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31.
Alberto Sturaro Giorgio Parvoli Lucio Doretti Cristina Bancomina Carlo Neonato 《Journal of mass spectrometry : JMS》1994,29(10):575-577
The determination of vinyl acetate at parts per 109 by volume (ppbv) levels in air by gas chromatography/mass spectrometry (GC/MS) was achieved by optimizing the GC conditions and choosing specific extraction solvents that are sufficiently pure. The ideal solvent should not give rise to fragment ions at m/z 43, in order to rule out any possible interference during the monitoring of the same vinyl acetate ion. Traces of acetone and butan-2-one in solvents suitable for this GC/MS determination may also mislead the detection of vinyl acetate. A 440 μl volume of a mixture of tetrachloroethylene containing 9% acetonitrile allowed the recovery of more than 90% of the ester with a detection limit of 1.5 ppbv in air together with a good linearity of response. 相似文献
32.
Francesco De Sarlo Antonio Guarna Andrea Goti Alberto Brandi 《Journal of organometallic chemistry》1984,269(2):115-121
Organomercury fulminates react with acetylene derivatives to give unstable 3-(organomercurio)isoxazoles, which isomerize to 2-cyanoenolates. These are hydrolyzed with hydrochloric acid to the corresponding enols and are cleaved by water at the double bond. With monosubstituted acetylenes, substitution at the free position by the organomercury residue is predominant. 相似文献
33.
34.
Alberto Avenoza Jesús H Busto Carlos Cativiela Francisco Corzana Jesús M Peregrina María M Zurbano 《The Journal of organic chemistry》2002,67(2):598-601
A short and efficient asymmetric synthesis of the (6R,7S)-7-tert-butoxycarbonylamino-2-ketocarbacepham is described. The key step involves the hetero Diels-Alder reaction of the benzylimine derived from the enantiomer of Garner's aldehyde with Danishefsky's diene. 相似文献
35.
We have carried out an ab initio self-consistent-field molecular orbital study of the structures and relative bond strengths of some monoamine derivatives of cubane, azacubane, and 1,3-diazacubane. Our focus has been on the effect of the NH2 group on the strengths of the endocyclic strained bonds in these molecules, and, in particular, on the conformation dependence of this effect. Our results show a consistent bond-weakening observed in one [and only one] C-C or C-N bond adjacent to the site of NH2 substitution. We find that the particular bond that is weakened is in all cases essentially coplanar with the C-NH2 bond and the position of the most negative electrostatic potential of the amine nitrogen. This direction-specific bond-weakening is viewed as an example of the anomeric effect. 相似文献
36.
37.
Poschalko A Lancelot N Marin J Larras V Limal D Elbayed K Raya J Piotto M Briand JP Guichard G Bianco A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(18):4532-4537
A novel resin called DEUSS (perdeuterated poly(oxyethylene)-based solid support) has been prepared by anionic polymerization of deuterated [D4]ethylene oxide, followed by cross-linking with deuterated epichlorohydrin. DEUSS can be suspended in a wide range of solvents including organic and aqueous solutions, in which it displays a high swelling capacity. As measured by proton HRMAS of the swollen polymer, the signal intensity of the oxyethylene protons is reduced by a factor of 110 relative to the corresponding nondeuterated poly(oxyethylene)poly(oxypropylene) (POEPOP) resin, thus facilitating detailed HRMAS NMR studies of covalently linked molecules. This 1H NMR invisible matrix was used for the solid-phase synthesis of peptides, oligoureas, and a series of amides as well as their characterization by HRMAS NMR spectroscopy. On-bead NMR spectra of high quality and with resolution comparable to that of liquid samples were obtained and readily interpreted. The complete absence of the parasite resin signals will be of great advantage, for example, for the optimization of multistep solid-phase stereoselective reactions, and for the conformational study of resin-bound molecules in a large variety of solvents. 相似文献
38.
Dariush Habibollahzadeh Jane S. Murray Michael Grodzicki Jorge M. Seminario Peter Politzer 《International journal of quantum chemistry》1992,42(2):267-272
The C? H bond dissociation energy of acetylene was computed by both ab initio approaches and density functional theory in a local density approximation (DFT–LDA ). Structures and energies for acetylene and its dissociation products (the ethynyl and hydrogen radicals) are presented and compared. Using directly computed HCCH and HCC· energies and the exact H· value, the DFT–LDA calculations are found to yield C? H dissociation energies ranging from 129 to 131 kcal/mol, in good agreement with recent experimental and the highest level theoretical results. The DFT–LDA results show little dependence upon the computational procedure used to obtain geometries. 相似文献
39.
Valentina Domenici Carlo Alberto Veracini Katalin Fodor-Csorba Giacomo Prampolini Ivo Cacelli Andrjia Lebar Bostjan Zalar 《Chemphyschem》2007,8(16):2321-2330
The orientational properties of the banana-shaped liquid crystal 4-chloro-1,3-phenylenebis{4-[4'-(10-undecenyloxy)]benzoyloxy} benzoate (ClPbis11BB) are reported in the nematic phase under the effect of an external magnetic field. A new hypothesis, which states that the central ring of the aromatic core is oriented perpendicularly to the external magnetic field, is proposed. In support of this hypothesis, a series of studies based on (2)H NMR spectroscopy, both in the bulk and in solution, are discussed. (2)H NMR measurements on three selectively deuterium-labelled isotopomers are presented, together with DFT results from B3LYP/cc-pvDz calculations performed on the aromatic core. The rather flat shape of the investigated intramolecular energy surface allows for several different conformations to be populated, the computed magnetic susceptibilities of which are consistent with the proposed hypothesis of peculiar orientation of banana-shaped molecules. Moreover, the orientation of the magnetic susceptibility tensor is shown to be strongly dependent on the internal conformation of the banana-shaped molecules. 相似文献
40.
Reguera Edilso Fernández-Bertrán José Balmaseda Jorge 《Transition Metal Chemistry》1999,24(6):648-654
Evidence in the literature on the synthesis of ferrous ferricyanide is critically discussed. Pyrolysis and pressure effects on Prussian Blue lead to ferrous ferrocyanide together with decomposition by-products, and not to ferrous ferricyanide. The latter compound could be a precursor in the formation of Turnbull's Blue or an excited state of Prussian Blue, but it is not a stable chemical species. 相似文献