Use of iterative natural orbital method for calculating energy barrier to predissociation of the first excited states of NH3 (3s1,3 A″2)

The barrier to predissociation of the first excited states of NH₃ (3s^1,3 A) is computed from a truncated first-order wave function using the iterative natural orbital method. The results show that the barrier is lower than when calculated with RHF and the CI wave functions, where single and double replacements to a truncated virtual space are carried out. These findings are used to interpret experimental data.     

Voltammetric Determination of 3‐Nitrofluoranthene and 3‐Aminofluoranthene at Boron Doped Diamond Thin‐Film Electrode

The voltammetric behavior of 3‐nitrofluoranthene and 3‐aminofluoranthene was investigated in mixed methanol‐water solutions by differential pulse voltammetry (DPV) at boron doped diamond thin‐film electrode (BDDE). Optimum conditions have been found for determination of 3‐nitrofluoranthene in the concentration range of 2×10^?8–1×10^?6 mol L^?1, and for determination 3‐aminofluorathnene in the concentration range of 2×10^?7–1×10^?5 mol L^?1, respectively. Limits of determination were 3×10^?8 mol L^?1 (3‐nitrofluoranthene) and 2×10^?7 mol L^?1 (3‐aminofluoranthene).     

Steric Effects on Reaction Rates. VI. Application of a New MM2 Force-Field for Carbenium Ions to Solvolysis Rates of Secondary p-Toluenesulfonates

An empirical force-field for carbenium ions has been incorporated in Allinger's MM2 programme. Structural parameters of secondary carbenium ions calculated by this method are compared with those obtained with Schleyer's BIGSTRN calculations. The strain changes occurring upon solvolysis of secondary p-toluenesulfonates are evaluated by means of this force-field and correlated with the rate constants for solvolysis. The equation for correlation of acetolysis, relative to cyclohexyl p-toluenesulfonate, of 28 k_c substrates is ΔG = 0.67 ΔE_st - 0.20 (r = 0.958).     

The Photochemical Synthesis and Denitrogenation of 8,9-diazadibenzo[c,e]isobullvalene

Starting from dibenzo[a, c]cyclooctene (4) and 4-methyl-3H-1,2,4-triazol-3,5(4H)-dione (MTAD), the strained skeleton of the title azo compound 1 is assembled in a tandem photo-Diels-Alder addition/di-π-methane rearrangement sequence. The synthesis is completed by a stepwise hydrolytic oxidation of the ensuing triazolidine-dione 2 with nickel peroxide. Thermolysis of 1 in benzene solution is shown to be governed by an initial 1,3-dipolar cycloreversion which leads, via an intermediate diazo compound 11 , to cyclobuta[1]phenanthrene 8 and two further carbene-derived C₁₆H₁₂ products. Photolysis of 1 at 350 nm leads in modest yield (12%), via a diazenyl diradical, to an unstable bridged bicyclobutane 10 (dibenzooctavalene). MNDO calculations suggest the latter to have a rapidly inverting, twisted structure of C₂ symmetry.     

Fluorine core esca linewidths in CH3F and CF4

Ab initio calculations within the Hartree-Fock formation have been carried out on potential energy surfaces of the ground and the F_1s hole states of CH₃F and CF₄ in order to investigate linewidths of their ESCA spectra. The calculations show that potential energy surfaces of both hole states have dissociative character and can be approximated by straight lines in the region of interest. A simple formula for the ESCA fwhm linewidth is derived which yields results in good agreement with experiment. Theoretically derived relaxation and Koopmans' energies have been investigated as a function of geometry.     

Factor analysis of 27Al MAS NMR spectra for identifying nanocrystalline phases in amorphous geopolymers

Nanostructured materials offer enhanced physicochemical properties because of the large interfacial area. Typically, geopolymers with specifically synthesized nanosized zeolites are a promising material for the sorption of pollutants. The structural characterization of these aluminosilicates, however, continues to be a challenge. To circumvent complications resulting from the amorphous character of the aluminosilicate matrix and from the low concentrations of nanosized crystallites, we have proposed a procedure based on factor analysis of ²⁷Al MAS NMR spectra. The capability of the proposed method was tested on geopolymers that exhibited various tendencies to crystallize (i) completely amorphous systems, (ii) X‐ray amorphous systems with nanocrystalline phases, and (iii) highly crystalline systems. Although the recorded ²⁷Al MAS NMR spectra did not show visible differences between the amorphous systems (i) and the geopolymers with the nanocrystalline phase (ii), the applied factor analysis unambiguously distinguished these materials. The samples were separated into the well‐defined clusters, and the systems with the evolving crystalline phase were identified even before any crystalline fraction was detected by X‐ray powder diffraction. Reliability of the proposed procedure was verified by comparing it with ²⁹Si MAS NMR spectra. Factor analysis of ²⁷Al MAS NMR spectra thus has the ability to reveal spectroscopic features corresponding to the nanocrystalline phases. Because the measurement time of ²⁷Al MAS NMR spectra is significantly shorter than that of ²⁹Si MAS NMR data, the proposed procedure is particularly suitable for the analysis of large sets of specifically synthesized geopolymers in which the formation of the limited fractions of nanocrystalline phases is desired. Copyright © 2013 John Wiley & Sons, Ltd.     

Organic semiconductors in potentiometric gas sensors

Solid-state potentiometric sensors based on the chemical modulation of the work function of organic semiconductors are discussed. The theory of the chemical work function modulation is briefly reviewed. There are several sensor configurations, in which this transduction principle can be employed. First is the Kelvin probe, second is the chemically sensitive field-effect transistor in which the conventional metal gate of the silicon-based transistor has been replaced by an organic semiconductor. Chemical modulation of work function enters also into the operation of the third type of sensor discussed in this review, on “organic field-effect transistor”. It is shown that in reality such sensors are “field-modulated chemiresistors”, rather than potentiometric sensors.     

Silver Trifluoromethanesulfonate(Triflate) Activation of Trichloroacetimidates in Glycosylation Reactions

Abstract

Chemical syntheses of biologically active oligosaccharides, glycolipids and glycopeptides requires efficient stereospecific glycosylation reactions.² One of the most effective glycosylation methods involves activation of anomeric imidates, particularly mchloroacetimidates, by Lewis acids such as boron trifluoride etherate (BF₃·OEt₂), mmethylsilyl mfluoromethanesulfonate (TMSOTF)³ and mfluoromethanesulfonic anhydride.⁴ In a recent example from this laboratory, BF₃·OEt₂, has been used to promote the glycosylation of methyl 2,3,6-tri-O-benzoyl-B-D-galactopyranoside (I)⁵ with 2-deoxy-2-phthalimido-3,4,6-tri-O-acetyl-B-D-galactopyranosyl mchloroacetimidate (I): see Scheme 1. The expected β1-4-linked disaccharide III was obtained in 40% yield. The yield was so low since both the α-anomer and a 1-3-linked disaccharide were formed as by products, the latter in particularly large quantities (cf. Ref.⁷). The 1-3 disaccharide could be formed from a product of acid-catalyzed 3,4-migration of the benzoyl group which is not surprising, considering the cis relationship of the 3,4-hydroxyl groups in galactose.⁸ In fact, when the glycosylation reaction was quenched before all unreacted alcohol was consumed, the chromatographic fraction corresponding to the starting alcohol II contained at least three different tribenzoates (as shown by NMR analysis).⁹ Other promoters, ZnBr₂ ¹⁰ and TMSOTF, led to lower yields and more complicated mixtures than BF₃·OEt₂.     

In vitro stable isotope labeling for discovery of novel metabolites by liquid chromatography–mass spectrometry: Confirmation of γ-tocopherol metabolism in human A549 cell

A general approach for discovering novel catabolic metabolites from a parent biocompound was developed and validated on the metabolism of γ-tocopherol in human A549 cell. The method is based on LC–MS analysis of in vitro stable isotope-labeled metabolites and assumes that a parent compound and its metabolites share a common functional group that can be derivatized by well-documented reagents. In this method, two equal aliquots of extracted metabolites are separately derivatized with isotope-coded (heavy) and non-isotope-coded (light) form of derivatizing reagent, mixed at 1:1 ratio and analyzed using LC–MS. The metabolites with common functional group are then easily recognized by determination of a chromatographically co-eluted pair of isotopomers (MS doublet peaks) with similar peak intensities and mass difference corresponding to the mass difference between heavy and light form of derivatization reagent. The feasibility of this approach was demonstrated and validated by the identification of products of γ-tocopherol catabolism in human A549 cell culture media using N-methyl-nicotinic acid N-hydroxysuccinimide ester (C1-NANHS) and N-methyl-d3-nicotinic acid N-hydroxysuccinimide ester (C1-d3-NANHS) derivatizing reagent. Overall four γ-tocopherol metabolites were identified including 9′-COOH, 11′-COOH, 13′-COOH and 13′-OH. In addition, the developed LC–MS method can also be used for the fast and sensitive quantitative analysis of γ-tocopherol and other forms of vitamin E related compounds.