Switch From Pauli-Lowering to LUMO-Lowering Catalysis in Brønsted Acid-Catalyzed Aza-Diels-Alder Reactions

Brønsted acid-catalyzed inverse-electron demand (IED) aza-Diels-Alder reactions between 2-aza-dienes and ethylene were studied using quantum chemical calculations. The computed activation energy systematically decreases as the basic sites of the diene progressively become protonated. Our activation strain and Kohn-Sham molecular orbital analyses traced the origin of this enhanced reactivity to i) “Pauli-lowering catalysis” for mono-protonated 2-aza-dienes due to the induction of an asynchronous, but still concerted, reaction pathway that reduces the Pauli repulsion between the reactants; and ii) “LUMO-lowering catalysis” for multi-protonated 2-aza-dienes due to their highly stabilized LUMO(s) and more concerted synchronous reaction path that facilitates more efficient orbital overlaps in IED interactions. In all, we illustrate how the novel concept of “Pauli-lowering catalysis” can be overruled by the traditional concept of “LUMO-lowering catalysis” when the degree of LUMO stabilization is extreme as in the case of multi-protonated 2-aza-dienes.     

A PYRIDYL-TRIAZINE COMPLEX OF Co(II): SPECTROSCOPIC,STRUCTURAL, AND DOCKING STUDIES

Journal of Structural Chemistry - A new cobalt(II) complex of 5-phenyl-3-(pyridin-2-yl)-1,2,4-triazine (PPTA), [Co(PPTA)2(NO3)2] (1), is prepared and identified by the elemental analysis, FTIR...     

Synthesis and Optoelectronic Properties of a Quinoxalino-Phenanthrophenazine (QPP) Extended Tribenzotriquinacene (TBTQ)

A six-step synthesis towards a tribenzotriquinacene (TBTQ) bearing three quinoxalinophenanthrophenazine (QPP) units is presented. The optoelectronic properties are investigated and the effect of the three-dimensional arrangement of the individual QPP planes is examined using optical spectroscopy, electrochemical analysis and quantum-chemical calculations.     

Magnetic State of Layered Cobalt Chalcogenides Co7Se8 and Co7Te8

JETP Letters - The structural and magnetic properties of the Co7Te8 layered compound have been studied for the first time using X-ray diffraction, measurements of the magnetic susceptibility, and...     

Synthesis and Investigation of Piezophotocatalytic Properties of Polyvinylidene Fluoride Nanofibers Modified with Titanium Dioxide

Moscow University Chemistry Bulletin - A piezoactive nanofiber composite PVDF film with TiO2 nanoparticles included in it was synthesized by electrospinning. The composition, morphology and...     

AN EXPLORATION OF STRUCTURALLY CHARACTERIZED HETERO-TRINUCLEAR [Cu(II)2Ca(II)] FLEXIBLE-BIS(SALAMO)-TYPE COMPLEX

Journal of Structural Chemistry - A new bis(salamo)-type ligand has been designed and synthesized, and its hetero-trinuclear [Cu(II)2Ca(II)] complex has been successfully prepared. It is...     

Experiments on crack identification in cantilever beams

In this paper we present an experimental investigation of the identification of crack location and size. By providing the first three natural frequencies through vibration measurements, curves of crack equivalent stiffness versus crack location are plotted, and the intersection of the three curves predicts the crack location and size. In the experiments, the cracked specimens were made using a wire-cut electrical discharge machine, and the cantilever beams were excited next to the free end by means of an impulse force hammer. In order to obtain the accurate natural frequencies from the transient signal measured, the method of zoom fast Fourier transform is adopted to improve frequency resolution. From experimental results, it is observed that the identification errors of crack location and size are less than 2% and 4%, respectively. The effectiveness of crack identification through vibration measurements is verified.     

The influence of radiation on the temperature profile of an induced discharge in argon at atmospheric pressure

This study presents a quantitative evaluation of the influence of radiation upon the parameters of a long induced discharge in argon at atmospheric pressure. Radiation heat transfer in the optically transparent tin plasma layers with thickness of the order of several centimeters) and reabsorption spectral intervals is examined. Experimental error produced by inaccurate knowledge of the transfer and optical characteristics of the plasma is evaluated. The effect of convection on radial temperature profile is evaluated.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiztki, No. 6, pp. 35–43, November–December, 1971.     

Methods and correlations for the prediction of quenching rates on hot surfaces

A substantial quantity of experimental data on rewetting, much of which has not been previously reported, is analysed using the results of calculations of the two-dimensional conduction processes taking place in the walls of tubes, which have been used to simulate the cladding of nuclear fuel elements. Correlations giving the quenching heat-transfer coefficient and sputtering temperature are proposed as a result of the analysis. These correlations may be combined with the previously reported conduction analysis to predict rewetting rates under a wide range of conditions. The new data include falling film rewetting rates measured for a range of system pressures (1–15 bars), initial wall temperatures (200–650°C), coolant mass flowrates (3–50 g sec^?1) and subcoolings (0–90°C). Measurements have also been made of rewetting rates by bottom flooding of both saturated and subcooled water at atmospheric pressure.