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991.
Single crystals of Zr3Al3C5—a carbide previously reported with the formula ZrAlC2−x—were isolated from a sample prepared by reaction of ZrC with an excess of aluminum. The carbides ScAl3C3and UAl3C3were synthesized from the elemental components by arc-melting. The crystal structures of these three compounds were redetermined from four-circle X-ray diffractomter data. In the original structure determination of ZrAlC2−x, the metal positions were found to form close-packed layers in the space groupP63/mmc, while the carbon atoms were assumed to occupy 5/6 of the octahedral voids at random. The present structure determination in the space groupP63/mc(R=0.024 for 519 structure factors and 23 variable parameters) shows that all carbon positions are fully occupied and one has a trigonal bipyramidal aluminum coordination. The structures of ScAl3C3and UAl3C3also have originally been determined in the space groupP63/mmc. The present structure refinements in the space groupP63mc(ScAl3C3:R=0.031 for 282Fvalues and 16 variables; UAl3C3:R=0.029 for 217Fvalues and 16 variables) essentially confirms the structures with the exception of one aluminum site. In all of these structures the metal atoms are arranged in close-packed layers and together with the previously reported structure of U2Al3C4they form a homologous series with the general formulaT1+nAl3C3+n, wheren=0, 1, 2 for ScAl3C3, U2Al3C4, and Zr3Al3C5, respectively. The packing of the metal atoms is represented by the Zhdanov symbols (4)2, (5)2, and (6)2. The arrangement of the aluminum atoms is very similar to that of the binary carbide Al4C3, while the other metal atoms form a cubic stacking sequence, as it is found in the binary carbidesTC with NaCl type structure. 相似文献
992.
Barrett AG Blaney F Campbell AD Hamprecht D Meyer T White AJ Witty D Williams DJ 《The Journal of organic chemistry》2002,67(9):2735-2750
The total syntheses of eight members of the palmarumycin family have been achieved, with identification of the absolute stereochemistry for three of these natural products. In addition, the ras-farnesyl transferase inhibitor (-)-preussomerin G has been synthesized, achieving the first enantioselective route for accessing this family of natural products. Highlights of the synthetic work include an asymmetric epoxidation of a cyclic enone in excellent yield and enantiomeric excess and a potentially biomimetic oxidative spirocyclization for the introduction of the bis-spiroketal array unique to the preussomerin natural products. 相似文献
993.
Ternary Chlorides with Trigonal-Bipyramidal Clusters: [M5(C2)]Cl9 (M = La? Pr) The chlorides [M5(C2)]Cl9 (M = La? Pr) are obtained by metallothermic reduction of the respective trichlorides MCl3 with caesium in the presence of the lanthanide metal and carbon in sealed niobium ampoules at 800°C. They contain trigonal-bipyramidal clusters [M5(C2)] crystallizing with the triclinic crystal system. Only seven of the nine edges of the trigonal bipyramids are brigded by chloride (Cli). Each cluster is surrounded by twelve terminal ligands (Cla) so that units of the composition [M5(C2)Cl7i]Cl12a have to be considered. These are connected not only via Cli–a and Cla–a–a bridges. Rather, Cla–a (one linear and one bent) and Cli–i bridges are also observed. 相似文献
994.
995.
The kinetics of the reactions of eight nitroalkyl anions (nitronate anions) with benzhydrylium ions and quinone methides in DMSO and water were investigated photometrically. The second-order rate constants were found to follow a Ritchie constant selectivity relationship with slightly smaller selectivities than those observed previously for other carbanions and O or N nucleophiles. Evaluation of the kinetic data by the correlation equation log k (20 degrees C) = s(N + E) yields the nucleophilicity parameters (N), which allow a comparison of the nucleophilicities of nitronates with those of other classes of compounds. Although the aliphatic nitronates 1a-c are more nucleophilic than the aromatic representatives 1d-h in DMSO, hydration reduces the nucleophilicities of aliphatic nitronates by a factor of 1 million, which is considerably greater than the reduction of the reactivities of the aromatic nitronates with the consequence that aromatic nitronates are more nucleophilic in water than aliphatic ones. The nucleophilic reactivities of nitronates are only slightly affected by substituent variation in DMSO and even less so in aqueous solution, which is considered to be the reason for the unusual rate equilibrium relationships, the so-called nitroalkane anomaly. Outer-sphere electron transfer does not occur in any of the reactions that were investigated. 相似文献
996.
Stefan Matile Thorsten Hansen Achim Stocker Wolf-Dietrich Woggon 《Helvetica chimica acta》1994,77(4):1087-1098
The four peripheral ester moieties of the title compound 5 are regioselectively hydrolyzed under mild conditions in the presence of β -cyclodextrin ( 10 ), providing a new entry to tetraphenylporphyrin derivatives with differently substituted Ph groups in the meso-positions. UV, 1H-NMR and mass spectroscopy of the inclusion complex 9 of 2,6-O-dimethyl-β-cyclodextrin ( 12 ) and 5 indicate a stoichiometry of 2:1 for 10/5 . Moreover, calculations confirm NOE experiments consistent with the fact that the cyclodextrins 10 and 12 approach the Ph groups of the porphyrin with the small opening of the cavity (primary face). 相似文献
997.
Hongwei Chen Rolf Brandt Rolf Bandur Thorsten Hoffmann 《Frontiers of Chemistry in China》2006,1(2):119-129
In this contribution, iodine chemistry in the Marine Boundary Layer (MBL) is introduced. A series of methodologies for the
measurements of iodine species in the gas and particle phases of the coastal atmosphere has been developed. Iodine species
in the gas phase in real air samples has been determined in two field campaigns at the west coast of Ireland, indicating that
gaseous iodo-hydrocarbons and elemental iodine are the precursors of new particle formation. Particulate iodine speciation
from the same measurement campaigns show that the non-water-soluble iodine compounds are the main iodine species during the
marine particle formation. A seaweed-chamber experiment was performed, indicating that gaseous I2 is one of the important precursors that lead to new particle formation in the presence of solar light in the ambient air
at the coastal tidal area. 相似文献
998.
Felder T Schalley CA Fakhrnabavi H Lukin O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(19):5625-5636
Mass spectrometry, in particular MALDI-MS, has often been used as a valuable means to characterize dendritic molecules with respect to their molecular masses. Also, it is a valuable tool for analyzing potential defects in their structure which result from incomplete synthetic steps. This article presents a comparison of ESI and MALDI mass spectrometric experiments on dendrimers persulfonylated at their periphery. While the ESI mass spectra easily permit impurities and defects to be identified and thus provide evidence for sample purity, reactions with acidic matrices occur during the MALDI process. The resulting defects are identical to those expected from incomplete substitution. Thus, in these cases, MALDI-MS yields false negative results. With mass-selected, ESI-generated ions, collision experiments were performed in an FT-ICR mass spectrometer cell to provide detailed insight into the fragmentation patterns of the various dendrimers. Different fragmentation patterns are observed depending on the exact structure of the dendrimer. Also, the nature of the charge is important. The fragmentation reactions for protonated species differ much from those binding a sodium or potassium ion. These differences can be traced back to different sites for binding H+ versus Na+ or K+. Tandem MS experiments on mass-selected dendrimer ions with defects can be used to distinguish different types of defects. A concise structural assignment can thus be made on the basis of these experiments. Even mixtures of two isobaric defect variants with the same elemental composition can be identified. 相似文献
999.
Christe KO Boatz JA Gerken M Haiges R Schneider S Schroer T Tham FS Vij A Vij V Wagner RI Wilson WW 《Inorganic chemistry》2002,41(16):4275-4285
SO(2) solutions of azide anions are bright yellow, and their Raman spectra indicate the presence of covalently bound azide. Removal of the solvent at -64 degrees C from CsN(3) or N(CH(3))(4)N(3) solutions produces yellow (SO(2))(2)N(3)(-) salts. Above -64 degrees C, these salts lose 1 mol of SO(2), resulting in white SO(2)N(3)(-) salts that are marginally stable at room temperature and thermally decompose to the corresponding azides and SO(2). These anions were characterized by vibrational and (14)N NMR spectroscopy and theoretical calculations. Slow loss of the solvent by diffusion through the walls of a sealed Teflon tube containing a sample of CsSO(2)N(3) in SO(2) resulted in white and yellowish single crystals that were identified by X-ray diffraction as CsSO(2)N(3).CsSO(3)N(3) with a = 9.542(2) A, b = 6.2189(14) A, c = 10.342(2) A, and beta = 114.958(4) degrees in the monoclinic space group P2(1)/m, Z = 2, and Cs(2)S(2)O(5).Cs(2)S(2)O(7).SO(2), respectively. Pure CsSO(3)N(3) was also prepared and characterized by vibrational spectroscopy. The S-N bond in SO(2)N(3)(-) is much weaker than that in SO(3)N(3)(-), resulting in decreased thermal stability, an increase in the S-N bond distance by 0.23 A, and an increased tendency to undergo rotational disorder. This marked difference is due to SO(3) being a much stronger Lewis acid (pF(-) value of 7.83) than SO(2) (pF(-) value of 3.99), thus forming a stronger S-N bond with the Lewis base N(3)(-). The geometry of the free gaseous SO(2)N(3)(-) anion was calculated at the RHF, MP2, B3LYP, and CCSD(T) levels. The results show that only the correlated methods correctly reproduce the experimentally observed orientation of the SO(2) group. 相似文献
1000.