Thursday, June 07, 2007

Aaron's Journal Update (J Phys Chem B and C, Macromolecules, Organometallics)

Not too much SSNMR to be found in J Phys Chem B or C this time around.

-Marek Pruski and Ganapathy. Nice work on HETCOR and CP/CPMG of MCM-41 systems

J. Phys. Chem. B, 111 (15), 3877 -3885, 2007. 10.1021/jp067417x S1520-6106(06)07417-7 Web Release Date: March 28, 2007
Characterization of Covalent Linkages in Organically Functionalized MCM-41 Mesoporous Materials by Solid-State NMR and Theoretical Calculations
Jerzy W. Wiench, Yamini S. Avadhut, Niladri Maity, Sumit Bhaduri,* Goutam Kumar Lahiri, Marek Pruski,* and Subramanian Ganapathy*
Abstract: The covalent linkages formed during functionalization of MCM-41 mesoporous molecular sieves with five chloroalkylsilanes ((EtO)3Si(CH2Cl), (MeO)3Si(CH2CH2CH2Cl), Cl3Si(CH2CH2CH3), Cl2Si(CH3)(CH2Cl) and Cl2Si(CH3)2) have been investigated using high-resolution solid-state NMR spectroscopy and DFT calculations. Structural information was obtained from 1H-13C and 1H-29Si heteronuclear (HETCOR) NMR spectra, in which high resolution in the 1H dimension was obtained by using fast MAS. The 1H-13C HETCOR results provided the assignments of 1H and 13C resonances associated with the surface functional groups. Sensitivity-enhanced 1H-29Si HETCOR spectra, acquired using Carr-Purcell-Meiboom-Gill refocusing during data acquisition, revealed the identity of 29Si sites (Qn, Tn, and Dn) and the location of functional groups relative to these sites. Optimal geometries of local environments representing the Qn, Tn and Dn resonances were calculated using molecular mechanics and ab initio methods. Subsequently, DFT calculations of 29Si, 13C, and 1H chemical shifts were performed using Gaussian 03 at the B3LYP/6-311++G(2d,2p) level. The theoretical calculations are in excellent accord with the experimental chemical shifts. This work illustrates that state-of-the-art spectroscopic and theoretical tools can be used jointly to refine the complex structures of inorganic-organic hybrid materials.

- Gillian Goward. Studies of the structure and dynamics of benzimidazole salts

J. Phys. Chem. B, 111 (20), 5602 -5609, 2007. 10.1021/jp071471b S1520-6106(07)01471-X Web Release Date: May 2, 2007
A Solid-State NMR Study of Hydrogen-Bonding Networks and Ion Dynamics in Benzimidazole Salts
Jason W. Traer, James F. Britten, and Gillian R. Goward*
Abstract: On the basis of our solid-state NMR characterization of dynamics in two model salts, we draw the analogy to the fuel cell membrane candidate, phosphoric acid-doped poly(benzimidazole), and conclude that phosphate anion dynamics contribute to long-range proton transport, whereas the mobility of the polymer itself is not a contributing factor. This is contrasted with emerging membrane candidates, which rely on fully covalently bonded acid donors and acceptors, and target high-temperature PEM fuel cell operation in the absence of liquid electrolyte. The hydrogen-bonding structures of benzimidazolium phosphate and benzimidazolium methane phosphonate are established using X-ray diffraction paired with solid-state 1H DQF NMR. By comparing the dynamics of the phosphate and methane phosphonate anions with the dynamics of imidazolium and benzimidazolium cations, the relative importance of these processes in proton transport is determined. The imidazolium cation is known to undergo two-site ring reorientation on the millisecond time scale. In contrast, it is shown here that the benzimidazolium rings are immobile in analogous salts, on a time scale extending into the tens of seconds. Therefore, we look to the phosphate anions and demonstrate that the time scale of tetrahedral reorientation is comparably fast (50 ms). Moreover, the 31P CODEX NMR data clearly indicate a four-site jump process. In contrast, the methane phosphonate undergoes a three-site jump on a slower time scale (75 ms). A mechanism for a zigzag pathway of proton transport through the phosphonate salt crystallites is developed based on the 31P CODEX and 1H variable-temperature MAS NMR data.

-129Xe NMR of mesoporous materials

J. Phys. Chem. C, 111 (15), 5694 -5700, 2007. 10.1021/jp066163c S1932-7447(06)06163-2
Probing Xe Exchange in Delaminated Zeolites by Hyperpolarized 129Xe NMR
M.-A. Springuel-Huet,* F. Guenneau, A. Gédéon, and A. Corma
Abstract: MCM-22 and ferrierite zeolites and the corresponding delaminated materials, ITQ-2 and ITQ-6, respectively, have been studied by 129Xe NMR of adsorbed xenon and by nitrogen adsorption at 77 K. One-dimensional 129Xe NMR spectra of delaminated materials show additional lines compared with those of the parent zeolites. For ITQ-2, these new lines are attributed to Xe exchanging between the adsorbed phase and the intercrystallite gaseous phase, with the extent of exchange depending on the particle size, the distribution of which is discrete. For ITQ-6, the additional lines are attributed to the presence of mesopores in the particles. Two-dimensional NMR spectra of ITQ-6 samples under magic angle spinning conditions and continuous flow of hyperpolarized Xe allowed us to investigate the exchange between different sites and to obtain information on the delamination process.

-C13 diffusion spectroscopy of CO molecules on a Pt surface. Eric Oldfield

J. Phys. Chem. C, 111 (19), 7078 -7083, 2007. 10.1021/jp066803b S1932-7447(06)06803-8 Coverage Dependence of CO Surface Diffusion on Pt Nanoparticles: An EC-NMR Study
Takeshi Kobayashi, Panakkattu K. Babu, Jong Ho Chung, Eric Oldfield,
* and Andrzej Wieckowski
Abstract:We have studied the effects of CO surface coverage on the diffusion rates of CO adsorbed on commercial Pt-black in sulfuric acid media by using 13C electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy in the temperature range 253-293 K. The temperature range chosen for these measurements was such that the electrolyte is in a liquid-like and liquid environment. For CO coverage between = 1.0 and 0.36, the CO diffusion coefficients (DCO) follow a typical Arrhenius behavior and both the activation energies (Ed) as well as the pre-exponential factors ( ) show CO coverage dependence. For partially CO covered samples, Ed decreases linearly with increasing CO coverage, indicating that the repulsive CO-CO interactions exert a stronger influence on the coverage dependence of the activation energy than does the nature of the CO adlayer structure. On the other hand, shows an exponential decrease with increasing CO coverage, consistent with the free site hopping model [Gomer, R. Rep. Prog. Phys. 1990, 53, 917] as the major mechanism for surface diffusion of CO at partial coverages, unlike the situation found with a fully CO covered surface [Kobayashi et al., J. Am. Chem. Soc., 2005, 127, 14164]. Overall, these results are of interest since they improve our understanding of the surface dynamics of molecules at electrochemical interfaces, and may help facilitate better control of fuel cell reactions in which the presence of surface CO plays a crucial role in controlling electrocatalytic reaction rates.

-Melt state 13C NMR of polymers.

Macromolecules, 40 (9), 3505 -3509, 2007. 10.1021/ma070377q S0024-9297(07)00377-4
Microstructural Analysis of Insoluble Polyolefins by Melt-State 13C NMR at Very High Temperatures
Wei Hu,
Hideaki Hagihara, and Toshikazu Miyoshi*
Introduction: For polyolefins, their macroscopic properties, namely, crystallinity, crystallization temperature, lamellar thickness, and mechanical properties, are closely related to their microstructural parameters such as molecular weight, molecular weight distribution, stereoregularity, regiodefect concentration, branch length, and concentration.1-5 Therefore, the control of such microstructural parameters is industrially and scientifically important for producing various types of polyolefins with different macroscopic properties.1,3,4 The analysis of the microstructural parameters is also an important topic for newly synthesized polymers, and 13C solution-state NMR analysis has been successfully applied to characterizing locally heterogeneous structures such as end groups, regiodefects, stereodefects,6-11 and branch structures.12,13 On the other hand, solution-state 13C NMR analysis always suffers from the sensitivity of small signals corresponding to minor defect structures. Furthermore, by this technique, it is impossible to apply insoluble polymers such as cross-linked polyolefins and C3 branch poly( -olefins) such as isotactic poly(3-methyl-1-butene) (iP3M1B)14-17 and isotactic poly(3-methyl-1-penentene) (iP3M1P).18 For such polymers, microstructural analysis cannot be realized by solution-state NMR. In these cases, fractionation by solvent extraction has been utilized as a simple and practical means of performing a rough microstructural analysis. Also, solution-state NMR was applied to the analysis of the soluble fractions and the decomposition of insoluble parts by heat treatment,18 or of model compounds of oligomers,15,16 or a sample with a low tacticity.17
To overcome sensitivity and solubility problems, melt-state NMR has also been developed for the microstructural analysis of polymers.
19-22 In the melt state, rapid dynamics in the molten state considerably averages out 13C line widths broadened because of anisotropic interactions such as heteronuclear 1H-13C dipolar interactions and chemical shift anisotropy and because of conformations and disorders of chain packing. Furthermore, the combined use of typical solid-state NMR techniques, magic-angle sample spinning (MAS), and 1H dipolar decoupling (DD) in the detection period further decreases the line widths. Consequently, high-resolution NMR is feasible with the solid-state NMR apparatus at high temperatures above Tm. Melt-state NMR analysis requires no solvent, and therefore, a high polymer density (a high filling factor) within the NMR coil induces a large enhancement in signal intensities. Recently, Pollard et al. have optimized experimental times using a high filling factor and a transient nuclear Overhauser enhancement (NOE) effect and concluded that the sensitivity enhancement for melt-state 13C NMR is 30 times higher than that for solution-state NMR.21 Very recently, Klimke et al. have further investigated spectral resolution and sensitivity of melt-state NMR using different NMR rotor sizes (4 and 7 mm) and those by various 1H DD methods at various magnetic fields from 300 to 700 MHz.22 They concluded that the application of a high magnetic field of 500 MHz and a 7 mm probe head provides the best sensitivity and that decoupling effectively increases spectral resolution and allows a full observation of free induction decay (FID). Consequently, they successfully evaluated branch content with a concentration of 0.01% and copolymer contents by melt-state NMR analysis.22

-More ­13C NMR of Polymers

Macromolecules, 40 (10), 3615 -3623, 2007. 10.1021/ma062689j S0024-9297(06)02689-1
Identification of Oxidation Products in Selectively Labeled Polypropylene with Solid-State 13C NMR Techniques
Daniel M. Mowery,* Roger L. Clough, and Roger A. Assink
Abstract: Oxidatively degraded polypropylene (PP) samples, with selective 13C labeling of the three carbon sites on the PP chain (tertiary, secondary, and methyl carbons), have been analyzed with a suite of one- and two-dimensional solid-state 13C nuclear magnetic resonance (NMR) experiments that have been used to assign several 13C resonances attributed to oxidation-induced functional groups. These NMR techniques, several of which were recently developed, included dipolar dephasing for MAS speeds 10 kHz, chemical shift anisotropy (CSA) filtering, SUPER NMR to separate quasi-static CSA patterns, and 1H-13C heteronuclear correlation (HETCOR). In the course of the study, it has been demonstrated that NMR experiments which utilize the 13C CSA for resonance identification can be sensitive to sample temperature as a result of molecular motion-induced averaging of the CSA. These experiments have allowed hemiketal groups to be identified for the first time to our knowledge in oxidized PP. Possible mechanisms for the formation of hemiketals and other functional groups have been discussed.

-Microwave spectroscopy used to determine the structure of a metallocene.

Organometallics, 26 (8), 2070 -2076, 2007. 10.1021/om061027f S0276-7333(06)01027-2
Microwave Spectra and Gas-Phase Structural Parameters of Bis( 5-cyclopentadienyl)tungsten Dihydride
Brandon S. Tackett, Chandana Karunatilaka, Adam M. Daly, and Stephen G. Kukolich* Abstract: Microwave spectra for 11 isotopomers of bis( 5-cyclopentadienyl)tungsten dihydride ((C5H5)2WH2) were recorded in the 5-14 GHz region using a Flygare-Balle-type pulsed beam spectrometer. Spectra arising from four tungsten isotopomers of both the (C5H5)2WH2 and (C5H5)2WHD species and three W isotopomers for the (C5H5)2WD2 complex have been measured. The ~250 b-type transition frequencies assigned for these near-prolate asymmetric top molecules were accurately described ( fit = 2-4 kHz) using the rotational parameters A, B, and C and one centrifugal distortion constant, J. The small value obtained for J indicates a fairly rigid structure. From a least-squares fit using the resulting 33 rotational constants to obtain the molecular structure, we were able to determine the W-H bond length, r(W-H) = 1.703(2) Å, the H-W-H bond angle, (H-W-H) = 78.0(12) , the W-Cp centroid distance, r(W-Cp) = 1.940(8) Å, the angle made by the Cp centroids with tungsten, (Cp-W-Cp) = 155(2) , and the average C-C bond length, r(C-C) = 1.429(8) Å. The hydrogen atom separation is r(H-H) = 2.14(2) Å, indicating that this is clearly a "classical dihydride" rather than an " 2-dihydrogen" complex. The WH2 moiety parameters determined from Kraitchman's equations (r(W-H) = 1.682(2) Å, (H-W-H) = 78.6(2), r(H-H) = 2.130(2) Å) agree well with the least-squares results. Furthermore, the re parameters obtained from DFT calculations agree well with the experimental r0 structural parameters. To our knowledge, this work marks the first microwave study of a bent-metallocene complex. The present measurements were made with a pulsed-beam Fourier transform spectrometer employing a homodyne-type detection system, and this configuration is described. This homodyne system greatly simplifies the microwave circuit, with no apparent loss in sensitivity.

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