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511.
Michel Mons Evan G. Robertson Lavina C. Snoek John P. Simons 《Chemical physics letters》1999,310(5-6)
The structures of 2-phenylethanol and its 1:1 water complexes have been investigated by UV–UV holeburning and IR–UV ion-dip spectroscopy, coupled with ab initio computation. The most populated molecular conformer is stabilized by an intramolecular π-type H-bond and its rotational band contours suggest the incidence of vibronic coupling involving motion of the side chain. Its 1:1 water complexes are associated with two distinct structures – water binds either as a proton donor or an acceptor. In the latter, the intramolecular H-bond is disrupted and the water molecule inserts between the OH and the aromatic ring. A second, extended anti conformer can also be detected. 相似文献
512.
Klokkenburg M Lutz M Spek AL van der Maas JH van Walree CA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(15):3544-3554
The synthesis, structure, and electronic properties of a series of cross-conjugated p-phenylenevinylidene oligomers with one to four double bonds are reported. The X-ray crystal structure of the compound with two double bonds reveals a nonplanar conformation with torsion angles about the C(phenylene)-C(vinylidene) and C(phenyl)-C(vinylidene) formal single bonds of 39.5(2) degrees and 30.5(2) degrees, respectively. Admixture of quinoid character in the ground state is observed. Infrared and Raman spectroscopy do not provide a clear picture of the degree of electron delocalization in the series, since the C=C stretching mode does not adequately reflect the C=C bond order and has a local nature. In contrast, electronic spectra and electrochemical data, as well as AM1 and PPP/SCF calculations, reveal that the cross-conjugated compounds basically behave as linearly pi-conjugated systems in the sense that molecular orbitals are delocalized over the entire structure and systematically change in energy. The electronic interaction between the repeating units is, however, not very strong, which has the consequence that spatial extension of the molecular orbitals does not lead to a red shift of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) electronic transition. This is related to the feature that the modest narrowing of the HOMO-LUMO gap with the chain length is accompanied by a relatively large reduction of electron repulsion. This finding implies that care should be taken in the use of electronic spectra for the evaluation of conjugation phenomena. 相似文献
513.
Asbestos fibers, of the chrysotile variety, were coated with a thin polyamide film by an in situ polycondensation technique. Ionomer-based composites were prepared containing the so-modified asbestos fibers in a random in-plane orientation; results of testing the tensile properties of these asbestos/polyamide/ionomer composites are presented. Parameters investigated comprise the asbestos content in the composite and the polyamide content deposited on asbestos. A significant improvement in the tensile performance was established, especially at the intermediate polyamide content of 3.4 phr. The behavior is discussed in terms of possible interactions between the phases present in the composite material. 相似文献
514.
The ability of the monensin-producing organism Streptomyces cinnamonensis to bioconvert the (E)-and (Z)-isomers of nerolidol (= 3,7,1 1-trimethyldodeca-1,6,10-trien-3-ol) into new oxygenated products has been investigated. When a 3H-labelled racemic form of each sesquiterpene was added to fermentations of S. cinnamonensis, several new 3H-labelled products could be detected. Two products were isolated from bioconversion of (E)-nerolidol, the amide 8 and the 9 (Scheme 2), whereas four products were isolated from the bioconversion of (Z)-nerolidol, the epoxydiol 10 , triols 11 and 12 , and the tetrahydrofuryl alcohol 13 (Scheme 4). Products 9 – 13 were obtained as a 1 : 1 mixture of diastereoisomers, and 12 was shown to arise by the overall anti addition of two OH groups to the trisubstituted (Z)-double bond of (Z)-nerolidol. Both isomers of nerolidol as well as the acetylene 7 are inhibitors of monensin production in shake cultures of S. cinnamonensis. 相似文献
515.
Pamela M. St. John Rainer D. Beck Robert L. Whetten 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):226-228
We review the processes which have been observed from collisions between alkali-halide clusters and solid surfaces. Soft impact of nanocrystalline NanF n?1 + clusters against solid surfaces causes them to cleave along the lowest energy (100) plane. At higher collision energies (Ei>1 eV/atom), an evaporative cascade occurs which is characteristic of a transformation of the nanocrystal to a molten state. Efficient F? transfer from the cluster to the surface can occur for the larger clusters (>60 atoms) scattering from Si(111), in direct competition with the cleaving channel at low energies. In this regime, strong bonds can form between the F? and silicon surface. The reaction probability increases with cluster size indicating that an impact-initiated shock wave is needed to enhance the reactive process. 相似文献
516.
Whereas the thermospray mass spectra of most compounds consist of only the pseudo-molecular ion with little fragmentation, the thermospray mass spectra of arteether (a cyclic endoperoxide) and its metabolites are relatively complex. Assignments of structures to individual fragments from normal spectra was particularly ambiguous because of uncertainties as to which fragments arose from ammonium ion or methanol adducts. In this study, these assignments could be resolved through the comparison of the regular spectrum with the deuterium-exchange spectrum (in an ND4O2CCH3–CD3OD–D2O mobile phase) achieved using ‘sandwiched slug’ injection technique. The mass spectra of arteether and four of its metabolites all showed [M + ND4]+ pseudo-molecular ions with greater than 91% H/D exchange, indicating a high efficiency with a minimal use of deuterated mobile phase. Most fragments showed H/D exchange rates in the 70–90% range and the isotope shift of individual spectral lines (ΔM) was found to be extremely useful in determining the structure of the fragment. 相似文献
517.
Edgardo Laborde John S. Kiely Lawrence E. Lesheski Mel C. Schroeder 《Journal of heterocyclic chemistry》1991,28(1):191-198
A convergent synthesis of 1,8-naphthyridine antibacterials bearing a carbon-carbon bonded, acyclic or cyclic vinyl substituent at the C-7 position has been achieved. The synthetic methodology is based upon the palladium-catalyzed cross coupling of a 7-chloro-1,8-naphthyridine with an appropriately substituted organotin reagent. 相似文献
518.
Raman spectra of molten NaClO3 and KClO3 and of aqueous solutions of these salts were measured over the frequency interval from 50 to 1200 cm−1|. Infrared emission spectra of the molten chlorates and of chlorate-nitrate mixtures were recorded, and absorption spectra of aqueous sodium and potassium chlorate also were determined. The ν3(e) and ν4(e) modes of ClO−3 were split in the molten salt and aqueous solution spectra, and a single, weak band was observed between ca. 80 and 200 cm− in the Raman spectra of molten NaClO3 and KClO3. 相似文献
519.
Polyacrylamide gel electrophoresis was used to resolve as many as three protein components from incubation mixtures containing the inhibitor, 5-fluoro-2′-deoxyuridylate, the cofactor, 5,10-methylene tetrahydrofolate, and thymidylate synthetase. In a series of mixtures containing excess 5,10-methylenetetrahydrofolate and constant levels of thymidylate synthetase, the relative amounts of the protein components were shown to be dependent on the concentration of the inhibitor. Evidence is presented which suggests that the three protein components correspond to (1) native enzyme, (2) an inhibitor-cofactor-enzyme complex in a 1:1:1 molar ratio, and (3) an inhibitorcofactorenzyme complex in a 2:2:1 molar ratio, respectively. Ternary complexes of thymidylate synthetase are stable to gel filtration and are shown to undergo a relatively slow rate of breakdown on storage at 25 °C. 相似文献
520.
The oxymercuration—demercuration (OM—DM) of representative trimethylsilylalkenes in aqueous tetrahydrofuran was investigated. Vinyltrimethylsilane gives only the anti-Markovnikov alcohol, 2-trimethylsilylethanol in 90% yield. Allyltrimethylsilane gives mainly allylmercuric acetate which reacts further to give a 1,3-dimercurated-2-propanol. This species reacts slowly with allyltrimethylsilane to give allylmercuric acetate. 3-Buten-1-yltrimethylsilane gives only the Markovnikov product, 1-trimethylsilyl-3-butanol in 99% yield. cis-1-Propenyl-trimethylsilane gives 1-trimethylsilyl-2-propanol in 44% yield along with cleavage products. The formation of one of these products, 2-propanol, was found by PMR to arise from the OM—DM of cis-1-propenylmercuric acetate which gives the 1,1-dimercurated-2-propanol. While the oxymercuration of cis-1-propenyl-mercuric acetate is regiospecific giving exclusive placement of the mercury α to the silicon, it is completely nonstereospecific giving equal amounts of diastereomeric adducts. The OM—DM of 2-propenyl trimethylsilane (XIX) leads only to desilylation and oxidation products. Possible reasons for the diverse behavior of these silylated alkenes are discussed. 相似文献