Aaron's journal searches

-General interest 31P NMR. Nice distance measurements via spin diffusion.

Chem. Mater., 18 (26), 6333 -6338, 2006. 10.1021/cm0613410 S0897-4756(06)01341-X Web Release Date: November 24, 2006
Detection of Nanometer-Scale Mixing in Phosphate-Glass/Polyamide-6 Hybrids by 1H-31P NMR
A. Rawal, K. Urman, J. U. Otaigbe,* and K. Schmidt-Rohr*
Abstract: The size of phosphate-glass (Pglass) particles dispersed in a polyamide-6 (PA6) matrix by melt blending has been characterized by 1H-31P solid-state nuclear magnetic resonance (NMR). 1H spin diffusion from ~75% of the 1H in the glass to the polyamide is observed within 50 ms, indicating proximity on a 30 nm scale. Fast dephasing of a quarter of the 31P magnetization by dipolar couplings to polyamide protons in 31P{1H} heteronuclear recoupling with dephasing by strong homonuclear interactions of protons (HARDSHIP) NMR shows that ~25% of the Pglass is within 0.5 nm from the polyamide. This is confirmed by 1H-31P heteronuclear correlation NMR spectra with inverse T2,H filtering, which document relatively fast (1 ms) cross polarization from PA6 protons, identified by their upfield chemical shift and short transverse relaxation time T2,H, to a significant fraction of 31P in the glass. The 31P spectrum associated with the polyamide 1H reveals that the phosphate sites near the polyamide matrix are chemically altered but differently than previously observed in Pglass-polyethylene hybrids, where no such contact was proven. As expected, the 31P sites that cross-polarize from the polyamide protons also exhibit pronounced dephasing in 31P{1H} HARDSHIP experiments. HARDSHIP experiments after cross polarization and 31P spin diffusion experiments indicate that the ~25% of phosphate that is within 0.5 nm from the polyamide is not dispersed in the polymer but on the surface of ~10 nm diameter Pglass particles. This study represents the first conclusive evidence of intimate mixing of the hybrid components. Further, it suggests that this is an excellent model system for exploring new routes for driving organic polymers and inorganic glass to self-assemble into useful organic/inorganic hybrid materials

-Interesting study on a drug inside microporous materials

Chem. Mater., 18 (26), 6382 -6390, 2006. 10.1021/cm061551c S0897-4756(06)01551-1 Web Release Date: December 19, 2006
Solid-State NMR Study of Ibuprofen Confined in MCM-41 Material
Thierry Azaïs,* Corine Tourné-Péteilh, Fabien Aussenac, Niki Baccile, Cristina Coelho, Jean-Marie Devoisselle,* and Florence Babonneau
Abstract: Ibuprofen (an anti-inflammatory drug that is a crystalline solid at ambient temperature) has been encapsulated in MCM-41 silica matrices with different pore diameters (35 and 116 Å). Its behavior has been investigated by magic angle spinning (MAS) 1H, 13C, and 29Si solid-state NMR spectroscopy at ambient and low temperature. This study reveals an original physical state of the drug in such materials. At ambient temperature, ibuprofen is not in a solid state (crystalline or amorphous) and is extremely mobile inside the pores, with higher mobility in the largest pores (116 Å). The interaction between ibuprofen and the silica surface is weak, which favors fast drug release from this material in a simulated intestinal or gastric fluid. The quasi-liquid behavior of ibuprofen allows the use of NMR pulse sequences issued from solution-state NMR, such as the INEPT sequence, to characterize these solid-state samples. The solid-state MAS NMR study shows that the proton of the carboxylic acid group of ibuprofen is in a chemical exchange at ambient temperature. Furthermore, at low temperature (down to 223 K), NMR spectroscopy results show that ibuprofen is able to crystallize inside the largest pores (116 Å), whereas a glassy state is obtained for the smallest ones (35 Å).

-91Zr and 29Si studies of surface coated metal oxides. A couple of cites of our work on 91Zr. NMR done by Klaus Muller

Chem. Mater., 18 (25), 6019 -6030, 2006. 10.1021/cm061403m S0897-4756(06)01403-7 Structural Evolution upon Thermal Heating of Nanostructured Inorganic-Organic Hybrid Materials to Binary Oxides MO2-SiO2 (M = Hf, Zr) as Evaluated by Solid-State NMR and FTIR Spectroscopy
Lidia Armelao, Silvia Gross,* Klaus Müller,* Giuseppe Pace, Eugenio Tondello, Otgontuul Tsetsgee, and Andrea Zattin
Abstract:
The structural and compositional evolution of inorganic-organic silica-based hybrid materials and their conversion to mixed oxides, consisting of host silica with variable amounts of zirconia or hafnia (1:1 Si:M, 40:1 Si:M, M = Zr, Hf), was studied by multinuclear solid-state NMR spectroscopy, FTIR spectroscopy, and thermogravimetry. It is shown that the as-prepared materials, composed of M4O2(OMc)12 oxoclusters (M = Zr, Hf; OMc = methacrylate) and methacryloxymethyltriethoxysilane (MAPTMS), are not completely polymerized. Rather, a finite number of double bonds remain independent of the actual sample composition. Thermolysis between 300 and 500 C is accompanied by drastic changes in the structural composition, as reflected by the complete loss of all organic components. The incorporation of oxoclusters is found to catalyze the cleavage of Si-C bonds. At the same time, continuous alterations in the inorganic/oxide part are registered. The degree of condensation, as derived from the 29Si NMR data, increases steadily with calcination temperature and is found to depend on the oxocluster amount. Thus, for the samples with the highest oxocluster content, a pronounced temperature dependence for the degree of condensation is observed that is absent for the samples with a small oxocluster content or for pure MAPTMS. 91Zr NMR measurements on the sample with the highest zirconium content indicate that upon calcination at 1000 C, only a small amount of crystalline zirconia with tetragonal coordination exists. The major zirconia fraction distributed in the silica matrix is therefore in an amorphous state.


-Rod, 55Mn studies on Manganese Carbonyls

Inorg. Chem., 45 (21), 8492 -8499, 2006. 10.1021/ic0608445 S0020-1669(06)00844-5
Ultrahigh-Field NMR Spectroscopy of Quadrupolar Transition Metals: 55Mn NMR of Several Solid Manganese Carbonyls
Kristopher J. Ooms, Kirk W. Feindel, Victor V. Terskikh, and Roderick E. Wasylishen*
Abstract: 55Mn NMR spectra acquired at 21.14 T ( L(55Mn) = 223.1 MHz) are presented and demonstrate the advantages of using ultrahigh magnetic fields for characterizing the chemical shift tensors of several manganese carbonyls: 5-CpMn(CO)3, Mn2(CO)10, and (CO)5MnMPh3 (M = Ge, Sn, Pb). For the compounds investigated, the anisotropies of the manganese chemical shift tensors are less than 250 ppm except for 5-CpMn(CO)3, which has an anisotropy of 920 ppm. At 21.14 T, one can excite the entire mI = 1/2 mI = -1/2 central transition of 5-CpMn(CO)3, which has a breadth of approximately 700 kHz. The breadth arises from second-order quadrupolar broadening due to the 55Mn quadrupolar coupling constant of 64.3 MHz, as well as the anisotropic shielding. Subtle variations in the electric field gradient tensors at the manganese are observed for crystallographically unique sites in two of the solid pentacarbonyls, resulting in measurably different CQ values. MQMAS experiments are able to distinguish four magnetically unique Mn sites in (CO)5MnPbPh3, each with slightly different values of iso, CQ, and Q.


-Solid-state 119Sn NMR

Inorg. Chem., 45 (22), 9132 -9136, 2006. 10.1021/ic060983m S0020-1669(06)00983-9
Solid-State 119Sn NMR and Mössbauer Spectroscopy of "Distannynes": Evidence for Large Structural Differences in the Crystalline Phase
Geoffrey H. Spikes, Jason R. Giuliani, Matthew P. Augustine, Israel Nowik, Rolfe H. Herber, and Philip P. Power*
Abstract: The "distannynes" Ar'SnSnAr' (Ar' = C6H3-2,6(C6H3-2,6-Pri2)2) and Ar*SnSnAr* (Ar* = C6H3-2,6(C6H2-2,4,6-Pri3)2) were examined by solid-state 119Sn NMR and Mössbauer spectroscopy. The two compounds display substantially different spectroscopic parameters, while differing only in the absence (Ar'SnSnAr') or presence (Ar*SnSnAr*) of a para-Pri group in the flanking aryl rings of their terphenyl substituents. The spectroscopic differences can be interpreted in terms of a more trans-bent geometry and a longer Sn-Sn bond for Ar*SnSnAr* in comparison to the wider Sn-Sn-C angle (125.24(7) ) and shorter Sn-Sn bond length (2.6675(4)Å) determined from the crystal structure of Ar'SnSnAr'. The differences are consistent with previously published calculations by Nagase and Takagi for Ar*SnSnAr*.



-23Na, 27Al, and 19F SSNMR. WIEN2K calculations.

Inorg. Chem., 45 (25), 10215 -10223, 2006. 10.1021/ic061348j S0020-1669(06)01348-6
Multinuclear High-Resolution NMR Study of Compounds from the Ternary System NaF-CaF2-AlF3: from Determination to Modeling of NMR Parameters
C. Martineau,* M. Body, C. Legein, G. Silly, J.-Y. Buzaré, and F. Fayon
Abstract:27Al and 23Na NMR satellite transition spectroscopy and 3Q magic-angle-spinning spectra are recorded for three compounds from the ternary NaF-CaF2-AlF3 system. The quadrupolar frequency Q, asymmetry parameter Q, and isotropic chemical shift iso are extracted from the spectrum reconstructions for five aluminum and four sodium sites. The quadrupolar parameters are calculated using the LAPW-based ab initio code WIEN2k. It is necessary to perform a structure optimization of all compounds to ensure a fine agreement between experimental and calculated parameters. By a comparison of experimental and calculated values, an attribution of all of the 27Al and 23Na NMR lines to the crystallographic sites is achieved. High-speed 19F NMR MAS spectra are recorded and reconstructed for the same compounds, leading to the determination of 18 isotropic chemical shifts. The superposition model developed by Bureau et al. is used, allowing a bijective assignment of the 19F NMR lines to the crystallographic sites.



-Dave Bryce, 31P NMR.

Inorg. Chem., 45 (25), 10293 -10299, 2006. 10.1021/ic061021i S0020-1669(06)01021-4
A Chelate-Stabilized Ruthenium( -pyrrolato) Complex: Resolving Ambiguities in Nuclearity and Coordination Geometry through 1H PGSE and 31P Solid-State NMR Studies
Heather M. Foucault, David L. Bryce,* and Deryn E. Fogg*
Abstract: Reaction of RuCl2(PPh3)3 with LiNN' (NN' = 2-[(2,6-diisopropylphenyl)imino]pyrrolide) affords a single product, with the empirical formula RuCl[(2,6-iPr2C6H3)N=CHC4H3N](PPh3)2. We identify this species as a -pyrrolato complex, [Ru(NN')(PPh3)2]2( -Cl)2 (3b), rather than mononuclear RuCl(NN')(PPh3)2 (3a), on the basis of detailed 1D and 2D NMR characterization in solution and in the solid state. Retention of the chelating, -bound iminopyrrolato unit within 3b, despite the presence of labile (dative) chloride and PPh3 donors, indicates that the chelate effect is sufficient to inhibit isomerization of 3b to a piano-stool, -pyrrolato structure. 2D COSY, SECSY, and J-resolved solid-state 31P NMR experiments confirm that the PPh3 ligands on each metal center are magnetically and crystallographically inequivalent, and 31P CP/MAS NMR experiments reveal the largest 99Ru-31P spin-spin coupling constant (1J(99Ru,31P) = 244 ± 20 Hz) yet measured. Finally, 31P dipolar-chemical shift spectroscopy is applied to determine benchmark phosphorus chemical shift tensors for phosphine ligands in hexacoordinate ruthenium complexes.



-Massiot has some crazy 19F NMR specra!!

Inorg. Chem., 45 (26), 10636 -10641, 2006. 10.1021/ic061339a S0020-1669(06)01339-5
19F High Magnetic Field NMR Study of -ZrF4 and CeF4: From Spectra Reconstruction to Correlation between Fluorine Sites and 19F Isotropic Chemical Shifts
C. Legein,* F. Fayon, C. Martineau, M. Body, J.-Y. Buzaré, D. Massiot, E. Durand, A. Tressaud, A. Demourgues, O. Péron, and B. Boulard
Abstract:High magnetic field and high spinning frequency one- and two-dimensional one-pulse MAS 19F NMR spectra of -ZrF4 and CeF4 were recorded and reconstructed allowing the accurate determination of the 19F chemical shift tensor parameters for the seven different crystallographic fluorine sites of each compound. The attributions of the NMR resonances are performed using the superposition model for 19F isotropic chemical shift calculation initially proposed by Bureau et al. (Bureau, B.; Silly, G.; Emery, J.; Buzaré, J.-Y. Chem. Phys. 1999, 249, 85-104). A satisfactory reliability is reached with a root-mean-square (rms) deviation between calculated and measured isotropic chemical shift values equal to 1.5 and 3.5 ppm for -ZrF4 and CeF4, respectively.



-27Al and 19F SSNMR

Inorg. Chem., 45 (26), 10807 -10814, 2006. 10.1021/ic061493x S0020-1669(06)01493-5
F/Cl-Exchange on AlCl3-Pyridine Adducts: Synthesis and Characterization of trans-Difluoro-tetrakis-pyridine-aluminum-chloride, [AlF2(Py)4]+Cl-
Anton Dimitrov, Detlef Heidemann, and Erhard Kemnitz*
Abstract: Whereas liquid CCl3F reacts with solid AlCl3 exothermically under chlorine-fluorine-exchange already above -20 C, no reaction takes place between CCl3F and the pyridine complexes of AlCl3 (AlCl3·Py, AlCl3·2Py, or AlCl3·3Py) up to 100 C. The desired chlorine by fluorine substitution on the monomer AlCl3-pyridine adducts occurs, however, easily using Me3SiF as fluorinating agent. By reacting AlCl3·3Py with Me3SiF (even up to 10-fold stoichiometric excess) in pyridine as a solvent, only two of the three Cl atoms can be substituted by fluorine, leading in good yield to the new "mixed aluminum halide", AlF2Cl·4Py. Actually, it represents the first example of a stable solid donor-acceptor adduct of an aluminum-III halide with two different halogens of defined stoichiometry. It was characterized by multinuclear solid-state NMR (27Al and 19F), IR spectroscopy, as well as single-crystal structure analysis. The new compound has an ionic solid-state structure with helical trans-octahedral [(Py)4AlF2]+ cations and isolated Cl- anions. The comparison of its 27Al MAS solid-state NMR spectra with those of a compound bearing the analogous [(Py)4AlCl2]+ cation reveals an extreme increase in the quadrupolar coupling constants, from 0.24 MHz in case of the chlorine cation to about 16 MHz in case of the new [(Py)4AlF2]+ cation.




-Some cool 14N spectra, Hans Jakobsen of course

Inorg. Chem., 45 (26), 10873 -10881, 2006. 10.1021/ic061197k S0020-1669(06)01197-9
Probing Crystal Structures and Transformation Reactions of Ammonium Molybdates by 14N MAS NMR Spectroscopy
Anders R. Hove, Henrik Bilds e, J rgen Skibsted, Michael Brorson, and Hans J. Jakobsen*
Abstract:The unique high-resolution feature offered by 14N magic-angle spinning (MAS) NMR spectroscopy of ammonium ions has been used to characterize the crystal structures of various ammonium molybdates by their 14N quadrupole coupling parameters, i.e., CQ, the quadrupole coupling constant, and Q, the asymmetry parameter. Two polymorphs of diammonium monomolybdate, (NH4)2MoO4, recently structurally characterized by single-crystal X-ray diffraction (XRD) and named mS60 and mP60, show distinct but different 14N MAS NMR spectra from each of which two sets of characteristic 14N CQ and Q values have been obtained. Similarly, the well-characterized ammonium polymolybdates (NH4)2Mo2O7, (NH4)6Mo7O24·4H2O, and (NH4)6Mo8O27·4H2O also give rise to distinct and characteristic 14N MAS NMR spectra. In particular, it is noted that simulation of the experimental (NH4)6Mo7O24·4H2O spectrum requires an iterative fit with six independent NH4+ sites. For the slow spinning frequencies employed ( r = 1500-3000 Hz), all 14N MAS NMR spectra of the ammonium molybdates in this study are fingerprints of their identity. These different 14N MAS NMR fingerprints are shown to be an efficient tool in qualitative and quantitative assessment of the decomposition of (NH4)2MoO4 in humid air. Finally, by a combination of the 14N and 95Mo MAS NMR experiments performed here, it has become clear that a recent report of the 95Mo MAS spectra and data for the mS60 and mP60 polymorphs of (NH4)2MoO4 are erroneous because the sample examined had decomposed to (NH4)2Mo2O7.



-Ripmeester and Igor

J. Phys. Chem. B, 111 (3), 491 -495, 2007. 10.1021/jp0667994 S1520-6106(06)06799-X
39K NMR of Solid Potassium Salts at 21 T: Effect of Quadrupolar and Chemical Shift Tensors
Igor L. Moudrakovski* and John A. Ripmeester
Abstract:39K Solid State NMR spectra (static and magic angle spinning (MAS)) on a set of potassium salts measured at 21.14 T show that the chemical shift range for K+ ions in diamagnetic salts is well in excess of 100 ppm contrary to previous assumptions that it was quite small. Inequivalent potassium sites in crystals can be resolved through differences in chemical shifts, with chemically similar sites showing differences of over 10 ppm. The quadrupolar coupling constants obtained from MAS and solid echo experiments on powders cover the range from zero for potassium in cubic environments in halides to over 3 MHz for the highly asymmetric sites in K2CO3. Although the quadrupolar effects generally dominate the 39K spectra, in several instances, we have observed subtle but significant contributions of chemical shift anisotropy with values up to 45 ppm, a first such observation. Careful analysis of static and MAS spectra allows the observation of the various chemical shift and quadrupole coupling tensor components as well as their relative orientations, thereby demonstrating that high-field 39K NMR spectroscopy in the solid state has a substantial sensitivity to the local environment with parameters that will be of considerable value in materials characterization and electronic structure studies.




-Solid-state 15N NMR to observe phase transitions

J. Phys. Chem. B, 111 (4), 677 -681, 2007. 10.1021/jp0649347 S1520-6106(06)04934-0
High Resolution 15N NMR of the 225 K Phase Transition of Ammonia Borane (NH3BH3): Mixed Order-Disorder and Displacive Behavior
O. Gunaydin-Sen, R. Achey, N. S. Dalal,* A. Stowe, and T. Autrey
Department of Chemistry and Biochemistry and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida, 32306, and Pacific Northwest National Laboratory, Richland, Washington 99352
Received: August 1, 2006
In Final Form: November 16, 2006
Abstract:
We report high resolution 15N NMR probing of the solid-solid phase transition of 15N-labeled ammonia borane (NH3BH3) at 225 K. Both the 15N isotropic chemical shift ( iso) and the spin-lattice relaxation rate (T1-1) exhibited strong anomalies around 225 K. The analysis of T1-1 using the Bloembergen, Purcell, and Pound model showed that the motional correlation time, , increased from about 1 to 100 ps and the corresponding Arrhenius activation energy increased from 6 to 14.5 kJ/mol on going through the transition toward lower temperatures. The temperature dependence of iso was interpreted by an extension of the Bayer model. The time scale of the underlying motion was found to be in a reasonable agreement with the T1-1 data. These results imply that the NH3 rotor motion plays a pivotal role in the transition mechanism and that the transition is of both order-disorder and displacive type




-Multi-nuclear NMR study. REDOR with quadrupoles.

J. Phys. Chem. B, 111 (5), 968 -978, 2007. 10.1021/jp065201p S1520-6106(06)05201-1
Structural Description of the Na2B4O7-Na3AlF6-TiO2 System. 2. A Multinuclear NMR Approach of Melts and Solids
Pierre Florian, Elena M. Anghel, and Catherine Bessada*
Abstract:A quantitative structural investigation of pseudo-binary Na2B4O7-[Na3AlF6-TiO2]11 was carried out by NMR spectroscopy. We applied high-resolution solid-state NMR on quenched samples and in situ high-temperature NMR spectroscopy on molten compositions to obtain a more accurate description of the melts. Consistent with our previous findings, two compositional ranges can be evidenced. Below 50 mol % Na2B4O7, the TiIV TiVI valence conversion prevails, leading to the formation of B(O,F)3 species at the expense of B(O,F)4 ones and a low average coordination number of aluminum; above 50 mol %, the system behaves mainly like the binary Na2B4O7-Na3AlF6 where Na3AlF6 is playing a network former role. The dominant oxyfluoro-species evidenced by double-resonance NMR in the quenched samples are BO2F2, BO3F, BO2F, BOF2, NaO7F, NaO6F2, and AlO4F with sodium in various coordination states, Na(O,F)8, Na(O,F)7, and Na(O,F)6. F-Ti bonds were found to be almost negligible whereas F-Al2, F-B, F-Na, and F-Al-Na bonds were clearly observed in the solid state



-27Al and 29Si studies of Mesopourous Silicates, Marek Pruski

J. Phys. Chem. C, 111 (3), 1480 -1486, 2007. 10.1021/jp0659913 S1932-7447(06)05991-7
Mesoporous Aluminum Silicate Catalyst with Single-Type Active Sites: Characterization by Solid-State NMR and Studies of Reactivity for Claisen Rearrangement Reactions
Yang Cai, Rajeev Kumar, Wei Huang, Brian G. Trewyn, Jerzy W. Wiench, Marek Pruski,* and Victor S.-Y. Lin*
Abstract:A mesoporous aluminum silicate (Al-MS) material has been successfully synthesized via a base-catalyzed co-condensation reaction. Solid-state NMR studies showed that the aluminum atoms are tetrahedrally coordinated and are stable up to at least 400 C. The double resonance 27Al-29Si NMR experiment provided first direct evidence that all Al atoms are surrounded by Si atoms forming Al-O-Si bonds in the framework. The silicon to aluminum atomic ratio at the surface (Si/Al)s is estimated at around 100, which exceeds the overall Si/Al ratio of 60. Reactivity study of the mesoporous aluminosilicate shows an excellent catalytic reactivity for the Claisen rearrangement of allyl phenyl ether despite the low surface concentration of Al



-27Al of Al2O3. Of interest to Joel?

J. Phys. Chem. C, 111 (4), 1579 -1583, 2007. 10.1021/jp0651945 S1932-7447(06)05194-6
27Al Triple-Quantum Magic-Angle Spinning Nuclear Magnetic Resonance Characterization of Nanostructured Alumina Materials
Hae Jin Kim,* Hyun Chul Lee, and Jae Sung Lee
Abstract:The nanostructured transition aluminas were synthesized from an aluminum alkoxide precursor in the presence of various surfactants under hydrothermal conditions without any organic additives. The synthesized nanostructured alumina materials showed different morphologies depending on surfactants with high surface area, thermal stability, and aluminum sites of different coordination. The 27Al triple-quantum (3Q) magic-angle spinning (MAS) NMR results showed that the samples prepared with cationic and nonionic surfactants consisted of mainly -Al2O3 phase and a small amount of -Al2O3 phase due to the presence of the distorted tetrahedral site, but that the samples with anionic and neutral surfactants were predominantly in the -Al2O3 phase. The experimental and calculated values of the electric field gradient (EFG) definitely suggested that cation vacancies were preferentially located at the octahedral sites in -Al2O3 whereas at the tetrahedral sites in -Al2O3.



-More SSNMR of zeolites

J. Phys. Chem. C, 111 (4), 1830 -1839, 2007. 10.1021/jp066498u S1932-7447(06)06498-3
Characterization of Cr-MCM-41 and Al,Cr-MCM-41 Mesoporous Catalysts for Gas-Phase Oxidative Dehydrogenation of Cyclohexane
Maria Lezanska,* Grzegorz S. Szymanski, Piotr Pietrzyk, Zbigniew Sojka, and Johannes A. Lercher
Abstract:The effects of the Cr concentration in hydrothermally synthesized Cr-MCM-41 and the impact of various postsynthesis treatments including grafting with alumina were investigated. Using physicochemical characterization by XRD, diffuse reflectance UV-vis spectroscopy, EPR spectroscopy, 29Si MAS NMR spectroscopy, H2 TPR, and acidity measurements, formation of mono- and dichromate surface species as well as CrV, dispersed CrIII, and clustered CrIII was detected, confirming the high speciation and valence versatility of the surface chromium. The roles of each species in the preparation stage, calcination, leaching, and grafting are discussed. Leaching resulted in removal of all isolated CrIII species, suggesting that CrIII is attached to the MCM-41 surface via silanol groups. The resulting materials exhibited low acidity, with both Lewis and Br nsted acid sites being present. The oxidative dehydrogenation (ODH) of cyclohexane was catalyzed by Cr-MCM-41 with a conversion of up to 25.7% in the temperature range of 533-633 K. The CrIII sites have been concluded to be responsible for cyclohexane ODH, but total activity was influenced by the presence of easily reducible CrVI at the beginning of the reaction. During the reaction, high-valence Cr ions were reduced to clustered Cr2O3 species.



- 13C SSNMR of methane hydrates.

J. Phys. Chem. C, 111 (5), 2341 -2346, 2007. 10.1021/jp066536+ S1932-7447(06)06536-8
NMR Investigation of Methane Hydrate Dissociation
Arvind Gupta, Steven F. Dec,* Carolyn A. Koh, and E. D. Sloan, Jr.
Abstract:The methane hydrate dissociation mechanism was studied on the molecular scale using 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Analysis of time-resolved 13C MAS NMR spectra for methane Structure I (sI) hydrate dissociation obtained by raising the temperature above the hydrate equilibrium conditions permitted the cage occupancy and pressure to be determined during this process. The relationship between NMR methane gas chemical shift, pressure, and temperature was developed, which allows estimating the system pressure in the sealed glass tube sample at any temperature. The large to small cage occupancy ratio remained constant during hydrate dissociation suggesting that there is no preferential dissociation of hydrate cavities and the whole unit cell decomposes during dissociation. This decomposition rate is virtually the same for both the large and the small cages in methane hydrate on a per cage basis. The similar decomposition rate of both cage types shows that the sI methane hydrate dissociation mechanism differs from the formation process where preferential formation of specific hydrate cages has been observed.



-J Phys Chem C: A Zeolite Journal

J. Phys. Chem. C, 111 (6), 2520 -2524, 2007. 10.1021/jp066578y S1932-7447(06)06578-2
Distribution of Aluminum in the Periodical Building Units of Faujasites
Tam s I. Kor nyi* and J nos B.Nagy
Abstract: Four NH4-Y faujasites (FAU) with different Si/Al ratios were characterized by 27Al and 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The quantitative contributions of the Al sites, defect-free Si(nAl), and silanol groups to the different double-six-ring (D6R) periodical building units (PerBUs) of the zeolite framework were calculated from the various Si/Al ratios and relative 27Al and 29Si NMR signal intensities. Four different PerBU systems were assumed in the FAU structure, but only the distribution of PerBUs containing three, two, or one Al atom pairs in body-diagonal positions of D6Rs followed the composition of the framework with appropriate precision in the studied composition range (Si/Al = 2.6-19.5). Contrary to BEA, FER, and MOR zeolites, the PerBUs of FAU do not contain Al atoms in unpaired "lone" positions.




-15N NMR of organic species interacting with TiO2, Raftery

J. Phys. Chem. C, 111 (6), 2738 -2748, 2007. 10.1021/jp0652289 S1932-7447(06)05228-9
15N Solid State NMR and EPR Characterization of N-Doped TiO2 Photocatalysts
Enrique A. Reyes-Garcia, Yanping Sun, Karla Reyes-Gil, and Daniel Raftery*
Abstract:The solid-state nuclear magnetic resonance (SSNMR) analysis and structural characterization of N-doped TiO2 nanoparticle and monolayer materials suitable for visible photocatalysis is reported. The SSNMR analysis of 15N-doped TiO2 synthesized using 15N-urea before calcination indicates formation of various amino functionalities of the type NH, NH2, NH3, and probably NH4+, while the NMR spectrum of the yellow powder that results from high-temperature calcination shows that these nitrogen species oxidize to form nitrate. These and nonisotopically labeled materials were characterized also by X-ray diffraction, UV-vis, and electron paramagnetic resonance spectroscopy. Monitoring the oxidation of 1,2-13C-trichloroethylene (TCE), photochemical activity was confirmed by monitoring the production of CO2 by 13C NMR analysis. The powdered form of N-doped TiO2 proved to be a highly efficient visible light catalyst by oxidizing all of the TCE to chlorinated alkyl and acyl containing molecules along with CO2. The performance of a TiO2-N-TiO2/porous Vycor 7930 borosilicate glass monolayer catalyst was in turn evaluated by monitoring the photochemical oxidation of ethanol and acetone with UV-vis light, and it was found to perform better than TiO2-only monolayers. Direct nitridation of TiO2 powders and monolayers also was conducted to compare the 15N SSNMR and to discern if there is a nitridic bond in these materials. The SSNMR results provide more evidence supporting the hypothesis that the nitrogen atom in N-doped TiO2 is present in interstitial sites when N is in a highly oxidized state.




-Calculation of 19F and 29Si shifts

J. Phys. Chem. C, 111 (9), 3584 -3590, 2007. 10.1021/jp065695q S1932-7447(06)05695-0
Calculation of 19F and 29Si NMR Shifts and Stabilities of F- Encapsulating Silsesquioxanes
J. A. Tossell*
Abstract:Hartree-Fock and density functional theory techniques have been used to calculate the structures, the stabilities in aqueous and toluene solutions, and the 19F and 29Si NMR shifts of F- encapsulating double-ring geometry silsesquixoanes DnR with n = 3-6. We find for the fluorides that the encapsulation free energy is most negative for the double four-ring (D4R) cage, in which the most stable geometry has F- in the center. For n = 5, 6 (and for an opened version of the double five-ring, D5R, cage), the most stable geometry has F- bonded to a single apex Si atom, with a bond distance around 1.76 Å. The 19F NMR is somewhat deshielded relative to that of the Si(OH)4F- monomer for all of the apex bonded F- DnR species and is very strongly deshielded for the central F- encapsulating DnR species with n = 3, 4. The Si is slightly deshielded by the presence of F- for the DnR with central F- ions, while in the apex-bonded larger double rings, n = 5, 6, the Si directly bonded to F- is substantially shielded. The energetic results help to explain why fluorides, as mineralizing agents, increase the yield of zeolites and mesoporous silicas with D4R rings, and the NMR results provide a means for ascertaining double-ring size and the position of F- within the double-ring cage.




- 2H NMR of molecules inside zeolites (Andy?)

J. Phys. Chem. C, 111 (11), 4393 -4403, 2007. 10.1021/jp066959b S1932-7447(06)06959-7
Dynamics of Linear n-C6-n-C22 Alkanes Inside 5A Zeolite Studied by 2H NMR
Daniil I. Kolokolov, Sergei S. Arzumanov, Alexander G. Stepanov,* and Hervé Jobic*
Abstract:The dynamic behavior of deuterated analogues of linear alkanes, n-C6-n-C22, adsorbed in zeolite 5A has been studied by deuterium solid-state NMR (2H NMR). Temperature dependences of spin-lattice (T1) and spin-spin (T2) relaxation times of the deuterium located in the CD3 groups of the adsorbed n-alkanes were rationalized on the basis of a model derived for the motion of n-alkanes located in the pores of the zeolite. The model implies that the adsorbed molecules consist of two ensembles: diffusing (or stretched) and temporarily blocked from diffusion (or coiled). The possible intramolecular motions for the alkane chains were taken into account based on both the finite size of the zeolite cage and the allowable hydrocarbon chain conformations. The coiled molecules are involved in two modes of motion: isotropic reorientation and intramolecular conformational isomerization, whereas the stretched molecules are additionally involved in a diffusion process. Dynamics parameters for different modes of motion and a proportion of the blocked and stretched molecules were derived from the analysis of relaxation data. The estimated proportion of the diffusing molecules correlates with the alkanes diffusivities earlier obtained by neutron spin echo measurements.




-19F-27Al HETCOR

J. Phys. Chem. C, 111 (12), 4495 -4498, 2007. 10.1021/jp070739w S1932-7447(07)00739-X
Direct Solid-State NMR Observation of Tetrahedral Aluminum Fluorides in Zeolite HY Fluorinated by Ammonium Fluoride
Hsien-Ming Kao* and Yi-Chen Liao
Abstract:19F to 27Al CPMAS and 27Al{19F} 2D HETCOR NMR experiments provide direct evidence of the formation of tetrahedral Al-F species in zeolite HY fluorinated by an aqueous solution of ammonium fluoride at 80 C. On the basis of these NMR observations, a NMR peak assignment for the 19F signals at -173 and -182 ppm to the tetrahedral Al-F species corresponding to an 27Al signal at 50 ppm is made for the first time.



-13C-17O REAPDOR Spectroscopy

Macromolecules, 40 (5), 1363 -1365, 2007. 10.1021/ma062835l S0024-9297(06)02835-X
13C-17O REAPDOR NMR as a Tool for Determining Secondary Structure in Polyamides
Terry Gullion,* Kazuo Yamauchi, Michi Okonogi, and Tetsuo Asakura*
Secondary structure of polyamides is determined largely by hydrogen bonding between the carbonyl oxygen and the amide proton. Nylon-6, with its and crystalline forms, is the classic example of a synthetic polyamide where hydrogen bonding has a significant influence on structure, and the secondary structure of natural proteins and peptides is very much governed by hydrogen bonding.1 X-ray crystallography is the primary tool for determining structures in crystalline macromolecules, but not all polyamides can be suitably prepared for X-ray diffraction experiments. For such samples solid-state NMR has been widely used for structural studies, including 13C chemical shift analysis, 13C-13C spin-diffusion, and REDOR experiments.2 Because the carbonyl oxygen serves as the proton acceptor and, consequently, plays such an important role in hydrogen bonding and formation of the secondary structure, we have begun to explore 17O NMR as a structural tool in polyamides. This paper illustrates 13C-17O REAPDOR NMR as a structural tool for characterizing polyamides.




-Gillian Goward, studies of Nafion/SiO2 systems

Macromolecules, 40 (5), 1529 -1537, 2007. 10.1021/ma0621876 S0024-9297(06)02187-5
Proton Dynamics of Nafion and Nafion/SiO2 Composites by Solid State NMR and Pulse Field Gradient NMR
G. Ye, C. A. Hayden, and G. R. Goward*
Abstract:
Proton mobilities in Nafion and Nafion/SiO2 composites have been studied using high-resolution solid-state MAS NMR. High-resolution solid-state 1H NMR show that low concentrations of TEOS or short permeation times are necessary to allow complete hydrolysis of TEOS in Nafion. Incomplete hydrolysis of TEOS leaves residual ethyl groups on the surface of silica, which not only reduces the amount of water adsorbed by silica but also blocks the pathway of proton transport in the Nafion/SiO2 composites. The diffusion coefficients established using PFG NMR show that the best Nafion/SiO2 composite can be obtained from synthesis with a low concentration of TEOS in a methanol solution. This composite gives a higher diffusion coefficient than pure Nafion under dry conditions, although no differentiation in performance is observed when the membranes are hydrated. 29Si NMR shows that this composite has a high ratio of Q3/Q4 sites, consistent with a small particle size and many surface hydroxyl groups. Together, these data demonstrate the role of high-surface-area SiO2 particles in trapping water and building a pathway for structural (Grotthuss mechanism) proton diffusion. Good proton transport under low relative humidity is the holy grail of the PEM-FC community, and this molecular level study shows how conditions can be iteratively optimized to target desirable structure-property relationships.



-13C SSNMR studies of spider silk

Macromolecules, 40 (6), 1995 -2001, 2007. 10.1021/ma062452n S0024-9297(06)02452-1
Molecular Disorder and Structure of Spider Dragline Silk Investigated by Two-Dimensional Solid-State NMR Spectroscopy
Isabelle Marcotte, Jacco D. van Beek, and Beat H. Meier*
Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland
Received October 24, 2006
Revised Manuscript Received December 18, 2006
Abstract:
We have studied the molecular disorder in (13C)Ala- and (13C)Gly-labeled dragline silk from Nephila edulis through the distributions of isotropic chemical shifts in one- and two-dimensional 13C-13C NMR spectra obtained under MAS. The alanine residues were found to be present in two distinct major structural environments, in agreement with previous studies, where -sheets and 31-helices were found. The distributions in chemical shift within each residue were found to be uncorrelated, even between spins in the same amino acid residue, and were of similar widths for both the crystalline and noncrystalline parts. Upon long-term mechanical stretching of the silk, the NMR spectra showed no significant changes in conformation nor changes in the degree of disorder but did show increasing structural damage to the silk threads.