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MAS NMR Spectroscopy (ma + nmr_spectroscopy)
Selected AbstractsConformation analysis and molecular mobility of ethylene and tetrafluoroethylene copolymer using solid-state 19F MAS and 1H , 19F CP/MAS NMR spectroscopyMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2004Keitaro Aimi Abstract The changes in the conformation and molecular mobility accompanied by a phase transition in the crystalline domain were analyzed for ethylene (E) and tetrafluoroethylene (TFE) copolymer, ETFE, using variable-temperature (VT) solid-state 19F magic angle spinning (MAS) and 1H , 19F cross-polarization (CP)/MAS NMR spectroscopy. The shifts of the signals for fluorines in TFE units to higher frequency and the continuing decrease and increase in the T1,F values suggest that conformational exchange motions exist in the crystalline domain between 42 and 145 °C. Quantum chemical calculations of magnetic shielding constants showed that the high-frequency shift of TFE units should be induced by trans to gauche conformational changes at the CH2CF2 linkage in the E,TFE unit. Although the 19F signals of the crystalline domain are substantially overlapped with those of the amorphous domain at ambient probe temperature (68 °C), they were successfully distinguished by using the dipolar filter and spin-lock pulse sequences at 145 °C. The dipolar coupling constants for the crystalline domain, which can be estimated by fitting the dipolar oscillation behaviors in the 1H , 19F CP curve, showed a significant decrease with increasing temperature from 42 to 145 °C. This is due to the averaging of 1H19F dipolar interactions originating from the molecular motion in the crystalline domain. The increase in molecular mobility in the crystalline domain was clearly shown by VT T1,F and 1H , 19F CP measurements in the phase transition temperature range. Copyright © 2004 John Wiley & Sons, Ltd. [source] Bifunctional Heterogeneous Catalysis of Silica,Alumina-Supported Tertiary Amines with Controlled Acid,Base Interactions for Efficient 1,4-Addition ReactionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 41 2009Ken Motokura Dr. Abstract We report the first tunable bifunctional surface of silica,alumina-supported tertiary amines (SA,NEt2) active for catalytic 1,4-addition reactions of nitroalkanes and thiols to electron-deficient alkenes. The 1,4-addition reaction of nitroalkanes to electron-deficient alkenes is one of the most useful carbon,carbon bond-forming reactions and applicable toward a wide range of organic syntheses. The reaction between nitroethane and methyl vinyl ketone scarcely proceeded with either SA or homogeneous amines, and a mixture of SA and amines showed very low catalytic activity. In addition, undesirable side reactions occurred in the case of a strong base like sodium ethoxide employed as a catalytic reagent. Only the present SA-supported amine (SA,NEt2) catalyst enabled selective formation of a double-alkylated product without promotions of side reactions such as an intramolecular cyclization reaction. The heterogeneous SA,NEt2 catalyst was easily recovered from the reaction mixture by simple filtration and reusable with retention of its catalytic activity and selectivity. Furthermore, the SA,NEt2 catalyst system was applicable to the addition reaction of other nitroalkanes and thiols to various electron-deficient alkenes. The solid-state magic-angle spinning (MAS) NMR spectroscopic analyses, including variable-contact-time 13C cross-polarization (CP)/MAS NMR spectroscopy, revealed that acid,base interactions between surface acid sites and immobilized amines can be controlled by pretreatment of SA at different temperatures. The catalytic activities for these addition reactions were strongly affected by the surface acid,base interactions. [source] Drug Self-Templated Synthesis of Ibuprofen/Mesoporous Silica for Sustained ReleaseEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 19 2006Fengyu Qu Abstract The synthesis of Ibuprofen/mesoporous silica has been achieved by a drug self-templated one-step co-condensation process. The drug template of Ibuprofen, a co-template of 3-aminopropyltriethoxysilane (APTES), and tetraethoxysiliane (TEOS) were self-assembled into Ibuprofen/mesoporous silica in a neutral aqueous reaction system. This drug/mesoporous silica was characterized by powder XRD diffraction, FTIR spectroscopy, N2 adsorption/desorption, TEM, and 29Si/MAS and 13C/MAS NMR spectroscopy. The Ibuprofen/mesoporous silica system gives a well-sustained release profile. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Thermal Behavior of Tetrahydropyran-Intercalated VOPO4: Structural and Dynamics StudyEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2007Klára Melánová Abstract The thermal behavior of tetrahydropyran-(THP-)intercalated VOPO4 was probed by an extensive combination of experimental methods (XRD, DSC, FTIR, solid-state NMR) and quantum chemical calculations. Two temperature-induced transitions were detected and all polymorphs exhibit a high degree of molecular order and tight packing of THP in VOPO4. The first reversible thermal transition at around 100 °C was attributed to boat/chair conformation changes of the THP molecules. Most probably, a low-temperature boat conformation of the guest molecules present in the interlayer space of VOPO4 changes to a high-temperature chair conformation. This rearrangement of the THP molecules was confirmed by variable-temperature 13C CP/MAS NMR spectroscopy. Quantum chemical calculations using a B3LYP functional and 6-31G(d) basis set also support this idea. The second change at around 140 °C is probably caused by a weakening of the donor,acceptor bond between the oxygen molecule of THP and the vanadium atom of the host and the formation of a disorder in packing of the THP molecules. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Stereoselective Complex Formation between Polybutadiene and Cyclodextrins in BulkMACROMOLECULAR RAPID COMMUNICATIONS, Issue 11 2008Yukari Kuratomi Abstract Polybutadienes (PBs) are found to form inclusion complexes with cyclodextrins (CDs) stereoselectively to give crystalline compounds in bulk. These complexes have been isolated and characterized by means of 1H NMR and 13C CP/MAS NMR spectroscopy, and X-ray diffraction. Although , -CD did not form inclusion complexes with any kinds of PBs in aqueous solutions, , -CD did form inclusion complexes with PBs having 1,4- cis - and 1,4- trans -butadiene units in bulk by heating at 100,°C. On the other hand, PB having 79% of a 1,2-structure did not form inclusion complexes with , -CD. The yield of the inclusion complexes increases with an increase in the content of the 1,4- cis -structure of PB and decreases with the molecular weights of the PBs. [source] Conformation analysis and molecular mobility of ethylene and tetrafluoroethylene copolymer using solid-state 19F MAS and 1H , 19F CP/MAS NMR spectroscopyMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2004Keitaro Aimi Abstract The changes in the conformation and molecular mobility accompanied by a phase transition in the crystalline domain were analyzed for ethylene (E) and tetrafluoroethylene (TFE) copolymer, ETFE, using variable-temperature (VT) solid-state 19F magic angle spinning (MAS) and 1H , 19F cross-polarization (CP)/MAS NMR spectroscopy. The shifts of the signals for fluorines in TFE units to higher frequency and the continuing decrease and increase in the T1,F values suggest that conformational exchange motions exist in the crystalline domain between 42 and 145 °C. Quantum chemical calculations of magnetic shielding constants showed that the high-frequency shift of TFE units should be induced by trans to gauche conformational changes at the CH2CF2 linkage in the E,TFE unit. Although the 19F signals of the crystalline domain are substantially overlapped with those of the amorphous domain at ambient probe temperature (68 °C), they were successfully distinguished by using the dipolar filter and spin-lock pulse sequences at 145 °C. The dipolar coupling constants for the crystalline domain, which can be estimated by fitting the dipolar oscillation behaviors in the 1H , 19F CP curve, showed a significant decrease with increasing temperature from 42 to 145 °C. This is due to the averaging of 1H19F dipolar interactions originating from the molecular motion in the crystalline domain. The increase in molecular mobility in the crystalline domain was clearly shown by VT T1,F and 1H , 19F CP measurements in the phase transition temperature range. Copyright © 2004 John Wiley & Sons, Ltd. [source] Long-Range Correlations between Aliphatic 13C Nuclei in Protein MAS NMR Spectroscopy,ANGEWANDTE CHEMIE, Issue 31 2009Marvin Ein hocheffizienter Polarisationstransfer in MAS-NMR-Experimenten (MAS: Rotation im magischen Winkel) ist möglich durch eine Kombination von alternierenden 13C-Markierungen und bandselektiver Radiofrequenz(RF)-getriebener Wiedereinkopplung (BASE RFDR). Dabei wird die Bandselektivität schwacher RF-Strahlung genutzt, um die homonucleare Dipolwechselwirkung wieder einzuführen. [source] Direct Synthesis of Zr-Containing Hybrid Periodic Mesoporous Organosilicas with Tunable Zirconium ContentEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 35 2007Shang-Ru Zhai Abstract Highly ordered Zr-containing periodic mesoporous organosilicas (ZrPMO) with different Zr/Si ratios were successfully synthesized, for the first time, by employing a ZrOCl2/NaCl combination as the promoting agent and by simply adjusting the molar ratio of the zirconium species to the organosilica precursor; no addition of mineral acids was necessary. The effect of preparation parameters on the structural and textural properties were carefully investigated by using different ratios of NaCl/Si and Zr/Si. It was found that both salts are essential for this system and highly ordered ZrPMOs can be prepared within fairly wide Si/Zr ratios (5,100) while keeping the NaCl/Si ratio constant. To prove the effectiveness of this synthetic pathway with a higher Zr incorporation, ZrPMO materials were also synthesized under strongly acidic conditions for the purpose of comparison. The synthesized ZrPMO materials were thoroughly characterized by ICP-AES, SAXS, N2 sorption, TEM, SEM, 13C CP/29Si MAS NMR spectroscopy, XPS, and TGA. Elemental analyses show that the amount of Zr incorporated into ZrPMO, which was synthesized under mild conditions, is greater than that obtained in a strongly acidic environment, and the Zr content, with a Si/Zr ratio up to 12, is close to that in the initial gel composition. A plausible assembly mechanism based on the synergistic effect of both "nonhydrolyzable" (NaCl) and "hydrolyzable" (ZrOCl2) inorganic salts was discussed in detail, where the "salting out" effect and self-generated acidity from both inorganic salts, respectively, are believed to be key factors for the formation of ordered SBA-15-type ZrPMO materials under the synthetic conditions.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Incorporation of a (Cyclopentadienyl)molybdenum Oxo Complex in MCM-41 and Its Use as a Catalyst for Olefin EpoxidationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2004Marta Abrantes Abstract The tricarbonyl complex [(,5 -C5H4 -COOMe)Mo(CO)3Cl] was prepared from the reaction of sodium (methoxycarbonyl)cyclopentadienide, (C5H4 -CO2Me)Na, with (Bu4N)[Mo(CO)5I]. Heating the ester with 3-(triethoxysilyl)propylamine gave the amide derivative {[,5 -C5H4 -CONH-C3H6Si(OEt)3]Mo(CO)3Cl}. The functionalised tricarbonyl complex was immobilised in the ordered mesoporous silica MCM-41 with a loading of 13 wt.-% Mo (1.4 mmol·g,1) by carrying out a grafting reaction in dichloromethane. Powder X-ray diffraction and nitrogen adsorption,desorption analysis indicated that the structural integrity of the support was preserved during the grafting and that the channels remained accessible, despite significant reductions in surface area, pore volume and pore size. The success of the coupling reaction was confirmed by 29Si and 13C (CP) MAS NMR spectroscopy. A supported dioxo complex of the type [(,5 -C5H4R)MoO2Cl] was subsequently prepared by oxidative decarbonylation of the tethered tricarbonyl complex using tert -butyl hydroperoxide (TBHP). The oxidised material is an active catalyst for the liquid phase epoxidation of cyclooctene with TBHP as the oxygen source. Similar catalytic results were obtained using the tethered tricarbonyl complex directly as a pre-catalyst since fast oxidative decarbonylation occurs under the reaction conditions used. For both systems, the desired epoxide was the only product and the initial activities were about 13 mol·molMo,1·h,1. The solid catalysts were recycled several times. Some activity was lost between the first and second runs but thereafter tended to stabilise. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Novel Phosphovanadate Layered Structure Assembled from a Tetrametallic Cubane-Like VV ClusterEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2004Fa-Nian Shi Abstract A novel phosphovanadate layered structure intercalated by 4,4,-bipyridinium cations, (C10H10N2)[(VO2)4(PO4)2], was synthesised under hydrothermal conditions and its crystal structure determined using single-crystal X-ray diffraction. The anionic [(VO2)4(PO4)2]n2n, perforated layers are strongly hydrogen bonded to the interlayer 4,4,-bipyridinium cations and are assembled by an unprecedented secondary tetrametallic VV building unit constructed from a distorted cubane-like {V4O4} cluster. The compound was further characterised by IR, Raman, 1H, 1H- 13C, 31P and 51V MAS NMR spectroscopy, and by thermal and elemental analysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Crystal structures and cation ordering in Cs2MgSi5O12, Rb2MgSi5O12 and Cs2ZnSi5O12 leucitesACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2009A. M. T. Bell The crystal structures of the leucite analogues Cs2MgSi5O12, Cs2ZnSi5O12 and Rb2MgSi5O12 have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with 29Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs2CdSi5O12, thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. 29Si MAS NMR shows five distinct Si T sites for Cs2MgSi5O12 and Rb2MgSi5O12, but six Si T sites for Cs2ZnSi5O12. The refined structures for Cs2MgSi5O12 and Rb2MgSi5O12 were determined with complete T -site ordering, but the refined structure for Cs2ZnSi5O12 was determined with partial disorder of Mg and Si over two of the T sites. [source] New Insights into the Formation and Reactivity of Molecular Organostannonic AcidsCHEMISTRY - AN ASIAN JOURNAL, Issue 1 2010S. Usman Ahmad Abstract The controlled base hydrolysis of 2,6-Mes2C6H3SnCl3 (1; Mes=mesityl) provided 2,6-Mes2C6H3Sn(OH)Cl2,H2O (2) and the trinuclear organostannonic acid trans -[2,6-Mes2C6H3Sn(O)OH]3 (3), respectively. In moist C6D6, 3 reversibly reacts with water to give the monomeric organostannonic acid 2,6-Mes2C6H3Sn(OH)3 (3,a). The reaction of 3 with (tBu2SnO)3, Ph2PO2H, and NaH, gives rise to the multinuclear hypercoordinated organostannoxane clusters [tBu2Sn(OH)OSnR(OH)2OC(OSntBu2OH)2(O)SnR(OH)(H2O)]2 (5), [RSn(OH)2(O2PPh2)]2 (6), and Na3(RSn)4O6(OH)3 (7), respectively (R=2,6-Mes2C6H3). The characterization of the new compounds is achieved by multinuclear NMR spectroscopy and electrospray mass spectrometry in solution and 119Sn MAS NMR spectroscopy, IR spectroscopy, and X-ray crystallography in the solid-state. [source] Synthesis of Large-Pore Urea-Bridged Periodic Mesoporous OrganosilicasCHEMISTRY - AN ASIAN JOURNAL, Issue 4 2009Ling Gao Abstract Bringing order: A new class of periodic mesoporous organosilicas (PMOs) with a urea-bridged organosilica precursor under acid-catalyzed and inorganic-salt-assisted conditions was obtained. The large-pore hybrid materials have ordered mesostructure with uniform pore size distributions, which can be seen from the TEM images. In this article we report the successful synthesis of a new class of periodic mesoporous organosilicas (PMOs) with a urea-bridged organosilica precursor under acid-catalyzed and inorganic-salt-assisted conditions. The large-pore hybrid materials have an ordered mesostructure with uniform pore size distributions, excellent thick framework walls, thermal stability, and specific functional groups. The composition of the mesoporous organosilicas was characterized by FTIR spectroscopy, 13C cross-polarization magic-angle spinning (CP,MAS) NMR spectroscopy, and 29Si MAS NMR spectroscopy. [source] | ||||||||||