TT Conformation Research Articles

Cation Conformational Changes of 1-Butyl-3-methylimidazolium Halides at High Pressures

The cation structural changes of two 1-butyl-3-methylimidazolium halides, [BMIM]Cl and [BMIM]Br, are investigated by in situ Raman spectroscopy, infrared spectroscopy, and synchrotron X-ray diffraction as a function of hydrostatic pressure. The [BMIM]+ cations of [BMIM]Cl take the TT conformation under ambient conditions, undergo significant changes at 3.4 GPa, which is related to the potential formations of some new cation conformations, and take another conformation from 6.8 to 35.6 GPa. Correspondingly, crystalline [BMIM]Cl has a structure transition at 3.4 GPa, originating directly from the cation conformational changes. The [BMIM]+ cations totally transform into the GT conformer, if the diamond anvil cell-loaded sample is treated at 423 K for 12 h in a furnace before being cooled down to room temperature. It is the first time to observe the complete conformational transition from one conformer (TT) to another (GT) for the [BMIM]+ salts in experiments. The cations preserve the GT conformation as pressure reaches 44.6 GPa for heat-treated [BMIM]Cl. As a comparison, without and with the heat treatment, the [BMIM]+ cations of [BMIM]Br keep the GT conformation at high pressures up to 29.6 GPa, as it is a relatively stable state. Moreover, a notable photoluminescence emerges at high pressures over 20.0 GPa and is maintained in the decompression process, as a result of the pressure-induced polymerization of a small portion of the [BMIM]+ cations.

Linoleic acid and its potassium and sodium salts: A combined experimental and theoretical study

Linoleic acid (cis, cis-9,12-octodecadienoic acid) is the main polyunsaturated -omega 6- essential fatty acid. The conformational behaviour of linoleic acid (LA) in the gas phase was investigated by means of density functional theory (DFT). The structures of conformers of LA were fully optimized by using the B3LYP/6-311++G(d,p) method. The theory showed that the tttttts′CssCs′tt conformation of LA (conformer I) is the more stable than the other conformations.Fourier Transform Infrared (FTIR) and micro-Raman spectra of pure LA in liquid form were recorded in the region 4000–450 and 3500–100 cm−1, respectively. The DFT calculations on the molecular structure and vibrational spectra of the dimer form of most stable conformer of LA were also performed using the same method. The assignment of the vibrational modes was made based on calculated potential energy distributions (PEDs). The simulated spectra of dimer form of LA are in reasonably good agreement with the experimental spectra.The sodium and potassium salts of LA were synthesized and characterized by FTIR and Raman spectroscopy, X-ray diffraction and DFT calculations. Several molecular and electronic properties of LA and its salts such as HOMO-LUMO energies, chemical hardness and electronegativity were also calculated and interpreted.

Microinjection of poly(vinylidene fluoride) in the presence of various additives

In this study, microinjections of poly(vinylidene fluoride) (PVDF) in the presence of polar additives were investigated. The experimental results showed that compared with the static state, the strong stress during microinjection was more conducive to generate β‐PVDF with electroactive property because it could promote the transition from trans‐gauche–trans‐gauche conformation to all‐trans zigzag (TT) conformation. However, because metastable TT conformation easily relaxed back to trans‐gauche–trans‐gauche conformation with low energy, α‐PVDF coexisted with β‐PVDF in the microinjected pure PVDF. With the presence of polar additives, TT conformation formed during the microinjection was stabilized via special interaction between their polar groups and >CF2 groups of PVDF, which was beneficial to the formation of β‐PVDF. As a result, PVDF microparts with rich β‐phases were prepared under the synergistic effects of external force field and special interaction. J. VINYL ADDIT. TECHNOL., 24:103–108, 2018. © 2016 Society of Plastics Engineers

Characterization of two AgI coordination polymers of the flexible ligand 1,3-bis(4-pyridyl)propane

Two new Agi coordination polymers having 1-D waved chain and 2-D brick wall structures have been successfully prepared and characterized; the bpp ligands exhibit a rare TT conformation.

Copper(II) Coordination Polymers Generated from 1,3‐Bis(4‐pyridyl)propane: Synthesis, Structures, and Spectroscopic Studies

AbstractThis paper describes the synthesis and structural characterization of two new one‐dimensional coordination polymers involving 1,3‐bis(4‐pyridyl)propane ligand named {(μ‐NCS)2[Cu(bpp)]·dmf}n (1) and {[Cu(mebpy)(bpp)(dmso)](ClO4)2·dmso}n (2) (bpp = 1,3‐bis(4‐pyridyl)propane and mebpy = 4,4'‐dimethyl‐2,2'‐bipyridine). These compounds were characterized by means of elemental analysis, thermal analysis (TG/DTA), vibrational spectroscopy (IR and Raman) and had their structures determined by single‐crystal X‐ray diffraction analysis. Compound 1 consists of CuN4 coordination spheres with bpp ligands in the unusual GG conformation coordinated in bridges between CuII centers to form a wavelike 1‐D coordination polymer. On the other hand, in compound 2 the bpp ligand adopts the TT conformation in bridging mode extending the 1D cationic polymeric chain.

Preferred side‐chain conformation of arginine residues in a triple‐helical structure

The crystal structure of the triple-helical peptide (Pro-Hyp-Gly)3 -Pro-Arg-Gly-(Pro-Hyp-Gly)4 (POG3-PRG-POG4) was determined at 1.45 Å resolution. POG3-PRG-POG4 was designed to permit investigation of the side-chain conformation of the Arg residues in a triple-helical structure. Because of the alternative structure of one of three Arg residues, four side-chain conformations were observed in an asymmetric unit. Among them, three adopt a ttg(-) t conformation and the other adopts a tg(-) g(-) t conformation. A statistical analysis of 80 Arg residues in various triple-helical peptides showed that, unlike those in globular proteins, they preferentially adopt a tt conformation for χ1 and χ2 , as observed in POG3-PRG-POG4. This conformation permits van der Waals contacts between the side-chain atoms of Arg and the main-chain atoms of the adjacent strand in the same molecule. Unlike many other host-guest peptides, in which there is a significant difference between the helical twists in the guest and the host peptides, POG3-PRG-POG4 shows a marked difference between the helical twists in the N-terminal peptide and those in the C-terminal peptide, separated near the Arg residue. This suggested that the unique side-chain conformation of the Arg residue affects not only the conformation of the guest peptide, but also the conformation of the peptide away from the Arg residue.

Clarification of cross-linkage structure in boric acid doped poly(vinyl alcohol) and its model compound as studied by an organized combination of X-ray single-crystal structure analysis, Raman spectroscopy, and density functional theoretical calculation.

When boric acid (BA) is added to poly(vinyl alcohol) (PVA), a chemical reaction occurs to form the cross-linkages between the amorphous PVA chains. The local structural change caused by this reaction has been clarified concretely from the microscopic level on the basis of the X-ray-analyzed crystal structure, Raman spectra, and ab initio density functional theory using a model compound produced by the reaction between pentanediol (PENT) and boric acid (PENT-BA). The PENT-BA compound was found to take the TT and TG conformations in the methylene segmental parts depending on the stereoregularity of the PENT molecule itself, meso and racemo configurations, respectively. These two conformations give the Raman bands at the different positions. By comparison of the Raman spectra between the PVA-BA and PENT-BA model compounds, the local structures of PVA chains connected to BA molecules have been derived concretely: the syndiotactic PVA parts in the amorphous region form the TG-type ring structure with the 3-coordinate boron atom, where T and G are trans and gauche conformers, respectively. On the other hand, the isotactic PVA part takes the TT conformation when it forms a ring with boron atom. The thus-created rings are hydrogen-bonded to form a dimer, which plays a role as cross-linkage between the neighboring PVA chain segments in the amorphous region.

Elucidation of conformational characteristics and configurational properties of poly((R)-3-hydroxybutyrate) by ab initio statistical mechanics

Conformational free energies and geometrical parameters derived from ab initio molecular orbital (MO) calculations for monomeric and dimeric model compounds of poly((R)-3-hydroxybutyrate) (PHB) were introduced into the refined rotational isomeric state (RIS) scheme to yield the characteristic ratio, configurational entropy, configurational internal energy, bond conformations and averaged geometrical parameters of PHB. The reliability of the MO calculations was confirmed through comparison with 1H and 13C nuclear magnetic resonance experiments for the monomeric model compound. The characteristic ratio (5.60) derived from the refined RIS calculations for the unperturbed PHB chain at 25 °C is in satisfactory agreement with the experimental values (6.1–6.3). The crystalline PHB chain is known to adopt the ttg+g+ conformation in the repeating unit. A previous study revealed that a trimeric (R)-3-hydroxybutyrate substrate bound on a PHB depolymerase lies in the ttg+g+ and tg−tt conformations. In our MO calculations, the former conformation is the most stable, and the latter is metastable but further stabilized by intermolecular C=O⋯H−N and C=O⋯H−O hydrogen bonds on the enzyme.

Investigation of the crystalline structure of PVDF in PVDF/PMMA/graphene polymer blend nanocomposites

AbstractIn this work, we report the preparation of poly(vinylidene fluoride)/poly methylmethacrylate (PVDF/PMMA)/graphene polymer blend nanocomposites via synthesis of PMMA/graphene as a masterbatch through in situ polymerization. The PMMA/graphene masterbatch compounded with PVDF by solution mixing in different ratios. The compounding was followed by solution casting to form polymer blend nanocomposites. Solution cast films were subjected to thermal treatments at three different temperatures. The crystalline structure of thermally treated samples was studied with X‐ray diffraction spectroscopy and Differential Scanning Calorimetric (DSC) analysis. Results indicated PMMA chains persuade the β crystalline form in PVDF but cannot stabilize them in elevated temperature; however, graphene sheets due to restricting effect on TT conformation chains are able to stabilize them. DSC data revealed the graphene sheets can increase the crystallinity of PVDF and also act as nucleating agents. Transmission Electron Microscopy demonstrated coexistence of the different stacking orders of graphene sheets in both masterbatch and polymer blend nanocomposite. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

Fabrication of well‐developed nano‐ribbons composed of hierarchically grown isotactic poly(methyl methacrylate) crystals on substrates

AbstractWell‐developed nano‐ribbons composed of isotactic poly(methyl methacrylate) (it‐PMMA) were successfully fabricated on mica via a simple solution‐casting method. Typical morphologies with about 0.6 nm thickness and 30–40 nm widths, thermal stability, and alternative structural analyses suggested that nano‐ribbons were composed of two‐dimensional folded chain crystals of it‐PMMA. Typically, nano‐ribbons were developed by incubating tetrahydrofuran (THF) solutions at 20 °C for at least 2 months, more than equi‐amounts of water were added to incubated THF solutions, and solutions were cast onto mica. It was found that, after the aforementioned incubation of THF solutions, it‐PMMA chains adopted trans‐trans (tt) conformations, which are precursors for it‐PMMA crystals, suggesting that THF is a unique solvent for it‐PMMA. By adding water, a poor solvent for it‐PMMA, to THF solutions, it‐PMMA aggregates formed with several hundreds of nanometer sizes, further promoting an increase in the population of the tt conformation. Nano‐ribbons were similarly formed on silicon wafer substrates, suggesting that hydrophilic substrates were essential for the formation of nano‐ribbons. Interestingly, a modulation of the above described method, with the slight evaporation of THF from a THF/water solution before casting onto mica, succeeded in the development of epitaxially adsorbed nano‐ribbons. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3098–3110, 2009

Detailed Structures and Mechanism of Polymer Solvation

In the present study, we simulated a model system, PE in biphenyl, to gain the insight into the detailed solvation structures and the molecular mechanism of polymer chain solvation. Using atomistic molecular dynamics (MD) simulation, it was found that when the biphenyl is far from PE chain or in the bulk, the dihedral angle of the two rings in the solvent molecule are approximately 32 degrees. But, the dihedral angel is about 27 degrees when the biphenyls are very close to the PE chain. In the first solvation shell, the orientation angle of the biphenyl long axis to the chain segment backbone was found to be enhanced around two values: approximately 0 and approximately 60 degrees. The detailed solvation structures found here include all dyad conformations (TT, TG, TG', GT, GG, GG', G'T, G'G, and G'G') and vary as a function of the distance between PE chain and biphenyls in the first solvation shell. The closer the the solvent molecule to the PE segment, the higher the TT conformation fraction response is. The other dyad conformations such as TG, GG', etc. undergo different decreases, respectively. This study shows that the solvation even in the Theta condition makes the overall size expansion or the chain stretched. Such a cooperative change was examined here and found not due to generating or losing a conformational state but due to a change in conformational distribution. This change occurs in the middle location of the chain instead of the chain end locations.

Synthesis, Crystal Structure, and Optical Limiting Effect of a Cu(I) Coordination Polymeric Cluster Bridged by Chiral-carbon Skeleton Bipyridyl Ligand

Abstract Solution reaction of Cu(I) iodide with chiral-carbon skeleton ligand 2,3-di(4-pyridyl)-2,3-butanediol (meso-dpb) in DMSO generated a novel 2D coordination polymeric cluster [(CuI)4(dpb)4(DMSO)4(CH3OH)4(H2O)]n (1), in which the dpb exhibits a TT conformation and the adaptable Cu2I2 rhomboid cluster unit facilitates the formation of a final noninterpenetrating motif. The nonlinear optical limiting property of 1 was also studied at 532 nm. The limiting threshold was determined to be 0.42 J cm−2 and this value is three times better than 1.6 J cm−2 of C60.

Theoretical Conformational Analysis in the Determination of Productive Conformations of Substrates for Acetylcholinesterase and Butyrylcholinesterase

All the equilibrium conformations of 34 analogues of acetylcholine (ACh) with the general formula R-C(O)O-Alk-N+(CH3)3 are calculated by the method of molecular mechanics. In the series R-C(O)O-(CH2)2-N+(CH3)3, a reliable correlation is found between the molecular volume of the substrate and the rate of its hydrolysis by acetylcholinesterase (AChE); the absence of such a correlation is demonstrated for butyrylcholinesterase (BChE). Theoretical conformational analysis confirms that the completely extended tt conformation of ACh is productive for the hydrolysis by AChE, which agrees with the results of X-ray analysis of AChE. AChE is shown to hydrolyze only those substrates that form equilibrium conformers compatible in the mutual arrangement of trimethylammonium group, carbonyl carbon, and carbonyl oxygen with the tt conformation of ACh; in this case, the rate of substrate hydrolysis depends on the total population of these conformers. A reliable correlation was found between the population of the semifolded (tg-) conformation of the choline moiety of substrate molecules and the rate of their BChE hydrolysis. In a series of CH3-C(O)O-Alk-N+(CH3)3, the rate of BChE hydrolysis is demonstrated to depend on the total population of conformations compatible in the mutual arrangement of functionally important atoms with the tg- conformation of ACh. The tg- conformation of ACh is concluded to be productive for BChE hydrolysis. Similar orientations of the substrate molecules relative to the catalytic triads of both AChE and BChE are proven to coincide upon the substrate productive sorption in their active sites. It is hypothesized that the sorption stage is rate-limiting in cholinesterase hydrolysis and the enzyme hydrolyzes the ACh molecule in its energetically favorable conformation.

Three interpenetrated frameworks constructed by long flexible N, N′-bipyridyl and dicarboxylate ligands

Three interpenetrated polymeric networks, {[Co(bpp)(OH-BDC)] · H 2O} n ( 1) [Ni(bpp) 1.5(H 2O)(OH-BDC)] n ( 2) and {[Cd(bpp)(H 2O)(OH-BDC)] · 2H 2O} n ( 3), have been prepared by hydrothermal reactions of 1,3-bis(4-pyridyl)propane (bpp), 5-hydroxyisophthalic acid (OH-H 2BDC), with Co(NO 3) 2 · 6H 2O, Ni(NO 3) 2 · 6H 2O and Cd(NO 3) 2 · 4H 2O, respectively. Single-crystal X-ray diffraction analyses reveal that the three compounds all exhibit interpenetrated but entirely different structures. Compound 1 is a fourfold interpenetrated adamantanoid structure with water molecules as space fillers, in which bpp adopts a TG conformation ( T = trans, G = gauche). Compound 2 is an interdigitated structure from the interpenetrated long arms of one-dimensional molecular ladders, while bpp in 2 adopts both TT and TG conformations. Compound 3 is a twofold interpenetrated three-dimensional network from a one-dimensional metal-carboxylate chain bridged by TG conformational bpp. The hydrogen bonding interactions in 1– 3 further stabilize the whole structural frameworks and play critical roles in their constructions.

On a novel tentative stereochemical microstructure based explanation of β relaxation of poly(vinylchloride) (PVC)

A set of PVC samples with various stereochemical microstructures, especially the mmmr and the mmmmrx ( x = m or r placement) repeating stereosequences, termini of isotactic sequences, has been prepared whether by changing the temperature of polymerization or by S N2 substitution reaction which was demonstrated to occur through a highly stereospecific mechanism. The stereochemical microstructures were accurately analysed by 13C NMR spectroscopy. For purposes of stating the microstructure/β relaxation relationship, the mechanical behaviour of all samples was studied by a dynamic mechanical thermal analyser (DMTA). The results are discussed on the ground of earlier fundamental contributions within the framework of the property/stereochemical microstructure relationships as abundantly studied in our laboratory. Basically it is shown that β relaxation decreases in both temperature and intensity with the content of the aforementioned microstructures, thereby suggesting that relaxation to stem from the enhanced free volume and favoured rotational motion facilities of the mmmr pentad under GTGTTG − TT conformation.

Theoretical Determinations of the Ambient Conformational Distribution and Unimolecular Decomposition of n-Propylperoxy Radical

The conformational distribution and unimolecular decomposition pathways for the n-propylperoxy radical have been generated at the CBS-QB3, B3LYP/6-31+G and mPW1K/6-31+G levels of theory. At each of the theoretical levels, the 298 K Boltzmann distributions and rotational profiles indicate that all five unique rotamers of the n-propylperoxy radical can be expected to be present in significant concentrations at thermal equilibrium. At the CBS-QB3 level, the 298 K distribution of rotamers is predicted to be 28.1, 26.4, 19.6, 14.0, and 11.9% for the gG, tG, gT, gG', and tT conformations, respectively. The CBS-QB3 C-OO bond dissociation energy (DeltaH298 K) for the n-propylperoxy radical has been calculated to be 36.1 kcal/mol. The detailed CBS-QB3 potential energy surface for the unimolecular decomposition of the n-propylperoxy radical indicates that important bimolecular products could be derived from two 1,4-H transfer mechanisms available at T < 500 K, primarily via an activated n-propylperoxy adduct.

A Local Chain Conformation‐Based Approach to Physical Ageing Fundamentals of Poly(vinyl chloride)

AbstractSummary: Poly(vinyl chloride) (PVC) samples prepared at various temperatures have been submitted to a substitution reaction for purposes of selectively removing the GTTG–TT conformations of the mmr termini of long isotactic sequences. The physical ageing of all samples has been studied by means of differential scanning calorimetry techniques. Surprisingly, a straight relation between the loss of enthalpy and the above GTTG–TT content, which pass through a minimum, has been found. Such a novel relationship should further lead to better understanding of the reasons why polymeric materials experience nonequilibrium states which are known to cause physical ageing.Dependence of the area of the endothermic peak (Qt) on the degree of the substitution of PVC (sample X) in cyclohexanone solution.imageDependence of the area of the endothermic peak (Qt) on the degree of the substitution of PVC (sample X) in cyclohexanone solution.

A stereochemical microstructure‐based attempt to explain the dielectric behavior of poly(vinyl chloride) (PVC)

AbstractA poly(vinylchloride) (PVC) obtained by bulk polymerization at 70 °C was modified through substitution reaction with sodium benzenethiolate (NaBT) in cyclohexanone solution at 25 °C, at various conversions up to 15 mol %. According to earlier work, this allows to change, in a controlled way, the stereochemical microstructure in terms of mainly the content of mmr (meso, meso, racemic) tetrad termini of isotactic sequences of at least one heptad in length, and of its likely chain conformations. The samples were all studied by Dielectric Relaxation Spectroscopy, which allows to measure the real and the imaginary parts of the complex dielectrical constant in a frequency range between 1 and 106 Hz, and a temperature range between 100 and 600 K. Most of the changes in the physical quantities that can be analyzed by this technique were found to parallel the changes in stereochemical microstructure with substitution extent. The temperature of the maximum of the dipolar losses, which relates to the temperature range where α relaxation occurs, appeared to increase somewhat up to 0.7% molar conversion. Then the tendency is towards stabilization. The relaxation time τ(T) as deduced from the Havriliak‐Negami (NH) function, tends to decrease between 0 and 0.7 substitution percent, and to increase afterwards. The evolution of the Ngai's β parameter with substitution degree is also highly illustrative. It clearly decreases up to 0.7% and then it increases rapidly up to 7% and slowly afterwards. These changes are discussed by taking into account that 0.7 and 7% substitution extents agree with the removal of mmr under GTTG−TT conformation and of the same structure under GTGTTT conformation, respectively. From the results, some original correlations between stereochemical microstructure and molecular dynamic behaviors are proposed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2337–2347, 2004

Synthesis and structural characterization of 3-thienyl alkyl imides

Five new thiophene imides having the structures C 4H 3SC(O)N(H)C(O)R (where R=Me, Et, Pr, t-Bu, 1– 4 ) and C 4H 3SCH 2C(O)N(H)C(O)CH 3 5 have been prepared. Control over the orientation of the carbonyl groups was achieved by the introduction of a bulky group in the terminal position of the side chain, and molecular modeling calculations indicated that the energy difference between the ct and tt conformations is small. The acidity of 1– 4 decreases from Me to t-Bu, and, although 1– 4 will not undergo oxidative polymerization, the introduction of a single spacer CH 2 group allows 5 to polymerize.

One- and two-dimensional metal–dicyanamido complexes with a flexible bridging co-ligand: structural and magnetic propertiesElectronic supplementary information (ESI) available: a scheme of the different bridging network topologies of dca complexes. See http://www.rsc.org/suppdata/dt/b2/b212886a/

Five coordination-polymers of formula [M(dca)2(bpp)]∞ [bpp is the flexible 1,3-bis(4′-pyridyl)propane ligand, dca = dicyanamido anion, M = Mn (1), Co (2), Zn (3), and Cd (4a and 4b)] have been synthesized and structurally characterized. Compounds 1, 2, 3 and 4a are isomorphous and consist of uniform chains in which adjacent metal ions are triply linked by two 1,5-dca and one bpp bridges and each metal ion is octahedrally coordinated with a trans geometry. The M(dca)2 chain motif adopts a sinusoidal topology with the M(dca)2M rings in a boat conformation, to adapt to the arc-shaped bpp ligand, which here acts as an intrachain bridge and takes an unusual GG conformation. Compound 4b is thermodynamically more stable than 4a. The structure of 4b consists of extended 2D coordination layers in which neighboring metal ions are linked by a bpp or two 1,5-dca bridges and the metal ion is in a cis-octahedral environment. The M(dca)2 chain motif adopts a zigzag topology with the M(dca)2M rings in the usual flat conformation, and the bpp ligand takes a TT conformation and acts as an “arch bridge” between adjacent chains to yield a corrugated (4,4) layer. Each layer is interpenetrated with two adjacent layers, generating an interesting 3D polycatenane. The temperature dependence of the magnetic susceptibility has been investigated for the Mn(II) and Co(II) species. 1 behaves as a 1D Heisenberg Mn(II) chain with a rather weak intrachain antiferromagnetic interaction, and 2 exhibits the complicated behavior characteristic of polynuclear Co(II) species with an octahedral coordination geometry.