra. Other characteristic fragmentation pathways occurring in the EI and CI spectra of these compounds are outlined.