Wednesday, June 25, 2008

J Phys Chem B, Phys Chem C, Organometallics and Macromolecules Update

J. Phys. Chem. B, 112 (22), 6747–6752, 2008. 10.1021/jp711417t
Local Structure of Magnesium in Silicate Glasses: A 25Mg 3QMAS NMR Study
Keiji Shimoda,
* Takahiro Nemoto,§ and Koji Saito
Abstract: We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.

J. Phys. Chem. B, 112 (25), 7363–7369, 2008. 10.1021/jp800769u
Solvation and Aggregation of N,N′-Dialkylimidazolium Ionic Liquids: A Multinuclear NMR Spectroscopy and Molecular Dynamics Simulation Study
Richard C. Remsing, Zhiwei Liu, Ivan Sergeyev, and Guillermo Moyna*
Abstract: The solvation and aggregation of the ionic liquid (IL) 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl) in water and dimethylsulfoxide (DMSO) were examined by analysis of 1H and 35/37Cl chemical shift perturbations and molecular dynamics (MD) simulations. Evidence of aggregation of the IL n-butyl chains in aqueous environments at IL concentrations of 75−80 wt% was observed both in the NMR experiments and in the MD simulations. The studies also show that [C4mim]Cl behaves as a typical electrolyte in water, with both ions completely solvated at low concentrations. On the other hand, the data reveal that the interactions between the [C4mim]+ and Cl− ions strengthen as the DMSO content of the solutions increases, and IL-rich clusters persist in this solvent even at concentrations below 10 wt%. These results provide an experimentally supported atomistic explanation of the effects that these two solvents have on some of the macroscopic properties of [C4mim]Cl. The implications that these findings could have on the design of IL-based solvent systems are briefly discussed.

J. Phys. Chem. C, 112 (21), 7818–7828, 2008. 10.1021/jp710981g Web Release Date: May 8, 2008
Aggregation of Borate Salts in Hydrocarbon Solvents†
B. Endeward,‡§ P. Brant,∥ R. D. Nielsen,‡ M. Bernardo,‡⊥ K. Zick,# and H. Thomann*‡
Abstract: The aggregation of ammonium borate salts in hydrocarbon was investigated by 1H, 19F, and 11B pulsed field gradient (PFG) NMR. The molecular self-diffusion coefficients of the borate salts, [C(C6H5)3][B(C6F5)4], [N(CH3)2(n-C18H37)2][B(C6F5)4], [N(n-C18H37)4][B(C6F5)4], a neutral borane compound, and a siloxane model compound were measured in toluene, cyclohexane, hexane, and a solvent of mixed alkanes. Diffusion coefficients were determined from the echo attenuation of the stimulated echo pulsed field gradient NMR signal as described by Stejskal and Tanner. In all of the samples studied, the echo decay was observed to be a pure exponential decay, corresponding to a single diffusion coefficient within the resolution of the NMR experiment. The hydrodynamic radius of an equivalent diffusing sphere was calculated from the experimental diffusion coefficients using the Stokes−Einstein relation. We found that the neutral borane and siloxane model compounds are monomeric (nonaggregated) in both the aliphatic and aromatic solvents. In contrast, the borate salts exist as simple ion pairs in aromatic solvents, and as larger aggregates in aliphatic solvents for concentrations above approximately 1 mM. In the aliphatic solvents ion pair aggregate numbers are found which range from 5 1 to 11 2 ion pairs. Energy minimized structures of ion-pair multiplets were obtained using molecular mechanics simulations, and were used to establish the dependence of molecular volume on aggregate size. The aggregation of ions in nonpolar solvents with low dielectric constant is consistent with the known chemistry of electrolyte solutions. Ethylene−propylene copolymerizations were carried out in hexanes diluent where 0−2 molar equivalents of [N(n-C18H37)4][B(C6F5)4] was added to a metallocene catalyst and the concentration of the metallocene catalyst was held constant at 0.2 µM. Catalyst productivity decreases and ethylene incorporation increases with increasing ratio of [N(n-C18H37)4][B(C6F5)4] to metallocene catalyst. From these observations, it is inferred that, even under typical catalytic conditions, the ammonium borate salt is in close contact with the metallocene catalyst during polymerization.

J. Phys. Chem. C, 112 (21), 7895–7899, 2008. 10.1021/jp711727c
13C and 15N NMR and ab Initio/GIAO-CCSD(T) Study of the Structure of Mono-, Di-, and Triprotonated Guanidine, Urea, and Thiourea

George A. Olah,* G. K. Surya Prakash, and Golam Rasul
Abstract: Structures and 13C and 15N NMR chemical shifts of mono- and diprotonated guanidine, urea, and thiourea were studied by the ab initio/GIAO-SCF, GIAO-MP2, and GIAO-CCSD(T) methods. Calculated 15N NMR chemical shifts by these methods correlate very well with the experimental data. Also the calculated 13C NMR chemical shifts of the ions agree well with the available experimental values. Possible solvent effects on the 15N NMR chemical shifts in superacid media are also discussed. Structures and 15N and 13C NMR chemical shifts of yet experimentally unknown triprotonated guanidine, urea, and thiourea trications were also calculated.

J. Phys. Chem. C, 112 (22), 8481–8485, 2008. 10.1021/jp800486n
Hydrogen Sorption Behavior of the ScH2−LiBH4 System: Experimental Assesment of Chemical Destabilization Effects
Justin Purewal,
* Son-Jong Hwang, Robert C. Bowman, Jr.,§ Ewa Rönnebro, Brent Fultz, and Channing Ahn
Abstract: The hydrogen storage reaction ScH2 + 2LiBH4 → ScB2 + 2LiH + 4H2 (8.91 wt %), based on the thermodynamic destabilization of LiBH4, is predicted to have a reaction enthalpy of ΔH300K = 34.1 kJ/mol H2. The isothermal kinetic desorption behavior in this system was measured. At temperatures up to 450 °C, less than 5 wt % H2 is released, which is only half of the theoretical capacity. Powder X-ray diffraction data indicate that LiBH4 has decomposed into LiH in the final desorption product, but the data provide no evidence that ScH2 has participated in the reaction. Magic angle spinning NMR (MAS NMR) results do not show that the expected ScB2 equilibrium product phase has formed during desorption. While the addition of 2 mol % TiCl3 catalyst does improve desorption kinetics at 280 °C, it does not otherwise assist the destabilization reaction. The calculated reaction enthalpy suggests that this system should be of interest at moderate temperatures, but the large heats of formation of the reactant phases in this system appear to play a critical role in determining overall kinetics. Furthermore, the formation of a Li2B12H12 intermediate phase was determined by MAS NMR, which is an undesirable stable product if reaction reversibility is to be accomplished.

J. Phys. Chem. C, 112 (23), 8575–8586, 2008. 10.1021/jp711137c
Directly Probing the Metal Center Environment in Layered Zirconium Phosphates by Solid-State 91Zr NMR
Zhimin Yan, Christopher W. Kirby, and Yining Huang
Abstract: Layered zirconium phosphates (ZrPs) and their derivatives potentially have many important applications. In the past, these materials have been mainly characterized by X-ray diffraction and 31P MAS NMR. Their metal centers have not been directly probed by solid-state NMR spectroscopy. In this work, we present solid-state 91Zr NMR spectra acquired at several magnetic fields for several representative layered zirconium phosphates including α-Zr(HPO4)2·H2O, γ-Zr(PO4)(H2PO4)·2H2O, Zr(NH4PO4)2·H2O, and Zr2(NaPO4)4·6H2O. The NMR interaction tensors were extracted from the spectra. The results indicate that the 91Zr spectra are sensitive not only to the relatively small distortion in ZrO6 polyhedron but also to the difference in the geometry of Zr(OP)6 units in these materials. We show that 91Zr quadrupolar coupling constants (CQ) correlate well with several angular distortion parameters reflecting the deviation from a perfect ZrO6 octahedron such as distortion index, shear strain, and mean O−Zr−O angle. The relationships between CQ and structural parameters related to the Zr(OP)6 unit including the mean Zr−P distance and Zr−O−P angle also appear to exist. The theoretical calculations at both restricted Hartree−Fock and density functional levels were performed on model clusters to establish the relationships of various structural parameters with 91Zr EFG tensors, and the calculation results are consistent with the empirical correlations. For the related layered zirconium phosphates whose structures are unknown or poorly described, we have shown that 91Zr NMR can be used to directly obtain structural information on the local environment around the metal centers, which is complementary to that obtained from powder XRD and 31P MAS NMR, as demonstrated by a novel meso-lamellar ZrP as an example.

J. Phys. Chem. C, 112 (25), 9145–9154, 2008. 10.1021/jp711398h
Dynamics and Disorder in Surfactant-Templated Silicate Layers Studied by Solid-State NMR Dephasing Times and Correlated Line Shapes
Sylvian Cadars,
Nicolas Mifsud, Anne Lesage, Jan Dirk Epping,§ Niklas Hedin, Bradley F. Chmelka, and Lyndon Emsley*
Abstract: Surfactant-templated layered silicates are shown to possess complex compositional, structural, and dynamic features that manifest rich and interrelated order and disorder at molecular length scales. Temperature-dependent 1D and 2D solid-state 29Si NMR measurements reveal a chemical-exchange process involving the surfactant headgroups that is concomitant with reversible broadening of 29Si NMR line shapes under magic-angle-spinning (MAS) conditions at temperatures in the range 205−330 K. Specifically, the temperature-dependent changes in the 29Si transverse dephasing times T2′ can be quantitatively accounted for by 2-fold reorientational dynamics of the surfactant headgroups. Variable-temperature analyses demonstrate that the temperature-dependent 29Si shifts, peak broadening, and 2D 29Si{29Si} correlation NMR line shapes are directly related to the freezing of the surfactant headgroup dynamics, which results in local structural disorder within the silicate framework.

J. Phys. Chem. C, 112 (25), 9267–9271, 2008. 10.1021/jp800873c
Determining the Diameter of Functionalized Single-Walled Carbon Nanotubes with 13C NMR: A Theoretical Study
Eva Zurek,
* Chris J. Pickard, and Jochen Autschbach*§
Abstract: NMR chemical shifts of N–R-functionalized (n, 0) single-walled carbon nanotubes (SWNTs) with n = 11, 13−17 were computed from first principles. The chemical shifts of functionalized carbons at a bond diagonal to the SWNT axis are strongly dependent upon the C−C distance in the C−NR−C moiety. This distance, in turn, is sensitive to the SWNT radius. Monitoring these shifts could therefore help to determine the diameter distribution within a sample. Proton shifts are also reported.

J. Phys. Chem. C, 112 (25), 9486–9492, 2008. 10.1021/jp802631u
Role of Pentacoordinated Al3+ Ions in the High Temperature Phase Transformation of γ-Al2O3
Ja Hun Kwak, Jianzhi Hu, Adrienne Lukaski,
Do Heui Kim, János Szanyi, and Charles H. F. Peden*
Abstract: In this work, the structural stability of γ-alumina (γ-Al2O3) was investigated by a combination of XRD and high-resolution solid-state 27Al MAS NMR at an ultrahigh magnetic field of 21.1 T. XRD measurements show that γ-Al2O3 undergoes a phase transition to θ-Al2O3 during calcination at 1000 °C for 10 h. The formation of the θ-Al2O3 phase is further confirmed by 27Al MAS NMR; additional 27Al peaks centered at 10.5 and ~78 ppm were observed in samples calcined at this high temperature. Both the XRD and NMR results indicate that, after calcination at 1000 °C for 10 h, the ratio of the θ-Al2O3 phase to the total alumina in samples modified by either BaO or La2O3 is significantly reduced in comparison with γ-Al2O3. 27Al MAS NMR spectra revealed that the reduction in the extent of θ-Al2O3 formation was highly correlated with the reduction in the amount of pentacoordinated aluminum ions, measured after 500 °C calcination, in both BaO- and La2O3-modified γ-Al2O3 samples. These results strongly suggest that the pentacoordinated aluminum ions, present exclusively on the surface of γ-Al2O3, play a critical role in the phase transformation of γ-Al2O3 to θ-Al2O3. The role of the modifiers, in our case BaO or La2O3, is to convert the pentacoordinated aluminum ions into octahedral ones, thereby improving the thermal stabilities of the samples. Oxide additives, however, seem to have little, if any beneficial effect on preventing reductions in specific surface areas that occurred during high-temperature (≤1000 °C) calcination

ASAP Organometallics, ASAP Article, 10.1021/om800260c
6Li NMR Study of the Equilibrium between Methyllithium and n-Butyllithium in THF. Influence of a Third Partner, a Chiral Lithium Amide
Franck Paté,
Hassan Oulyadi,* Anne Harrison-Marchand, and Jacques Maddaluno*
Abstract: A 6Li NMR structural study of mixtures of labeled methyllithium, n-butyllithium, and 3-aminopyrrolidine lithium amide has been achieved in THF at −78 °C. In the first part, the study was focused on the mixed aggregates formed between methyllithium and n-butyllithium. The spectral data suggest that the two reactants form a series of tetramers (MeLi)4(n-BuLi)4−n, of which abundance depends on the proportion of the two alkyllithiums following a purely statistical distribution. The labeled chiral lithium amide was next introduced in the sample, and the competitive formation of the amide−methyllithium and amide−butyllithium mixed dimers was monitored in situ. The NMR spectra show a juxtaposition of signals typical of the entities characterized separately, suggesting that there is no preferential formation of one aggregate over the others.

Organometallics, 27 (12), 2756–2760, 2008. 10.1021/om800096f
Homoleptic Complexes of Bisstannylenes with Nickel(0): Synthesis, X-Ray Diffraction Studies, and 119Sn NMR Investigations
Alexander V. Zabula, Tania Pape, Alexander Hepp, and F. Ekkehardt Hahn
Abstract: The benzannulated bisstannylenes linked by a CH2C(CH3)2CH2 (3) or 1,3-(CH2)2C6H4 bridging unit (4) react with [Ni(cod)2] to give the intensely colored homoleptic Ni0 complexes 5 and 6. X-ray diffraction and 119Sn NMR studies for the complex of the bisstannylene with the CH2C(CH3)2CH2 linker 5 reveal the presence of four identical tin atoms. For the nickel complex 6 of the bisstannylene ligand, which contains a 1,3-(CH2)2C6H4 bridging unit, two types of tin atoms with different coordination environments were found both in solution and in the solid state.

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