Mass spectral fragmentation patterns of heterocycles. VI. Carbazole and 1,8-dideuteriocarbazole

The mass spectral fragmentation of carbazole was reinvestigated using metastable ion studies, exact mass measurements and 1,8-dideuteriocarbazole. 1,8-Dideuteriocarbazole was prepared by successive metalations and deuterations of carbazole. The ¹H- and ¹³C-nmr spectra of 1,8-dideuteriocarbazole confirmed earlier assignments for carbazole.     

Examination of phase eouilibrium of the Ca(NO3)2-H2O system with quasi-isothermal-quasi-isobaric thermogravimetry and DTA

By means of the quasi-isothermal-quasi-isobaric thermogravimetry (Q-TG) technique ideal experimental conditions can be realized. This provides a possibility to compare the measured data with data from other phase equilibrium (solubility, tension) examinations.On the example of the Ca(NO₃)₂-H₂O system it is demonstrated that the phase diagrams can be used to interpret thermoanalytical (especially Q-TG) curves, or conversely, the results of thermoanalytical (Q-TG, DTA) examinations can serve for the control of equilibrium data of phase diagrams or to supply missing data.

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Zusammenfassung Mit Hilfe des quasi-isothermischen-quasi-isobaren thermogravimetrischen (Q-TG) Verfahrens können ideale Versuchsbedingungen realisiert werden. Das ermöglicht es, die gemessenen Daten mit von anderen Phasengleichgewicht-Untersuchungen (Lösbarkeit, Tension) stammenden Daten zu vergleichen.Durch das Beispiel des Ca(NO₃)₂-H₂O-Systems wurde es gezeigt, dass die Phasendiagramme zur Erklärung von thermoanalytischen (insbesondere Q-TG) Kurven können angewandt werden, oder umgekehrt, die Ergebnisse der thermoanalytischen (Q-TG, DTA) Untersuchungen als Kontrolle der Gleichgewichtsdaten der Phasendiagramme dienen können, oder die fehlenden Daten liefern können.

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- (Q-) . ( , ). Ca(NO₃)₂-H₂O , ( Q-) , .

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The authors are indebted to Prof. E. Pungor for valuable discussions, and thank Mrs. M. Kiss and Miss I. Fábián for their technical assistance.     

Synthesis of coumarins by ring-closing metathesis using Grubbs’ catalyst

A novel generally applicable synthesis of coumarins from phenolic substrates utilizing ring-closing metathesis is described. This sequence involves O-allylation of phenols followed by ortho-Claisen rearrangement, subsequent based-induced isomerization affording 2-(1-propenyl)phenols, acylation with acryloyl chloride, and finally ring-closing metathesis (RCM) with Grubbs’ second generation catalyst.     

Synthesis and properties of allenic natural products and pharmaceuticals

Nowadays, about 150 natural products comprising an allenic or cumulenic structure are known. The chemistry of these compounds has turned out to be a very attractive and prolific area of interest: advances in the isolation and characterization of new allenic natural products have led to the establishment of efficient synthetic procedures which in many cases also open up an access to enantiomerically pure target molecules. Inspired by the intriguing biological activities of many allenic natural products, allene moieties are now systematically introduced in pharmacologically active classes of compounds (steroids, prostaglandins, amino acids, nucleosides). The functionalized allenes thus obtained often exhibit impressive activities as mechanism-based enzyme inhibitors, cytotoxic, or antiviral agents. A prerequisite for further developments in this field is the efficient stereoselective synthesis of allene derivatives.     

Synthesis and structural characterisation of a novel tris-methylene bridged compound (NO) 4Fe2 Se(μ-CH2)3

The tris-methylene bridged compound (NO)₄Fe₂Se(μ-CH₂)₃ has been isolated. It has been characterised by IR and ¹H, ¹³C, and ⁷⁷Se NMR spectroscopy. Its structure has been determined by single-crystal X-ray diffraction methods. The structure consists of a heavy atom triangle consisting of one Se and two Fe atoms. The Fe-Fe and the two Fe-Se edges are bridged by methylene groups.     

Influence of localized excited states on the transition moment directions of charge transfer complex absorptions

The influence of localized excited (LE) states on the spectroscopy of charge transfer (CT) complexes has been examined for a series of complexes formed between methyl-substituted benzene donors and 1,2,4,5-tetracyanobenzene as acceptor in 1,2-dichloroethane and octanenitrile solvents. A molecular orbital model was used to describe the appearance of multiple CT absorption bands that occur in the spectra of these complexes. The influence of LE states in these CT absorptions was explored using time-resolved linear dichroism spectroscopy where the direction of the CT transition moment vector (TMV) was used to probe the magnitude of intensity borrowing. The TMV directions for each of the observed CT transitions within the absorption spectra were determined for several complexes. In some cases, the observed CT transitions were interpreted as being pure CT transitions; in others the observed transitions are influenced significantly by a LE transition. The correlation between the TMV directions and the transition energy suggests that the magnitude of intensity borrowing is influenced not only by the energy difference between the CT and LE transitions but also by the specific character of the transitions under consideration.     

Obtusin,a unique bromine-containing polycyclic ketal from the red marine alga Laurenciaobtusa

The structure of a unique bromine-containing ketal, obtusin ($\text{1}$), has been determined by spectral and X-ray crystallographic techniques. Obtusin is a natural component of the Mediteranean red alga $\text{Laurencia}$$\text{obtusa}$.     

Deprotection of Thioacetals Using K2S2O8/[bmim]Br as a Mild and Efficient Reagent under Solvent-Free Conditions

Summary. A straightforward and effective procedure for the deprotection of thioacetals to the corresponding carbonyl compounds using potassium persulfate and the ionic liquid [bmim]Br under solvent-free conditions is reported. A variety of aliphatic and aromatic 1,3-dithiolanes or 1,3-dithanes was deprotected to the corresponding carbonyl derivatives by this procedure.     

Spectroscopy and quantum chemical modeling reveal a predominant contribution of excitonic interactions to the bathochromic shift in alpha-crustacyanin, the blue carotenoprotein in the carapace of the lobster Homarus gammarus

To resolve the molecular basis of the coloration mechanism of alpha-crustacyanin, we used (13)C-labeled astaxanthins as chromophores for solid-state (13)C NMR and resonance Raman spectroscopy of [6,6',7,7']-(13)C(4) alpha-crustacyanin and [8,8',9,9',10,10',11,11',20,20']-(13)C(10) alpha-crustacyanin. We complement the experimental data with time-dependent density functional theory calculations on several models based on the structural information available for beta-crustacyanin. The data rule out major changes and strong polarization effects in the ground-state electron density of astaxanthin upon binding to the protein. Conformational changes in the chromophore and hydrogen-bond interactions between the astaxanthin and the protein can account only for about one-third of the total bathochromic shift in alpha-crustacyanin. The exciton coupling due to the proximity of two astaxanthin chromophores is found to be large, suggesting that aggregation effects in the protein represent the primary source of the color change.     

Correlating second harmonic optical responses of single Ag nanoparticles with morphology

Femtosecond laser excited second harmonic (SH) activity from single Ag nanoparticles is reported. A correlation of SH single-particle measurements with high-resolution imaging of particle morphology by TEM was achieved by creating position markers on an optical and electron transparent substrate (Si3N4 thin film, approximately 100 nm). We compared the SH activity of single Ag nanoparticles (nanospheres versus nanorods) and cluster structures (composed of two or multiple particles, e.g., dimers and trimers). The direct correlation of single-particle structures and SH activity, spectral and power dependence, strongly suggests one-photon resonant driven nonlinear oscillator response mechanism.