Synthesis and characterization of propylene and butylene bridged fac-tricarbonylrhenium(I) biscarbene complexes

Nine new complexes of the type fac-[(CO)(3)ReBr(L)], where L represents a chelating bis-NHC ligand consisting of two alkylene (propylene and butylene) bridged (R') N-substituted (R) imidazoline moieties (1,1'-di-R-3,3'-R'-diimidazolin-2,2'-diylidene), were synthesized. Spectroscopic methods, elemental analysis, mass spectrometry and X-ray crystallography were employed to characterize the complexes both in solution and in solid state. The complexes were compared to the analogous complexes with smaller bridge lengths in order to reveal differences and similarities. Depending on the bridge length and the ligand size in some cases dinuclear complexes were obtained as small scale side products.     

Recent developments in ion detection techniques for Penning trap mass spectrometry at TRIGA-TRAP

The highest precision in the determination of nuclear and atomic masses can be achieved by Penning trap mass spectrometry. The mass value is obtained through a measurement of the cyclotron frequency of the stored charged particle. Two different approaches are used at the Penning trap mass spectrometer TRIGA-TRAP for the mass determination: the destructive Time-Of-Flight Ion Cyclotron Resonance (TOF-ICR) technique and the non-destructive Fourier Transform Ion Cyclotron Resonance (FT-ICR) method. New developments for both techniques are described, which will improve the detection efficiency and the suppression of contaminations in the case of TOF-ICR. The FT-ICR detection systems will allow for the investigation of an incoming ion bunch from a radioactive-beam facility on the one hand, and for the detection of a single singly charged ion in the Penning trap on the other hand.     

Online ATR-IR investigations and mechanistic understanding of the carbonylation of epoxides - the selective synthesis of lactones or polyesters from epoxides and CO

In situ ATR-IR spectroscopy is applied as a powerful tool to study the factors that control the reaction of epoxides with carbon monoxide in the presence of [Lewis acid]⁺[Co(CO)₄]⁻ salts. Based on these investigations, a consistent mechanistic scheme is presented, comprising the main lactone and polyester products and minor components, e.g., acetone and crotonic acid derivatives. β-Alkoxy-acyl-cobalttetracarbonyl species are shown to be key intermediates from which two reaction routes start in dependence of the applied Lewis acid (LA). Labile LA-alkoxy combinations favor a backbiting process of the oxygen function on the Co-acyl bond, primarily producing lactone products. More stable LA-alkoxy entities are unreactive toward PO conversion and afford a polymerization reaction after the addition of a nucleophile. In that case, the Lewis acid remains bonded to the chain end.     

Reactivity of Yttrium Carboxylates Toward Alkylaluminum Hydrides

Yttrocene‐carboxylate complex [Cp*₂Y(OOCAr^Me)] (Cp*=C₅Me₅, Ar^Me=C₆H₂Me₃‐2,4,6) was synthesized as a spectroscopically versatile model system for investigating the reactivity of alkylaluminum hydrides towards rare‐earth‐metal carboxylates. Equimolar reactions with bis‐neosilylaluminum hydride and dimethylaluminum hydride gave adduct complexes of the general formula [Cp*₂Y(μ‐OOCAr^Me)(μ‐H)AlR₂] (R=CH₂SiMe₃, Me). The use of an excess of the respective aluminum hydride led to the formation of product mixtures, from which the yttrium‐aluminum‐hydride complex [{Cp*₂Y(μ‐H)AlMe₂(μ‐H)AlMe₂(μ‐CH₃)}₂] could be isolated, which features a 12‐membered‐ring structure. The adduct complexes [Cp*₂Y(μ‐OOCAr^Me)(μ‐H)AlR₂] display identical ¹J(Y,H) coupling constants of 24.5 Hz for the bridging hydrido ligands and similar ⁸⁹Y NMR shifts of δ=?88.1 ppm (R=CH₂SiMe₃) and δ=?86.3 ppm (R=Me) in the ⁸⁹Y DEPT45 NMR experiments.     

Time-of-flight mass and photoelectron spectroscopy study of LaC n -

Mass spectra of LaC _n ^- were taken by using a laser-vaporization source (LVS) and pulsed arc cluster ion source (PACIS) applied to La-carbon composite rods (1:130 atomic ratio). The mass spectrum using the LVS with annealing procedure has shown several magic numbers for LaC _n ^- (n = 44, 50, 60, and 70), whereas only small LaC _n ^- (up to n = 14) have been observed in the mass spectrum using the PACIS. Photoelectron spectra of some of these have been measured using a magnetic-bottle type time-of-flight electron spectrometer. These results indicate the exsistence of a few conformational isomers for small La-containing carbon cluster negative ions LaC _n ^- (n = 5–8).     

Amide-Amide and Amide-Water Hydrogen Bonds: Implications for Protein Folding and Stability

Amide-amide hydrogen bonds have been implicated in directing protein folding and enhancing protein stability. Inversion transfer (13)C NMR spectroscopy and IR spectroscopy were used to compare the ability of various amide solvents and of water to alter the rate of the cis-trans isomerization of the prolyl peptide bond of Ac-Gly-[β,δ-(13)C]Pro-OMe and the amide I vibrational mode of [(13)C=O]Ac-Pro-OMe. The results indicate that secondary amides are significantly weaker hydrogen bond donors than is formamide or water. These results are most consistent with models for protein folding in which the formation of secondary structure is a cooperative process that follows hydrophobic collapse. These results also suggest that a hydrogen bond between a main-chain oxygen and an asparagine or glutamine sidechain may contribute more to protein stability than does a main-chain-main-chain hydrogen bond.     

The magnetic rotation spectrum of iodine monobromide

The MRS of IBr in the visible region of the spectrum has been studied at high resolution and a rotational and vibrational analysis is reported. The spectrum consists of short runs in J′ for several neighboring vibrational states of the mixed B, ${}^3\text{Π}{}_0{}^{\mathrm{+}}$, and $\text{B}\text{?}\text{′}$, 0⁺ electronic states. These results imply that only a small number of closely related rotational-vibrational states of the combined system have sufficiently long lifetimes to provide the sharp lines required for the appearance of a MRS. The values observed extend to higher energies similar results reported by Selin for the absorption spectrum.