Thursday, August 07, 2008

J Phys Chem B, J Phys Chem C, Organometallics and Macromolecules, End of June to Beginning of August

J. Phys. Chem. C, 112 (26), 9808–9821, 2008. 10.1021/jp8010348
7Li NMR Spectroscopy and Multiquantum Relaxation as a Probe of the Microstructure and Dynamics of Confined Li+ Cations: An Application to Dense Clay Sediments

Patrice Porion,* Anne Marie Faugère, and Alfred Delville*
Centre de Recherche sur la Matière Divisée, CNRS - Université d’Orléans, 1b rue de la Férollerie, 45071 Orléans Cedex 02, France

Abstract: 7Li NMR spectroscopy and relaxometry are exploited to obtain structural and dynamical information within heterogeneous systems resulting from aqueous dispersion of clay platelets. In addition to the specific orientation of the clay platelets detected by the splitting of the 7Li resonance line, multiquantum relaxation measurements are used to determine the relative contributions from the quadrupolar and dipolar relaxation mechanisms responsible for the detected broadening of the 7Li resonance line. Intrinsic spectral densities are further extracted from the variation of these apparent relaxation rates as a function of the orientation of the clay sediment within the static magnetic field. A semiquantitative interpretation of the intrinsic quadrupolar and dipolar relaxation behavior is performed by a multiscale modeling of the Li+ ions’ diffusion in relation to the structure of the clay dispersions. Equivalent structural and dynamical investigations can be performed within a broad class of material containing electrically charged solid/liquid interfaces by exploiting the sensitivity of 7Li NMR spectroscopy.

J. Phys. Chem. C, 112 (26), 9962, 2008. 10.1021/jp802847m
Comment on “27Al, 47,49Ti, 31P, and 13C MAS NMR Study of VX, GD, and HD Reactions with Nanosize Al2O3, Conventional Al2O3 and TiO2, and Aluminum and Titanium Metal”

George W. Wagner* and Boris Itin

Wagner et al.1 recently reported 47,49Ti MAS NMR spectra for TiO2 (anatase) at 14, 17.5, and 21 T to assess anticipated line-narrowing for these nuclei at high magnetic field. As quadrupolar nuclei, 49Ti (I = 7/2, Q = 0.24) and 47Ti (I = 5/2, Q = 0.29) are susceptible to broadening by the second-order quadrupolar interactions that diminish with respect to the dominant NMR Zeeman interaction at higher magnetic fields.2 Although the lineshapes of both 49Ti and 47Ti were clearly evident in the 14 and 17.5 T spectra, only the 49Ti peak was detected in the 21 T spectrum because of the poor signal-to-noise ratio. By 21 T, the 49Ti resonance had narrowed to less than half of its width at 14 T, but the extent of narrowing of the 47Ti could not be ascertained.
As shown in Figure
1, much better signal-to-noise 47,49Ti MAS NMR spectra have now been obtained3 for anatase at 21 T, which allow the line shape of the much broader 47Ti resonance to be observed. The better sensitivity was evidently provided by use of a larger-diameter 5 mm probe rather than the previous 3.2 mm probe.1 Spectra previously reported1 at 14 and 17.5 T are also shown for comparison. At 21 T, the line width of the 47Ti peak narrows to less than half of its width at 14 T, in agreement with the results previously noted1 for the sharper 49Ti resonance.

J. Phys. Chem. C, 112 (27), 9996–10003, 2008. 10.1021/jp801992d

Tuning Metal Nanostructures in Mesoporous Silica by a Simple Change of Metal Complexes and by Reduction with Grafted Imines and Hemiaminals
Youwei Xie, Sean Quinlivan, and Tewodros Asefa*

Abstract: We report synthetic methods to produce silver (Ag) or gold (Au) monodisperse nanoparticles with different loadings as well as Ag and Au nanorods with tunable aspect ratios and optical properties. This is achieved inside externally functionalized mesoporous SBA-15 material by utilizing a simple change of the type of metal complex that is reduced with imines or hemiaminals that were anchored within the channel walls of the mesoporous material. The imine and hemiaminal groups were produced by reacting grafted 3-aminopropyl groups with CH3CHO or HCHO, respectively. Upon using lower concentrations of aqueous AgNO3 or ammonical [Ag(NH3)2]NO3 solution as the metal source, Ag monodisperse nanoparticles formed. Interestingly, when the concentration of ammonical [Ag(NH3)2]NO3 solution was increased, this resulted exclusively in nanoparticles with a higher loading and with no formation of nanorods; while increasing the concentration of aqueous AgNO3 solution resulted in silver nanorods with higher aspect ratios and tunable optical absorption and color. A possible mechanism for the growth of nanoparticles versus nanorods in the mesoporous materials by using ammonical instead of aqueous silver complexes, respectively, was proposed. The synthesis of gold nanoparticles or tunable gold nanorods was also carried out by changing the concentration of aqueous HAuCl4 in the same material. The synthesis of Ag nanostructures with grafted imines and Au nanostructures with grafted imines and hemiaminals are to be reported for the first time. The functionalized mesoporous materials, the Ag and Au nanoparticles, and nanorods inside the mesoporous materials were characterized by UV−vis absorption and reflectance spectroscopy, small-angle and wide-angle X-ray diffraction, solid-state NMR spectroscopy, and transmission electron microscopy.

J. Phys. Chem. C, 112 (28), 10462–10471, 2008. 10.1021/jp800721e

Structural Role of Fluoride in the Ion-Conducting Glass System B2O3−PbO−LiF Studied by Single- and Double-Resonance NMR

A. S. Cattaneo,†‡ R. P. Lima,§ C. E. Tambelli,§ C. J. Magon,§ V. R. Mastelaro,§ A. Garcia,§ J. E. de Souza,§ A. S. S. de Camargo,§ C. C. de Araujo,† J. F. Schneider,§ J. P. Donoso,§ and H. Eckert*†

Abstract: The local structure of an ion-conducting glass with nominal composition 50B2O3−10PbO−40LiF has been investigated by complementary 7Li, 11B, 19F, and 207Pb single- and double-resonance experiments. The results give insight into the structural role of the lithium fluoride additive in borate glasses: (1) LiF is seen to actively participate in the network transformation process contributing to the conversion of three- into four-coordinate boron units, as shown by 11B single-resonance as well as by 11B{19F} and 19F{11B} double-resonance experiments. (2) 19F signal quantification experiments suggest substantial fluoride loss, presumably caused by formation of volatile BF3. A part of the fluoride remains in the dopant role, possibly in the form of small LiF-like cluster domains, which serve as a mobile ion supply. (3) The extent of lithium−fluorine and lead−fluorine interactions has been characterized by 7Li{19F} and 207Pb{19F} REDOR and SEDOR experiments. On the basis of these results, a quantitative structural description of this system has been developed.

J. Phys. Chem. C, 112 (29), 10899–10908, 2008. 10.1021/jp803037u
Evidence of Multiple Cation Site Occupation in Zeolite NaY with High Si/Al Ratio

Laurent Gueudré, Anne Agathe Quoineaud, Gerhard Pirngruber, and Philibert Leflaive*

Abstract: 23Na MAS NMR and 23Na 2D MQMAS NMR spectra of dehydrated NaY zeolite with different Si/Al ratios were analyzed. Particularly, the cation distribution was examined in both as synthesized NaY with a Si/Al ratio of 2.7 and a dealuminated NaY with a framework Si/Al ratio of 8.6, prepared by ion-exchange of a dealuminated NH4Y zeolite CBV712. This is, to the best of our knowledge, the first experimental investigation of the cation distribution in dealuminated NaY. Careful analysis of 23Na MAS NMR and 23Na 2D MQMAS NMR spectra of dealuminated NaY revealed the occupation of at least three different sites, contrary to what was expected from previous computational studies where site II was the only occupied site. Moreover, two new features could be observed. First, a split of the contribution of site I into two relatively close and similar narrow peaks in the MQMAS NMR spectra, and second a new narrow signal, interpreted in terms of fast cation movement inside the sodalite cage. This work also allows a clear identification of the contribution of site II to the 23Na MAS and MQMAS NMR spectra of as synthesized NaY (Na52Y). This was hardly possible in previous 23Na MAS NMR studies of NaX or NaY due to strong overlap of the broad quadrupolar pattern of site II with the one of the cations located in site I′.

-13C Solid-State NMR
J. Phys. Chem. B, 112 (29), 8443–8446, 2008. 10.1021/jp804447r

Spectroscopic Observation of Critical Guest Concentration Appearing in tert-Butyl Alcohol Clathrate Hydrate

Youngjune Park, Minjun Cha, Woongchul Shin, Huen Lee,* and John A. Ripmeester*

Abstract: The tert-butyl alcohol (TBA) is the most hydrophobic of the simple alcohol and by itself does not form a clathrate hydrate with water. A genuine clathrate hydrate is synthesized by exposing a gaseous guest to solid TBA + H2O powders. Here, we examine three consecutive spectroscopic approaches of (1) the occurrence of a “free” OH stretching band (ν(OH)) signal of TBA molecules representing an absence of hydrogen bonding between the host water and guest TBA, (2) a tuning effect for creating fresh cages via the rearrangement of the host−water lattice, and finally (3) the existence of a critical guest concentration (CGC) that appears only when the TBA concentration is dilute. The present findings from this simple three-step approach can be extended to other alcoholic guest species with the specific modifications to provide the new insights into inclusion chemistry.

-14N CQ measured via EPR
J. Phys. Chem. B, 112 (29), 8549–8557, 2008. 10.1021/jp800222c

Electron Spin Resonance Investigation of Microscopic Viscosity, Ordering, and Polarity in Nafion Membranes Containing Methanol−Water Mixtures

Jamie S. Lawton, Eugene S. Smotkin, and David E. Budil*

Abstract: Electron spin resonance (ESR) was used to monitor the local environment of 2,2,6,6-tetramethyl-4-piperidone N-oxide (Tempone) spin probe in water and methanol mixtures in solution and in Li+ ion exchanged Nafion 117 membranes. Solution spectra were analyzed using the standard fast-motion line width parameters, while membrane spectra were fitted using the microscopic order macroscopic disorder (MOMD) slow-motional line shape program of Freed and co-workers. The 14N hyperfine splitting, aN, which reflects the local polarity of the nitroxide probe, decreases with increasing methanol concentration, consistent with the decrease in solvent polarity. The polarity depended only weakly on composition in the Nafion membrane, but was noticeably more temperature-dependent. The microviscosity of the membrane aqueous phase as reflected by the rotational correlation time (τc) of the probe, was nearly 2 orders of magnitude longer in the membrane than in solution and varied by an order of magnitude over the composition range studied. The probe exhibits significant local ordering in the aqueous phase of Nafion membranes that is diminished with increasing methanol concentration.

J. Phys. Chem. B, 112 (30), 9079–9090, 2008. 10.1021/jp711640p

High-Field EPR and ESEEM Investigation of the Nitrogen Quadrupole Interaction of Nitroxide Spin Labels in Disordered Solids: Toward Differentiation between Polarity and Proticity Matrix Effects on Protein Function

A. Savitsky, A. A. Dubinskii, M. Plato, Y. A. Grishin,§ H. Zimmermann, and K. Möbius*

Abstract: The combination of high-field electron paramagnetic resonance (EPR) with site-directed spin labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in revealing subtle changes of the polarity and proticity profiles in proteins enbedded in membranes. This information can be obtained by orientation-selective high-field EPR resolving principal components of the nitroxide Zeeman (g) and hyperfine (A) tensors of the spin labels attached to specific molecular sites. In contrast to the g- and A-tensors, the 14N (I = 1) quadrupole interaction tensor of the nitroxide spin label has not been exploited in EPR for probing effects of the microenvironment of functional protein sites. In this work it is shown that the W-band (95 GHz) high-field electron spin echo envelope modulation (ESEEM) method is well suited for determining with high accuracy the 14N quadrupole tensor principal components of a nitroxide spin label in disordered frozen solution. By W-band ESEEM the quadrupole components of a five-ring pyrroline-type nitroxide radical in glassy ortho-terphenyl and glycerol solutions have been determined. This radical is the headgroup of the MTS spin label widely used in SDSL protein studies. By DFT calulations and W-band ESEEM experiments it is demonstrated that the Qyy value is especially sensitive to the proticity and polarity of the nitroxide environment in H-bonding and nonbonding situations. The quadrupole tensor is shown to be rather insensitive to structural variations of the nitroxide label itself. When using Qyy as a testing probe of the environment, its ruggedness toward temperature changes represents an important advantage over the gxx and Azz parameters which are usually employed for probing matrix effects on the spin labeled molecular site. Thus, beyond measurenments of gxx and Azz of spin labeled protein sites in disordered solids, W-band high-field ESEEM studies of 14N quadrupole interactions open a new avenue to reliably probe subtle environmental effects on the electronic structure. This is a significant step forward on the way to differentiate between effects from matrix polarity and hydrogen-bond formation.

-Some solution NMR for the solids blog

J. Phys. Chem. B, 112 (30), 9174–9181, 2008. 10.1021/jp8030545

Remarkable Metal Counterion Effect on the Internucleotide J-Couplings and Chemical Shifts of the N−H···N Hydrogen Bonds in the W−C Base Pairs

Huifang Li, Robert I. Cukier, and Yuxiang Bu*

Abstract: The effects of metal ion binding on the 2hJNN-coupling and δ(1H)/Δδ(15N) chemical shifts of N−H···N H-bond units in internucleotide base pairs were explored by a combination of density functional theory calculations and molecular dynamics (MD) simulations. Results indicate that the NMR parameters vary considerably upon cation binding to the natural GC or AT base pairs, and thus can be used to identify the status of the base pairs, if cation-perturbed. The basic trend is that cation perturbation causes 2hJNN to increase, Δδ(15N) to decrease, and δ(1H) to shift upfield for GC, and in the opposite directions for AT. The magnitudes of variation are closely related to the Lewis acidity of the metal ions. For both base pair series (Mz+GC and Mz+AT), these NMR parameters are linearly correlated among themselves. Their values depend strongly on the energy gaps (ΔELP→σ*) and the second-order interaction energies (E(2)) between the donor N lone pair (LPN) and the acceptor σ*N−H localized NBO orbitals. In addition, the 2hJNN changes are also sensitive to the amount of σ charge transfer from LPN to σ*N−H NBOs or from the purine to the pyrimidine moieties. The different trends are a consequence of the different H-bond patterns combined with the polarization effect of the metal ions in the cationized Mz+AT series, Mz+ ← A → T, and the cationized GC series, Mz+ ← G ← C. The predicted cation-induced systematic trends of 2hJNN and δ(15N,1H) in N−H···N H-bond units may provide a new approach to the determination of H-bond structure and strength in Watson−Crick base pairs, and provide an alternative probe of the heterogeneity of DNA sequences.

J. Phys. Chem. B, 112 (32), 9625–9629, 2008. 10.1021/jp801128x Web Release Date: July 17, 2008

Study of the Adsorption of Organic Molecules on Transition Metal Exchanged Zeolites via Solid State NMR. Part 1: Theoretical Aspects

Kristof Houthoofd,* Piet J. Grobet, and Pierre A. Jacobs

Abstract: A theoretical study is performed of the 1H relaxation behavior of an organic molecule that is adsorbed in a zeolite exchanged with transition metal ions. An expression is derived for the spin−lattice and spin−spin relaxation rate constant of a uniform proton ensemble, while the occurrence of molecular exchange is taken into account. The influence is analyzed of an approach of a uniform proton ensemble toward the nearest neighboring paramagnetic ion system. It is shown that there exists a signal extinction effect, for which the critical distance was estimated.

J. Phys. Chem. B, 112 (32), 9630–9640, 2008. 10.1021/jp801133x

Study of the Adsorption of Organic Molecules on Transition Metal Exchanged Zeolites via Solid State NMR. Part 2: Adsorption of Organic Molecules on Zeolite NaX, CaX, and CaCoX

Kristof Houthoofd,* Piet J. Grobet, and Pierre A. Jacobs

Abstract: Literature [Denayer et al. Microporous Mesoporous Mater. 2007, 103, 1 and Denayer et al. Microporous Mesoporous Mater. 2007, 103, 11] shows that zeolite NaX exchanged with Ca2+ and Co2+ ions is able to remove cyclopentadiene (CPD) impurities from a 1-octene feed with high selectivity. In the present work, the adsorption of dicyclopentadiene (DCPD), CPD, 1-octene, and n-octane on zeolite X, exchanged with Ca2+ and/or Co2+ ions, has been investigated via 1H magic-angle spinning (MAS) NMR spectroscopy. The liquid adsorbate was dosed under inert atmosphere in an MAS rotor filled with dry adsorbent, at a pore filling degree of 70%. Next, the evolution in time was recorded of the 1H MAS NMR spectrum and the 1H spin−lattice and spin−spin relaxation times of the adsorbed components. For the various adsorbate−adsorbent systems, a plot is made of the signal intensity versus the square root of the contact time. It is found that, over the considered time interval, Fickian diffusion takes place. On the basis of the change in time of the spin−lattice relaxation time, a transport diffusion coefficient ranging between 1 and 2 × 10−15 m2·s−1 is calculated. Moreover, there appear to be two sorption regimes, with different diffusivities. A comparison is made between the 1H spin−lattice relaxation behavior of DCPD, 1-octene, and n-octane, indicating that 1-octene and n-octane are located closer to the paramagnetic ions than DCPD. The average distance between the adsorbate molecules and the paramagnetic ions is derived from relaxometric data. By analyzing the chemical shifts of the resonance lines, it is found that the π-interaction of CPD and 1-octene is stronger than that of DCPD.

Macromolecules, 41 (12), 4290–4295, 2008. 10.1021/ma8002483

NMR Characterization of Low Hard Segment Thermoplastic Polyurethane/Carbon Nanofiber Composites

Daniel S. Powers,† Richard A. Vaia,† Hilmar Koerner,‡ Jennifer Serres,§ and Peter A. Mirau*†

Abstract:Solid-state proton nuclear magnetic resonance (NMR) has been used to investigate the structure and dynamics of a thermoplastic polyurethane elastomer (TPE) filled with carbon nanofibers (CNF’s) for shape-memory applications. The TPE soft segments are above their glass transition temperature (Tg) at ambient temperature and give rise to relatively narrow (~2 kHz) signals in the solid-state proton spectrum. The introduction of CNF’s leads to a concentration-dependent shifting and broadening of the signals, while the proton spin−lattice and spin−spin relaxation times are not significantly altered, showing that the broadening is inhomogeneous and related to the difference in magnetic susceptibility between the TPE and the CNF’s. Proton spin diffusion experiments reveal the onset of stress-induced crystallinity as the samples are stretched to 60%, and stretching to 1000% leads to crystallization at the CNF surface and increased separation between the CNF’s and the mobile amorphous phase of the TPE. The implications for the mixing of polymers and CNF’s are considered.

Macromolecules, 41 (14), 5313–5321, 2008. 10.1021/ma702771s

From Simple Liquid to Polymer Melt. Glassy and Polymer Dynamics Studied by Fast Field Cycling NMR Relaxometry: Low and High Molecular Weight Limit

S. Kariyo, A. Brodin,§ C. Gainaru,§ A. Herrmann,§ H. Schick,§ V. N. Novikov, and E. A. Rössler*§

Abstract: Fast field cycling (FFC) NMR is applied to study the dispersion of the 1H spin−lattice relaxation in the low molecular weight glass-formers o-terphenyl, tristyrene, and oligomeric polybutadiene (PB, with M/gmol−1 = 355 and 466) over a broad temperature range (203−401 K). Differing from previous FFC NMR works, we analyze the relaxation data in the susceptibility form ω/T1(ω), and applying frequency−temperature superposition, master spectra are obtained covering up to 8 decades in frequency. In all cases solely the glassy dynamics (α-process) determines the relaxation behavior, and the Rouse unit is estimated to MR @ 500 g/mol. The time constant τα(T) in the range 10−11−10−6 s is extracted, which agrees well with those measured at the same time by dielectric spectroscopy. For the high molecular weight PB (M/gmol−1 = 56 500, 87 000, 314 000, and 817 000) pronounced polymer effects are observed at low frequencies (ωτα ≪ 1) which are isolated from the total spectrum by subtracting the “glass spectrum” as obtained from low molecular PB. We argue that unless the underlying α-relaxation is properly accounted for, the apparent power law spectrum does not reflect the actual polymer dynamics.

Macromolecules, 41 (14), 5322–5332, 2008. 10.1021/ma702758j

From Simple Liquid to Polymer Melt. Glassy and Polymer Dynamics Studied by Fast Field Cycling NMR Relaxometry: Rouse Regime

S. Kariyo, A. Brodin,§ C. Gainaru,§ A. Herrmann,§ J. Hintermeyer,§ H. Schick,§ V. N. Novikov, and E. A. Rössler*§

Abstract: We apply fast field cycling NMR to study the dispersion of the 1H spin−lattice relaxation time T1(ω) of linear 1,4-polybutadienes with molecular weight M (g/mol) ranging from M = 355 to 817 000. By this, the crossover from glassy dynamics through Rouse to reptation becomes accessible. Analyzing the data in the susceptibility form ω/T1(ω) and applying frequency−temperature superposition, spectra extending over up to 8 decades in ω are obtained. Characteristic polymer spectra are revealed when the underlying glassy dynamics are accounted for. Instead of describing the unentangled melt by the full Rouse mode spectrum, the emergence of a limited number of modes is identified which saturates when entanglement sets in. A quantitative analysis yields the molecular weight of a Rouse unit MR @ 500, and the entanglement weight Me @ 2000, at which first entanglement effects are observed. Moreover, the dynamic order parameter S(M) and the behavior of the terminal time τmax(M) are obtained. Both quantities allow to identify three dynamic regimes, namely simple liquid, Rouse, and reptation dynamics. The temperature dependence of the segmental relaxation time τs(T) coincides with the corresponding dielectric relaxation times which were measured additionally, and the M dependence of the glass transition temperature Tg shows distinctive kinks at MR and Me, indicating that glassy dynamics are modified by polymer dynamics.

Organometallics, 27 (15), 3633–3634, 2008. 10.1021/om800399x

Quantitative Tin Loading Determination of Supported Catalysts by 119Sn HRMAS NMR using a Calibrated Internal Signal (ERETIC)
Vanja Pinoie, Monique Biesemans,
* and Rudolph Willem

Abstract: As an alternative to tedious elemental analysis, tin loading of a grafted organotin catalyst is quantified using an electronic reference (ERETIC) in 119Sn HRMAS NMR. The results are in excellent agreement with elemental analysis data and display a significantly higher precision.

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