The mechanism of the gas-phase elimination kinetics of the β,γ-unsaturated aldehyde 2,2–dimethyl-3-butenal: a theoretical study

The study on the mechanism of the gas-phase elimination or thermal decomposition kinetics of 2, 2-dimethyl-3-butenal has been carried out by using theoretical calculation at MP2, combined ab initio CBSQB3 and DFT (B3LYP, B3PW91, MPW1PW91, PBEPBE, PBE1PBE, CAMB3LYP, M06, B97d) levels of theory. A good reasonable agreement between experimental and calculated parameters was obtained by using CAMB3LYP/6-311G(d,pd) calculations. The contrasted calculated parameters against experimental values suggested decarbonylation reaction to proceed through a concerted five-membered cyclic transition state type of mechanism, involving the hydrogen transfer from the carbonyl carbon to the gamma carbon, consistent with observed kinetic isotope effect. The breaking of alpha carbon–carbonyl carbon bond to produce carbon monoxide is 50% advanced in the transition state. The reaction mechanism may be described as a concerted moderately non-synchronous process. Examination of the Atoms in Molecules (AIM) analysis of electron density supports the suggested mechanism.     

Electrocyclic [1,5] hydrogen shift in the thermal elimination kinetics of phenyl acetate and p-tolyl acetate in the gas phase: a density functional theory study

The kinetics and mechanisms of thermal decomposition of phenyl acetate and p-tolyl acetate in the gas phase were studied by means of electronic structure calculations using density functional theory methods: B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), B3PW91/6-31G(d,p), B3PW91/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p), PBE/6-31G(d,p) and PBE/6-31++G(d,p). Two possible mechanisms have been considered: mechanism A is a stepwise process involving electrocyclic [1,5] hydrogen shift to eliminate ketene through concerted six-membered cyclic transition-state structure, followed by tautomerisation of cyclohexadienone or by 4-methyl cyclohexadienone intermediate to give the corresponding phenol. Mechanism B is a one-step concerted [1,3] hydrogen shift through a four-membered cyclic transition-state geometry, to produce ketene and phenol or p-cresol. Theoretical calculations showed reasonable agreement with experimental activation parameters when using the Perdew, Burke and Ernserhof (PBE)functional, through the stepwise [1,5] hydrogen-shift mechanism. For mechanism B, large deviation for the entropy of activation was observed. No experimental data were available for p-tolyl acetate; however, theoretical calculations showed similar results to phenyl acetate, thus supporting the stepwise mechanism for both phenyl acetate and p-tolyl acetate.     

Electron energy-loss spectroscopy of anomalous plutonium behavior in nuclear waste materials

Plutonium-enriched layer has been observed in corroded spent uranium oxide fuel (CSNF). These Pu-enriched regions were examined with analytical transmission electron microscopy combined with electron energy-loss spectroscopy (EELS). The enriched region also contained U, Am, Ru, Zr, but only minor enrichment of rare earth elements. The Pu, possibly as Pu(V) according to EELS measurements, was dispersed within re-precipitated uranium oxide (identified as U3O8) nano-crystals between U(VI) secondary phases and the CSNF surface. The U, Pu, and Am enrichment was observed in the corrosion products with tests on different nuclear fuels. This may have implications for the long-term behavior of CSNF under storage in a geologic waste repository. Furthermore, there may be an increased potential for the generation of Pu-bearing colloids from this type of weathered CSNF.     

Isolation, biological activity evaluation, structure elucidation, and total synthesis of eliamid: a novel complex i inhibitor

Eliamid is a secondary metabolite isolated from two bacterial strains. This molecule features a linear polyketide backbone terminated by a tetramic acid amide moiety. Among other biological activities, eliamid shows a high and specific cytostatic action on human lymphoma and cervix carcinoma cell lines. The 2,4-anti relative configuration of the C-2,C-4-dimethyl substituted amide fragment was assigned by means of Breit's rule. The absolute configuration of all stereocenters was determined by a combination of degradation methods, structural similarity analysis and total synthesis. The stereogenic centers were introduced by vinylogous Mukaiyama aldol reaction and two consecutive Myers alkylations. The use of pentafluorophenyl ester as acylation agent allowed the efficient formation of tetramic acid amide. The longest linear sequence in the synthesis consist of 13?steps and proceeds with 12?% overall yield. Differential spectroscopy experiments with beef heart submitochondrial particles established that eliamid is a potent inhibitor of the NADH-ubiquinone oxidoreductase complex. Additionally, biosynthesis of eliamid was investigated by feeding experiments with (13) C-labeled precursors.     

Protecting-group-free synthesis of glycosyl 1-phosphates

Glycosyl 1-phosphates enriched in the α-anomer are obtained without the use of protecting groups in two steps starting from the free hemiacetal. Condensation of free hemiacetals with toluenesulfonylhydrazide yields a range of glycosylsulfonohydrazide donors which can be oxidized using cupric chloride in the presence of phosphoric acid and the coordinating additive 2-methyl-2-oxazoline to give useful yields of the fully deprotected glycosyl 1-phosphates.     

Ruthenium‐Catalyzed Functionalization of Pyrroles and Indoles with Propargyl Alcohols

Several ruthenium‐catalyzed atom‐economic transformations of propargyl alcohols with pyrroles or indoles leading to alkylated, propargylated, or annulated heteroaromatics are reported. The mechanistically distinct reactions are catalyzed by a single ruthenium(0) complex containing a redox‐coupled dienone ligand. The mode of activation regarding the propargyl alcohols determines the reaction pathway and depends on the alcohols’ substitution pattern. Secondary substrates form alkenyl complexes by a 1,2‐hydrogen shift, whereas the transformation of tertiary substrates involves allenylidene intermediates. 1‐Vinyl propargyl alcohols are converted by a cascade allylation/cyclization sequence. The environmentally benign processes are of broad scope and allow the selective synthesis of highly functionalized pyrroles and indoles generating water as the only waste product.     

Crossing points of distributions and a theorem that relates them to second order stochastic dominance

We state formal definitions for crossing points in pairs of distributions and give a detailed proof of a theorem that relates those points to the second order stochastic dominance (SSD). The theorem states that the fulfillment of the area balance conditions for SSD at the t values that correspond to crossing points, and at the limit t→∞, is a necessary and sufficient condition for its fulfillment at all t: {−∞<t<∞}, as required for the existence of SSD. We provide examples for the application of the theorem in the case of continuous distributions, including a continuous counter example to prove that the Mean-Variance criterion is not sufficient to state preferences under risk aversion.     

Enhanced sonochemical degradation of azure B dye by the electroFenton process

The degradation of azure B dye (C₁₅H₁₆ClN₃S; AB) has been studied by Fenton, sonolysis and sono-electroFenton processes employing ultrasound at 23 kHz and the electrogeneration of H₂O₂ at the reticulated vitreous carbon electrode. It was found that the dye degradation followed apparent first-order kinetics in all the degradation processes tested. The rate constant was affected by both the pH of the solution and initial concentration of Fe²⁺, with the highest degradation obtained at pH between 2.6 and 3. The first-order rate constant decreased in the following order: sono-electroFenton > Fenton > sonolysis. The rate constant for AB degradation by sono-electroFenton is ∼10-fold that of sonolysis and ∼2-fold the one obtained by Fenton under silent conditions. The chemical oxygen demand was abated ∼68% and ∼85% by Fenton and sono-electroFenton respectively, achieving AB concentration removal over 90% with both processes.     

Effects of the induced birefringence in photorefractive crystals on speckle optical vortices

In this work, we show that it is possible to change the properties of an optical vortex formed in a speckle pattern by the control of an external electric field applied to a sillenite-type photorefractive material in a non-holographic configuration. To show it, a scheme that allows the recovering of both, the polarization state and the phase distribution of an optical field was implemented. Changes in the polarization state of the light in the neighborhood of the vortex were observed with the application of an external electric field. Likewise, changes in the phase structure around the vortex and displacements of the vortices themselves were measured. These displacements have a fairly linear dependence on the applied field to the photorefractive crystal. Experimental results are qualitatively explained with the theoretical treatment of non-holographic recording in photorefractive crystals.