A comment on “Accurate ab initio determination of binding energies for rare-gas dimers by basis set extrapolation”

In this work we comment on an extrapolation scheme presented by Lee in Theoretical Chemistry Accounts, which is based on an extrapolation of energy differences instead of actual energies. In particular, we show that a very similar scheme had been introduced already in 1999, and used to estimate the MP5, CCSDT and FCI complete basis set limits of He₂. Comment to the article “Accurate ab initio determination of binding energies for rare-gas dimers by basis set extrapolation” by J.S. Lee, Theor Chem Acc (2005) 113: 87–94     

Increased sensitivity of an enzyme immunoassay (ELISA) for the determination of triazine herbicides by variation of tracer incubation time

Immunoassays for triazine herbicides were tested for their reaction to the variation of the tracer incubation time. By application of a sequential technique the measuring range of atrazine could be expanded to five decades and the total duration of the test could be reduced to about 30 min. In an optimized version a lower detection limit of 9 pmol/l (2 ng/l) was achieved. The detection limit of a sensitive immunoassay for terbuthylazine is also below the concentration limit demanded of the German drinking water regulation (100 ng/l) and reaches 130 pmol/l (30 ng/l). Short tracer incubation times did not lead to increased cross-reactivities in contrast to theoretical models [1, 2]. Different mechanisms, which could cause a shift of the center point of the calibration curve, are discussed, including kinetic considerations.Nomenclature ametryn 2-(ethylamino)-4-(isopropylamino)-6-(methylthio)-1,3,5-triazine - atrazine 2-(chloro)-4-(ethylamino)-6-(isopropylamino)-1,3,5-triazine - deethylatrazine 2-(amino)-4-(chloro)-6-(isopropylamino)-1,3,5-triazine - DMSO dimethylsulfoxide - DOC dissolved organic carbon - ELISA enzyme-linked immunosorbent assay - glyme 1,2-dimethoxyethane - hydroxyatrazine 2-(ethylamino)-4-(hydroxy)-6-(isopropylamino)-1,3,5-triazine - PBS phosphate buffered saline - propazine 2-(chloro)-4,6-bis(isopropylamino)-1,3,5-triazine - simazine 2-(chloro)-4,6-bis(ethylamino)-1,3,5-triazine - terbuthylazine 2-(tert-butylamino)-4-(chloro)-6-(ethylamino)-1,3,5-triazine - TLC thin-layer chromatography - TMB 3,3,5,5-tetramethylbenzidine - tracer enzyme (peroxidase) labeled hapten     

Ab initio calculation of the equilibrium geometry and the 13C, 17O, 23Na and 7Li NMR shielding tensors of the systems Na+/CO and Li+/CO

Using ab initio SCF calculations two minima have been found in Na⁺/CO and Li⁺/CO. The minimum corresponding to CO…Na⁺ is slightly deeper than in Na⁺…CO. The minimum corresponding to CO…Li⁺ is deeper than in Li⁺…CO. The ¹³C, ¹⁷O, ²³Na and ⁷Li NMR shielding tensors were computed with the coupled Hartree—Fock method with large gaussian basis sets.     

Reaction monitoring of enzyme-catalyzed ester cleavage by time-resolved fluorescence and electrospray mass spectrometry: method development and comparison

Two complementary methods for reaction monitoring of the esterase-catalyzed cleavage of bis(2-pyridylmethyl)(2-acetoxyphenyl)amine are developed and compared. While enzyme-amplified lanthanide luminescence (EALL) allows for the time-resolved fluorescence determination of the intrinsically non-fluorescent product, both substrate and product of the enzymatic reaction may be determined simultaneously by electrospray mass spectrometry (ESI-MS). Excitation wavelength for the Tb(III) complex of the reaction product is 297 nm and emission was detected at 545 nm, which is the characteristic emission wavelength of the terbium(III) ion. In contrast to other EALL techniques, the presented method allows for the direct monitoring of an enzymatic conversion without any further sample preparation (e.g., rebuffering). For the mass spectrometric measurements the mass traces were set to m/z=306, 328, 348, and 370 for the protonated ester, the resulting phenol and their sodium adducts, respectively.     

Insight into the partial oxidation of propene: the reactions of 2-propen-1-ol on clean and O-covered Mo(110)

The reactions of 2-propen-1-ol (allyl alcohol) were studied on clean and O-covered Mo(110) to understand the effect of resonance stabilization and the presence of surface oxygen on reaction selectivity. Propene is the only gaseous hydrocarbon product evolved from allyl alcohol reaction on O-covered Mo(110). Water and dihydrogen are also produced, along with a small amount of adsorbed carbon. We estimated, using X-ray photoelectron spectroscopy, that approximately 70% of the 0.11 ML of 2-propen-1-ol that reacts forms propene. In contrast, the dominant reaction pathway on the clean surface is nonselective decomposition to adsorbed carbon and hydrogen, leading to a 23% selectivity for propene formation. On both clean and O-covered Mo(110), X-ray photoelectron spectroscopy and infrared spectroscopy identify allyloxy as the reaction intermediate yielding propene. These results are discussed in the context of propene oxidation and periodic trends in reactivity.     

The pyrochlore family -- a potential panacea for the frustrated perovskite chemist

Many known complex oxides of general formula A(2)B(2)X(7) adopt the pyrochlore structure, a key structure-type that has been shown to demonstrate a vast range of useful physical properties. Areas currently of much interest with respect to pyrochlores, include metal-insulator transitions, magnetic frustration/spin ices, magnetoresistance, superconductivity, ferroelectrics, O/F ionic conductivity, mixed conductivity, pigments and catalysis. We present some recent results on three types of pyrochlore materials that show unusual magnetic, optical and electronic behaviours associated with subtle structural and compositional changes. High-resolution powder neutron diffraction studies of the superconducting Cd(2)Re(2)O(7) and the ferroelectric Cd(2)Nb(2)O(7) have been undertaken on material cooled below room temperature. Both Cd(2)Re(2)O(7) and Cd(2)Nb(2)O(7) exhibit small structure distortions, in each case involving a distortion from a cubic unit cell, on cooling below approximately 180 K and possible models that can be used to describe the low-temperature structures and associated atomic displacements are developed and described in this article. A range of materials of the general formula Ca(1-x)Ln(x)TaO(2-x)N(1+x), x= 0.5 and x= 1, Ln = La-Yb have been synthesised and shown to adopt pyrochlore and/or perovskite structures. The absorption spectra of these materials are discussed in terms of their structures and compositions.