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The crystal structure of the ζ2‐phase Al3Cu4‐δ was determined by means of X‐ray powder diffraction: a = 409.72(1) pm, b = 703.13(2) pm, c = 997.93(3) pm, space group Imm2, Pearson symbol oI24‐3.5, RI = 0.0696. ζ2‐Al3Cu4‐δ forms a distinctive a × √3a × 2c superstructure of a metal deficient Ni2In‐type‐related structure. The phase is meta‐stable at ambient temperature. Between 400 °C and 450 °C it decomposes into ζ1‐Al3Cu4 and η2‐AlCu. Entropic contributions to the stability of ζ2‐Al3Cu4‐δ are reflected in three statistically or partially occupied sites. 相似文献
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G. Raju V. Ramesh R. Srinivas G. V. M. Sharma B. Shoban Babu 《Journal of mass spectrometry : JMS》2010,45(6):651-663
Two new series of Boc‐N‐α,δ‐/δ,α‐ and β,δ‐/δ,β‐hybrid peptides containing repeats of L ‐Ala‐δ5‐Caa/δ5‐Caa‐L ‐Ala and β3‐Caa‐δ5‐Caa/δ5‐Caa‐β3‐Caa (L ‐Ala = L ‐alanine, Caa = C‐linked carbo amino acid derived from D ‐xylose) have been differentiated by both positive and negative ion electrospray ionization (ESI) ion trap tandem mass spectrometry (MS/MS). MSn spectra of protonated isomeric peptides produce characteristic fragmentation involving the peptide backbone, the Boc‐group, and the side chain. The dipeptide positional isomers are differentiated by the collision‐induced dissociation (CID) of the protonated peptides. The loss of 2‐methylprop‐1‐ene is more pronounced for Boc‐NH‐L ‐Ala‐δ‐Caa‐OCH3 (1), whereas it is totally absent for its positional isomer Boc‐NH‐δ‐Caa‐L ‐Ala‐OCH3 (7), instead it shows significant loss of t‐butanol. On the other hand, second isomeric pair shows significant loss of t‐butanol and loss of acetone for Boc‐NH‐δ‐Caa‐β‐Caa‐OCH3 (18), whereas these are insignificant for its positional isomer Boc‐NH‐β‐Caa‐δ‐Caa‐OCH3 (13). The tetra‐ and hexapeptide positional isomers also show significant differences in MS2 and MS3 CID spectra. It is observed that ‘b’ ions are abundant when oxazolone structures are formed through five‐membered cyclic transition state and cyclization process for larger ‘b’ ions led to its insignificant abundance. However, b1+ ion is formed in case of δ,α‐dipeptide that may have a six‐membered substituted piperidone ion structure. Furthermore, ESI negative ion MS/MS has also been found to be useful for differentiating these isomeric peptide acids. Thus, the results of MS/MS of pairs of di‐, tetra‐, and hexapeptide positional isomers provide peptide sequencing information and distinguish the positional isomers. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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L. C. R. Andrade J. A. Paixo M. J. M. De Almeida F. M. Fernandes Roleira E. J. Tavares da Silva 《Acta Crystallographica. Section C, Structural Chemistry》2005,61(3):o131-o133
The title compounds, both C23H34O5, are the 5α and 5β configurations of two diacetate epimers. The 5β‐diacetate crystallizes in an hexagonal structure, unusual for steroid molecules. The unit cell has an accessible solvent volume of 358 Å3, responsible for clathrate behaviour. The 5β‐epimer also features some shorter than average bond lengths in the 3α,4β‐acetoxy groups. The conformations of the molecules of both epimers are compared with those obtained through abinitio quantum chemistry calculations. Cohesion of the crystals can be attributed to van der Waals and weak molecular C—H⋯O interactions. 相似文献
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Erwann Jeanneau Nathalie Audebrand Daniel Louër 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(9):1012-1013
Crystals of anhydrous cadmium oxalate, β‐[Cd(C2O4)], have been synthesized hydrothermally and the crystal structure solved using single‐crystal X‐ray diffraction data. The Cd and oxalate ions lie about independent inversion centres. The structure consists of a three‐dimensional framework built from sheets of cadmium octahedra linked together by oxalate groups. 相似文献
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Gilles Wallez Sylvie Jaulmes Ammy Elfakir Jean‐Paul Souron Michel Quarton 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(11):i107-i109
The ambient‐temperature form of dithallium sulfate, β‐Tl2SO4, is similar to β‐K2SO4 and is characterized by isolated sulfate tetrahedra and two different thallium sites with coordination numbers 9 and 11. All the atoms, except one O atom, lie on mirror planes. In spite of there being a high concentration of Tl+ cations, the stereochemical activity of the 6s2 pairs is low, similar to that of isotypic Tl2XO4 compounds (X = Cr and Se). This behaviour is the consequence of both weak Tl—O bonds and strong X—O bonds, because in a Tl—O—X linkage the electronic cloud of the O2− anion is strongly distorted and displaced towards X, resulting in a low negative charge in the face of the Tl atom. Consequently, the Coulombic repulsions between the lone pair and the O2− anions are weak. All of the Tl2XO4 compounds exhibit the same open packing of A+ cations and [XO4]2− anions as their isotypic alkali counterparts. 相似文献
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Feng H. Lu 《Rapid communications in mass spectrometry : RCM》2009,23(19):3144-3150
A method for online simultaneous δ2H and δ18O analysis in water by high‐temperature conversion is presented. Water is injected by using a syringe into a high‐temperature carbon reactor and converted into H2 and CO, which are separated by gas chromatography (GC) and carried by helium to the isotope ratio mass spectrometer for hydrogen and oxygen isotope analysis. A series of experiments was conducted to evaluate several issues such as sample size, temperature and memory effects. The δ2H and δ18O values in multiple water standards changed consistently as the reactor temperature increased from 1150 to 1480°C. The δ18O in water can be measured at a lower temperature (e.g. 1150°C) although the precision was relatively poor at temperatures <1300°C. Memory effects exist for δ2H and δ18O between two waters, and can be reduced (to <1%) with proper measures. The injection of different amounts of water may affect the isotope ratio results. For example, in contrast to small injections (100 nL or less) from small syringes (e.g. 1.2 µL), large injections (1 µL or more) from larger syringes (e.g. 10 µL) with dilution produced asymmetric peaks and shifts of isotope ratios, e.g. 4‰ for δ2H and 0.4‰ for δ18O, probably resulting from isotope fractionation during dilution via the ConFlo interface. This method can be used to analyze nanoliter samples of water (e.g. 30 nL) with good precision of 0.5‰ for δ2H and 0.1‰ for δ18O. This is important for geosciences; for instance, fluid inclusions in ancient minerals may be analyzed for δ2H and δ18O to help understand the formation environments. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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Giuditta Bartalucci Charles Delroy Stuart Fisher Madeleine Helliwell Synnve Liaaen‐Jensen 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(3):o128-o131
13‐cis‐β,β‐Carotene, C40H56, crystallizes with a complete molecule in the asymmetric unit, whereas 15‐cis‐β,β‐carotene, also C40H56, has twofold symmetry about an axis through the central bond of the polyene chain. The polyene methyl groups are arranged on one side of the polyene chains for each molecule and the 6‐s‐cisβ end groups, with the cyclohexene rings in half‐chair conformations, are twisted out of the planes of the polyene chains by angles ranging from 41.37 (17) to 52.2 (4)°. The molecules in each structure pack so that the arms of one occupy the cleft of the next, and there is significant π–π stacking of the almost‐parallel polyene chains of the 15‐cis isomer, which approach at distances of 3.319 (1)–3.591 (1) Å. 相似文献
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Grald Lelais Peter Micuch Delphine Josien‐Lefebvre Francesco Rossi Dieter Seebach 《Helvetica chimica acta》2004,87(12):3131-3159
The Ser, Cys, and His side chains play decisive roles in the syntheses, structures, and functions of proteins and enzymes. For our structural and biomedical investigations of β‐peptides consisting of amino acids with proteinogenic side chains, we needed to have reliable preparative access to the title compounds. The two β3‐homoamino acid derivatives were obtained by Arndt–Eistert methodology from Boc‐His(Ts)‐OH and Fmoc‐Cys(PMB)‐OH (Schemes 2–4), with the side‐chain functional groups' reactivities requiring special precautions. The β2‐homoamino acids were prepared with the help of the chiral oxazolidinone auxiliary DIOZ by diastereoselective aldol additions of suitable Ti‐enolates to formaldehyde (generated in situ from trioxane) and subsequent functional‐group manipulations. These include OH→OtBu etherification (for β2hSer; Schemes 5 and 6), OH→STrt replacement (for β2hCys; Scheme 7), and CH2OH→CH2N3→CH2NH2 transformations (for β2hHis; Schemes 9–11). Including protection/deprotection/re‐protection reactions, it takes up to ten steps to obtain the enantiomerically pure target compounds from commercial precursors. Unsuccessful approaches, pitfalls, and optimization procedures are also discussed. The final products and the intermediate compounds are fully characterized by retention times (tR), melting points, optical rotations, HPLC on chiral columns, IR, 1H‐ and 13C‐NMR spectroscopy, mass spectrometry, elemental analyses, and (in some cases) by X‐ray crystal‐structure analysis. 相似文献
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A one‐step transformation of γ‐ and δ‐(spiro)lactones into γ,δ‐ and δ,ε‐unsaturated aldehydes with an excess of formic acid in the vapor phase over a supported manganese catalyst is described for the first time. The scope and limitations of this new reaction are shown with different lactones as substrate, and a mechanistic rationale is proposed. 相似文献
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Rui M. A. Pinto Jorge A. R. Salvador Jos A. Paixo 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(5):o279-o282
In the title compounds, C21H30O4, (I), and C23H34O4, (II), respectively, which are valuable intermediates in the synthesis of important steroid derivatives, rings A and B are cis‐(5β,10β)‐fused. The two molecules have similar conformations of rings A, B and C. The presence of the 5β,6β‐epoxide group induces a significant twist of the steroid nucleus and a strong flattening of the B ring. The different C17 substituents result in different conformations for ring D. Cohesion of the molecular packing is achieved in both compounds only by weak intermolecular interactions. The geometries of the molecules in the crystalline environment are compared with those of the free molecules as given by ab initio Roothan Hartree–Fock calculations. We show in this work that quantum mechanical ab initio methods reproduce well the details of the conformation of these molecules, including a large twist of the steroid nucleus. The calculated twist values are comparable, but are larger than the observed values, indicating a possible small effect of the crystal packing on the twist angles. 相似文献
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Wenhui Zhang Allen G. Oliver Henry M. Vu John G. Duman Anthony S. Serianni 《Acta Crystallographica. Section C, Structural Chemistry》2012,68(12):o502-o506
Methyl β‐D‐mannopyranosyl‐(1→4)‐β‐D‐xylopyranoside, C12H22O10, (I), crystallizes as colorless needles from water, with two crystallographically independent molecules, (IA) and (IB), comprising the asymmetric unit. The internal glycosidic linkage conformation in molecule (IA) is characterized by a ϕ′ torsion angle (O5′Man—C1′Man—O1′Man—C4Xyl; Man is mannose and Xyl is xylose) of −88.38 (17)° and a ψ′ torsion angle (C1′Man—O1′Man—C4Xyl—C5Xyl) of −149.22 (15)°, whereas the corresponding torsion angles in molecule (IB) are −89.82 (17) and −159.98 (14)°, respectively. Ring atom numbering conforms to the convention in which C1 denotes the anomeric C atom, and C5 and C6 denote the hydroxymethyl (–CH2OH) C atom in the β‐Xylp and β‐Manp residues, respectively. By comparison, the internal glycosidic linkage in the major disorder component of the structurally related disaccharide, methyl β‐D‐galactopyranosyl‐(1→4)‐β‐D‐xylopyranoside), (II) [Zhang, Oliver & Serriani (2012). Acta Cryst. C 68 , o7–o11], is characterized by ϕ′ = −85.7 (6)° and ψ′ = −141.6 (8)°. Inter‐residue hydrogen bonding is observed between atoms O3Xyl and O5′Man in both (IA) and (IB) [O3Xyl...O5′Man internuclear distances = 2.7268 (16) and 2.6920 (17) Å, respectively], analogous to the inter‐residue hydrogen bond detected between atoms O3Xyl and O5′Gal in (II). Exocyclic hydroxymethyl group conformation in the β‐Manp residue of (IA) is gauche–gauche, whereas that in the β‐Manp residue of (IB) is gauche–trans. 相似文献
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《中国化学》2018,36(4):311-320
A 5πe carbonyl activation mode is reported on the basis of photo‐induced single‐electron‐transfer (SET) oxidation of a secondary enamine. The resultant β‐enaminyl radical intermediate was trapped by a wide range of Michael acceptors, producing β‐alkylation products of β‐ketocarbonyls in a highly efficient manner. 相似文献
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The utility of diphenylphosphonoacetamides [(PhO)2P(O)CH2CONRR′] as Horner–Wadsworth–Emmons reagents was examined with five different patterns of substitution upon the amide nitrogen atom ( 2a : R, R′ = CH2Ph; 2b : R = CH2Ph, R′ = H; 2c : R = Me, R′ = OMe; 2d : R, R′ = Ph; 2e : R, R′ = (CH2)4). The reaction of 2a was found to be Z‐selective for aromatic aldehydes with selectivities up to 95:5. Reagent 2b led to reasonable selectivity for both benzaldehyde (85:15) and 3‐phenylpropionaldehyde (87:13), while 2c was somewhat effective for only the latter alkyl aldehyde (83:17). Compounds 2d and 2e exhibited slightly lower selectivities compared with 2a . © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:515–523, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20054 相似文献
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Lamine Hamdellou Olivier Hernandez Jean Meinnel 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(9):o557-o560
The structures of 4‐dimethylamino‐β‐nitrostyrene (DANS), C10H12N2O2, and 4‐dimethylamino‐β‐ethyl‐β‐nitrostyrene (DAENS), C12H16N2O2, have been solved at T = 100 K. The structure solution for DANS was complicated by the presence of a static disorder, characterized by a misorientation of 17% of the molecules. The molecule of DANS is almost planar, indicating significant conjugation, with a push–pull effect through the styrene skeleton extending up to the terminal substituents and enhancing the dipole moment. As a consequence of this conjugation, the hexagonal ring displays a quinoidal character; the lengths of the C—N [1.3595 (15) Å] and C—C [1.448 (2) Å] bonds adjacent to the benzene ring are shorter than single bonds. The molecules are stacked in dimers with antiparallel dipoles. In contrast, the molecule of DAENS is not planar. The ethyl substituent pushes the nitropropene group out of the benzene plane, with a torsion angle of −21.9 (3). Nevertheless, the molecule remains conjugated, with a shortening of the same bonds as in DANS. 相似文献