Laser spectroscopy of Si3C

The C 1B1<--X 1A1 band system of the potential interstellar species Si3C has been recorded in a silane/acetylene discharge by resonant two-color two-photon ionization spectroscopy. The origin band is located near 24,925 cm-1 (3.09 eV). Several other features in the spectrum are assigned to progressions in the Si-Si stretching modes as well as to sequence and hot band transitions. The assignment was facilitated by ab initio calculations, which also indicate that this is the strongest electronic transition of Si3C in the visible region of the spectrum. Features in the spectrum are broadened considerably (ca. 10 cm-1), and suggest an excited state lifetime of a few picoseconds. Possible reasons for the short-lived nature of the excited state are discussed.     

The measurement of high proton affinities for a variety of metallic compounds: a new approach by flame-ion mass spectrometry

A small amount (≤ 10⁻⁶ mol fraction) of four alkaline earth metals, tin and yttrium were introduced into five, premixed, fuel-rich, H₂–O₂–N₂ flames at atmospheric pressure in the temperature range 1820–2400 K. Aqueous salt solutions of the metals were sprayed into the premixed flame gas as an aerosol using an atomizer technique. Ions in a flame were observed by sampling flame gas through a nozzle into a mass spectrometer. The concentrations of the major neutral metallic species present in the flame were calculated from thermodynamic data currently available. The principal metallic ions observed were AOH⁺ (A = Mg, Ca, Sr, Ba, Sn) and A(OH)₂⁺ (A = Y), formed initially by proton transfer to AO and OAOH from H₃O⁺, a natural flame ion. Except for Mg, the ions were also produced by chemi-ionization processes. By adjusting the concentration(s) of the salt solution in the atomizer, it was found that a pair of ions could be brought into equilibrium within the time scale of the flame; the pairs included H₃O⁺ with a metal ion or two metallic ions. Because water is a major product of combustion, a very large difference in proton affinity PA⁰(AO) − PA⁰(H₂O) ≤ 490 kJ mol⁻¹ (117 kcal mol⁻¹) could be attempted for the proton transfer equilibrium. Using PA⁰(H₂O) = 691.0 kJ mol⁻¹ (165.2 kcal mol⁻¹) as a reference base to anchor the proton affinity scale, ion ratio measurements led to proton affinity PA⁰ values of 766, 912, 1004, 1184, 1201, and 1222 kJ mol⁻¹ (183, 218, 240, 283, 287, and 292 kcal mol⁻¹) corrected to 298 K for OYOH, SnO, MgO, CaO, SrO, and BaO, respectively; of these, only the value for OYOH has not been reported previously. If it is assumed that the neutral thermodynamic data are correct (although some appear to be in error), the uncertainties in the PA results reported here are ± 21 kJ mol⁻¹ (5 kcal mol⁻¹). The realization that these equilibria can be achieved in flames provides a new approach to consolidate and build the high end of the proton affinity ladder, primarily of metallic species which are not accessible at lower temperatures.     

Quantitative millimetre wavelength spectrometry at pressures approaching atmospheric: II. Determination of oxygen at atmospheric pressure

A millimetre wavelength (MMW) Fabry-Perot cavity spectrometer described in earlier work has been applied to the measurement of oxygen absorption at 60 GHz and atmospheric pressure in a gas matrix of nitrogen. The spectrometer has also been modified such that the MMW source is stabilised by a sub-harmonic microwave signal transmitted by an infrared carrier on a single mode telecommunications fibre optic. This is a step towards developing an instrument comprising minimal electronic components that can perform MMW spectrometry remotely. Oxygen determinations were achieved by monitoring the change in the quality factor (Q) of a resonant Fabry-Perot cavity due to the presence of an absorbing sample. The MMW absorption of the sample was determined by incrementing the frequency modulation (FM) deviation of the source frequency scanning the cavity resonance profile. The response curve of absorption signal versus fraction of oxygen in nitrogen was found to be linear throughout the working range of 1-100% O₂ (v/v) in N₂ with a slope of (1.407±0.007)×10⁻⁴ m⁻¹ (% O₂)⁻¹. The detection limit (3× standard deviation of the background) was found to be ∼0.8% (v/v). The MMW technique employed is advantageous since, unlike common MMW techniques, there is no vacuum requirement. Application of this method, to the monitoring of oxygen in gas mixtures of practical importance, is proposed. Values of the oxygen spectral absorption coefficients of lines between 55 and 60 GHz were measured at reduced pressure and found to be within ±2% of previous literature values. A pressure correction coefficient for O₂ absorption at 60 GHz in the 45-121 kPa range was obtained and found to be (1.354±0.014)×10⁻⁴ m⁻¹ kPa⁻¹.     

LAMMA mass spectra of β-diketone,bis(benzoylacetone)ethylenediimine and bis(salicylidene)ethylenediimine complexes of some transition metals

Complexes of 2,4-pentanedione (acetylacetone, acac), [Cu(acac)₂], [VO(acac)₂] and [CO(acac)₃], and the chromium(III) derivative of 3-methyl-2,4-pentanedione (methylacetylacetone, meac), [Cr(meac)₃], the ligands bis(benzoylacetone)ethylenediimine and bis(salicylidene)ethylenediimine, and their cobalt(II), nickel(II) and copper(II) chelates were analysed by laser desorption mass spectrometry (LAMMA) and compared to electron impact (EI) results. The positive ion LAMMA spectra generally reveal mostly small fragments, although metal cationization peaks are seen for most complexes. Negative ion LAMMA produce carbon clusters and some structurally important fragments.     

Arene transition metal chemistry : III. Arene exchange phenomena

Arene exchange between free arene and an arene—transition metal complex is reviewed with respect to structure, thermodynamics, kinetics and reaction mechanism. Catalysis of such arene exchange is also examined. Experimental results are presented for arene exchange between arene and the arene—metal complexes: η⁶-arene-1,5-cyclooctadieneruthenium, the cationic η⁶-arene-1,5-cyclooctadieneiridium and η⁶-arenetricarbonylmolybdenum. Mechanistic features of these exchange reactions are discussed.     

Selective ortho-cleavage of methoxymethyl- and 4-methoxybenzyl ethers

Iodine in methanol has been found to be an effective catalyst system for the cleavage of alkoxymethyl ethers. This catalyst system is particularly useful for the selective removal of ortho-methoxymethyl- and ortho-(4-methoxybenzyl) ethers in the presence of their para-counterparts. Further investigation of various metal salts in methanol for their ability to cleave alkoxymethyl ethers revealed high-valent salts as effective catalysts, presumably through the release of small quantities of HCl.     

Scope and mechanism of palladium-catalyzed amination of five-membered heterocyclic halides

Detailed studies have been conducted to determine the activity of palladium catalysts for the amination of five-membered heterocyclic halides and to determine the factors that control the scope of this reaction. Palladium-catalyzed aminations of the electron-rich furanyl, thiophenyl, and indolyl halides and of the related 2-halogenated thiazoles, benzimidazole, and benzoxazole have been shown to occur with a subset of amines. Various combinations of palladium precursors and P(t)Bu(3) were tested as catalysts for reaction of 3-bromothiophene with N-methylaniline, and the fastest reactions occurred with the Pd(I) dimer, [PdBr(P(t)Bu(3))](2). The fastest aminations of thiazoles, benzimidazoles, and benzoxazoles occurred with the combination of palladium trifluoroacetate and P(t)Bu(3) as catalyst.     

Electrospray tandem mass spectrometric analysis of zeaxanthin and its oxidation products

Carotenoids have been implicated in protection of the eye from light-mediated photo-toxicity caused by free radicals. Under conditions of normal oxidative stress the carotenoids serve as protective antioxidants; however, when the oxidative stress exceeds the antioxidant capacity, carotenoids can be oxidized into numerous cleavage products. The determination and identification of oxidized carotenoids in biological samples remains a major challenge due to the small sample size and low stability of these compounds. We investigated the reaction of various zeaxanthin cleavage products with O-ethyl hydroxylamine to evaluate their levels in a biological sample. For this, a sensitive and specific electrospray tandem mass spectrometry (ESI-MS/MS) was developed, avoiding the classical lower sensitive and specific HPLC-UV and fluorescence absorption methods. Protonated molecules [M + H](+) of carotenoids upon collision-induced dissociation produced a number of structurally characteristic product ions. A series of complicated clusters of product ions differing in 14 (CH(2))and 26 (C(2)H(2))Da was characteristic of the polyene chain of intact carotenoids. All carotenoid ethyl oximes of zeaxanthin cleavage products were characterized by the losses of 60 and 61 Da in their MS/MS spectra. Through the application of the LC/MS/MS method, we identified two oxime derivatives of 3-hydroxy-beta-ionone and 3-hydroxy-14'-apocarotenal with protonated molecules at m/z 252 and m/z 370 respectively, in a human eye sample.     

Synthetic studies towards congeners of phomactin A. Re-examination of the structure of Sch 49028

The total syntheses of the epoxy cyclic hemiacetal structures 8 and 9, which are isomeric with the structure 6 proposed for the phomactin known as Sch 49028 isolated from the marine fungus Phoma sp. are described. Neither of these structures showed spectroscopic data consistent with those reported for the purported natural product, adding credibility to the proposal that the structure Sch 49028 does not exist in nature and that its NMR spectroscopic data should have been assigned as phomactin A (1).     

Determination of the pesticides Chlorpropham,Propham, Propoxur,and Terbucarb by low temperature phosphorimetry

Summary Simple, direct, and sensitive low temperature phosphorimetric methods for the determination of Chlorpropham, Propham, Propoxur, and Terbucarb in EPA solvent are described. The minimum detectable amounts are 0.01 p. p. m. for Chlorpropham and Propham and 0.02 and 0.3 p. p. m. for Propoxur and Terbucarb respectively. The ranges of linearity of the analytical calibration graphs are 0.05-7.5, 0.05–10, 0.5–50, and 1–50 p. p. m. for Chlorpropham, Propham, Propoxur, and Terbucarb respectively. The procedure for Chlorpropham has been applied to the analysis of a solid commercial formulation.

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Bestimmung der Pesticide Chlorpropham, Propham, Propoxur und Terbucarb bei Tief-Temperatur-Phosphorimetrie

Zusammenfassung Einfache, direkte und empfindliche Methoden für die Bestimmung der genannten Pesticide in dem Lösungsmittel Diethylether-i-Pentan-abs. Ethanol wurden beschrieben. Die geringsten nachweisbaren Mengen sind 0,01 ppm für Chlorpropham und Propham sowie 0,02 und 0,3 ppm für Propoxur und Terbucarb. Die Eichkurven verlaufen zwischen 0,05 und 7,5, 0,05 und 10, 0,5 und 50 sowie zwischen 1 und 50 ppm für Chlorpropham, Propham, Propoxur bzw. Terbucarb linear. Das Verfahren für Chlorpropham wurde zur Analyse eines festen Handelsproduktes angewendet.