Conformational and substituent effects in the 19F spectra of C7-C10 straight-chain chlorofluoroalkanes

One- and two-dimensional fluorine NMR spectra of straight-chain chlorofluoroalkanes having 7-10 carbons were obtained and interpreted. Spectral assignments were based primarily on indirect spin-spin correlations. A scale of chlorine substituent effects was developed. The effects of asymmetric CFCl centers on chemical-shift differences in nearby CF2 groups were analyzed in terms of molecular conformational effects. Spin-spin coupling patterns of CF3 groups were elucidated.     

Conformational equilibrium and dynamic behavior of bis‐N‐triflyl substituted 3,8‐diazabicyclo[3.2.1]octane

Restricted rotation about the N–S partial double bonds in a bis‐N‐triflyl substituted 3,8‐diazabicyclo[3.2.1]octane derivative 1 has been frozen at low temperature (ΔG^≠ = 11.6 kcal mol^?1), and the existence of all four rotamers about the two N–S bonds, 3‐in,8‐in, 3‐in,8‐out, 3‐out,8‐in, and 3‐out,8‐out, respectively, proved experimentally by NMR spectroscopy and theoretically by DFT and MP2 calculations. Copyright © 2014 John Wiley & Sons, Ltd.     

Enhanced β‐phase in PVDF polymer nanocomposite and its application for nanogenerator

Harvesting energy from the ambient mechanical energy by using flexible piezoelectric nanogenerator is a revolutionary step toward achieving reliable and green energy source. Polyvinylidene fluoride (PVDF), a flexible polymer, can be a potential candidate for the nanogenerator if its piezoelectric property can be enhanced. In the present work, we have shown that the polar crystalline β‐phase of PVDF, which is responsible for the piezoelectric property, can be enhanced from 48.2% to 76.1% just by adding ZnO nanorods into the PVDF matrix without any mechanical or electrical treatment. A systematic investigation of PVDF‐ZnO nanocomposite films by using X‐ray diffractometer, Fourier transform infrared spectroscopy, and polarization‐electric field loop measurements supports the enhancement of β‐phase in the flexible nanocomposite polymer films. The piezoelectric constant (d₃₃) of the PVDF‐ZnO (15 wt%) film is found to be maximum of approximately −1.17 pC/N. Nanogenerators have been fabricated by using these nanocomposite films, and the piezoresponse of PVDF is found to enhance after ZnO loading. A maximum open‐circuit voltage ~1.81 V and short‐circuit current of 0.57 μA are obtained for 15 wt% ZnO‐loaded PVDF nanocomposite film. The maximum instantaneous output power density is obtained as 0.21 μW/cm² with the load resistance of 7 MΩ, which makes it feasible for the use of energy harvesting that can be integrated to use for driving small‐scale electronic devices. This enhanced piezoresponse of the PVDF‐ZnO nanocomposite film‐based nanogenerators attributed to the enhancement of electroactive β‐phase and enhanced d₃₃ value in PVDF with the addition of ZnO nanorods.     

Polymorphism and phase transformation in the dimethyl sulfoxide solvate of 2,3,5,6‐tetrafluoro‐1,4‐diiodobenzene

A new polymorph (form II) is reported for the 1:1 dimethyl sulfoxide solvate of 2,3,5,6‐tetrafluoro‐1,4‐diiodobenzene (TFDIB·DMSO or C₆F₄I₂·C₂H₆SO). The structure is similar to that of a previously reported polymorph (form I) [Britton (2003). Acta Cryst. E 59 , o1332–o1333], containing layers of TFDIB molecules with DMSO molecules between, accepting I…O halogen bonds from two TFDIB molecules. Re‐examination of form I over the temperature range 300–120 K shows that it undergoes a phase transformation around 220 K, where the DMSO molecules undergo re‐orientation and become ordered. The unit cell expands by ca 0.5 Å along the c axis and contracts by ca 1.0 Å along the a axis, and the space‐group symmetry is reduced from Pnma to P2₁2₁2₁. Refinement of form I against data collected at 220 K captures the (average) structure of the crystal prior to the phase transformation, with the DMSO molecules showing four distinct disorder components, corresponding to an overlay of the 297 and 120 K structures. Assessment of the intermolecular interaction energies using the PIXEL method indicates that the various orientations of the DMSO molecules have very similar total interaction energies with the molecules of the TFDIB framework. The phase transformation is driven by interactions between DMSO molecules, whereby re‐orientation at lower temperature yields significantly closer and more stabilizing interactions between neighbouring DMSO molecules, which lock in an ordered arrangement along the shortened a axis.     

Conformational variation and crystalline phase transformation of low syndiotactic polypropylene films in stretched and stress‐relaxed states

Low syndiotactic polypropylene (sPP; rrrr = 80%) films were isothermally crystallized at 0 °C (sample S₀) and 90 °C (sample S₉₀) for 65 h, respectively. Fourier transform infrared spectroscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction were used to characterize the structure transformation and orientation behavior of samples S₀ and S₉₀ at both stretched and stress‐relaxed states. It was found that stretching (λ = 0–700%) induces the transformation of the chain conformation from helical to trans‐planar form for both S₀ and S₉₀ films. The stretched S₀ and S₉₀ samples show well oriented trans‐planar chains as well as partially retained helices. Simultaneously, crystalline phase transformation occurs during the stretching and relaxing processes of the investigated sPP samples, i.e., stable form I crystals can be transformed into metastable form III or mesophase by stretching samples, and vice versa. For stretched S₀ sample, form III with trans‐planar conformation, which generally exists in highly stretched sPP, cannot be observed, even at higher strains. For sample S₉₀, however, stretching might induce the formation of both the form III crystals and mesophase with trans‐planar chains; releasing the tension, form III again gets converted into trans‐planar mesophase and form I crystals. In the stretched and stress‐relaxed states of samples S₀ and S₉₀, the difference of the delicate orientation behavior and relative content of chain conformation and crystalline form can be attributed to the different heat‐treating methods of the low syndiotacticity sPP. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2924–2936, 2005     

Impact of side‐chain length on the phase structures of P3ATs and P3AT:PCBM films as revealed by SSNMR and FTIR

It is known that poly(3‐alkylthiophene) (P3AT) side‐chain length notably influences the photovoltaic performances of relating devices. However, comprehensively study on its impact on the structures of P3ATs and their blends with [6, 6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) is insufficient. By using solid‐state NMR and FTIR techniques, four P3ATs and their PCBM blends are investigated in this work, focusing on the phase structures as modulated by side‐chain length. Recently, we revealed multiple crystalline main‐chain packings of packing a and b together with a mesophase in poly(3‐butylthiophene) (P3BT) films (DOI: 10.1021/acs.macromol.6b01828). Here, the semicrystalline structures are investigated on poly(3‐hexylthiophene) (P3HT), poly(3‐octylthiophene) (P3OT), and poly(3‐dodecylthiophene) (P3DDT) with traditional form I modification, where packing a and the amorphous phase are probed. Furthermore, crystallized side chain within packing a is detected in both P3OT and P3DDT films, which shows a FTIR absorption at 806 cm⁻¹. Structural studies are also conducted on P3AT:PCBM blends. Compared with the pure P3ATs, the polymer crystallinities of the blends show reduction of about 40% for P3OT and P3DDT, whereas only about 10% for P3HT. Moreover, in P3BT:PCBM and P3HT:PCBM, the crystalline polymers and PCBM are phase separated, while in P3OT:PCBM and P3DDT:PCBM, blend components are mostly miscible. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 751–761     

Monitoring Conformational Changes in the NDM‐1 Metallo‐β‐lactamase by 19F NMR Spectroscopy

The New Delhi metallo‐β‐lactamase (NDM‐1) is involved in the emerging antibiotic resistance problem. Development of metallo‐β‐lactamases (MBLs) inhibitors has proven challenging, due to their conformational flexibility. Here we report site‐selective labeling of NDM‐1 with 1,1,1‐trifluoro‐3‐bromo acetone (BFA), and its use to study binding events and conformational changes upon ligand–metal binding using ¹⁹F NMR spectroscopy. The results demonstrate different modes of binding of known NDM‐1 inhibitors, including L ‐ and D ‐captopril by monitoring the changing chemical environment of the active‐site loop of NDM‐1. The method described will be applicable to other MBLs and more generally to monitoring ligand‐induced conformational changes.     

Micro ATR‐FTIR study of the hydration state of poly(N‐tert‐butylacrylamide‐co‐acrylamide) copolymers during phase transition in aqueous media and in the mixture of water–methanol

Coil‐globule transition of poly(N‐tert‐butylacrylamide‐co‐acrylamide) P(NTBAM‐co‐AM) copolymers is investigated in the aqueous solution and in the mixture of water–methanol by micro ATR‐FTIR spectroscopy technique. In this study the microstructure and its changes in the hydration states of the distinct groups of these copolymers are investigated by micro ATR/FTIR technique. The results showed that by heating the solution above the LCST hydrogen bonding between C?O and water was decreased but the hydrogen bonding between polymeric chains increased, which prove the aggregation of polymer chain during phase separation. The chemical shifts of IR bands are also studied in the mixture of water–methanol. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 356–363, 2010     

Conformational Selection in Glycomimetics: Human Galectin‐1 Only Recognizes syn‐Ψ‐Type Conformations of β‐1,3‐Linked Lactose and Its C‐Glycosyl Derivative

The human lectin galectin‐1 (hGal‐1) translates sugar signals, that is, β‐galactosides, into effects on the level of cells, for example, growth regulation, and has become a model for studying binding of biopharmaceutically relevant derivatives. Bound‐state conformations of Galβ‐C‐(1→3)‐Glcβ‐OMe ( 1 ) and its βGal‐(1→3)‐βGlc‐OMe disaccharide parent compound were studied by using NMR spectroscopy (transferred (TR)‐NOESY data), assisted by docking experiments and molecular dynamics (MD) simulations. The molecular recognition process involves a conformational selection event. Although free C‐glycoside access four distinct conformers in solution, hGal‐1 recognizes shape of a local minimum of compound 1 , the syn‐Φ/syn‐Ψ conformer, not the structure at global minimum. MD simulations were run to explain, in structural terms, the observed geometry of the complex.     

Energy‐induced polymorphic changes in poly(vinylidene fluoride): How ultrasonication results in polymer with predominantly γ phase

Self‐polarized poly(vinylidene fluoride) (PVDF) films were prepared via solution crystallization technique wherein the polymorphism of the films was controlled from α phase (>85%) to γ phase (>90%) by varying the time of ultrasonication. On increasing ultrasonication time up to 60 min, γ phase crystallites were found to be self‐aligned in the matrix while an equal proportion of α and γ phases coexist in the PVDF films ultrasonicated for 120 min. The phase conversion as well as inversion was evident from Fourier transform infrared, X‐ray diffraction, and differential scanning calorimetry analyses. Microscopic images of films ultrasonicated for 60 min showed a scrolled lamellar morphology while those sonicated for 120 min showed mixture of scrolled lamellar and spherulitic morphology. With the help of computational studies, it is explained that a large amount of energy is required for transforming trans‐gauche‐trans‐gauche into trans‐trans‐trans‐gauche conformation which is provided by ultrasonication. The mechanism of γ phase formation is proposed based on the experimental and theoretical approaches. Our studies show that just by tuning the time of ultrasonication, PVDF films with various morphologies can be processed; either one with predominantly electroactive γ phase with superior electrical properties or one with equal proportion of α and γ phases with superior mechanical properties. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 40–50     

Conformational change of proline residues observed in swollen and shrunken phases: γ‐Ray synthesis and variable temperature circular dichroism spectra of a thermo‐responding polymer,poly(acryloyl‐Pro‐OMe)

Poly(acryloyl‐L ‐proline‐methyl ester) ( 1 ) has optically active side chain, and constitutes thermoresponsive hydrogels upon crosslinking. In this study, we have prepared uncrosslinked polymer of 1 with a 10‐kGy irradiation dose of γ‐ray. For this polymer, 1 , variable temperature circular dichroism (CD) and ¹H NMR spectra have been studied in the range of 0–30 °C. The intense CD spectrum at 0 °C suggests that the side chains in 1 have an ordered orientation. The CD intensity decreases gradually with increasing temperature. The decreased intensity of CD spectra indicates that the disordering occurs for the side‐chain orientation. The CD band shape changes discontinuously at 20 °C. In the ¹H NMR spectra, signals disappear above 20 °C. These spectral change at 20 °C indicate that the phase transition occurs at around 20 °C from swollen to shrunken phase. Even after the phase transition, the CD spectra are still changing with isochromic point at 212 nm. It appears that the side‐chain conformation is still changing from one state to the other state in the shrunken phase polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4524–4530, 2000     

Side‐chain conformational changes during the thermoshrinking process: γ‐Ray polymerization and spectroscopic study of uncrosslinked poly(methacryloyl‐Ala‐OMe)

Poly(methacryloyl‐L ‐alanine‐methyl ester) (1) has an optically active side chain and consists of thermoshrinking hydrogels upon crosslinking. We synthesized an uncrosslinked polymer of 1 by the γ‐ray polymerization method. For the prepared polymer, variable‐temperature circular dichroism (CD) and ¹H NMR spectra were studied, and we found conformational changes in the optically active side chains during the thermally induced phase transition. Intense CD spectra reveal ordered conformation in the side chain of 1 below the phase transition temperature (∼28 °C). A well‐resolved ¹H NMR spectrum of 1 at 0 °C shows that the conformational angles in the polymer side chain are fixed at low‐energy minima. With increasing temperature, the frozen side chain starts rotating vigorously and takes an unordered orientation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2671–2677, 2000     

1H,13C and 19F NMR data of N‐substituted 6‐(4‐methoxyphenyl)‐7H‐pyrrolo[2,3‐d]pyrimidin‐4‐amines in DMSO‐d6

Chemical shift assignment of seven N‐substituted 6‐(4‐methoxyphenyl)‐7H‐pyrrolo[2, 3‐d]pyrimidin‐4‐amines, six of which are fluorinated, have been performed based on ¹H, ¹³C, ¹⁹F, and 2D COSY, HMBC and HSQC experiments. Copyright © 2009 John Wiley & Sons, Ltd.     

Magnetism,optical absorbance,and 19F NMR spectra of nafion films with self‐assembling paramagnetic networks

Magnetization, optical absorbance, and ¹⁹F NMR spectra of Nafion transparent films as received and doped with Mn²⁺, Co²⁺, Fe²⁺, and Fe³⁺ ions with and without treatment in 1H‐1,2,4‐triazole (trz) have been studied. Doping of Nafion with Fe²⁺ and Co²⁺ and their bridging to nitrogen of triazole yields a hybrid self‐assembling paramagnetic system that exhibits interesting magnetic and optical properties. These include spin crossover phenomena between high‐spin (HS) and low‐spin (LS) states in Nafion‐Fe²⁺‐trz and Nafion‐Co²⁺‐trz accompanied by thermochromic effects in the visible range induced by temperature. A large shift of the magnetization curve induced by a magnetic field in the vicinity of the HS ? LS, ～220 K, observed for Nafion‐Fe²⁺‐trz has a rate of ～6 K/kOe, which is about three orders of magnitude larger than that in bulk spin crossover Fe²⁺ materials. Selective response of ¹⁹F NMR signals on doping with paramagnetic ions demonstrates that NMR can be used as spatially resolved method to study Nafion film with paramagnetic network. Both chemical shift and width of ¹⁹F NMR signals show that SO groups of Nafion, Fe or Co ions, and nitrogen of triazole are bonded whereas they form a spin crossover system. Based on a model of nanosize cylinders proposed for Nafion [K. Schmidt‐Rohr and Q. Chen, Nat Mater (2008), 75], we suggest that paramagnetic ions are located inside these cylinders, forming self‐assembling magnetically and optically active nanoscale networks. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 129–138, 2012     

A combined crystallographic,thermal, Raman and computational study on polymorphism and phase transition in 1‐(4‐hexyloxy‐3‐hydroxyphenyl)ethanone

The crystal structure of the triclinic polymorph of 1‐(4‐hexyloxy‐3‐hydroxyphenyl)ethanone, C₁₄H₂₀O₃, differs markedly from that of the orthorhombic polymorph [Manzano et al. (2015). Acta Cryst. C 71 , 1022–1027]. The two molecular structures are alike with respect to their bond lengths and angles, but differ in their spatial arrangement. This gives rise to quite different packing schemes, even if built up by similar chains having the hydroxy–ethanone O—H…O hydrogen‐bond synthon in common. Both phases were found to be related by a first‐order thermally driven phase transformation at 338–340 K, which is discussed in detail. The relative stabilities of both polymorphs are explained on the basis of both the noncovalent interactions operating in each structure and quantum chemical calculations. The polymorphic phase transition has also been studied experimentally by means of differential scanning calorimetry (DSC) experiments, conducted on individual single crystals, Raman spectroscopy and controlled heating under a microscope of individual single crystals, which were further characterized by powder and single‐crystal X‐ray diffraction.     

Simultaneous determination of triclosan,triclocarban, and transformation products of triclocarban in aqueous samples using solid‐phase micro‐extraction‐HPLC‐MS/MS

The presence of triclosan and triclocarban, two endocrine‐disrupting chemicals and antimicrobial agents, and transformation products of triclocarban, 1,3‐di(phenyl)urea, 1,3‐bis(4‐chlorophenyl)urea and 1,3‐bis(3,4‐dichlorophenyl)urea, in tap water, treated household drinking water, bottled water, and river water samples were investigated using solid‐phase micro‐extraction coupled with‐HPLC‐MS/MS, a rapid, green, and sensitive method. Factors influencing the quantity of the analytes extracted onto the solid‐phase micro‐extraction fiber, such as addition of salt, sample pH, extraction time, desorption time, and sample volume, were optimized using solid‐phase micro‐extraction‐HPLC‐MS/MS. The results showed that the method gave satisfactory sensitivities and precisions for analyzing sub‐part‐per‐trillion levels of triclosan, triclocarban, and transformation products of triclocarban in samples collected locally. The recoveries of analytes ranged from 97 to 107% for deionized water samples, and 99 to 110% for river water samples, and limits of detection were in the range of 0.32–3.44 and 0.38–4.67 ng/L for deionized water and river water samples, respectively. On average, the daily consumption of triclosan and triclocarban by an adult by consuming 2 liters of different types of drinking water were estimated to be in the range of 6.13–425 ng/day as a result of the concentrations of triclosan and triclocarban measured in this study.     

Interpreting 2hJ(F,N), 1hJ(H,N) and 1J(F,H) in the hydrogen‐bonded FH–collidine complex

Ab initio EOM‐CCSD calculations were performed to determine ¹⁹F,¹H, ¹⁹F,¹⁵N and ¹H,¹⁵N spin–spin coupling constants in model complexes FH–NH₃ and FH–pyridine as a function of the F—H and F—N distances. The absolute value of ¹J(F,H) decreases and that of ^1hJ(H,N) increases rapidly along the proton‐transfer coordinate, even in the region of the proton‐shared F—H—N hydrogen bond. In contrast, ^2hJ(F,N) remains essentially constant in this region. These results are consistent with the recently reported experimental NMR spectra of FH–collidine which show that ^1hJ(H,N) increases and ¹J(F,H) decreases, while ^2hJ(F,N) remains constant as the temperature of the solution decreases. They suggest that the FH–collidine complex is stabilized by a proton‐shared hydrogen bond over the range of experimental temperatures investigated, being on the traditional side of quasi‐symmetric at high temperatures, and on the ion‐pair side at low temperatures. Copyright © 2002 John Wiley & Sons, Ltd.     

Simultaneous analysis of current‐use pesticides and their transformation products in water using mixture‐sorbent solid phase extraction and high‐performance liquid chromatography–tandem mass spectrometry

Pesticides have posed significant threats to aquatic ecosystems, yet little is known about their transformation products. The challenge is to simultaneously analyze various pesticides and transformation products in water as they have distinct physicochemical properties. A mix‐mode solid phase extraction method was established to simultaneously analyze current‐use pesticides and their transformation products using a mixture of hydrophile–lipophile balance, weak anion, and cation exchange resins (2:1:1, w/w/w) in combination with high‐performance liquid chromatography and tandem mass spectrometry for chemical quantification. Neutral, acidic, and alkaline methanol were used as the elution solvent. Box‐Behnken design was applied to optimize extraction conditions. Optimal conditions were as follows: sorbent mass, 200 mg; volume of elution solvent, 5 mL × 3; pH 4. The method was validated for compounds at concentrations from 20 to 1000 ng/L in different types of water samples, with recovery being from 43.5 ± 3.1 to 141 ± 35%. Low method detection limits (0.02?5.6 ng/L) implied that the developed method was sensitive. Finally, the method was applied to monitor current‐use pesticides and their transformation products in natural waters. Frequent detection of transformation products of pesticides indicated that their contribution to aquatic risk should not be ignored.