Twisted Form Research Articles

Kinetics and Mechanistic Investigation of Hydrotalcite‐Catalyzed Melt Synthesis of Poly(ethylene terephthalate)

AbstractSummary: Hydrotalcite‐catalyzed polycondensation of BHET was studied by thermogravimetry to elucidate the kinetics. The reaction was found to follow second‐order kinetics with respect to hydroxyl end groups. The overall activation energy of the polycondensation was found to decrease with increasing catalyst concentration before it leveled out to the value of 93 kJ · mol−1 at high catalyst concentration. This is a result of the uncatalyzed reaction that takes place parallel to the catalyzed one. The activation energy of the uncatalyzed path was found to be 156 kJ · mol−1. IR spectroscopy and XRD showed that the monomer intercalates between the layers of hydrotalcite at the beginning of the reaction enabling the complexation of oxygen‐containing functional groups with the hydroxide groups of the catalyst. Based on these findings a polycondensation mechanism is proposed. One end group of the monomer is activated in the form of alkoxide that counterbalances the positive charge of the hydrotalcite layer. This alkoxide group attacks an ester carbonyl group fixed close to it, generating a new ester bond and a glycoxide species. The role of hydrotalcite is to activate the reactants, rendering the attacking hydroxyl group more nucleophilic and the ester carbonyl group more electrophilic, and at the same time fixing the reactant together in a favorable geometry.Intercalation of BHET in the gallery of HT layers in a twisted alkoxide form.magnified imageIntercalation of BHET in the gallery of HT layers in a twisted alkoxide form.

Vibrational spectral studies and the non‐linear optical properties of a novel NLO material L‐prolinium tartrate

AbstractVibrational spectral analysis of the novel non‐linear optical (NLO) material, L‐prolinium tartrate (LPT) was carried out using NIR‐FT‐Raman and FT‐IR spectroscopy. The density functional theoretical (DFT) computations have been performed at B3LYP/6–31G (d) level to derive equilibrium geometry, vibrational wavenumbers, intensities and first hyperpolarizability. The reasonable NLO efficiency, predicted for the first time in this novel compound, has been confirmed by Kurtz–Perry powder second‐harmonic generation (SHG) experiments. The charge‐transfer interaction between the pyrrolidine ring and the carbonyl group of the tartrate anion through the intramolecular ionic hydrogen bonds is confirmed by the simultaneous activation of ring modes in IR and Raman spectra. The splitting of the ring‐breathing mode, pseudo‐rotational ring puckering modes and the NH2 modes of the pyrrolidine ring lead to the conclusion that the pyrrolidine ring adopts a conformation intermediate between the envelope (bent) form and the half‐chair (twisted) form, resulting in the lowering of symmetry from C2 to Cs. The lowering of the methylenic stretching wavenumbers and the enhancement of the stretching intensities suggest the existence of the electronic effects of back‐donation in LPT. The positional disorder of the pyrrolidine ring, the presence of blue‐shifting H‐bonds as well as other non‐bonded interactions in LPT, low frequency H‐bond vibrations and the role of intramolecular charge transfer and the hydrogen bonds in making the molecule NLO active have been analysed on the basis of the vibrational spectral features. Copyright © 2006 John Wiley & Sons, Ltd.

Swelling behavior of the cellulose Iβ crystal models by molecular dynamics

The various crystal models of cellulose Iβ, each differing in crystal size, have been studied by computer simulation using the amber molecular-dynamics package and the GLYCAM parameters. The four types of crystal model were constructed by a combination of two base-plane sizes, consisting of either 24 or 48 chains and two chain lengths having either 10 or 20 residues. The base planes of the crystal models were composed by the edges of the [1, 1, 0], [1, −1, 0], and [1, 0, 0] crystal planes, where the [1, 1, 0] plane was assigned to the longest edge. The crystal models were soaked in water boxes to investigate their swelling behavior. Unexpectedly, the crystal models twisted quickly to form a slightly right-handed shape during the initial ∼50 ps and that, in a steady, swollen state, the twisted forms remained for the rest of the simulation time. In spite of such overall deformation, the inner part of the swollen model fairly reproduced the important structural features of the original crystal structure, such as the rotational positions of the substituent groups and the hydrogen-bonding scheme. On heating the crystal model up to 550 K, the twisted shape was conserved in most of the temperature range, while the initial conformations of the substituent groups deviated above ∼430 K, followed by appreciable disordering in chain sheets at higher temperatures. It is suggested that some internal tensions are involved within a chain sheet of the initial structure. In the course of swelling, some of these tensions were released to introduce a twisted shape in the crystal models.

Formation of Unexpected Selenium‐Rich Compounds by Selenation of the Very Bulky (PhMe2Si)3CLi:s‐Tetraselenane, a Novel Polyseleno Heterocycle

AbstractThe reaction of the highly crowded trisilylmethyllithium compound (PhMe2Si)3CLi with elemental selenium resulted in a variety of products, among them the triselane [(PhMe2Si)3CSe]2Se, the unexpected diselane [(PhMe2Si)2HCSe]2and the novel heterocycle [(PhMe2Si)2CSeSe]2. The constitution of the latter could be elucidated from the NMR spectroscopic data and was confirmed by the crystal structure which displays the C2Se4cycle in the twist form. Calculations were performed for the chair and twist conformers of the heterocycles [(H3Si)2CXX]2(X = O, S, Se, Te, Po).

Computational studies on the ground and excited states of BrOOBr

In this work, theoretical computations for the ground and excited states of BrOOBr have been performed at high-level ab initio molecular orbital theories. The ground-state geometries of BrOOBr in different forms (trans, cis, and twist form) have been optimized at the couple-cluster CCSD(T) level of theory with cc-pVTZ and aug-cc-pVTZ basis sets, which indicates that at CCSD(T)/cc-pVTZ level of theory, the twist form is 4.96 kcal/mol more stable than the trans form and 10.67 kcal/mol more stable than the cis form; at the CCSD(T)/aug-cc-pVTZ basis set the twist form is 4.33 kcal/mol more stable than the trans form and 9.54 kcal/mol more stable than the cis form. The vertical excitation energies and potential-energy curves for the singlet and triplet low-lying excited states of BrOOBr were calculated at both the complete active space self-consistent-field (CASSCF) level of theory and the multireference internally contracted configuration interaction (MRCI) level of theory. The differences of potential-energy curves at CASSCF and MRCI levels of theory are found for the BrOOBr excited states. At CASSCF level of theory, none of the BrOOBr excited states are bound. However, at MRCI level of theory, all the BrOOBr states studied in this work are bound or slightly bound at the Frank-Condon region. In addition, the scalar relativistic effect and the spin-orbital coupling effect on the vertical excitation energies of the electronic states of BrOOBr were estimated.

Two conformers of 10,11-dihydro-5H-dibenzo[a,d]cycloheptene spiro-linked with homobenzoquinone epoxide

The crystal structures of the two thermally equilibrated conformational isomers of the epoxide 1',5'-dimethylspiro[10,11-dihydro-5H-dibenzo[a,d]cycloheptene-5,8'-4'-oxatricyclo[5.1.0.0(3,5)]octane]-2',6'-dione, C23H20O3, have been determined by X-ray diffraction. In the tricyclic dione skeleton, the oxirane and cyclopropane rings adopt an anti structure with respect to the conjunct quinone frame. The spiro-linked 10,11-dihydro-5H-dibenzo[a,d]cycloheptene ring of the major isomer has a fairly twisted boat form, folding opposite to the adjoining cyclopropane methyl substituent, whereas the seven-membered ring of the minor isomer has an almost ideal twist-boat form, inversely folding to the side of the relevant methyl group. The conformational structures of these isomers have been compared with those of the corresponding isomers of the unepoxidized homobenzoquinone.

Raman and infrared studies supported by ab initio calculations for the determination of conformational stability, silyl rotational barrier and structural parameters of cyclohexyl silane

The Raman spectra (4000–100 cm −1) of liquid and solid and infrared spectra (4000–400 cm −1) of gaseous, liquid and solid cyclohexyl silane, c-C 6H 11SiH 3, have been recorded and assigned. These spectral data indicate the presence of two conformers in the fluid states. Variable temperature (21 to −71 °C) Raman spectra of the liquid were also recorded and by utilizing three conformer pairs, an enthalpy difference of 520±70 cm −1 (6.22±0.84 kJ/mol) was obtained with the chair-equatorial form the more stable conformer. It is calculated that at ambient temperature there is only 7.5±2.4% of the chair-axial form present in the liquid phase. Comparison with the spectra of the polycrystalline solid phase shows that the chair-equatorial conformer is the only form remaining in the solid. MP2 and DFT calculated conformational energy differences are slightly larger but in reasonable agreement with the experimental value. In addition, force constants, infrared intensities, Raman activities, depolarization ratios, scaled vibrational frequencies and potential energy distributions have been calculated from the MP2(full)/6–31G(d) results for both chair forms. These data support the complete vibrational assignment for the chair-equatorial form as well as the assignments for several of the fundamentals of the chair-axial form. By utilizing a series of sum and difference bands on the SiH stretching modes, the barrier to SiH 3 internal rotation has been determined to be 684±10 cm −1 (8.18±0.12 kJ/mol) for the chair-equatorial form. Estimated r 0 structural parameters have been obtained for both conformers by adjusting MP2(full)/6–311+G(d,p) structural predictions. Three additional twist forms (equatorial, axial and form III) were also predicted to be local minima since all calculated vibrational frequencies are real. However, all three twist forms are much higher in energy (2000–3000 cm −1) than the chair forms. Effects of electronegativity and steric effect on the conformational stability are compared among a series of mono-substituted cyclohexanes by NBO analyses of the donor-acceptor delocalization interactions.

Reconstruction of [formula omitted] supersymmetry from topological symmetry

The scalar and vector topological Yang–Mills symmetries on Calabi–Yau manifolds geometrically define consistent sectors of Yang–Mills D=4,6, N=1 supersymmetry, which fully determine the supersymmetric actions up to twist. For a CY2 manifold, both N=1, D=4 Wess and Zumino and super-Yang–Mills theory can be reconstructed in this way. A superpotential can be introduced for the matter sector, as well as the Fayet–Iliopoulos mechanism. For a CY3 manifold, the N=1, D=6 Yang–Mills theory is also obtained, in a twisted form. Putting these results together with those already known for the D=4,8, N=2 cases, we conclude that all Yang–Mills supersymmetries with 4, 8 and 16 generators are determined from topological symmetry on special manifolds.

Is solvated trans-azobenzene twisted or planar?

To arbitrate between previous conflicting theoretical and experimental results, a quantum-mechanical investigation of the ground-state geometry and UV spectrum of trans-azobenzene has been performed by using the Møller–Plesset approach and time-dependent density functional theory. It turns out that gas-phase trans-azobenzene is consistently predicted to be planar by the most accurate approaches. In solvent, the energy difference between planar and twisted trans-azobenzene is also negligible, although the twisted form tends to be favoured by a polar surrounding medium. These predictions are supported by comparisons between measured and predicted excitation energies.

회전압출다이를 사용한 헬리컬 핀붙이 원형단면 제품의 압출가공에 관한 연구

A new extrusion process of the circular section product with helical fins could be developed by rotating extrusion die. The twisting of extruded product is caused by the twisted conical die surface connecting the die entrance section and the die exit section linearly. But, until now, because the process has used fixed extrusion die, it needs high pressure in order to twist billet and form fin shape on the surface of billet. So, during extruding billet, in order not to twist billet, the extrusion die is needed to rotate itself instead of twisting of billet. It is known that it is possible to reduce extrusion load of product with helical fins by analysis and experiments using rotating die. And it is known that, through the extrusion load analysis by <TEX>$DEFORM^{TM}-3D$</TEX> software, optimal rotational velocity of rotating die can be obtained according to reduction ratio of area and twisted angle of die. And experiments and analysis using rotating extrusion die show that the twisted angle of product can be controlled by twisted angle of extrusion helical die and the rotational velocity of extrusion helical die.

The Dynamics of Intramolecular Excited State Relaxation of N-Anthryl Substituted Pyridinium Cations

N-(1-Anthryl)-2,4,6-trimethyl-pyridinium (I), N-(2-anthryl)-2,4,6-trimethyl-pyridinium (II) and 10-(1-anthryl)-1,2,3,4,5,6,7,8-octahydro-acridinium cations (III) with anomalously high fluorescence Stokes' shift have been investigated. Fluorescence kinetics analysis at various temperatures showed that in the range 293-77 K, the radiative deactivation rate constants (kf) increase by 5.5 to 30 times. The low-temperature time-resolved emission spectra of I-III were found to be consistent with the model: A--> A*<--> B* where A* is the local excited twisted form and B* is the relaxed more planar, bent conformer of the molecule. The rate constants of the excited relaxed state formation (k1) and back reaction (k-1) of compounds studied were estimated.

Vapor Phase Growth of Carbon Microcoils / Nanocoils

The carbon microcoils and carbon nanocoils were prepared by the catalytic pyrolysis of acetylene under the Ni and/or Fe-containing catalysts, and the growth pattern, morphology and growth mechanism of the carbon coils were examined in detail. The inner coil diameter of carbon microcoils are of several µm and coil gap from zero to several µm. The inner coil diameter of carbon nanocoils are from zero to several ten nm and coil gap from zero to several nm. The carbon microcoils are generally of double helix coils such as DNA while carbon nanocoils were single helix coils such as α-helix proteins, with spring-like or twisted forms. A catalyst grain was usually observed on the tip of carbon coil. The carbon nanocoils are almost amorphous and can be graphitized by the high temperature heat-treatment.

On the stability of the tangent bundle of a hypersurface in a Fano variety

Let $M$ be a complex projective Fano manifold whose Picard group is isomorphic to $\mathbb{Z}$ and the tangent bundle $TM$ is semistable. Let $Z \subset M$ be a smooth hypersurface of degree strictly greater than degree($TM$)$(\mathrm{dim}_{\mathbb{C}} Z-1)/(2\mathrm{dim}_{\mathbb{C}} Z-1)$ and satisfying the condition that the inclusion of $Z$ in $M$ gives an isomorphism of Picard groups. We prove that the tangent bundle of $Z$ is stable. A similar result is proved also for smooth complete intersections in $M$. The main ingredient in the proof of it is a vanishing result for the top cohomology of the twisted holomorphic differential forms on $Z$.

Failure of the Hasse Principle for Atkin–Lehner Quotients of Shimura Curves over Q

We show how to construct counter-examples to the Hasse principle over the field of rational numbers on Atkin–Lehner quotients of Shimura curves and on twisted forms of Shimura curves by Atkin–Lehner involutions. A particular example is the quotient of the Shimura curve X23∙107 attached to the indefinite rational quaternion algebra of discriminant 23∙107 by the Atkin–Lehner involution ω107. The quadratic twist of X23∙107 by Q( √ −23) with respect to this involution is also a counter-example to the Hasse principle over Q.

HOFER–ZEHNDER CAPACITY AND HAMILTONIAN CIRCLE ACTIONS

We introduce the Hofer–Zehnder Γ-semicapacity [Formula: see text] of a symplectic manifold (M,ω) (or Γ-sensitive Hofer–Zehnder capacity) with respect to a subset Γ⊂π1(M)[Formula: see text] and prove that given a geometrically bounded symplectic manifold (M,ω) and an open subset N⊂M admitting a Hamiltonian free circle action with order greater than two then N has bounded Hofer–Zehnder Γ-semicapacity, where Γ⊂π1(N) is the subgroup generated by the orbits of the action. We give several applications of this result. Using Biran's decomposition theorem, we prove the following: let (M2n,Ω) be a closed Kähler manifold (n&gt;2) with [Ω]∈H2(M,ℤ) and Σ a complex hypersurface representing the Poincaré dual of k[Ω], for some k∈ℕ. Suppose either that Ω vanishes on π2(M) or that k&gt;2. Then there exists a decomposition of M into an open dense subset E such that E\Σ has finite Hofer–Zehnder Γ-semicapacity and an isotropic CW-complex, where Γ⊂π1(E\Σ) is the subgroup generated by the obvious circle action on the normal bundle of Σ. Moreover, we prove that if (M,Σ) is subcritical then M\Σ has finite Hofer–Zehnder Γ-semicapacity. We also show that given a hyperbolic surface M and TM endowed with the twisted symplectic form ω0+π*Ω, where Ω is the Kähler form on M, then the Hofer–Zehnder Γ-semicapacity of the domain Uk bounded by the hypersurface of kinetic energy k minus the zero section M0 is finite if k&lt;1/2, where Γ⊂π1(Uk) is the subgroup generated by the fibers of SM. Finally, we consider the problem of the existence of periodic orbits on prescribed energy levels for magnetic flows. We prove that given any weakly exact magnetic field Ω on any compact Riemannian manifold M there exists a sequence of contractible periodic orbits of arbitrarily small energy, extending a previous result of Polterovich.

Synthesis, Conformation and PKC Isozyme Surrogate Binding of Indolinelactam‐Vs, New Conformationally Restricted Analogues of (‐)‐Indolactam‐V.

AbstractFor Abstract see ChemInform Abstract in Full Text.

AB INITIO MOLECULAR ORBITAL STUDY OF 1,2-DITHIANE AND 1,2,4,5-TETRATHIANE

Ab initio calculations at HF/6-31+G* level of theory for geometry optimization, and MP2/6-31+G*//HF/6-31+G* and B3LYP/6-31+G*//HF/6-31+G* levels for a single-point total energy calculation, are reported for the chair and twist conformations of 1,2-dithiane (1), 3,3,6,6-tetramethyl-1,2-dithiane (2), 1,2,4,5-tetrathiane (3), and 3,3,6,6-tetramethyl-1,2,4,5-tetrathiane (4). The C2 symmetric chair conformations of 1 and 2 are calculated to be 21.9 and 8.6 kJ mol−1 more stable than the corresponding twist forms. The calculated energy barriers for chair-to-twist processes in 1 and 2 are 56.3 and 72.8 kJ mol−1, respectively. The C2h symmetric chair conformation of 3 is 10.7 kJ mol−1 more stable than the twist form. Interconversion of these forms takes place via a C2 symmetric transition state, which is 67.5 kJ mol−1 less stable than 3-Chair. The D2 symmetric twist-boat conformation of 4 is calculated to be 4.0 kJ mol−1 more stable than the C2h symmetric chair form. The calculated strain energy for twist to chair process is 61.1 kJ mol−1.

Torsional profiles of protonated and metal-coordinated 2,2′-bipyridine

The rotational profiles of both the mono and di-protonated 2,2′-bipyridinium cations were calculated with ab initio Hartree Fock and density functional theoretical methods. The most stable form of the singly protonated bipyridine is the planar cisoid conformation with a weak, bent and strongly asymmetric NHN hydrogen brigde. The transoid conformer is predicted to exhibit the maximal charge delocalization in the coplanar arrangement, however, it is twisted due to sterical hindrance. The di-protonated species is most stable in a twisted (about 40°) transoid conformation with a very shallow potential minimum of the twisted cisoid form. Rotational profiles of transition metal complexes [bipyZn(H 2O) n] 2+ with n=2–4 and [bipyRu(CO) 4] 2+ leading from the monodentate transoid to the stable bidentate bipy complex are also calculated with density functional theory methods. There is no potential energy barrier between the coordinated cisoid and transoid bipy complexes with [bipyZn(H 2O) n ] 2+ and a small barrier is found for [bipyRu(CO) 4] 2+.

Synthesis, conformation and PKC isozyme surrogate binding of indolinelactam-Vs, new conformationally restricted analogues of (−)-indolactam-V

New conformationally restricted analogues of tumor promoter (−)-indolactam-V ( 1 ), indolinelactam-Vs ( 8 , 11 ) and their hexyl derivatives at position 1 or 7 ( 9 , 10 , 12 , 13 ), were synthesized from 1 . (3 R)-Indolinelactam-V ( 8 ) adopted a conformation similar to the twist form of 1 with a cis amide, while the conformation of (3 S)-indolinelactam-V ( 11 ) was close to that of the sofa form of 1 with a trans amide. 7-Hexyl derivatives of 8 and 11 ( 10 , 13 ) showed binding affinities for C1 domains of protein kinase C (PKC) isozymes compared to 1 , but exhibited little selectivity among these PKC isozymes. However, introduction of the hexyl group at position 1 of 8 and 11 significantly enhanced their binding selectivity for novel PKC isozymes. The best selectivity for novel PKC isozymes was observed in (3 S)-1-hexylindolinelactam-V ( 12 ) with a sofa-like conformation. These results suggest that a sofa-restricted analogue of 1 with a hydrophobic chain at an appropriate position would be a promising lead for designing agents with a high selectivity for novel PKC isozymes.

Elastic properties of poly(hydroxybutyrate) molecules.

Elasticity of various poly(hydroxybutyrate) (PHB) molecules of regular and irregular conformational structure was examined by the molecular mechanics (MM) calculations. Force - distance functions and the Young's moduli E were computed by stretching of PHB molecules. Unwinding of the 2(1) helical conformation H is characterized at small deformations by the Young's modulus E = 1.8 GPa. The H form is transformed on stretching into the highly extended twisted form E, similar to the beta-structure observed earlier by X-ray fiber diffraction. The computations revealed that in contrast to paraffins, the planar all-trans structure of undeformed PHB is bent. Hence, a PHB molecule attains the maximum contour length in highly straightened, but slightly twisted conformations. A dependence of the single-chain moduli of regular and disordered conformations on the chain extension ratio x was found. The computed data were used to analyze elastic response of tie (bridging) molecules in the interlamellar (IL) region of a semi-crystalline PHB. A modification of the chain length distribution function of tie molecules tau(N) due to secondary crystallization of PHB was conjectured. The resulting narrow distribution tau(N) comprises the taut tie molecules of higher chain moduli prone to overstressing. The molecular model outlined is in line with the macroscopically observed increase in the modulus and brittleness of PHB with storage time.