ENZYMATICALLY SYNTHESIZED POLYANILINE IN THE PRESENCE OF A TEMPLATE POLY(VINYLPHOSPHONIC ACID): A SOLID STATE NMR STUDY

Template guided enzymatic synthesis of conducting polyaniline (PANI) is a one-step reaction and more importantly, it is an environmentally friendly process. Understanding of the reaction and coupling mechanism at the molecular level is of paramount significance to improve its processability and conductivity. Solid-state NMR techniques are useful to investigate molecular structures of enzymatically synthesized polyaniline (PANI). The PANI sample in three different forms i.e., (a) as-synthesized, self-doped conducting form; (b) dedoped, base form and; (c) redoped, conducting form, are investigated by solid-state ¹³C and ¹⁵N CP/MAS NMR techniques. Solid-state NMR data analysis shows that the structural features of enzymatically synthesized PANI are similar to that of chemically synthesized PANI. The solid-state ¹³C CP/MAS NMR spectrum of the base form of PANI confirmed that benzenoid-quinoid repeating units are present in the backbone of the PANI polymer chain. The poly(vinylphosphonic acid) (PVP) template provides charge compensation during the chain growth of linear polyaniline. After the completion of template-guided synthesis of PANI, it is now possible that the PVP template can be completely removed from the complex by dedoping with aqueous NH₄OH. The detached PANI from the PANI-PVP complex can then be redoped to conducting form without the presence of the template. The conductivity of the PANI and PANI-PVP complex are of the same order of magnitude. The solid-state ¹⁵N CP/MAS NMR chemical shifts are sensitive to charge distribution on the nitrogens in the backbone. The solid-state ¹⁵N CP/MAS NMR spectrum of the base form of the enzymatically derived PANI sample showed the clear signature for benzenoid-quinoid repeating units in the polymer backbone.

     

The 19F shielding anisotropy of p-nitrofluorobenzene from nematic phase NMR

NMR experiments on p-nitrofluorobenzene dissolved in a nematic solvent are reported. It is shown that the ¹⁹F chemical shift tensor does not possess axial symmetry around the CF bond. For σ_aa ? σ_bb a value of 81 ppm, for σ_cc ? σ_bb a value of 156 ppm is determined.     

Effect of phenyl group on the structure and formation of transitional alumina from Al (OPh)<Subscript>3</Subscript>

The structure of freshly prepared Al(OPh)₃, its decomposition product, the hydrolyzed products and their structural evolution were investigated employing ²⁷Al MAS NMR spectroscopy, PXRD, TGA/DTA/DSC/FTIR techniques. In the ²⁷Al MAS NMR spectrum of the aluminium phenoxide, three signals with the chemical shift at 3.78, 21 and 45 ppm were observed. The chemical shift at 3.78 and 45 ppm revealed the presence of four and sixfold coordinated aluminum. The signal at 21 ppm corresponded to fivefold coordinated aluminium. When the aluminium phenoxide was directly decomposed in air at 600 °C, it resulted in amorphous product as evidenced from the PXRD pattern. The observed signals with chemical shifts at 10.1, 42, 73.6 ppm in the ²⁷Al MAS NMR spectrum indicated the presence of 6, 5 and 4 coordination for the aluminium atoms suggesting disordered transitional γ-alumina to be the product. The hydrolysis studies of Al(OPh)₃ with excess of water at 70 °C yielded bohemite (γ-AlOOH). The alumina obtained after dehydration at 600 °C was X-ray amorphous. The dehydrated product at 600 °C showed the presence of four and six coordinated aluminium atoms in the ²⁷Al MAS NMR spectrum confirming it to be ordered γ-Al₂O₃. Crystalline γ-Al₂O₃ was obtained on further heating at 800 °C.     

Proton magnetic shielding in malonic acid by multiple pulse techniques

The proton NMR in single crystals of malonic acid has been studied by multiple pulse line narrowing techniques. The nuclear magnetic shielding tensors σ⁽ⁱ⁾ of all protons in malonic acid could be determined from the spectra. There are two magnetically distinct carboxyl protons. The principal components of their shielding tensors are found to be σ⁽¹⁾_ZZ= ?0.8 ppm, σ⁽¹⁾_YY = ?19.2 ppm, σ⁽¹⁾_XX = ?21.8 ppm, and σ⁽²⁾_ZZ = ?1.0 ppm, σ⁽²⁾_ZZ = ?21.3 ppm relative to adamantane. The error limits are estimated to be ± 1 ppm. The most shielded directions lie along the hydrogen bond directions to within 8 degrees. The least shielded directions are essentially perpendicular to the plane of the carboxyl groups. Within experimental accuracy the shielding of the aliphatic protons is axially symmetric about the CH bond axes. The anisotropy Δσ = σ_? ? σ_⊥ is (4 ± 1) ppm. The gross features of the anisotropy of the carboxyl protons are shown to be governed by the diamagnetic effect.     

Investigating the two inequivalent NH2(CH3)2 ions in [NH2(CH3)2]2CuCl4 using magic angle spinning nuclear magnetic resonance

The structural change near the phase transition temperatures of [NH₂(CH₃)₂]₂CuCl₄ is discussed in terms of the chemical shifts and the spin-lattice relaxation times T_1ρ in the rotating frame for ¹H MAS NMR and ¹³C CP/MAS NMR. The ¹H T_1ρ undergoes molecular motion near the phase-transition temperature (T_C2 = 253 K). In addition, the two inequivalent [NH₂(CH₃)₂] (1) and [NH₂(CH₃)₂] (2) sites were distinguishable by the ¹³C chemical shift. And, the most significant change was observed at T_C2 for the ¹³C CP/MAS NMR spectrum; this temperature corresponds to a ferroelastic phase transition with different orientations.     

Polysiloxane-Modified Mesoporous Materials

We report the preparation of polysiloxane-modified mesoporous silica gels derived from the acid catalysed hydrolysis of tetraethoxysilane (TEOS) and oligomeric silanol terminated polydimethylsiloxane (PDMS) in the presence of the non-ionic surfactant, octaethylene glycol monohexadecyl ether. The gels were characterised using thermal gravimetric analysis (TGA), infra-red (IR) spectroscopy, X-ray diffraction (XRD) and ²⁹Si solid state cross-polarisation (CP) magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. TGA and IR spectroscopy showed the loss of surfactant after calcination and a decrease in the Si–OH band at 950 cm^–1 indicated further condensation had occurred. This was confirmed by the increase in Q⁴ at –110 ppm, in ²⁹Si MAS NMR spectroscopy, which also showed that calcination had led to the redistribution of PDMS forming a T species. XRD data showed ordering within the structure, with an initial d-spacing of 45 Å, decreasing to 35 Å after calcination.     

Application of cationic polymerization to grafting and coating of silica particles

The cationic polymerization of electron rich monomers such as vinyl ethers, vinyl furane, and cyclopentadiene on silica surfaces can be initiated by aryl methyl halides. The reactions yield always soluble polymers (by heterogeneous catalysis) and novel polymer/silica hybrid materials. The link between polymer and solid is caused by covalent Si-O-C bonds, by network formation of the polymers during the chain growth, or by a combination of both of them. The analysis of the polymer structures on the surface by ¹H MAS NMR spectroscopy in suspension and by solid state ¹³C CP MAS NMR spectroscopy is described. Proof of Si-O-C bonds via DRIFT spectroscopy and ¹³C CP MAS NMR spectroscopy is given. The most effective method of irreversibly linking the polymer to the silica surface is the network formation. Polyvinyl ethers are bound strongly to the surface, as can be shown by FTIR measurements, but the linkage is not stable due to the Si-O-C bonds' susceptibility to hydrolysis. Poly-cyclopentadienes (PCPD) are linked to the surface by Si-O-C bonds, which show an extraordinary high resistance to acids and bases. Si-O-C bond formation of poly-2-vinyl furane could not yet be detected by ¹³C CP MAS NMR spectroscopy and DRIFT spectroscopy. In this case the high degree of coating derives from the bifunctionality of 2-vinyl furane: it may undergo Friedel-Crafts-alkylation at the 5-position of the furane ring as well as chain polymerization via the vinyl group at the 2-position.     

Study of chemically inequivalent N(CH3)4 ions in [N(CH3)4]2ZnBr4 near the phase transition temperature using 1H MAS NMR, 13C CP/MAS NMR,and 14N NMR

The temperature dependences of the chemical shifts and intensities of ¹H, ¹³C, and ¹⁴N nuclei in tetramethylammonium tetrabromozincate, [N(CH₃)₄]₂ZnBr₄, were investigated using single-crystal nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR spectroscopy to elucidate the structural geometry near the phase transition temperature. Based on the analysis of the ¹³C cross-polarization (CP)/MAS NMR and ¹⁴N NMR spectra, the two chemically inequivalent N(1) (CH₃)₄ and N(2) (CH₃)₄ ions were distinguished. Furthermore, the ¹⁴N NMR spectrum at the phase transition temperature indicated the existence of the ferroelastic characteristics of the N(CH₃)₄ ions.     

Solid‐state cross‐polarization magic angle spinning 13C and 15N NMR characterization of Sepia melanin,Sepia melanin free acid and Human hair melanin in comparison with several model compounds

This paper presents the high‐resolution ¹³C and ¹⁵N cross‐polarization magic angle spinning (CP/MAS) NMR spectra of three natural melanin solids: Sepia officinalis melanin, Sepia officinalis melanin free acid (MFA) and Human hair melanin. The functional group characterization of Human hair melanin by NMR is the first to date and the ¹³C CP/MAS NMR spectra reported here show improved resolution of chemically inequivalent sites. The observed spectral regions of the solid melanin samples can be assigned to the postulated structural unit of the polymer chain of Sepia MFA derived from solution‐state NMR studies. To assist in the assignment of functional groups in the spectra, the solid‐state CP/MAS NMR spectra are compared with high‐resolution ¹³C and ¹⁵N CP/MAS spectra of four model compounds, L ‐dopa, dopamine, 2‐methoxycarbonyl‐3‐ethoxycarbonyl‐4‐methylpyrrole and ethyl 5,6‐dimethoxyindole‐2‐carboxylate. To aid further in the assignment of protonated and non‐protonated carbon atoms, CP contact time dependence and non‐quaternary carbon suppression (NQS) experiments were performed on the melanin samples. The ¹⁵N CP/MAS spectra of the melanin samples confirm the presence of indole and pyrrole units in the melanin polymer chain. The NMR peaks observed in all of the melanin samples are relatively broad, presumably owing to the presence of free radicals. Electron spin resonance (ESR) data shows that all three melanin samples contain localized free radicals (g = 2.007), with the Sepia melanin containing a 10‐fold higher free radical density than Human hair melanin. Copyright © 2003 John Wiley & Sons, Ltd.     

Natural Abundance 15N NMR by Dynamic Nuclear Polarization: Fast Analysis of Binding Sites of a Novel Amine‐Carboxyl‐Linked Immobilized Dirhodium Catalyst

A novel heterogeneous dirhodium catalyst has been synthesized. This stable catalyst is constructed from dirhodium acetate dimer (Rh₂(OAc)₄) units, which are covalently linked to amine‐ and carboxyl‐bifunctionalized mesoporous silica (SBA‐15?NH₂?COOH). It shows good efficiency in catalyzing the cyclopropanation reaction of styrene and ethyl diazoacetate (EDA) forming cis‐ and trans‐1‐ethoxycarbonyl‐2‐phenylcyclopropane. To characterize the structure of this catalyst and to confirm the successful immobilization, heteronuclear solid‐state NMR experiments have been performed. The high application potential of dynamic nuclear polarization (DNP) NMR for the analysis of binding sites in this novel catalyst is demonstrated. Signal‐enhanced ¹³C CP MAS and ¹⁵N CP MAS techniques have been employed to detect different carboxyl and amine binding sites in natural abundance on a fast time scale. The interpretation of the experimental chemical shift values for different binding sites has been corroborated by quantum chemical calculations on dirhodium model complexes.     

Synthesis and Characterization of UV-Curable Poly(dimethylsiloxane) Dimethacrylate

This paper presents a new route to the synthesis of UV-curable poly(dimethylsiloxane) dimethacrylate (PDMSDMA). PDMSDMA was essentially prepared by modification of poly(dimethylsiloxane), bis(3-aminopropyl) terminated (PDMS-NH₂) with methacrylic anhydride (MAA). The synthesized products were cured under UV in the presence of camphorquinone (CQ) used as a photoinitiator. The chemical structure of PDMSDMA samples was analyzed by FT-IR and ¹H-NMR spectroscopy. The ¹H-NMR spectrum of PDMSDMA revealed new peaks at 3.20 ppm, corresponding to methylene protons in  CH₂ NH , and 5.25 and 5.65 ppm, corresponding to vinylic protons in  NH CO CCH₃CH₂. The chemical structure of the cured products and the degree of curing were determined by solid state ¹³C CP/MAS NMR and FT-IR (Micro-ATR) spectroscopy. Various parameters, such as concentration of methacrylic anhydride, amount of camphorquinone, and curing time, were studied.     

Characterising lone-pair activity of lead(II) by 207Pb solid-state NMR spectroscopy: coordination polymers of [N(CN)2]- and [Au(CN)2]- with terpyridine ancillary ligands

A series of lead(II) coordination polymers containing [N(CN)₂]^? (DCA) or [Au(CN)₂]^? bridging ligands and substituted terpyridine (terpy) ancillary ligands ([Pb(DCA)₂] ( 1 ), [Pb(terpy)(DCA)₂] ( 2 ), [Pb(terpy){Au(CN)₂}₂] ( 3 ), [Pb(4′‐chloro‐terpy){Au(CN)₂}₂] ( 4 ) and [Pb(4′‐bromo‐terpy)(μ‐OH₂)_0.5{Au(CN)₂}₂] ( 5 )) was spectroscopically examined by solid‐state ²⁰⁷Pb MAS NMR spectroscopy in order to characterise the structural and electronic changes associated with lead(II) lone‐pair activity. Two new compounds, 2 and [Pb(4′‐hydroxy‐terpy){Au(CN)₂}₂] ( 6 ), were prepared and structurally characterised. The series displays contrasting coordination environments, bridging ligands with differing basicities and structural and electronic effects that occur with various substitutions on the terpyridine ligand (for the [Au(CN)₂]^? polymers). ²⁰⁷Pb NMR spectra show an increase in both isotropic chemical shift and span (Ω) with increasing ligand basicity (from δ_iso=?3090 ppm and Ω=389 ppm for 1 (the least basic) to δ_iso=?1553 ppm and Ω=2238 ppm for 3 (the most basic)). The trends observed in ²⁰⁷Pb NMR data correlate with the coordination sphere anisotropy through comparison and quantification of the Pb? N bond lengths about the lead centre. Density functional theory calculations confirm that the more basic ligands result in greater p‐orbital character and show a strong correlation to the ²⁰⁷Pb NMR chemical shift parameters. Preliminary trends suggest that ²⁰⁷Pb NMR chemical shift anisotropy relates to the measured birefringence, given the established correlations with structure and lone‐pair activity.     

Donorstabilized Monometaphosphates and Phosphonates- Structure,Bonding, and Reactivity

Abstract

Previoudy we reported on the preparative use of donorstabilized monometaphosphates, Py.PO₂Cl and Py.PS₂Cl[¹]. ³P MAS investigations as well as X-ray single crystal analysis have been carried out. The isotropic chemical shift of the sulfur compound shows the typical downfield shift (106 ppm) with respect to the oxygen compound The chemical shift anisotropy gives additional information. The relative large spans ω (Py.PO₂Cl: 401 ppm, Py.PS₂Cl: 461 ppm) [²] reveal strong deviations from tetrahedral symmetry (ω = 0 ppm) in agreement with the structural data. The values of the skew parametem K (0.64 and 0.71 resp.) reflect the trend to axial symmetry (k = ± l). X-ray diffraction and NMR data suggest a structural model corresponding to a concentration of double bond character on the two P-X bonds. This model is consistent with the extremely short P-X bond length (1.449 Å and 1.921 Å rasp.) as well as the large P-X-P bond angles (126.4°and 123.7° resp.).     

Cross-polarization with magic-angle spinning kinetics and impedance spectroscopy study of proton mobility,local disorder,and thermal equilibration in hydrogen-bonded poly(methacrylic acid)

The ¹H–¹³C cross-polarization with magic-angle spinning (CP MAS) kinetics was studied in poly(methacrylic acid) (PMAA) having the purpose to track the links between the local order in the main chain and the proton dynamics in peripheral hydrogen bond networks. The experimental CP MAS kinetic curves were analyzed applying the models of isotropic and anisotropic spin-diffusion with thermal equilibration. The fractal dimension D_p ≈ 3 was deduced that indicates that PMAA behaves as an isotropic 3D-system. No proton conductivity in the neat PMAA was deduced from the impedance spectroscopy data analyzing the frequency dependences of the complex dielectric permittivity. The value of local order parameter S = 0.70 for CH₂ in PMAA occupies an intermediate position between 0.63 and 0.85 deduced for CH₂ sites in the main chains of poly(vinyl phosphonic acid) and poly(2-hydroxyethyl methacrylate), that is, the true proton conductor and the polymer that contains the H-bond network, however, no proton conductivity, respectively.     

Polymorphie von Bis(dineopentoxyphosphorothioyl)diselenid – Korrelation von Röntgenstruktur und MAS‐NMR‐Daten

Polymorphism of Bis(dineopentoxyphosphorothioyl)diselenide – Correlation of X‐Ray Structure and MAS NMR Data The crystal structures of two polymorphs of the title compound were determined by single‐crystal X‐ray methods and refined both at room temperature and 250 K. A triclinic and a monoclinic phase were discovered and studied. Both modifications are centrosymmetrical layer structures. The numerically clearly significant differences were observed in unit cell volumes as well as in alternating disproportions of distances of atoms being chemically and crystallographically equivalent as a result of discontinuously distributed conformational changes along the single bonds. Phase transitions were not observed by cooling up to 240 K. Lowering temperatures single crystals of both phases decompose because of the considerable anisotropy of intermolecular interaction. The small differences of molecular structure produce slightly splitted ³¹P CP MAS NMR signals. A comparison of the chemical shifts from ¹³C CP MAS NMR spectra and from quantum‐chemical calculations leads to the conclusion that the inner rotation around CH₂–C_q bonds is not frozen in the solid state.     

In situ 13C solid-state polarization transfer NMR to follow starch transformations in food

Convenience food products tend to alter their quality and texture while stored. Texture-giving food components are often starch-rich ingredients, such as pasta or rice. Starch transforms depending on time, temperature and water content, which alters the properties of products. Monitoring these transformations, which are associated with a change in mobility of the starch chain segments, could optimize the quality of food products containing multiple ingredients. In order to do so, we applied a simple and efficient in situ ¹³C solid-state magic angle spinning (MAS) NMR approach, based on two different polarization transfer schemes, cross polarization (CP) and insensitive nuclei enhanced by polarization transfer (INEPT). The efficiency of the CP and INEPT transfer depends strongly on the mobility of chain segments—the time scale of reorientation of the CH-bond and the order parameter. Rigid crystalline or amorphous starch chains give rise to CP peaks, whereas mobile gelatinized starch chains appear as INEPT peaks. Comparing ¹³C solid-state MAS NMR experiments based on CP and INEPT allows insight into the progress of gelatinization, and other starch transformations, by reporting on both rigid and mobile starch chains simultaneously with atomic resolution by the ¹³C chemical shift. In conjunction with ¹H solid-state MAS NMR, complementary information about other food components present at low concentration, such as lipids and protein, can be obtained. We demonstrate our approach on starch-based products and commercial pasta as a function of temperature and storage.     

Dynamics and Crystallization in Polydimethylsiloxane Nanocomposites

Several routes were used to achieve silicon nanocomposites. The first and second one are the melt intercalation of polydimethylsiloxane (PDMS), which is a mechanical blending of the polymer in the molten state with the untreated inorganic filler or intercalated nanoparticles. The last one is an in situ polymerization, which previously requires the intercalation of hexamethylcyclotrisiloxane (D₃) followed by a subsequent polymerization step. We used synthetic mineral oxide HTiNbO₅ as nanofiller. These systems were investigated by differential scanning calorimetry (DSC) and solid state NMR in order to better understand the relation between the nanocomposites dynamics, and crystallisation. The efficiency of grafting reactions was studied by ²⁹Si CP/MAS NMR. The nature of the interfacial interactions seems to play the major role. Indeed, the nanocomposites 1 and 2 for which only physical interactions are expected do not exhibit any T_g deviation whereas the nanocomposite 3, for which chemical grafting is achieved, increases strongly the T_g. Crystallization is more sensitive to density and strength of interfacial interactions which are maximum for the pristine filler.     

Nuclear magnetic resonance study of the ferroelastic phase transition of order-disorder type in [N(C2H5)4]2CdCl4

This study uses nuclear magnetic resonance (NMR) techniques to examine the detailed changes in [N(C₂H₅)₄]₂CdCl₄ around its phase transition at the temperature T_C = 284 K. The chemical shifts and spin-lattice relaxation times in the rotating frame (T_1ρ) were determined from ¹H magic angle spinning (MAS) NMR and ¹³C cross-polarization (CP)/MAS NMR spectra. The two sets of inequivalent ¹H and ¹³C nuclei in CH₃ and CH₂ were distinguished. A ferroelastic phase transition was observed at T_C, without structural symmetry change. The phase transition is mainly attributed to the orientational ordering of the [N(C₂H₅)₄]⁺ cations, and the spectral splitting at low temperature is associated with different ferroelastic domains.     

Solid state NMR studies of syndiotactic polystyrene/ethylbenzene and poly(ethylene oxide)/LiCF3SO3 molecular complexes

Solvent dynamics and polymer-solvent interactions in syndiotactic (s) polystyrene (PS)/ethylbenzene (PhEt) clathrates, as well as polymer-salt interactions in the poly(ethylene oxide) (PEO)/LiCF₃SO₃ complex, were characterized by solid state ¹H and ¹³C NMR. ¹H static and ¹H MAS NMR spectra have shown that PhEt molecules in s-PS clathrates retain relatively large, but spatially anisotropic mobility. ¹³C CP/MAS (cross polarization/magic angle spinning) spectra and CP dynamics measured for s-PS-dg/PhEt system indicate that at least a part of PhEt molecules are intercalated between phenyl rings of s-PS. ¹³C CP/MAS NMR spectra show that PEO carbons in complex with LiCF₃SO₃ are more shielded in comparison to neat crystalline PEO. The results (distances) obtained from CP dynamics are in agreement with the published crystal structure of the PEO/LiCF₃SO₃ complex. ¹³C spin-lattice relaxation time measurements have shown that the mobility of PEO in the complex is lower than that in neat crystalline PEO.