Interaction of the perfluorinated polymer Nafion-H with water studied by proton broad-line NMR at 4 K and MAS NMR at room temperature

Broad-line¹H NMR at 4 K shows that, when Nafion-H interacts with water molecules at low concentrations, the only species formed is the hydronium ion. The chemical shift of the unsolvated hydronium ion in Nafion-H with 1 H₂O/SO ₃ ^– , determined by¹H MAS NMR at room temperature, is 10.4 ppm relative to external TMS.     

Comparison of the acidity of dealuminated and non-dealuminated HY zeolite by two1H-NMR methods: wide line at 4 K and high resolution MAS at 300 K

¹H-Wide-Line and MAS-NMR at 4 and 300 K, respectively, for HY zeolite samples, non-dealuminated and partially dealuminated with (NH₄)₂SiF₆ reveal significant differences.     

FTIR spectroscopic and1 H MAS NMR studies of the influence of lattice chemistry and structure on Brønsted acidity in zeolites

The influence of the Si/Al ratio, of the nature of the T-atom and of the pore size on the acidic strength of Brønsted sites in zeolites has been investigated using changes of the vibrational properties of Brønsted OH(OD) groups and a shift change of Brønsted protons in nuclear magnetic resonance upon adsorption of weak bases. Deuterated acetonitrile and trichloro-acetonitrile have been chosen to probe the acidic strengths of ZSM-5, FeZSM-5, mordenite and zeolite Y, which are often used as catalysts. From the results of the FTIR and 1 H MAS NMR studies it can be concluded that the chemical composition of the lattice dominates the acidic strength of the Brønsted sites in zeolites. Differences in structure or pore size play a much smaller role.     

Proton NMR studies on concentrated aqueous sulfuric acid solutions and Nafion-H

Sulfuric acid containing limited amounts of water, H₂SO_4.nH₂O with 0.23≤n≤4, has been studied by ¹H broad-line NMR at 4 K and MAS NMR at room temperature. The broad-line NMR spectra indicate the formation of H₃O⁺ and HSO₄ ^- ions. H₂SO_4.2H₂O is correctly written as H₃O⁺HSO₄ ^-.H₂O. The results are compared with the Nafion- H/water system. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interaction between CD3OH and a H-mordenite studied by 1H NMR, broad-line at 4 K and MAS at 300 K

The simulation of 4 K ¹H broad-line NMR spectra of CD₃OH interacting with the acidic OH groups of H-mordenite zeolite shows no CD₃OH₂ ⁺ ions but hydrogen-bonded complexes. For high CD₃OH concentrations, clusters of hydrogen-bonded methanol molecules are formed, also hydrogen-bonded to Brønsted acid sites of the zeolite. The 300 K ¹H MAS spectra prove that the desorbed H-mordenite sample contains also silanol groups in electrostatic interaction with the framework; these silanols do not interact with methanol under our experimental conditions.     

Brønsted acidity of amorphous silica–aluminas studied by 1H NMR

¹H broad-line (4 K) and MAS (room temperature) NMR have been used to study the acid strength of two amorphous silica–aluminas interacting or not with adsorbed water. The study is more difficult than for zeolites, because the acidic SiO(H)Al bridges are reversibly destroyed by dehydration. However, an acidity coefficient value (H₃O⁺ concentration per Brønsted acid site when one water molecule interacts with each Brønsted site) of 0.34±10%; has been determined. This value is equal to that obtained for H-faujasite and H-mordenite samples with Si/Al ratios high enough for maximum acid strength.     

Diffusional behavior of amino acids in solid-phase reaction field as studied by H pulsed-field-gradient spin-echo NMR method

The diffusion coefficients (D) of tert-butyloxycarbonyl-l-phenylalanine (Boc-Phe) in Merrifield network polystyrene gels, used as a solid-phase reaction field have been determined as a function of the amino acid concentration over the temperature range from 30 to 50 °C by means of the ¹H pulsed-field-gradient spin-echo NMR method. From these experimental results, it was found that the D value of Boc-Phe in DMF-d₇ solution, in DVB 1 and 2% cross-linked network polystyrene gels depends on the amino acid concentration. The D value of Boc-Phe·Cs(tert-butyloxycarbonyl-l-phenylalanine cesium) salt in the solid-phase reaction field under chemical reaction was determined at 50 °C. Further, it was found that the D value depends on the NMR observation time, that is the applied two field-gradient pulse interval. Details of its analysis were discussed.     

1H broad-line and MAS NMR: application to the study of acid sites of desilicated zeolite ZSM-5

Broad-line¹H NMR study of desilicated zeolite ZSM-5 was carried out as a function of the number of adsorbed water molecules in amounts lower than or equal to that of the Brønsted acid sites. The dissociation coefficient of the acid OH groups, currently associated with the acid strength, was shown not to be affected by the selective removal of Si from the zeolite ZSM-5 framework which resulted in more Brønsted acid sites per unit surface area. On the other hand, by using MAS NMR, bridging Brønsted acid sites hydrogen-bonded to the zeolite framework were identified on the anhydrous surface of the desilicated ZSM-5. Moreover, MAS NMR spectra of the desilicated zeolite partially rehydrated showed the presence of some Lewis acid sites.     

Reinforcement of model filled elastomers: characterization of the cross-linking density at the filler-elastomer interface by H NMR measurements

The cross-linking density at the filler-elastomer interface is analyzed by ¹H NMR measurements in model reinforced elastomers composed by grafted nanosilica particles and cross-linked ethylacrylate chains. We have focused our attention on the effect of introducing fillers on the relaxation of the bulk polymer matrix which is observed at long times (t>100 μs). Measurements performed at high temperature (T>T_g+120 K) have revealed that its relaxation is affected by the topological constraints existing at the particle surface. We deduce that the effect of particles in the bulk matrix can be interpreted as that of an homogeneous additional constraint density which increases proportionally to the surface area introduced in the matrix.     

Molecular model of Estonian kukersite kerogen evaluated by C MAS NMR spectra

The chemical structure of Estonian kukersite kerogen is evaluated using a simulation of ¹³C MAS NMR spectrum. A reasonable fit to the experimental NMR spectrum is obtained by assuming a model of the geomacromolecule with empirical formula C₄₂₁H₆₃₈O₄₄S₄NCl and a set of structural elements comprising mainly alkylated phenolic structures particularly alkyl-1,3-benzenediols and condensed alicyclic rings. From the presented model new views are coming up on the carbon skeleton of kerogen and constraints on the phenol formation pathways in the retorting process, i.e. up to 80% of methylene groups in kerogen are located in aliphatic chains and the complicated mixture of phenols in the retort oil seems to result mainly from the thermal conversion of alkyl-1,3-benzenediol units originally present in kerogen.     

Gasoline composition determined by H NMR spectroscopy

A fast and simple ¹H NMR spectroscopic method was developed for the concentration measurement of aromatic, olefinic and aliphatic fractions in gasoline samples. Simultaneously, individual components such as benzene, methyl tert-butyl ether and several dienes could be determined. The method relies on only a few, well-established assumptions about the molecular compositions and is therefore applicable to a broad range of gasolines. It is well suited for laboratories with high sample throughput since measurement time is short and all concentrations are determined within one NMR experiment. The method was applied to commercial gasolines and samples used in European round robin tests. Comparisons of NMR and round robin test results showed excellent agreement.     

Quantitative Si MAS NMR spectroscopy of cement and silica fume containing paramagnetic impurities

The low natural abundance and the long spin lattice relaxation time of ²⁹Si lead to long measurement times and/or low signal-to-noise ratios using ²⁹Si magic angle spinning NMR spectroscopy. By contrast, samples containing paramagnetic iron ions have much shorter relaxation times, making measurements up to seven times more efficient, but at the same time making quantitative analysis unreliable. To solve the problem, the spin-lattice relaxation times of ordinary Portland cement (opc) and silica fume with and without iron content has been determined with inversion recovery experiments. The effect of varying the spectrum repetition time on the quantitative analysis is demonstrated for mixtures of opc with silica fume. For opc and silica fume with iron impurities repetition times as short as 5 s has permitted accurate quantitative analysis of the silicates present in these materials.     

A novel method for determination of polyester end-groups by NMR spectroscopy

An efficient, convenient and quantitative method for characterising polyester end-groups is described. We have found that trichloroacetyl isocyanate (TAI) reacts rapidly and quantitatively with both carboxyl [C(O)OH] and hydroxyl (OH) chain ends to form derivatives that can be readily determined by ¹H NMR spectroscopy. The TAI capped end-groups give rise to characteristic imidic NH resonances in a normally clear region of the ¹H NMR spectrum [δ∼10-11.5 for C(O)-O-C(O)-NH-C(O)CCl₃ from C(O)OH, δ∼8-9 for O-C(O)-NH-C(O)CCl₃ from OH]. The method has been successfully applied to quantitative determination of the end-groups of a wide variety of oligomeric polyesters. It has also been applied to higher molecular weight polyesters including commercial, bottle grade, poly(ethylene terephthalate) (PET) and PET based copolyesters (e.g. PETG).     

A simple method for determining protic end-groups of synthetic polymers by H NMR spectroscopy

A simple method for the determination of protic end-groups (-XH) in synthetic polymers involves in situ derivatization with trichloroacetyl isocyanate (TAI) in an NMR tube and observation of the imidic hydrogens of the derivatized products [-X-C(O)-NH-COCCl₃] by ¹H NMR spectroscopy. In this paper, we report that the method is effective for the quantitative determination of hydroxy, primary amino and carboxy end-groups of polymers with . It may also be applied to detect chain ends in higher molecular weight polymers. The signals for the imidic (and, in the case of amines, amidic) hydrogens appear in a region (δ 7.5-11) that is clear of other signals in the case of most aliphatic polymers and many aromatic polymers such as polystyrene and poly(ethylene terephthalate). The method has been applied in the characterization of polymers formed by conventional and living radical polymerization (RAFT, ATRP, NMP), to end functional poly(ethylene oxide) and to polyethylene-block-poly(ethylene oxide). The method appears less effective in the case of sulfanyl end-groups. The chemical shift of the imidic hydrogen shows remarkable sensitivity to the microenvironment of chain end. Thus, the imidic hydrogens of TAI derivatized polyethylene-block-poly(ethylene oxide) [PE-(EO)_mOC(O)NHC(O)CCl₃] are at least partially resolved for m=0, 1, 2, 3 and ≧4 in the 400 MHz ¹H NMR spectrum. It is also sensitive to the chain end tacticity of, for example, amino-end-functional polystyrenes and thus to the relative configuration of groups removed from the chain-end by two or more monomer units. TAI derivatization also facilitates analysis of amine functional polymers by gel permeation chromatography (GPC) which is often rendered difficult by specific interactions between the amine group and the GPC column packing.     

17O MAS NMR study of 12-molybdophosphoric acid

¹⁷O MAS NMR spectra for¹⁷O enriched solid heteropoly acid H₃PMo₁₂O₄₀ are reported. The oxygen exchange between solid H₃PMo₁₂ ¹⁷O₄₀ and H₂ ¹⁶O vapor at 200–250°C is shown to be accompanied by fast mixing of terminal Mo=O and bridging Mo-O-Mo oxygens in the Keggin unit. The oxygen exchange in H₃PMo₁₂ ¹⁷O₄₀ is much faster than in H₃PW₁₂ ¹⁷O₄₀ in solution as well as in the gas phase.     

High-resolution F and H NMR of a vinylidenefluoride telomer

The reaction products of vinylidenefluoride (VDF) with methanol as a telogen have been analysed in the solution state by ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectroscopy. High-resolution ¹⁹F and ¹H NMR spectra were achieved using high-power ¹H and ¹⁹F decoupling, respectively, giving superior resolution and revealing previously unresolved signals of the vinylidenefluoride telomer (VDFT). ¹H and ¹⁹F homo- and hetero-nuclear scalar coupling constants are presented and the spectra of functional groups and reverse units (including the identification of short-chain structures) are discussed. Furthermore, the application of ¹⁹F or ¹H decoupling for the correct assessment of reverse-unit content and degree of polymerisation is demonstrated. This work highlights the need for high-resolution NMR spectroscopy to determine both the chemical structure and the composition of these important fluoropolymers.     

Surface complexes in zeolite-catalysed acylation reactions detected by13C MAS NMR spectroscopy

The state of the acylating agent acetyl chloride, adsorbed on a series of proton and metal ion exchanged zeolites X, Y (faujasite) and ZSM-5, was investigated by¹³C MAS NMR spectroscopy. The observed carbonyl signals were assigned to two species: chemisorbed acetyl chloride bound to lattice oxygen (signals near 182 ppm), and acetyl chloride complexed with counter cations in the lattice (signals near 172 ppm). In a few cases signals were observed which have been assigned to free acylium cation stabilized on the surface of the solid (signal 160–165 ppm). Experiments in which toluene was adsorbed on to ZnY pretreated with acetyl chloride showed the participation of various adsorbed species in the acylation reaction of toluene.     

Strain-induced crystallization of natural rubber as studied by high-resolution solid-state C NMR spectroscopy

A series of high-resolution solid-state ¹³C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the ¹³C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the ¹³C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. ¹³C spin-lattice relaxation time (T₁) and ¹H spin-spin relaxation time (T₂) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative ¹³C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.     

Zr K-Edge XAS and 29Si MAS NMR Studies on Hexagonal Mesoporous Zirconium Silicate

Hexagonal mesoporous silicate (HMS) molecular sieve containing zirconium species (Zr-HMS) has been synthesized using a neutral template of hexadecylamine with ethanol and water. A disordered channel structure with a uniform diameter of 44.1 Å was confirmed by powder X-ray diffraction after calcination. From N₂ adsorption/desorption measurement, it is found that Zr-HMS has a very high surface area of about 910 m²/g (BET) and a mesopore with a size of 21 Å (BJH). According to the X-ray absorption spectroscopy at the Zr K-edge, it is confirmed that Zr atoms are coordinated by six oxygen neighbors with a distance of 2.13 Å, which is shorter than the (Zr—O) bond distance found in ZrO₂ or ZrOCl₂·8H₂O. The ²⁹Si MAS-NMR spectrum for Zr-HMS shows two additional peaks around –94.6 and –107.2 ppm due to the Q³Zr and Q⁴Zr components, respectively, besides three peaks at –91.1, –101.5, and –111.3 ppm due to the silicate network. The relation between the pore and crystal structures and the local environment around Zr is discussed in detail.     

MAS NMR studies of ZSM-5 zeolites: Correlation to para selectivity and SEM observations

²⁷Al and²⁹Si MAS NMR studies of various ZSM-5 zeolites of different Si/Al ratio, crystal size synthesized with and without alkali metal salt addition by the hydrothermal technique were carried out. The NMR observations clearly showed that the technique can be employed to differentiate Si/Al ratio, framework tetrahedral content and the crystal size. The last observation has been confirmed by SEM and catalytic activity test for para ethylation of ethylbenzene. A good correlation was found between the catalytic activity and the data from NMR and SEM.