Time-resolved fluorescence of carbon nanotubes and its implication for radiative lifetimes

The temporal evolution of fluorescence from isolated single-wall carbon nanotubes (SWNTs) has been investigated using optical Kerr gating. The fluorescence emission is found to decay on a time scale of 10 ps. This fast relaxation arises from nonradiative processes, the existence of which explains the relatively low observed fluorescence efficiency in isolated SWNTs. From the measured decay rate and a determination of fluorescence quantum efficiency, we deduce a radiative lifetime of 110 ns.     

13C NMR spectroscopy of heteroaromatic compounds: A quantitative measure of electron deficiency and excessiveness

A quantitative measure, ^πΔ, for expressing the π-deficiency and π-excessiveness of heteroaromatic compounds has been developed through ¹³C NMR chemical shift studies. An example which correlates electrophilic reactivity in π-excessive systems with ^πΔ of these compounds is given. The following decreasing order of π-excessiveness and increasing π-deficiency of a number of heteroaromatic systems has been established (numbers in parentheses correspond to ^πΔ): tellurophene (1.83), selenophene (1.43), thiophene (1.35), furan (1.31), benzene (1.00), pyrazine (1.00) pyridine (0.89), pyrimidine (0.77), pyridazine (0.71), 1,2,4-triazine (0.64). These correlations also allow the prediction of ¹³C chemical shifts in substituted heteroaromatic compounds, where the ratio of the difference between a given substituted heterocyclic compound and its parent, and the identically substituted benzene derivative, has been determined.     

Zur lokalisierung funktioneller gruppen in steroiden mit hilfe der massenspektrometrie X : 5-Hydroxy-cholestane und 3,5-dihydroxy-cholestane sowie deren acetylderivate

Mass spectra of steroids with hydroxygroups in position 3 and 5 are characterized by ions produced by elimination of C-1 to C-4. These degradation reactions are especially important for steroids with a 5β-configurated hydroxygroup.In addition 3α,5β-dihydroxysteroids loose the carbon atoms of ring A together with C-6 in a structure specific cleavage process, enabling an unambigiuous determination of the configuration. Apart from these degradation reactions caused by the presence of functional groups derivatives of cholestanes show the well known cleavage processes of ring D. 3α,5-Dihydroxycholestanes may be differentiated from their 3β-isomeres by the preferential production of an ion of mass 247.     

On the Gabai-Eliashberg-Thurston Theorem

We present a new, completely three-dimensional proof of the fact, due to the combined work of Gabai and Eliashberg-Thurston, that every closed, oriented, connected, irreducible 3-manifold with nonzero second homology carries a universally tight contact structure.     

5‐Nitro‐N‐phenyl‐2‐thio­fur­amide

The title compound, C₁₁H₈N₂O₃S, crystallizes with two crystallographically independent mol­ecules, which are conformationally almost identical, per asymmetric unit. The dihedral angles between the phenyl and 2‐thio­fur­amide planes are 46.3 (1) and 47.0 (1)° for the first and second mol­ecule, respectively. Strong intramolecular N—H?O hydrogen bonds [N?O 2.664 (2) and 2.661 (2) Å] dictate an anti conformation of the C=S groups in relation to the furan‐O atoms.     

3‐[(2‐Oxo‐1‐naphthylidene)methyl­amino]­benzoic acid

The title compound, C₁₈H₁₃NO₃, exists as a keto­amino tautomer implying a fairly short N—H?O intramolecular hydrogen bond between the 2‐naphthalenone and amino moieties [N?O 2.531 (3) Å] which is enhanced by the π‐electron delocalization effect. The naphthald­imine and 3‐carboxy­phenyl fragments are inclined at an angle of 4.41 (7)°, so the mol­ecule is almost planar. The mol­ecules are connected by intermolecular O—H?O hydrogen bonds between the carboxy and keto O atoms, forming infinite chains around the twofold screw axes parallel to b .     

N‐(4‐Methyl­phenyl)‐N‐(5‐nitro­furfuryl)‐N‐prop‐2‐ynyl­amine

The title compound, C₁₅H₁₄N₂O₃, is the first example of a structurally determined tertiary amine with both N‐5‐nitro­furfuryl and N‐prop‐2‐ynyl moieties. The mol­ecule is not planar, i.e. the furan ring is inclined at an angle of 84.35 (4)° to the phenyl ring. The crystal structure is dominated by van der Waals forces. The terminal alkynyl group as the strongest C—H hydrogen‐bond donor is not involved in hydrogen‐bond formation.     

Thermal behavior of gamma-irradiated urea–formaldehyde composites based on the differently activated montmorillonite K10

Journal of Thermal Analysis and Calorimetry - This work investigated the thermal characteristics of irradiated composite materials formulated on the urea–formaldehyde resin (UF) and...     

A mixed integer program for partitioning graphs with supply and demand emphasizing sparse graphs

The focus of this paper is on finding optimal solutions for the problem of maximal partitioning of graphs with supply and demand (MPGSD) for arbitrary graphs. A mixed integer programming (MIP) model is developed for the problem of interest. We also present some specific constraints that can be used in the case of tree graphs. With the goal of lowering the computational cost for solving the underlying model, a preprocessing stage is included. It is used to produce additional constraints based on shortest paths in the graph. With the aim of exploring the effectiveness of the proposed MIP formulation we have performed computational experiments for general graphs and trees. The main objective of the tests is to observe the properties and sizes of supply/demand graphs that can be solved to optimality using the proposed approach in reasonable time. The conducted computational experiments have shown that the proposed method is especially suitable for sparse graphs.