A structural comparison of three different crystalline forms of poly(β‐propiolactone) (PPL) was carried out by wide‐angle X‐ray diffraction, Fourier‐transform infrared spectroscopy, and differential scanning calorimetry. The α‐form in a hot‐drawn and annealed film represents a 21 helix conformation. The β‐form in a cold‐drawn and annealed film represents a planar zigzag conformation. The γ‐form in an oriented sedimented mat of solution‐grown chain‐folded lamellar crystals also implies a planar zigzag conformation. The solution‐cast film depicts similar outlines with the γ‐form in lamellar crystals in all the experimental measurements, suggesting that the molecular chain in the solution‐cast film has a planar zigzag conformation. While elongation at break decreased, tensile strength and Young's modulus increased with an increase in the crystallinity, independent of the crystalline forms. The influence of the enzymatic degradation of these crystal structures has been investigated by using an extracellular PHB depolymerase purified from Ralstonia pickettii T1. The rate of degradation was in the order of β‐form > α‐form > solution‐cast (γ‐form) film, and the different surface morphologies after partial enzymatic degradation were observed in scanning electron micrographs. It is suggested that the crystal structure is one of the important factors for determining the rate of degradation together with crystallinity.
Enzymatic degradation profiles of poly(β‐propiolactone) films. 相似文献
Two new non‐metallic filled β‐manganese phases M2Ga6Te10 (M: Li, Na) are obtained as black, homogeneous, microcristalline samples as well as single crystals by direct reaction of the elements. According to the single crystal structure determinations both compounds crystallize in space group R32 (No. 155, Z = 2) with the lattice constants: a = 1436.9(2), c = 1759.0(4) pm (T = 180 K, Li2Ga6Te10) and a = 1458(1) pm, c = 1776.1(4) pm (T = 290 K, Na2Ga6Te10). Their structures are characterized by tetrahedral close packings of Te2–, corresponding to the arrangement of Mn atoms in β‐Mn. While Ga3+ ions are distributed in an ordered way over 12% of the tetrahedral holes, the M+ ions occupy all distorted octahedral (“metaprismatic”) holes. As the Li+ ions are too small they occupy off‐center positions inside the metaprisms. Positions with the strongest off‐centering can only be refined on the basis of a split model. MAS‐NMR measurements, including multiple quantum NMR, allowed the two different crystallographic M+ sites to be distinguished unambigously by separate 7Li and 23Na signals, respectively. The assignment of the NMR signals was supported by measurements of samples in which Li+ was partly substituted by larger cations (Sn2+, Pb2+). 相似文献
Heterometallic Cluster Complexes of the Types Re2(μ-PR2)(CO)8(HgY) and ReMo(μ-PR2)(η5-C5H5)(CO)6(HgY) (R = Ph, Cy; Y = Cl, W(η5-C5H5)(CO)3) Dinuclear complexes Re2(μ-H)(μ-PR2)(CO)8 and ReMo(μ-H)(μ-PR2)(η5-C5H5)(CO)6 (R = phenyl, cyclohexyl) were deprotonated and reacted as anions with HgCl2 to compounds of the both types Re2(μ-PR2)(CO)8HgCl) and ReMo(μ-PR2)(η5-C5H5)(CO)6(HgCl). The heterometallic three-membered cluster complexes correspond to an isolobal exchange of a proton against a cationic HgCl+ group. For one of the products ReMo(μ-PCy2)(η5-C5H5)(CO)6(HgCl) has been shown its conversion with NaW(η5-C5H5)(CO)3 to ReMo(μ-PCy2)(η5-C5H5)(HgW(η5-C5H5)(CO)3) under substitution of the chloro ligand, par example. The newly prepared compounds were characterized by means of IR, UV/VIS and 31P NMR data. A complete determination of the molecular structure by single crystal analyses was done in the case of Re2(μ-PCy2)(CO)8(HgCl) and of ReMo(μ-PCy2)(η5-C5H5)(CO)6(HgCl) which both are dimer because of the presence of an asymmetric dichloro bridge, and of ReMo(μ-PCy2)(η5-C5H5)(CO)6(HgW(η5-C5H5)(CO)3). The structural study illustrates through comparison the influence of various metal types on an interaction between centric and edge-bridged frontier orbitals in three-membered metal rings. 相似文献
The effect of gem‐dialkyl substituents on the backbone conformations of β‐amino acid residues in peptides has been investigated by using four model peptides: Boc‐Xxx‐β2,2Ac6c(1‐aminomethylcyclohexanecarboxylic acid)‐NHMe (Xxx=Leu ( 1 ), Phe ( 2 ); Boc=tert‐butyloxycarbonyl) and Boc‐Xxx‐β3,3Ac6c(1‐aminocyclohexaneacetic acid)‐NHMe (Xxx=Leu ( 3 ), Phe ( 4 )). Tetrasubstituted carbon atoms restrict the ranges of stereochemically allowed conformations about flanking single bonds. The crystal structure of Boc‐Leu‐β2,2Ac6c‐NHMe ( 1 ) established a C11 hydrogen‐bonded turn in the αβ‐hybrid sequence. The observed torsion angles (α(?≈?60°, ψ≈?30°), β(?≈?90°, θ≈60°, ψ≈?90°)) corresponded to a C11 helical turn, which was a backbone‐expanded analogue of the type III β turn in αα sequences. The crystal structure of the peptide Boc‐Phe‐β3,3Ac6c‐NHMe ( 4 ) established a C11 hydrogen‐bonded turn with distinctly different backbone torsion angles (α(?≈?60°, ψ≈120°), β(?≈60°, θ≈60°, ψ≈?60°)), which corresponded to a backbone‐expanded analogue of the type II β turn observed in αα sequences. In peptide 4 , the two molecules in the asymmetric unit adopted backbone torsion angles of opposite signs. In one of the molecules, the Phe residue adopted an unfavorable backbone conformation, with the energetic penalty being offset by a favorable aromatic interaction between proximal molecules in the crystal. NMR spectroscopy studies provided evidence for the maintenance of folded structures in solution in these αβ‐hybrid sequences. 相似文献
Chloroselenates with Di- and Tetravalent Selenium: 77Se-NMR-Spectra, Syntheses, and Crystal Structures of (PPh4)2SeCl6 · 2 CH2Cl2, (NMe3Ph)2SeCl6, (K-18-crown-6)2SeCl6 · 2 CH3CN, PPh4Se2Cl9, (NEt4)2Se2Cl10, (PPh4)2Se3Cl8 · CH2Cl2, and (PPh4)2Se4Cl12 · CH2Cl2 The title compounds were obtained from reactions of selenium and selenium tetrachloride with PPh4Cl, NEt4Cl, NMe3PhCl, or (K-18-crown-6)Cl in dichloromethane or acetonitrile. (PPh4)2Se3Cl8 · CH2Cl2 was also formed from GeSe, PPh4Cl and chlorine in acetonitrile. The 77Se-NMR spectra of the solutions show the presence of dynamical equilibria which, depending on composition, mainly contain SeCl2, SeCl4, Se2Cl2, SeCl62–, Se2Cl62–, and/or Se2Cl102–. Solutions of AsCl3 and (PPh4)2Se4 in acetonitrile upon chlorination with Cl2 or PPh4AsCl6 yielded (PPh4)2Se2Cl6, while (PPh4)2As2Se4Cl12 was the product after chlorination with SOCl2. According to the X-ray crystal structure analyses the ions SeCl62–, Se2Cl9–, and Se2Cl102– have the known structures with octahedral coordination of the Se atoms. The structure of the Se3Cl82– ion corresponds to that of Se3Br82– consisting of three SeCl2 molecules associated via two Cl– ions. (PPh4)2Se4Cl12 · CH2Cl2 is isotypic with the corresponding bromoselenate and contains anions in which three SeCl2 molecules are attached to a SeCl62– ion; there is a peculiar Se–Se interaction. 相似文献
Short peptides that fold into β‐hairpins are ideal model systems for investigating the mechanism of protein folding because their folding process shows dynamics typical of proteins. We performed folding, unfolding, and refolding molecular dynamics simulations (total of 2.7 μs) of the 10‐residue β‐hairpin peptide chignolin, which is the smallest β‐hairpin structure known to be stable in solution. Our results revealed the folding mechanism of chignolin, which comprises three steps. First, the folding begins with hydrophobic assembly. It brings the main chain together; subsequently, a nascent turn structure is formed. The second step is the conversion of the nascent turn into a tight turn structure along with interconversion of the hydrophobic packing and interstrand hydrogen bonds. Finally, the formation of the hydrogen‐bond network and the complete hydrophobic core as well as the arrangement of side‐chain–side‐chain interactions occur at approximately the same time. This three‐step mechanism appropriately interprets the folding process as involving a combination of previous inconsistent explanations of the folding mechanism of the β‐hairpin, that the first event of the folding is formation of hydrogen bonds and the second is that of the hydrophobic core, or vice versa. 相似文献
The reaction of PPh2Cl with orthomanganated acetophenone, 2′-CH3C(O)C6H4Mn(CO)4, gives Mn2(μ-η1,η1-Ph2PPPh2)(μ-Cl)2(CO)6. An X-ray structure determination [triclinic, space group P1 , a = 10.908(4) Å, b = 11.756(3) Å, c = 12.186(3) Å, α = 96.20(2)°, β = 99.51(2)°, γ = 96.52(2)°] shows two Mn(CO)3 groups held together by two bridging Cl ligands, and further bridged by a Ph2P? PPh2 group prepared in situ. 相似文献
The Chlorooxoarsenates(III) (PPh4)2[As4O2Cl10] · 2 CH3CN and (PPh4)2[As2OCl6] · 3 CH3CN (PPh4)2[As2Cl8] can be prepared from As2O3, SOCl2 and PPh4Cl in acetonitrile. Its oxidation with chlorine yields PPh4[AsCl6]. This was also obtained directly from arsenic, chlorine and PPh4Cl, (PPh4)2[As4O2Cl10] · 2 CH3CN being a side product; the latter was obtained with high yield from AsCl3, As2O3 and PPh4Cl in acetonitrile. By addition of PPh4Cl it was converted to (PPh4)2[As2OCl6] · 3 CH3CN. According to their X-ray crystal structure analyses, both crystallize in the triclinic space group P 1. The [As4O2Cl10]2– ion can be regarded as a centrosymmetric association product of two Cl2AsOAsCl2 molecules and two Cl– ions, each Cl– ion being coordinated with all four As atoms. In the [As2OCl6]2– ion the As atoms are linked via the O atom and two Cl atoms. 相似文献
The new materials CsPbBi(3)Te(6) and CsPb(2)Bi(3)Te(7) were discovered through reactions of CsBi(4)Te(6) with PbTe, whereas the isostructural materials CsSnBi(3)Te(6) and CsSn(2)Bi(3)Te(7) were discovered through corresponding reactions with SnTe. The compounds can also be prepared from stoichiometric mixtures of Cs(2)Te, Pb (Sn), Bi, and Te. The crystal structures show a layered architecture of NaCl-type slabs alternating with layers of Cs atoms. This group of compounds offers a new quaternary system, Cs-M-Bi-Te (M = Pb and Sn), available for thermoelectric investigations, including fine-tuning of compositions and doping. 相似文献
Chemical Vapour Transport of Intermetallic Systems. 7. Chemical Transport of Ni3Ge, Ni5Ge3, Ni(Ge)-mixed Crystal, CoSn, Co3Sn2, Cu41Sn11 (δ-phase), Cu10Sn3 (ζ-phase), and Cu(Sn)-mixed Crystals By means of GaI3 as transport agent some intermetallics in the Ni/Ge- and Co/Sn-system can be prepared by CVT-methods. Using Iodine Cu–Sn-compounds can be deposited in a similiar way. 相似文献
Reaction of Tin Chlorides with Polysulfides. Crystal Structures of (PPh4)2[SnCl2(S6)2], (PPh4)2[Sn4Cl4S5(S3)O], and (PPh4)2[SnCl6] · S8 · 2CH3CN . The reaction of PPh4[SnCl3] with Na2S4 in acetonitrile in the presence of small amounts of water yields (PPh4)2[Sn4Cl4S5(S3)O] and minor amounts of (PPh4)2[SnCl2(S6)2], PPh4Cl · 2S8 and (PPh4)2[SnCl6]. SnCl4 is partially reduced by (PPh4)2Sx, PPh4[SnCl3] and (PPh4)2[SnCl6] · S8 · 2CH3CN being produced. According to the X-ray crystal structure determination the [Sn4Cl4S5(S3)O]2?-ion consists of an O atom that is coordinated by four Sn atoms which in turn are liked with one another by five single S atoms and one S3 group. In the [SnCl2(S6)2]2?-ion the Sn atom is octahedrally coordinated by two Cl atoms in trans arrangement and by two chelating S6 groups. Octahedral [SnCl6]2? ions and S8 molecules in the crown conformation are present in (PPh4)4[SnCl6] · S8 · 2CH3CN. 相似文献
Whereas for a series of layered compounds with the general formula (GeTe)n(Sb2Te3)m the stoichiometry allows to predict the structure type and the average thickness of the hexagonal atom layers, these rules are not generally applicable for GeTe‐rich compounds like Ge4Sb2Te7. A 39R layer stacking is expected, however, single crystal diffraction studies reveal a 33R layered structure ( , a = 4.1891(5) Å, c = 62.169(15), R = 0.047) closely related to that of Ge3Sb2Te6. This is also corroborated by the average layer thickness that can be determined from the strong reflections of powder patterns and exhibits a direct relation to the structure type. Mixed occupancy of cation positions with Ge and Sb and possibly defects allow this unusual range of homogeneity. Bulk material of the kinetically stable compound can be synthesized by quenching stoichiometric melts of the pure elements and subsequent annealing. 相似文献
Cyclic Polyselenidoarsenates(III) and Polyselenidoantimonates(III): PPh4[Se5AsSe], PPh4[AsSe6–xS x ], (PPh4)2[As2Se6] · 2 CH3CN, and (PPh4)2[Se6SbSe]2 In acetonitrile, AsCl3 and sodiumphenolate formed Cl2AsOPh which then was reacted with PPh4Se5 and finally with Na2Se to yield PPh4[Se5AsSe]. With Na2S instead of Na2Se, PPh4[AsSe6–xSx] was obtained; the sulfur contents increased with increasing reaction temperature and time (x = 0.21 to 1.09). With PPh4Se2 instead of PPh4Se5, (PPh4)2[1,4-As2Se6] · 2 CH3CN and PPh4[Se5AsSe] were the products. With SbCl3 instead of AsCl3, (PPh4)2[Se6SbSe]2 formed. PPh4[Se5AsSe] can also be produced from As2Se3, PPh4Br, Na2Se and selenium in acetonitrile. The crystal structure of PPh4[SeAsSe5] is isotypic with PPh4[S5AsS] (X-ray structure analysis with 2414 observed reflexions, R = 0.038). The Se5AsSe– ion consists of a six-membered AsSe5 ring in chair conformation, and the As atom has an additional terminal Se atom. The compounds PPh4[AsSe6–xSx] have the same crystal structures, with sulfur atoms taking all selenium positions at random, but with a preference for the terminal position. The anion in (PPh4)2[As2Se6] · 2 CH3CN also has a six-membered ring structure in chair conformation, with two arsenic atoms in positions 1 and 4. The centrosymmetric anion in (PPh4)2[Se6SbSe]2 consists of a central Sb2Se2 ring, and a Se6 ligand is bonded in a chelating manner to each Sb atom (X-ray structure analysis with 2669 observed reflexions, R = 0.099). 77Se-NMR spectra are reported. 相似文献
Te(C6F5)4 was prepared from the reactions of TeCl4 or Te(C6F5)2Cl2 with Grignard reagents or AgC6F5 in moderate to good yields. Substitution reactions with Me3SiX (X = Cl, Br, OSO2CF3), with equimolar amounts of Br2, with AgNO3 and with H[BF4] or BF3·OEt2 yielded the Te(C6F5)3X derivatives (X = Cl, Br, OSO2CF3, NO3, BF4). Oxidation reactions of Cd, Hg, and Pd0 complexes led to Te(C6F5)2 and the corresponding bis(pentafluorophenyl) derivatives M(C6F5)2 (M = Cd, Hg, Pd) and with InBr to In(C6F5)2Br. From very slow hydrolysis of Te(C6F5)4 the oxide Te(C6F5)2O was prepared. The thermal decomposition, the NMR and mass spectra of the partially new compounds are discussed. The crystal structures of Te(C6F5)3Br (monoclinic, P21/a, Z = 4), [Te(C6F5)3][OSO2CF3] (monoclinic, P21/n, Z = 16) and [Te(C6F5)2O]2 (triclinic, P1¯, Z = 2) were determined. 相似文献
Synthesis and Crystal Structures of Li4?2xSr2+xB10S19 (x ≈ 0.27) and Na6B10S18. Two Novel Thioborates with Highly Polymeric Macro-tetrahedral Networks Li4?2xSr2+xB10S19 (x ≈ 0.27) and Na6B10S18 were prepared from the reaction of strontium sulfide and lithium sulfide (sodium sulfide) with boron and sulfur at 700°C in graphitized silica tubes. Li4?2xSr2+xB10S19 (x ≈ 0.27) crystallizes in the monoclinic space group P21/c with a = 10.919(2) Å, b = 13.590(3) Å, c = 16.423(4) Å, and β = 90.48(2)°, Na6B10S18 in the tetragonal space group I41/acd with a = 14.415(3) Å, c = 26.137(4) Å. Both structures contain supertetrahedral B10S20 units which are linked through tetrahedral corners to form a three-dimensional polymeric network in the case of Na6B10S18 and one-dimensional chains in the case of Li4?2xSr2+xB10S19 (x ≈ 0.27). All boron atoms are in tetrahedral BS4 coordination (B? S bond lengths vary from 1.879(5) to 1.951(5) Å (1.875(10) to 1.987(9) Å)). The strontium and lithium (sodium) cations are located within large channels formed by the anions. 相似文献
Two Representatives of the α-LiFeO2 Type: LiInO2 and α-LiYbO2 Single crystals of LiInO2 (well ground mixtures of ‘In2O’: KO2: Li2O = 1:1.8:1, Ag-tube, 22d, 520°C); and α-LiYbO2 (Yb2O3:Li2O = 1:1.1, Ni-tube, 30d, 1150°C) were prepared for the first time from mixtures of the binary oxides. The determination of the structure confirmed the α-LiFeO2 type. Both oxides crystallize in the space group I 41/amd (Z = 4); LiInO2: a = 431.2(1) pm c = 934.2(2) pm; c/a = 2.167; 123 Io(hkl), R = 3.6%, Rw = 2.9%. α-LiYbO2:a = 438.7(1) pm c = 1006.7(2) pm; c/a = 2.295, 130 Io(hkl), R = 4.4%, Rw = 3.6%. The Madelung part of lattice energy, MAPLE and characteristics of this kind of structure are discussed. 相似文献