Excitation energy effects in the photophysics of trans-stilbene in solution

Excitation of trans-stilbene with 306 nm pulses generates a narrower transient spectrum than with 265 nm. These spectra do not evolve in the same manner, suggesting that the spectral changes with 265 nm result from vibrational cooling. Spectral differences at longer times suggest there is a slow cross-relaxation component in the inhomogeneous distribution.     

Studies on poly(iminomethylenes)

A variety of simple alkyl and aryl isocyanides have been polymerized using 0.5% NiCl₂ in ethanol as a catalyst. The resulting poly(iminomethylenes) have been characterized by carbon-13 NMR spectroscopy and their polystyrene-equivalent molecular weights have been determined by gel permeation chromatography. Straight chain aliphatic isocyanides having from three to ten carbon atoms in the chain form readily solyble polymers having molecular weights (M_w) in the general range 10,000 to 30,000. Neopentyl isocyanide unlike tert-butyl isocyanide forms an insoluble polymer. A number of new soluble aryl isocyanide polymers have been obtained. However, aryl isocyanides having a single alkyl substituent (CH₃, C₂H₅, CF₃) in the ortho position give only insoluble polymers, whereas aryl isocyanides having alkyl substituents in both ortho positions (e.g., 2,6-(CH₃)₂C₆H₃NC and 2,4,6-(CH₃)₃C₆H₂NC) fail to polymerize under these conditions. The highest molecular weight soluble aryl isocyanide homopolymer is obtained from 3-CH₃OC₆H₄NC(M_w = 26,000). The trimethylsilyl substituted isocyanide (CH₃)₃SiCH₂CH₂NC has been obtained from LiCH₂NC and (CH₃)SiCH₂Cl and gives a brown soluble homopolymer with a molecular weight (M_w) of 19,000.     

Carbon paste electrodes modified with cation-exchange resin in differential pulse voltammetry

Chemically modified carbon paste eletrodes are prepared by incorporating appropriate quantities of a cation-exchange resin directly into the paste mixture. Ionic analytes can be preconcentrated on these electrodes by an ion-exchange reaction rather than electrolytic plating. Differential pulse voltammetry is used to quantify the accumulated ions. The response is characterized with respect to preconcentration period, bulk concentration, pH, paste composition, reproducibility, and other variables. Copper ion is used as a test system. The procedure exhibits good linearity for 6.25 × 10^?5?3.0 × 10^?4 M copper(II) ions and the peak current varies linearly with preconcentration time between 1 and 7 min for the conditions used.     

Mechanism of azole antifungal activity as determined by liquid chromatographic/mass spectrometric monitoring of ergosterol biosynthesis

A liquid chromatography/mass spectrometry (LC/MS) method for separation and characterization of ergosterol biosynthetic precursors was developed to study the effect of Posaconazole on sterol biosynthesis in fungi. Ergosterol biosynthetic precursors were characterized from their electron ionization mass spectra acquired by a normal-phase chromatography, particle beam LC/MS method. Fragment ions resulting from cleavage across the D-ring and an abundant M - 15 fragment ion were diagnostic for methyl substitution at C-4 and C-14. Comparison of the sterol profile in control and treated Candida albicans incubations showed depletion of ergosterol and accumulation of C-4 and C-14 methyl-substituted sterols following treatment with Posaconazole. These C-4 and C-14 methyl sterols are known to be incapable of sustaining cell growth. The results demonstrate that Posaconazole exerts its antifungal activity by inhibition of ergosterol biosynthesis. Furthermore, Posaconazole appears to disrupt ergosterol biosynthesis by inhibition of lanosterol 14alpha-demethylase.     

Insight into the reactivity of olefins in the Pauson-Khand reaction

The reactivity of different Co(2)(CO)(6)-acetylene complexes in the Pauson-Khand reaction is strongly dependent on the nature of the olefin. Theoretical calculations at both the DFT and ONIOM levels are concordant with experimental observations and suggest how the olefin-cobalt interactions in the complex could have a major effect on the relative reactivity of the olefins. This study rationalizes for the first time the experimentally observed reactivity differences of cyclohexene, cyclopentene, and norbornene.     

Anomalous behavior of the Nd1.84Ce0.16CuO4 superconductor

The as-prepared electron-doped system Nd_2–xCe_xCuO₄ (where x0.16) is not superconducting. It becomes superconducting only after removal of a minuscule amount of extraneous oxygen (0.02 of O per unit formula). Mössbauer effect studies were carried out for oxygenated and deoxygenated Nd_2–xCe_xCu(⁵⁷Co)O₄ with x=0.14, 0.16, and 0.18. The spectra show evidence of oxygen anions attaching to the probe⁵⁷Co in apical positions, to form 5- and 6-coordinated species. A conventional procedure for deoxygenation brings about little change in the Mössbauer spectra both above and below the optimal superconducting concentration; however, for x=0.16, a dramatic change is observed—a major fraction of the magnetically split five-coordinated species manifests itself as a paramagnetically relaxed doublet upon deoxygenation, which costitutes a microscopic measure of the superconducting volume fraction. This apparently anomalous behavior at x0.16, where the extraneous oxygen is more readily desorbed, may be related to an electronic and/or local structural change in the CuO₂ plane.