Borrowing hydrogen: iridium-catalysed reactions for the formation of C-C bonds from alcohols

Alcohols have been employed as substrates for C-C bond-forming reactions which involve initial activation by the temporary removal of hydrogen to form an aldehyde. The intermediate aldehyde is converted into an alkene via a Horner-Wadsworth-Emmons reaction, nitroaldol and aldol reactions. The 'borrowed hydrogen' is then returned to the alkene to form a C-C bond.     

Modelling and simulation of steam turbine processes: individual models for individual tasks

Within power plants, several physical, chemical and mechanical processes are conducted to transfer the energy, stored in fossil fuel, into electrical energy. This energy conversion is divided into several stages. Hitherto, the largest conventional power plants employ steam turbines as prime movers to drive a generator. Hence, a steam turbine is one module to convert heat energy into mechanical energy. And thus it is one link in the chain of energy conversions with the aim of generating electrical energy. Today, steam turbine industry faces numerous challenges concerning efficiency, commissioning time, start-up times, operation, availability, safety, cost-effectiveness, etc. Many of these tasks can be supported by simulating the transient operational behaviour of the turbine in advance. For example, the commissioning time can be shortened if the turbine controllers are initialized with well-tuned pre-set parameters; cost-effectiveness can be increased by setting aside unnecessary devices and exactly determining material specifications; safety may be increased by predicting the impacts of failures and thus taking the necessary precautions. Different tasks require different details regarding the employed turbine simulation model. Thus, the turbine controller may be well tuned with less complex simulation models of turbine, generator and electrical grid, whereas detailed studies of failures, mainly the transient behaviour which may lead to serious damages, may require detailed modelling of the turbine-internal thermodynamic processes. Here, a brief overview of models which simulate the transient thermodynamic behaviour of a steam turbine is presented. Three different approaches will be introduced and compared with respect to different operating situations. Also, special attention is directed towards the time dependence of critical states, mainly turbine speed and pressure development in certain areas. The first model is based on a simple, linear approach and is suitable of giving a quick overview. The second one incorporates more details and is useful if the operating point is close to the design point. Finally, the last model incorporates mass and energy balances as well as the major non-linearities. Hence it depicts the turbine behaviour over a large range of operating points.     

Conversion of alcohols into N-alkyl anilines via an indirect aza-Wittig reaction

Iridium catalysed oxidation of alcohols provides the aldehydes required for in situ aza-Wittig reactions and the so-formed imines are reduced to amines under the reaction conditions.     

Three-Dimensional Laser-Doppler Velocimeter Measurements in Swirling Turbulent Pipe Flow

The present work is an experimental study of the evolutioncharacteristics of swirl in the flow in a pipe at a moderately highReynolds number and over a wide range of swirl numbers characterisingthe swirl strength. The measurements are done by 3D LDV in a speciallydesigned facility that works on the principle of refractive-indexmatching. Swirl with the well-defined distribution of a constant angularvelocity is introduced into the test section of this facility by a swirlgenerating device in which a tube bundle in the pipe rotates about thepipe axis. The measurements have been processed to yield themean-velocity vector and the Reynolds stress tensor in this flow. Ananalysis of the measured evolution characteristics brings out a dominanteffect of swirl with a solid body rotation as the annihilation ofReynolds shear streses.     

Understanding the properties of isospectral points and pairs in graphs: The concept of orthogonal relation

The mathematical property orthogonal relationship is used in proving the fact that isospectrality, isocodality and isocoefficiency of vertices within a graph are all equivalent. The same is true for isospectrality, strict isocodality and strict isocoefficiency of pairs (including edges) within a graph, whereas the weak versions of the latter properties are necessary but not sufficient for isospectrality of pairs. Similarly, necessary and sufficient conditions for isospectrality of vertices and pairs in different graphs are derived. In all these proofs, the concept of orthogonal relation plays a major role in that it allows the use of tools of elementary linear algebra.     

Organotin and organogermanium linkers for simple, direct functionalization of polyoxotungstates

Fifteen Keggin-anion-derived polytungstates [TW11O39[MCH2CH2X]]n- (T = Si, Ge, Ga; M = Sn, Ge; X = COOH, COOCH3, CONH2, CN; n = 5, 6) were prepared in aqueous or aqueous-organic solution from the corresponding lacunary polytungstates and trichlorotin and -germanium precursors, and were isolated as caesium salts. The derivatized polytungstates were characterized by elemental analysis, multinuclear NMR spectroscopy, and cyclic voltammetry; they are stable in aqueous solution to pH 6-7. NMR spectroscopy revealed the presence of a second (beta1 or beta3) isomer in the tungstogallate derivatives. Acid hydrolysis of the ester and nitrile derivatives could be achieved without decomposition of the polytungstate moieties, and esterification and amidation of the carboxylate functions was straightforward using standard coupling techniques, e.g. the formation, isolation and characterization of [SiW11O39[Ge(CH2)2CONHCH2COOCH3]]5- from glycine methyl ester. Since the Cl3MCH2CH2X precursors are readily accessible by hydrostannation/germanation reactions with the corresponding alkenes, novel coupled polytungstates, such as [(SiW11O39GeCH2CH2COOCH2)4C]20- from pentaerythritol tetraacrylate, can also be prepared.     

Organic synthesis--art or science?

The LHASA rules for finding strategic bonds in polycyclic target structures are analyzed with respect to the following question: Do the strategic bonds tend to give the greatest simplification upon disconnection, as measured by recently introduced indices of molecular complexity? The answer is yes, at least for the more general rules. This result implies that the bonds most useful for retrosynthetic disconnection can now be identified by a simple calculation rather than by application of a body of rules. It is concluded that organic synthesis, as far as described by these rules, has a mathematical basis and consequently can be considered a science as well as an art.     

Walking backward: walk counts of negative order

A closed formula is derived for walk counts of negative order k in a graph or molecule, as defined recently by Lukovits and Trinajsti?. Some unexpected observations made by these authors easily follow from this formula. Gratifyingly, the formula is very similar to the one obtained earlier for usual walk counts. Moreover, while for walk counts of k --> + infinity the numerically largest eigenvalue of the adjacency matrix plays an important part, for walk counts of k --> - infinity the numerically smallest eigenvalue plays a corresponding part.     

Trace metal contents in certified reference sediments determined by nitric acid digestion and atomic absorption spectrometry

The trace metal (Cd, Cu, Pb, Zn, Fe) contents of certified reference sediments BCSS-1, MESS-1, SRM 1645 river sediment and SRM 1646 estuarine sediment) were determined by nitric acid digestion at 140°C without silicate lattice dissolution. All the metals studied except iron were completely recovered by this digestion method. About 20% of the iron was not recovered under the given conditions. The trace metal contents in SRM 1645 were also examined by using the hydroxylammonium chloride/acetic acid leaching procedure.     

Exploring the limits of graph invariant- and spectrum-based discrimination of (sub)structures

The limits of a recently proposed computer method for finding all distinct substructures of a chemical structure are systematically explored within comprehensive graph samples which serve as supersets of the graphs corresponding to saturated hydrocarbons, both acyclic (up to n = 20) and (poly)cyclic (up to n = 10). Several pairs of smallest graphs and compounds are identified that cannot be distinguished using selected combinations of invariants such as combinations of Balaban's index J and graph matrix eigenvalues. As the most important result, it can now be stated that the computer program NIMSG, using J and distance eigenvalues, is safe within the domain of mono- through tetracyclic saturated hydrocarbon substructures up to n = 10 (oligocyclic decanes) and of all acyclic alkane substructures up to n = 19 (nonadecanes), i.e., it will not miss any of these substructures. For the regions surrounding this safe domain, upper limits are found for the numbers of substructures that may be lost in the worst case, and these are low. This taken together means that the computer program can be reasonably employed in chemistry whenever one is interested in finding the saturated hydrocarbon substructures. As to unsaturated and heteroatom containing substructures, there are reasons to conjecture that the method's resolving power for them is similar.