A new model for the molecular conformation in polyoxymethylene crystals

Nonintegral indices are found for the extra meridional reflections in the x-ray diffraction of drawn polyoxymethylene. The index of one of the extra reflections is estimated at 00l: l = 2.830 ± 0.003. A new model for the molecular conformation which accounts for the nonintegral indices is proposed. The new model is a helix with defects at constant intervals along the helix axis. The defect is an unwinding of the helix by 20° around the helix axis. This defect is believed to be characteristic of crystals of helical polymers. The interval between the defects is estimated as 18 monomers. The crystal structure with these defects is consistent with precise measurements of the layer line positions.     

X-ray scattering from the long spacing of completely crystalline oligoethylene glycol di-n-alkyl ethers

Completely crystalline samples of oligoethylene glycol di-n-alkyl ethers, H(CH₂)_nO-(CH₂CH₂O)_m(CH₂)_nH, where m is 9 or 15 and n is in the range 12-18, were orientated in capillaries by slow crystallization in a temperature gradient. X-ray scattering from the long spacings of the oriented samples, rotated continuously through 360°, was concentrated on the equator of a Debye–Scherrer photograph and many orders of reflection could be seen (e.g., up to order 30). The intensities of these reflections were analyzed by use of a model electron density distribution through the layer structure. Thus it was shown that the central oxyethylene block, in helical conformation, is oriented normal to the layer end plane, while the end alkyl bolcks, in planar zigzag conformation, are tilted relative to the layer end plane at an angle in the range 70°–64°.     

Structure of a poly(2,5‐benzimidazole)/phosphoric acid complex

As‐cast films of poly(2,5‐benzimidazole) exhibit uniplanar orientation in which the planes of the aromatic rings lie parallel to the film surface. Upon doping with phosphoric acid, the original crystalline order is lost, but the doped film can be stretched to produce films with uniaxial orientation. After thermal annealing at 540 °C, nine Bragg reflections are resolved in the fiber diagram, and these are indexed by an orthorhombic unit cell with the dimensions a = 18.1 Å, b = 3.5 Å, and c = 11.4 Å, containing four monomer units of two chains. The absence of odd‐order 00l reflections points to a 2₁ chain conformation, which is probably planar so that the aromatic units can be stacked along the b axis. The water and phosphoric acid contents of the crystalline structure cannot be determined exactly because of the presence of extensive amorphous regions that probably have different solvation. The best agreement between the observed and calculated intensities is for an idealized structure containing two phosphoric acids and two water molecules per unit cell. However, the phosphoric acid is probably present mainly in the form of pyrophosphoric acid and its higher oligomers. In addition, the X‐ray data are consistent with a more disordered structure containing chains with random (up and down) polarity and a lack of c‐axis registry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2576–2585, 2004     

Synthesis of a Type-VIβ-Turn Peptide Mimetic and Its Incorporation into a High-Affinity Somatostatin Receptor Ligand

The synthesis of a cis-Phe-Pro dipeptide mimetic is described, which adopts a type-VIβ-turn conformation. In this mimetic, the α-positions of Phe and Pro are joined by a CH₂CH₂ bridge, thereby forming a fused bicyclic system, and fixing a geometry similar to that seen in cis-Phe-Pro units in protein crystal structures. The dipeptide mimetic 20 was synthesized in optically pure form starting from (R)-α-allylproline ( 6 ; Schemes 1, 3, and 4), with a free carboxylic acid and an Fmoc-protected N-terminus, thereby allowing its incorporation into linear and cyclic peptides using standard solid-phase methods. The mimetic 20 was incorporated into the cyclic somatostatin analogue cyclo(-Phe = Pro-Phe-D -Trp-Lys-Thr-), where Phe = Pro represents the mimetic. This analogue shows a high affinity (pIC₅₀ 8.6) for somatostatin receptors on rat-brain cortex membranes. Based on NMR studies in aqueous solution, likely low-energy conformations for this analogue were deduced using restrained dynamic simulated annealing. The conformations found, which include a distorted type-II′ turn at D -Trp-Lys, are similar to low-energy conformations deduced elsewhere for cyclo(-Phe-Pro-Phe-D -Trp-Lys-Thr-), as well as to those seen in crystal structures of the somatostatin analogue octreotide.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen—IXSterische Effekte in den Massenspekten der Stereoisomeren von Decalin-2,3-diolen,Decalin-2,4-Diolen und Ihen Dimethyläthern

The mass spectra of all stereoisomers of decalin-2,3-diol, the corresponding dimethyl ethers and of some deuterated derivatives are discussed. The mass spectra of isomeric decalin-2,3-diols differ only slightly in ion intensities. The mass spectra of the stereoisomeric 2,3-dimethoxy-decalins are nearly identical within the series of transand cisderivatives. A mass spectrometric identification of the stereoisomers of these compounds is therefore diffucult. Stereoselective eliminations from the molecular ion are not observed. The mass spectra -of stereoisomeric decalin-1,4-diols show characteristic differences in the intensities of the[M ? H₂O]⁺˙-ions, which can be related to the geometry of the molecules in a similiar mode as was the case with cyclohexane-1,4-diols, The sterechemical control of the elimination of H₂O from the molecular ions has been confirmed by deuterium labelling. The mass spectra of stereoismeric 1,4-dimethoxy-decalins also differ characteristically in the intensities of the [M ? CH₃OH]⁺˙ ions. Furthermore peak due to the [M ? CH₂O]⁺˙ ions are only observed in the mass spectra of those stereoisomers, which have at least one conformation with a short distance between the two methoxy. The stereospecifity of the CH₃OH- and CH₂O-eliminationjs has also been determined by deuterium labelling.     

Crystallographic report: 1,4‐Bis[(phenyldichlorostannyl)ethyl]benzene,p‐(Cl2PhSnCH2CH2)2C6H4

An ‘S’ conformation, stabilized by intramolecular C? H···π interactions, is found in centrosymmetric p‐(Cl₂PhSnCH₂CH₂)₂C₆H₄. The dinuclear species features distorted tetrahedral tin centres, with the greatest distortion manifested in the C? Sn? C angle of 134.32(16) °. Copyright © 2002 John Wiley & Sons, Ltd.     

Crystal structure of ionic and non-ionic surfactants based on polyoxyethylenated dodecanol: A13C-CP-MAS-NMR and x-ray investigation

Crystal structure of monodisperse non-ionic surfactants having the general formula C₁₂H₂₅O–(CH₂–CH₂–O)_nH (C₁₂E_n),n=7,9, 10, 15, 16 and ionic derivatives, C₁₂H₂₅–O–(CH₂–CH₂–O)_n–CH₂COONa (C₁₂E_nC),n=3,4,5,6,7,9, 12 has been investigated by¹³C-CP-MAS-NMR and x-ray diffraction. A structural model, in which aliphatic and polyether segments are segregated in bilayers stacking parallel to the elongation direction of the molecules, fits the experimental data for both series. The experimental values of the repeat distance along the stacking directiond(001) are linearly dependent onn and the slope is nearly equal to twice the repeat distance of7/2 helix conformation, which is typical for crystalline polyethyleneoxide. The values ofd(001) agree very well with the values expected for the C₁₂ segment in a planar zig-zag conformation, which is tilted with respect to the polyerther segment (7/2 helix conformation) in such a way that both the aliphatic and the polyether regions adopt the mass density of the corresponding crystalline compound. Two additional phases have been detected for C₁₂E_nC. One of them is probably characterized by the planar zig-zag conformation of the polyether chain. The meader model, previously proposed in the literature for surfactants containing polyethylene oxide segments is inconsistent with the obtained experimental data.     

[MPy4(NCO)2]-2Py clathrates (M = M(II) = Mn,Fe, Co,Ni, Cu,Zn, Cd; Py = pyridine)

The title compounds form an iso structural series and are isomorphic with other [MPy₄X₂]-2Py clathrates (XRD, KM4 diffractometer, cell parameters and space group Ccca from 17–80 reflections). In the clathrate [NiPy₄(NCO)₂]-2Py studied in detail (XRD, CAD-4 diffractom eter, λCuK_α, Ω/2θ scan mode, θ_max = 78‡, 990 strong reflections, 104 parameters, R = 0.053), the host molecule has 222 symmetry, and the twofold axes run along the coordination bonds. The transoctahedral environment of nickel consists of six nitrogen atoms of four pyridine and two isocyanate ligands. The coordination polyhedron is slightly distorted due to changes in the bond lengths. The molecule has a propeller conformation. The guest molecules lie in the cavities of the crystal structure in conformity with the van der Waals type of packing. The host complex [NiPy₄(NCO)₂] (XRD, CAD-4 diffractometer, 4615 strong reflections, 560 parameters, R-0.037) crystallizes in the triclinic crystal system (space group P1) with two independent asymmetric molecules in the unit cell. The molecular structure is analogous to that in the ciathrate phase, but the coordination angles are severely distorted; one of the molecules acquires a distorted propeller conformation, and the other, a centrosvmmetric conformation, which is less favorable. While being structurally identical, the [MPy₄(NCO)₂]-2Py clathrates differ heavily in the properties. The first four complexes dissociate to host complexes, and their thermal stability changes in the sequence Mn< Fe< Co< Ni; the Cu and Zn clathrates decompose in one step to dipyridine complexes with decomposition of host complexes. Decomposition of the Cd ciathrate follows one of these patterns depending on conditions. The results are compared with those for other known systems. Synthetic procedures are given. Translated fromZhurnal Strukturnoi Khimii, Vol. 40, No. 5, pp. 935–953, September–October, 1999.     

A solvent-dependent peptide spring unraveled by 2D-NMR

In this work, we introduce a 2D-NMR method to discriminate between the fully-extended and the 3₁₀-helical conformations for the C^α,α-diethylglycine homo-peptides in the solution phase. It is based on the observation of divergent cross-peak intensities in the NOESY spectra. In particular, any βCH₂(i-1)→NH(i) cross peak is more intense than the intraresidue βCH₂(i)→NH(i) cross peak when the peptide adopts the fully-extended conformation. In this 3D-structure a marked splitting of the chemical shifts of the two non-equivalent βCH₂ protons is also apparent. In contrast, an opposite trend of intensities of the same NOE cross-peaks indicates the occurrence of a 3₁₀-helical conformation. This 3D-structural shift is induced by a change in the nature of solvent.     

Morpholine‐2,5‐dione

In the title compound, C₄H₅NO₃, the morpholine ring adopts a boat conformation that is distorted towards twist–boat, the boat ends being the two Csp³ atoms of the ring. The mol­ecular packing is stabilized by the establishment of strong inter­molecular NH⋯OC hydrogen bonds, which give rise to centrosymmetric dimers, and a network of weak CH₂⋯OC hydrogen bonds, where each dimer inter­acts with eight neighbouring morpholine­dione rings.     

Crystallographic report: 1,4‐Bis[bis(trimethylsilyl)methyl‐dichlorostannylmethyldimethylsilyl]benzene,p‐{[(Me3Si)2CH]Sn(Cl)2CH2SiMe2}2C6H4

The dinuclear molecule of p‐{[(Me₃Si)₂CH]Sn(Cl)₂CH₂SiMe₂}₂C₆H₄ is centrosymmetric and adopts an ‘S’ conformation that is stabilized by intramolecular C? H···π interactions. The tin atom exists within a distorted tetrahedron defined by a C₂Cl₂ donor set. Copyright © 2002 John Wiley & Sons, Ltd.     

Effects on Coordination of Thioether Sites. 4. Structural Analysis of η3-Tripodal Ligand Manganese(I) Complexes

Crystal structures of a series of manganese(I) complexes containing tripodal ligands were determined. For [η³-{CH₃C(CH₂PPh₂)₂(CH₂SPh)-P,P′,S}Mn(CO)₃]PF₆ ( 1 ): a = 10.856(3) Å, b = 19.698(3) Å, c = 17.596(5) Å, β = 96.17(2)°, monoclinic, Z = 4, P2₁/c, R(F_o) = 0.068, R_w(F_o) = 0.055 for 3617 reflections with I_o > 2σ(I_o). For [η³-{CH₃C(CH₂PPh₂)(CH₂SPh)₂-P,P′,S}Mn(CO)₃]PF₆ ( 2 ): a = 9.890(2) Å, b = 20.403(4) Å, c = 10.269(3) Å, β = 117.44(2)°, monoclinic, Z = 2, P2_l, R(F_o) = 0.050, R_w(F_o) = 0.037 for 1760 reflections with I_o > 2σ(I_o). For [η³-{CH₃C(CH₂PPh₂)₂(CH₂S)-P,P′,S}Mn(CO)₃] ( 4 ): a = 8.191(7) Å, b = 10.495(3) Å, c = 19.858(6) Å, α = 99.61(2)°, β = 96.17(2)°, γ = 92.70(4)°, triclinic, Z = 2, P-I, R(F_o) = 0.048, R_w(F_o) = 0.039 for 2973 reflections with I_o > 2σ(I_o). There is no significant difference in the bond lengths of Mn-S bonds among three species in their crystal structures [2.325(2) Å in 1; 2.358(4) in 2; 2.380(2) in 4], but the better donating ability of thiolate in complex 4 appears on the lower frequencies of its carbonyl stretching absorptions.     

[Be3(μ3‐O)3(MeCN)6{Be(MeCN)3}3](I)6 – ein Berylliumkomplex mit Cyclo‐(Be3O3)‐Kern und sein Hydrolyseprodukt [Be(H2O)4](I)2·2MeCN

Single crystals of [Be₃(μ₃‐O)₃(MeCN)₆{Be(MeCN)₃}₃](I)₆·4CH₃CN ( 1 ·4CH₃CN) were obtained in low yield by the reaction of beryllium powder with iodine in acetonitrile suspension, which probably result from traces of beryllium oxide containing the applied beryllium metal. The compound 1 ·4CH₃CN forms moisture sensitive, colourless crystal needles, which were characterized by IR spectroscopy and X‐ray diffraction (Space group Pnma, Z = 4, lattice dimensions at 100(2) K: a = 2317.4(1), b = 2491.4(1), c = 1190.6(1) pm, R₁ = 0.0315). The hexaiodide complex cation 1 ⁶⁺consists of a cyclo‐Be₃O₃ core with slightly distorted chair conformation, stabilized by coordination of two acetonitrile ligands at each of the beryllium atoms and by a {Be(CH₃CN)₃}²⁺ cation at each of the oxygen atoms. This unique coordination behaviour results in coplanar OBe₃ units with short Be–O distances of 155.0 pm and 153.6 pm on average of bond lengths within the cyclo‐Be₃O₃ unit and of the peripheric BeO bonds, respectively. Exposure of compound 1 ·4CH₃CN to moist air leads to small orange crystal plates of [Be(H₂O)₄]I₂·2CH₃CN ( 3 ·2CH₃CN). According to the crystal structure determination (Space group C2/c, Z = 4, lattice dimensions at 100(2) K: a = 1220.7(1), b = 735.0(1), c = 1608.5(1) pm, β = 97.97(1)°, R₁ = 0.0394), all hydrogen atoms of the dication [Be(H₂O)₄]²⁺ are involved to form O–H ··· N and O–H ··· I hydrogen bonds with the acetonitrile molecules and the iodide ions, respectively. Quantum chemical calculations (B3LYP/6‐311+G**) at the model [Be₃(μ₃‐O)₃(HCN)₆{Be(HCN)₃}₃]⁶⁺ show that chair and boat conformation are stable and that the distorted chair conformation is stabilized by packing effects.     

13C-CP/MAS-NMR spectroscopy of tridentate diacidic ligands and their titanium(IV) chelates. molecular structure of bis[benzoylacetone thiobenzoylhydrazonato(2-)]titanium(iv)

A series of tridentate diacidic ligands having the donor set ONO(S) as well as the corresponding titanium(IV) chelates were studied by¹³C-CP/MAS-NMR spectroscopy. Structural changes caused by the complex formation are discussed. The crystal structure of bis[benzoylacetone thiobenzo-ylhydrazonato(2-)]titanium(IV) was determined by X-ray analysis as to be distorted trigonal prismatic. Orthorhombic space group Pca2_l,Z=4, 3304 observed independent reflections, unit-cell parameters at 298 K:a=22,808(2) å,b=13,073(4) å,c=10,450(4) å.     

Triterpenoide. XX. 3β‐Acetoxy‐12‐oxo‐18β‐olean‐ und 3β‐Acetoxy‐12,19‐dioxo‐9(11),13(18)‐oleandien‐28‐säure‐methyl­ester

The structures of methyl 3β‐acetoxy‐12‐oxo‐18β‐olean‐28‐oate [C₃₃H₅₂O₅, (I)] and methyl 3β‐acetoxy‐12,19‐dioxoolean‐9(11),13(18)‐dien‐28‐oate [C₃₃H₄₆O₆, (II)] are described. In (I), all rings are in the chair conformation, rings D and E are cis and the other rings trans‐fused. In compound (II), only rings A and E are in the chair conformation, ring B has a distorted chair conformation, ring C a distorted half‐boat and ring D an insignificantly distorted half‐chair conformation.     

Stereospecific polymerization of 9-fluorenyl methacrylate: Tacticity effects on the thermal and photophysical properties of the polymers

Poly(9-fluoreneyl methacrylate) was obtained through anionic polymerization with t-BuLi and t-BuMgBr and through radical polymerization with α,α′-azobisisobutyronitrile. Anionic polymerization with t-BuLi in tetrahydrofuran and radical polymerization afforded syndiotactic polymers (rr ∼ 90%), whereas anionic polymerization with Li and Mg initiators in toluene and CH₂Cl₂ led to isotactic polymers. The thermal and photophysical properties of the polymers were examined. A syndiotactic polymer tended to show higher glass transition and decomposition temperatures than an isotactic polymer. However, polymers with different tacticities were not likely to assume specific, distinctive conformations such as a helix or a π-stacked conformation in solution. An isotactic polymer showed stronger interactions in a CH₂Cl₂ solution with 2,4,7-trinitro-9-fluorenylidenemalononitrile, an electron-acceptor molecule, than a syndiotactic polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4656–4665, 2004     

Syntheses and Structures of F6XeNCCH3 and F6Xe(NCCH3)2

Acetonitrile and the potent oxidative fluorinating agent XeF₆ react at ?40 °C in Freon‐114 to form the highly energetic, shock‐sensitive compounds F₆XeNCCH₃ ( 1 ) and F₆Xe(NCCH₃)₂?CH₃CN ( 2 ?CH₃CN). Their low‐temperature single‐crystal X‐ray structures show that the adducted XeF₆ molecules of these compounds are the most isolated XeF₆ moieties thus far encountered in the solid state and also provide the first examples of Xe^VI? N bonds. The geometry of the XeF₆ moiety in 1 is nearly identical to the calculated distorted octahedral (C_3v) geometry of gas‐phase XeF₆. The C_2v geometry of the XeF₆ moiety in 2 resembles the transition state proposed to account for the fluxionality of gas‐phase XeF₆. The energy‐minimized gas‐phase geometries and vibrational frequencies were calculated for 1 and 2 , and their respective binding energies with CH₃CN were determined. The Raman spectra of 1 and 2 ?CH₃CN were assigned by comparison with their calculated vibrational frequencies and intensities.     

Tetraphenylphosphonium-trichloroplumbat(II), PPh4PbCl3 · CH3CN

Tetraphenylphosphonium Trichloroplumbate(II), PPh₄PbCl₃ · CH₃CN PPh₄PbCl₃ · CH₃CN was obtained by reaction of PbCl₂ and PPh₄Cl in acetonitrile. It was also formed along with (PPh₄)₂Se₂Cl₆ when PbSe was treated with chlorine in the presence of PPh₄Cl. Its crystal structure was determined by X-ray diffraction (R = 0.029 for 4186 reflections). The triclinic crystals contain PbCl₃^– ions that are associated to polymer chains. Each Pb atom has distorted square pyramidal coordination; the pyramids share two opposite basal edges. The chloro bridges are rather asymmetrical.     

CRYSTAL AND MOLECULAR STRUCTURE OF BIS(TRIPHENYLBENZYLPHOSPHONIUM) TETRABROMOCADMATE

Abstract

The molecular and crystal structure of bis(triphenylbenzylphosphonium)tetrabromocadmate has been determined by x-ray diffractometer data. Crystals are triclinic, space-group Pl with two formula units in a unit-cell of dimensions a = 12.506(6), b = 10.471(5), c = 18.396(13) Å, α = 93.07(4)°, β = 105.75(5)°, γ = 92.58(4)°. The structure was solved by direct and Fourier methods and refined by least-squares techniques to R = 0.061 for 3723 independent observed reflections. The structure consists of tetrabromocadmate (II) anions and triphenylbenzylphosphonium cations, both with a quasi-perfect tetrahedral symmetry around the cadmium and phosphorus atoms. The most significant average bond distances are: Cd-Br, 2.588(2) Å, P[sbnd]C (Phen), 1.794(5) Å and P[sbnd]CH₂, 1.806(6) Å. The P[sbnd]C (Phen) bonds are in slightly distorted staggered conformation (gauche-, gauche +, and trans) in respect of the C (Phen)-CH₂ bonds of the benzyl residues. The interatomic distances between the ions correspond to the usual Van der Waals distances.     

Darstellung und Kristallstrukturen von neutralen und kationischen Kupfer(I)-Gemischtligandkomplexen mit Derivaten des Biimidazols und Triphenylphosphan

Preparation and Crystal Structures of Neutral and Cationic Copper(I) Mixed Ligand Complexes with Triphenylphosphane and Derivatives of Biimidazole Eight triphenylphosphanecopper(I) complexes with bibenzimidazole, tetramethylbiimidazole or tetrahydrobiimidazole were prepared and characterized so far as possible by elemental analysis, IR, ¹H-NMR and ³¹P-NMR spectra. The crystal structures of two complexes with bibenzimidazole were determined. [Cu(bbimH₂)(PPh₃)₂]Cl · CH₂Cl₂: Reaction of CuCl with bibenzimidazole in fused triphenylphosphane or [CuCl(PPh₃)₃] with bibenzimidazole in CH₂Cl₂. Space group P 1, Z = 2, 6440 observed independent reflections, R = 0.064 for refletions with I > 2σ(I). Lattice parameters at 203 K: a = 983.6; b = 1348.9; c = 1805.5 pm; α = 77.24; β = 80.90; γ = 85.81°. The crystal structure is built up by monomeric molecules with distorted tetrahedral coordination of the copper atom (CuN₂P₂) and bibenzimidazole as bidentate ligand. The chloride ion is linked by H-bonds with the NH groups of the bibenzimidazole. [{Cu(PPh₃)₂}₂(μ-bbim)] · 2 CH₂Cl₂: Reaction of [CuCl(PPh₃)₃] with the dipotassium salt of bibenzimidazole in CH₃OH/CH₂Cl₂. Space group P 1, Z = 1, 7192 observed independent reflections, R = 0.057 for reflections with I > 2σ(I). Lattice parameters at 203 K: a = 1334.1; b = 1386.8; c = 1443.7 pm; α = 107.51; β = 103.35; γ = 113.74°. The crystal structure is built up by centrosymmetric molecules with distorted tetrahedral coordination of the copper atoms (CuN₂P₂) and bibenzimidazolate(2–) as tetradentate bridging ligand.