Solid-State NMR Literature Blog: December 2008

Tuesday, December 23, 2008

J. Am. Chem. Soc., 2008, 130 (51), pp 17502–17508

2H Solid-State NMR of Ruthenium Complexes

Bernadeta Walaszek, Anna Adamczyk, Tal Pery, Xu Yeping†, Torsten Gutmann, Nader de Sousa Amadeu, Stefan Ulrich†, Hergen Breitzke, Hans Martin Vieth, Sylviane Sabo-Etienne§, Bruno Chaudret§, Hans-Heinrich Limbach† and Gerd Buntkowsky

Abstract
The 2H solid-state NMR spectra of the transition metal complexes Tp*RuD(THT)2 (1a), Tp*RuD(D2)(THT) (1b), Tp*RuD(D2)2 (1c), Cp*RuD3(PPh3) (2) and RuD2(η2-D2)2(PCy3)2 (3) have been measured in a wide temperature range. These compounds were chosen as potential model systems for hydrogen surface species in Ru-nanoparticles. The deuterium quadrupolar coupling constants Qcc and asymmetry parameters were extracted by 2H NMR line-shape analysis. The Qcc values of the deuterons bound to the metal vary between 13 kHz and 76 kHz. In addition all spectra show that some of the deuterium is incorporated into carbon positions exhibiting quadrupolar coupling constants in the range of 134 kHz to 192 kHz. The room temperature spectra contain an additional weak very narrow line which was assigned to deuterons exhibiting a high mobility. These deuterons are attributed to crystallographic impurity and partially to D2 molecules which lost by the complexes. The temperature where their motion is quenched and the types of these motions depend on the chemical structure. We propose to use the values of the quadrupolar coupling constants measured in order to characterize different hydrogen species on the surface of Ru-nanoparticles.

Friday, December 19, 2008

Special Issue Coordination Chemistry Reviews: Applications of NMR to Inorganic and Organometallic Chemistry

Thursday, December 18, 2008

J Phys Chem C Issue 50

Methane Activation over Zn-Modified MFI Zeolite: NMR Evidence for Zn−Methyl Surface Species Formation
Yuriy G. Kolyagin†, Irina I. Ivanova*†, Vitaly V. Ordomsky†, Antoine Gedeon‡ and Yuri A. Pirogov§
J. Phys. Chem. C, 2008, 112 (50), pp 20065–20069

Abstract: The early stages of methane activation over Zn-modified H-MFI catalysts obtained by high-temperature reaction with zinc vapor have been studied by 13C MAS NMR in situ. Methane 99.9% enriched with 13C was used as labeled reactant. The spectroscopic data pointed to formation of zinc methyl species at ambient temperature just after methane adsorption onto the zeolite sample. The results suggest that methane activation occurs via dissociative adsorption over acid−base Zn−O pairs involving Zn2+ cations and negatively charged oxygen atoms of the zeolite framework. The nature of the sites responsible for the dissociation is discussed.

Wednesday, December 17, 2008

European Journal of Pharmaceutical Sciences, Volume 34, Issues 2-3, 3 July 2008, Pages 140-148

Anticancer cisplatin interactions with bilayers of total lipid extract from pig brain: A 13C, 31P and 15N solid-state NMR study

Magnus Jensena and Willy Nerdal

Abstract
Cisplatin (cis-diamminedichloroplatinum(II)) is used in chemotherapy and it is well established that cisplatin forms platinum-DNA adducts that initiate tumor cell death. Drawbacks are side effects such as neurotoxicity and cellular cisplatin resistance and it is possible that part of these effects are linked to cisplatin interaction with lipids and the phospholipid bilayer. 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of total lipid extract from pig brain with and without cisplatin show that the phosphatidylserine (PS) carboxyl resonance disappears in presence of cisplatin and that a new resonance of similar intensity appears at 185.5 ppm. Thus, indicating cisplatin interaction with the PS head-group. Static and MAS 31P NMR spectra of lipid extract with and without cisplatin show that the phospholipids to a large extent reside in a bilayer environment in pure lipid extract, and that the presence of cisplatin promotes isotropic and/or hexagonal lipid phases.

Monday, December 15, 2008

J. Am. Chem. Soc., 2008, 130 (50), pp 17148–17153

Competition Saturation Transfer Difference Experiments Improved with Isotope Editing and Filtering Schemes in NMR-Based Screening

Krisztina Fehér†, Patrick Groves‡§, Gyula Batta⊥, Jesús Jiménez-Barbero§, Claudia Muhle-Goll†◼ and Katalin E. Kövér

Abstract
Competition binding experiments are used in NMR-based screenings to match up to the binding site with that of a known ligand and to determine the strength of the interaction, thus providing a ranking of hits according to receptor affinity. These competition titration experiments must use a reference ligand for which the binding site on the receptor and the affinity of the interaction is known. These experiments rely on the observation of separate signals of the reference and hit compounds, which is very often hampered by signal overlap. Here, we present a combination of isotope editing and filtering schemes with saturation transfer difference (STD) experiments that allows the separation of the STD signals of the labeled reference ligand from that of the natural abundance hit compound even in the case of severe signal overlap. Thus, the measurement of well-defined titration curves in competition STD titration experiments is feasible and allows the quantitative determination of binding constants. Note that the method requires the availability of the reference ligand in an NMR-active, stable-isotope-labeled form.

Tuesday, December 09, 2008

J Phys Chem B - October to December

Solvation of Carbohydrates in N,N′-Dialkylimidazolium Ionic Liquids: A Multinuclear NMR Spectroscopy Study
Richard C. Remsing†, Gonzalo Hernandez‡§, Richard P. Swatloski∥, Walter W. Massefski‡, Robin D. Rogers*∥ and Guillermo Moyna*†
J. Phys. Chem. B, 2008, 112 (35), pp 11071–11078

Abstract: The solvation of carbohydrates in N,N′-dialkylimidazolium ionic liquids (ILs) was investigated by means of 13C and 35/37Cl NMR relaxation and 1H pulsed field gradient stimulated echo (PFG-STE) diffusion measurements. Solutions of model sugars in 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl), 1-allyl-3-methylimidazolium chloride ([CC2mim]Cl), and 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) were studied to evaluate the effects of cation and anion structure on the solvation mechanism. In all cases, the changes in the relaxation times of carbon nuclei of the IL cations as a function of carbohydrate concentration are small and consistent with the variation in solution viscosities. Conversely, the 35/37Cl and 13C relaxation rates of chloride ions and acetate ion carbons, respectively, have a strong dependency on sugar content. For [C2mim][OAc], the correlation times estimated from 13C relaxation data for both ions reveal that, as the carbohydrate concentration increases, the reorientation rate of the anion decreases faster than that of the cation. Although not as marked as the variations observed in the relaxation data, similar trends were obtained from the analysis of cation and, in the case of [C2mim][OAc], anion self-diffusion coefficients of the sugar/IL systems. Our results show that the interactions between the IL cation and the solutes are nonspecific, confirm that the process is governed by the interactions between the IL anion and the carbohydrate, and, more importantly, indicate no change in the solvation mechanism regardless of the structure of the anion.

Effects of Salt and Nanoparticles on the Segmental Motion of Poly(ethylene oxide) in Its Crystalline and Amorphous Phases: 2H and 7Li NMR Studies
M. Vogel*†, C. Herbers and B. Koch
J. Phys. Chem. B, 2008, 112 (36), pp 11217–11226

Abstract: We use 2H NMR to investigate the segmental motion of poly(ethylene oxide) (PEO) in neat and nanocomposite materials that do and do not contain salt. Specifically, in addition to a neat low-molecular-weight PEO, we study mixtures of this polymer with TiO2 nanoparticles and LiClO4. To characterize the polymer dynamics over a wide range of time scales, we combine 2H NMR spin−lattice relaxation, line-shape, and stimulated-echo analyses. The results consistently show that the presence of nanoparticles hardly affects the behavior of the polymer, while addition of salt leads to substantial changes; e.g., it reduces the crystallinity. For neat PEO and a PEO−TiO2 mixture, stimulated-echo spectroscopy enables measurement of rotational correlation functions for the crystalline phase. Analysis of the decays allows us to determine correlation times, to demonstrate the existence of a nonexponential relaxation, which implies a high complexity of the polymer dynamics in the crystal, and to show that the reorientation can be described as a large-angle jump. For a PEO−TiO2−LiClO4 mixture, we use 2H and 7Li NMR to study the polymer and the lithium dynamics, respectively. Analysis of the 7Li spin−lattice relaxation reveals a high lithium ionic mobility in this nanocomposite polymer electrolyte. The 7Li stimulated-echo decay is well described by a stretched exponential extending over about 6 orders of magnitude, indicating that a broad and continuous distribution of correlation times characterizes the fluctuations of the local lithium ionic environments.

Nuclear Magnetic Shielding of the 113Cd(II) Ion in Aqua Solution: A Combined Molecular Dynamics/Density Functional Theory Study
Xin Li*†‡, Zilvinas Rinkevicius†, Yaoquan Tu§, He Tian‡ and Hans Ågren†
J. Phys. Chem. B, 2008, 112 (36), pp 11347–11352

Abstract: We present a combined molecular dynamics simulation and density functional theory investigation of the nuclear magnetic shielding constant of the 113Cd(II) ion solvated in aqueous solution. Molecular dynamics simulations are carried out for the cadmium−water system in order to produce instantaneous geometries for subsequent determination of the nuclear magnetic shielding constant at the density functional theory level. The nuclear magnetic shielding constant is computed using a perturbation theory formalism, which includes nonrelativistic and leading order relativistic contributions to the nuclear magnetic shielding tensor. Although the NMR shielding constant varies significantly with respect to simulation time, the value averaged over increasing number of snapshots remains almost constant. The paramagnetic nonrelativistic contribution is found to be most sensitive to dynamical changes in the system and is mainly responsible for the thermal and solvent effects in solution. The relativistic correction features very little sensitivity to the chemical environment, and can be disregarded in theoretical calculations when a Cd complex is used as reference compound in 113Cd NMR experiments, due to the mutual cancelation between individual relativistic corrections.

Pore Structure, Thinning Effect, and Lateral Diffusive Dynamics of Oriented Lipid Membranes Interacting with Antimicrobial Peptide Protegrin-1: 31P and 2H Solid-State NMR Study
Sungsool Wi* and Chul Kim
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061
J. Phys. Chem. B, 2008, 112 (36), pp 11402–11414
Abstract: Membrane pores that are induced in oriented membranes by an antimicrobial peptide (AMP), protegrin-1 (PG-1), are investigated by 31P and 2H solid state NMR spectroscopy. We incorporated a well-studied peptide, protegrin-1 (PG-1), a β-sheet AMP, to investigate AMP-induced dynamic supramolecular lipid assemblies at different peptide concentrations and membrane compositions. Anisotropic NMR line shapes specifying toroidal pores and thinned membranes, which are formed in membrane bilayers by the binding of AMPs, have been analyzed for the first time. Theoretical NMR line shapes of lipids distributed on the surface of toroidal pores and thinned membranes reproduce reasonably well the line shape characteristics of our experimentally measured 31P and 2H solid-state NMR spectra of oriented lipids binding with PG-1. The lateral diffusions of lipids are also analyzed from the motionally averaged one- and two-dimensional 31P and 2H solid-state NMR spectra of oriented lipids that are binding with AMPs.

Oxygen-17 Nuclear Magnetic Resonance Study of the Structure of Mixed Cation Calcium−Sodium Silicate Glasses at High Pressure: Implications for Molecular Link to Element Partitioning between Silicate Liquids and Crystals
Sung Keun Lee*†, George D. Cody‡, Yingwei Fei‡ and Bjorn O. Mysen‡
J. Phys. Chem. B, 2008, 112 (37), pp 11756–11761

Abstract: The structure of silicate glasses and the corresponding liquids at high pressure and their structure−property relations remain difficult questions in modern physical chemistry, geochemistry, and condensed matter physics. Here we report high- resolution solid-state O-17 3QMAS NMR spectra for mixed cation Ca−Na silicate glasses quenched from melts at high pressure up to 8 GPa. The spectra provide the experimental evidence for the varying pressure-dependence in two different types of nonbridging oxygen (NBO) environments (i.e., Na−O−Si and mixed {Ca,Na}−O−Si) in the single glass composition. The percentage of NBO drops significantly with increasing pressure and is a complex function of melt composition, including cation field strength of network modifying cations. A decrease in NBO fraction with pressure is negatively correlated with the element partitioning coefficient between crystals and liquids at high pressure.

High-Resolution Characterization of Liquid-Crystalline [60]Fullerenes Using Solid-State Nuclear Magnetic Resonance Spectroscopy
Sergey V. Dvinskikh†‡, Kazutoshi Yamamoto†, David Scanu§, Robert Deschenaux§ and Ayyalusamy Ramamoorthy*†

J. Phys. Chem. B, 2008, 112 (39), pp 12347–12353
Abstract: Liquid-crystalline materials containing fullerenes are valuable in the development of supramolecular switches and in solar cell technology. In this study, we characterize the liquid-crystalline and dynamic properties of fullerene-containing thermotropic compounds using solid-state natural abundance 13C NMR experiments under stationary and magic angle spinning sample conditions. Chemical shifts spectra were measured in isotropic, liquid-crystalline nematic and smectic A and crystalline phases using one-dimensional 13C experiments, while two-dimensional separated local-field experiments were used to measure the 1H−13C dipolar couplings in mesophases. Chemical shift and dipolar coupling parameters were used to characterize the structure and dynamics of the liquid-crystalline dyads. NMR data of fullerene-containing thermotropic liquid crystals are compared to that of basic mesogenic unit and mesomorphic promoter compounds. Our NMR results suggest that the fullerene−ferrocene dyads form highly dynamic liquid-crystalline phases in which molecules rotate fast around the symmetry axis on the characteristic NMR time scale of ∼10−4 s.

An Unexpected Phase Transition during the [2 + 2] Photocycloaddition Reaction of Cinnamic Acid to Truxillic Acid: Changes in Polymorphism Monitored by Solid-State NMR
Ryan C. Nieuwendaal, Marko Bertmer† and Sophia E. Hayes*
Department of Chemistry and Center for Materials Innovation, Washington University, 1 Brookings Drive, Saint Louis, Missouri 63108
J. Phys. Chem. B, 2008, 112 (41), pp 12920–12926

Abstract: We have detected a phase transition during the progress of the solid-state [2 + 2] photocycloaddition reaction of α-trans-cinnamic acid. The reaction was monitored using 13C CPMAS experiments as a function of irradiation time of the parent α-trans-cinnamic acid, which forms the product dimer, α-truxillic acid. UV light centered at 350 nm was used for photoirradiation, which is in the “tail” of the absorption band of cinnamic acid. Two different crystal polymorphs of α-truxillic acid are observed (P21/n and C2/c) at different stages of conversion of the parent crystal, assigned through 13C NMR and powder X-ray diffraction. The two polymorphs showed clear, distinguishable patterns in the 13C NMR spectra: a 2-peak versus 3-peak pattern corresponding to sites on the 4-membered sp3 hybridized ring in the photoproduct. A phase transition is observed midway through the reaction, which we have assigned to the conversion of the P21/n polymorph to the C2/c polymorph of α-truxillic acid. The packing energy of the resultant mixed crystal of cinnamic acid and truxillic acid changes during the course of the photoreaction, which allows for the C2/c polymorph of truxillic acid to appear. Both phases have been confirmed via X-ray powder diffraction. Two techniquesdifferential scanning calorimetry and solid-state CPMAS NMR using increasingly fast rotational frequenciesdemonstrate that the P21/n phase is metastable.

Hydrogen NMR of H2−TDF−D2O Clathrate
Lasitha Senadheera† and Mark S. Conradi*†‡
Departments of Physics, and Chemistry, Washington University, One Brookings Drive, Saint Louis, Missouri 63130
J. Phys. Chem. B, 2008, 112 (44), pp 13695–13700

Abstract: The three-component clathrate H2−TDF−D2O offers hydrogen storage at lower pressure, but with reduced weight fraction of H2, compared to H2−H2O clathrate. In H2−TDF−D2O, H2 resides exclusively and singly in the small cages of structure II, allowing the rotational behavior of H2 in this nominally uniform environment to be probed. Here we report NMR measurements of the H2 line shape and relaxation times T1, T2, and T1ρ. The principal differences in the results, compared to the H2−D2O binary system, are the dips in T2 and T1ρ near 28 K due to thermally activated reorientation of TDF molecules, line-narrowing and decreases in T2 and T1ρ near 175 K due to D2O reorientations and diffusion, and the apparent absence of H2 diffusion between small cages.

Furanose Dynamics in the HhaI Methyltransferase Target DNA Studied by Solution and Solid-State NMR Relaxation
Dorothy Echodu†, Gil Goobes†§, Zahra Shajani†⊥, Kari Pederson†, Gary Meints†∥, Gabriele Varani†‡ and Gary Drobny*†
J. Phys. Chem. B, 2008, 112 (44), pp 13934–13944
Abstract: Both solid-state and solution NMR relaxation measurements are routinely used to quantify the internal dynamics of biomolecules, but in very few cases have these two techniques been applied to the same system, and even fewer attempts have been made so far to describe the results obtained through these two methods through a common theoretical framework. We have previously collected both solution 13C and solid-state 2H relaxation measurements for multiple nuclei within the furanose rings of several nucleotides of the DNA sequence recognized by HhaI methyltransferase. The data demonstrated that the furanose rings within the GCGC recognition sequence are very flexible, with the furanose rings of the cytidine, which is the methylation target, experiencing the most extensive motions. To interpret these experimental results quantitatively, we have developed a dynamic model of furanose rings based on the analysis of solid-state 2H line shapes. The motions are modeled by treating bond reorientations as Brownian excursions within a restoring potential. By applying this model, we are able to reproduce the rates of 2H spin−lattice relaxation in the solid and 13C spin−lattice relaxation in solution using comparable restoring force constants and internal diffusion coefficients. As expected, the 13C relaxation rates in solution are less sensitive to motions that are slower than overall molecular tumbling than to the details of global molecular reorientation, but are somewhat more sensitive to motions in the immediate region of the Larmor frequency. Thus, we conclude that the local internal motions of this DNA oligomer in solution and in the hydrated solid state are virtually the same, and we validate an approach to the conjoint analysis of solution and solid-state NMR relaxation and line shapes data, with wide applicability to many biophysical problems.

Carboxylic Acid-Doped SBA-15 Silica as a Host for Metallo-supramolecular Coordination Polymers†
D. Akcakayiran‡, D. Mauder§, C. Hess∥⊥, T. K. Sievers#, D. G. Kurth#, I. Shenderovich§, H.-H. Limbach§ and G. H. Findenegg*‡
J. Phys. Chem. B, 2008, 112 (46), pp 14637–14647

Abstract: The adsorption of a metallo-supramolecular coordination polymer (Fe−MEPE) in the cylindrical pores of SBA-15 silica with pure and carboxylic acid (CA) carrying pore walls has been studied. Fe−MEPE is an intrinsically stiff polycation formed by complexation of Fe(II)−acetate with an uncharged ditopic bis-terpyridine ligand. The adsorption affinity and kinetics of the Fe−MEPE chains is strongly enhanced when the pore walls are doped with CA, and when the pH of the aqueous medium or temperature is increased. The initial fast uptake is connected with a decrease of pH of the aqueous solution, indicating an ion-exchange mechanism. It is followed by a slower (presumably diffusion-controlled) further uptake. The maximum adsorbed amount of Fe−MEPE in the CA-doped material corresponds to a monolayer of Fe−MEPE chains disposed side-by-side along the pore walls. The stoichiometry of Fe−MEPE in the pores (determined by XPS) was found to be independent of the loading and similar to that of the starting material. The mean chain length of Fe−MEPE before and after embedding in the CA-doped matrix was studied by solid-state 15N NMR using partially 15N-labeled Fe−MEPE. It is shown that the average chain length of Fe−MEPE is reduced when the complex is incorporated in the pores.

Order and Dynamics of a Liquid Crystalline Dendrimer by Means of 2H NMR Spectroscopy
Valentina Domenici*†, Mario Cifelli†, Carlo Alberto Veracini†, Natalia I. Boiko‡, Elena V. Agina‡ and Valery P. Shibaev‡
J. Phys. Chem. B, 2008, 112 (47), pp 14718–14728

Abstract: A complete Deuterium NMR study performed on partially deuterated liquid crystalline carbosilane dendrimer is here reported. The dendrimer under investigation shows a SmA phase in a large temperature range from 381 to 293 K, and its mesophasic properties have been previously determined. However, in this work the occurrence of a biphasic region between the isotropic and SmA phases has been put in evidence. The orientational order of the dendrimer, labeled on its lateral mesogenic units, is here evaluated in the whole temperature range by means of 2H NMR, revealing a peculiar trend at low temperatures (T < 326 K). This aspect has been further investigated by a detailed analysis of the 2H NMR spectral features, such as the quadrupolar splitting, the line shape, and the line-width, as a function of temperature. In the context of a detailed NMR analysis, relaxation times (T1 and T2) have also been measured, pointing out a slowing down of the dynamics by decreasing the temperature, which determines from one side the spectral changes observed in the NMR spectra, on the other the observation of a minimum in the T1.

A Joined Theoretical−Experimental Investigation on the 1H and 13C NMR Signatures of Defects in Poly(vinyl chloride)
Philippe d’Antuono, Edith Botek and Benoît Champagne
Joris Wieme, Marie-Françoise Reyniers and Guy B. Marin
Peter J. Adriaensens and Jan M. Gelan
J. Phys. Chem. B, 2008, 112 (47), pp 14804–14818

Abstract: 1H and 13C chemical shifts of PVC chains have been evaluated using quantum chemistry methods in order to evidence and interpret the NMR signatures of chains bearing unsaturated and branched defects. The geometrical structures of the stable conformers have been determined using molecular mechanics and the OPLS force field and then density functional theory with the B3LYP functional and the 6-311G(d) basis set. The nuclear shielding tensor has been calculated at the coupled-perturbed Kohn−Sham level (B3LYP exchange-correlation functional) using the 6-311+G(2d,p) basis set. The computational scheme accounts for the large number of stable conformers of the PVC chains, and average chemical shifts are evaluated using the Maxwell−Boltzmann distribution. Moreover, the chemical shifts are corrected for the inherent and rather systematic errors of the method of calculation by employing linear regression equations, which have been deduced from comparing experimental and theoretical results on small alkane model compounds containing Cl atoms and/or unsaturations. For each type of defect, PVC segments presenting different tacticities have been considered because it is known from linear PVC chains that the racemic (meso) dyads are characterized by larger (smaller) chemical shifts. NMR signatures of unsaturations in PVC chains have been highlighted for the internal CHCH and CHCCl units as well as for terminal unsaturations like the chloroallylic CHCHCH2Cl group. In particular, the 13C chemical shifts of the two sp2 C atoms are very close for the chloroallylic end group. The CH2 and CHCl units surrounding an unsaturation present also specific 13C chemical shifts, which allow distinguishing them from the others. In the case of the proton, the CH2 unit of the CHClCH2CClCH segment presents a larger chemical shift (2.6−2.7 ppm), while some CHCl units close to the CHCH unsaturations appear at rather small chemical shifts (3.7 ppm). The CH2Cl and CHClCH2Cl branches also display specific signatures, which result in large part from modifications of the equilibrium conformations and their reduced number owing to the increased steric interactions. These branches lead to the appearance of 13C peaks at lower field associated either to the CH unit linking the CH2Cl and CHClCH2Cl branches (50 ppm) or to the CHCl unit of the ethyl branches (60 ppm). The corresponding protons resonate also at specific frequencies: 3.5−4.0 ppm for the CH2Cl branch or 3.8−4.2 ppm for the terminal unit of the CHClCH2Cl branch. Several of these signatures have been detected in the experimental 1H and 13C NMR spectra and are consistent with the reaction mechanisms.

Acid−Base Interactions and Secondary Structures of Poly-l-Lysine Probed by 15N and 13C Solid State NMR and Ab initio Model Calculations
Alexandra Dos‡, Volkmar Schimming‡, Sergio Tosoni§ and Hans-Heinrich Limbach*‡
J. Phys. Chem. B, 2008, 112 (49), pp 15604–15615

Abstract: The interactions of the 15N-labeled amino groups of dry solid poly-l-lysine (PLL) with various halogen and oxygen acids HX and the relation to the secondary structure have been studied using solid-state 15N and 13C CPMAS NMR spectroscopy (CP = cross polarization and MAS = magic angle spinning). For comparison, 15N NMR spectra of an aqueous solution of PLL were measured as a function of pH. In order to understand the effects of protonation and hydration on the 15N chemical shifts of the amino groups, DFT and chemical shielding calculations were performed on isolated methylamine−acid complexes and on periodic halide clusters of the type (CH3NH3+X−)n. The combined experimental and computational results reveal low-field shifts of the amino nitrogens upon interaction with the oxygen acids HX = HF, H2SO4, CH3COOH, (CH3)2POOH, H3PO4, HNO3, and internal carbamic acid formed by reaction of the amino groups with gaseous CO2. Evidence is obtained that only hydrogen-bonded species of the type (Lys−NH2···H−X)n are formed in the absence of water. 15N chemical shifts are maximum when H is located in the hydrogen bond center and then decrease again upon full protonation, as found for aqueous solution at low pH. By contrast, halogen acids interact in a different way. They form internal salts of the type (Lys−NH3+X−)n via the interaction of many acid−base pairs. This salt formation is possible only in the β-sheet conformation. By contrast, the formation of hydrogen-bonded complexes can occur both in β-sheet domains as well as in α-helical domains. The 15N chemical shifts of the protonated ammonium groups increase when the size of the interacting halogen anions is increased from chloride to iodide and when the number of the interacting anions is increased. Thus, the observed high-field 15N shift of ammonium groups upon hydration is the consequence of replacing interacting halogen atoms by oxygen atoms.

Monday, December 08, 2008

J. Phys. Chem. C - October to December

J. Phys. Chem. C, 112 (31), 11744–11750, 2008. 10.1021/jp803180v Density Functional Study of the 13C NMR Chemical Shifts in Single-Walled Carbon Nanotubes with Stone−Wales Defects
Eva Zurek,† Chris J. Pickard,‡ and Jochen Autschbach*§
Abstract: The 13C NMR chemical shifts of (7,0), (8,0), (9,0), and (10,0) single-walled carbon nanotubes (SWNTs) with Stone−Wales (SW) defects have been studied computationally using a gauge-including projector-augmented plane-wave (GIPAW) density functional theory (DFT) method. A SW-defect substantially broadens the NMR signal of a particular tube, however, in general the average shift of the non-defect carbons does not differ greatly from that of the pristine species. “Parallel” orientations of the defect site yields shifts at around 150−160 ppm from atoms in the defect site which are separated from the rest of the NMR signal. Therefore, the results indicate that 13C NMR might be able to detect the presence of, and perhaps even quantify the concentration of SW defects found in SWNTs. Differences in the NMR obtained for two defect orientations are analyzed by comparing the shifts of the defect atoms with those of planar and bent structures of the azupyrene molecule. Representative visualizations for the shielding tensors of the (8,0) SWNT with and without defects are also reported.

J. Phys. Chem. C, 112 (31), 11869–11874, 2008. 10.1021/jp802162h Kinetics of H/D Exchange for n-Butane on Zeolite H-ZSM-5 Studied with 1H MAS NMR In Situ
Sergei S. Arzumnov,† Alexander G. Stepanov,*† and Dieter Freude*‡
Abstract: The kinetics of hydrogen (H/D) exchange between Brønsted acid sites of zeolite H-ZSM-5 and deuterated n-butanes (n-butane-d10 and n-butane-1,1,1,4,4,4-d6) has been monitored by 1H magic-angle spinning (MAS) NMR spectroscopy in situ within the temperature range of 423−448 K. The initial part of the kinetics is defined mainly by the hydrogen exchange, whereas the final part is strongly influenced by the chemical transformation of the alkane. Analysis of the initial part has been performed on the basis of consecutive, parallel, and cyclic kinetic schemes of the H/D exchange. It has been found that both the methyl and methylene groups of n-butane are directly involved in the exchange with acidic SiOHAl groups of the zeolite. No intramolecular hydrogen exchange between the methyl and the methylene groups of the adsorbed n-butane has been detected. Similar rates of the direct exchange of either the methyl or methylene group with acidic SiOHAl groups and the apparent activation energy of 108 kJ mol−1 are rationalized in terms of the carbonium ion mechanism of the exchange with the involvement of a pentacoordinated carbon atom in a transition state.

J. Phys. Chem. C, 112 (31), 11893–11900, 2008. 10.1021/jp802928n 1H and 7Li NMR Pulsed Gradient Spin Echo Measurements and Multiscale Modeling of the Water and Ionic Mobility within Aqueous Dispersions of Charged Anisotropic Nanoparticles
Patrice Porion,* Anne Marie Faugère, and Alfred Delville*
Abstract: The mobility of the water molecules and the neutralizing lithium counterions are simultaneously determined in dense dispersions of charged anisotropic nanoplatelets by exploiting 1H and 7Li pulsed gradient spin echo NMR measurements, respectively. The strong difference between the measured solvent and ionic mobility results from the specific lithium/clay electrostatic coupling responsible for the ionic condensation of the lithium counterions on the basal surface of these charged nanoplatelets. Such a property should be exploited to enhance the retention capacity of clay sediments used as diffusion barriers for the storing of ionic waste. A multiscale modeling of the ionic condensation and diffusion processes is used to semiquantitatively interpret these measurements by relating the dynamical property of the diffusing probes (water molecule and lithium counterion) and the organization of the clay dispersion.

J. Phys. Chem. C, 112 (31), 11901–11906, 2008. 10.1021/jp803003k
Reactions of VX, GD, and HD with Nanotubular Titania
George W. Wagner,*† Qiang Chen,‡ and Yue Wu‡
Abstract: Reactions of VX [O-ethyl-S-(2-diisopropylethylamino)ethyl methylphosphonothioate], GD (pinacolyl methylphosphonofluoridate), and HD [bis(2-chloroethyl) sulfide] have been examined with nanotubular titania (NTT) using 31P and 13C MAS NMR. All three agents hydrolyze on NTT with the reaction of VX being notably fast (t1/2 < 30 min), approaching the rate achievable with liquid decontaminants. 31P MAS NMR reveals that VX is adsorbed within the NTT tubules and/or its titania layers, perhaps providing optimum conditions for its hydrolysis by water sandwiched between the layers. Consistent with the availability of copious water on NTT, HD is hydrolyzed to its CH-TG sulfonium ion. GD hydrolysis is similarly efficient, with its products—PMPA and HF—attacking the titania structure to form a titanophosphonate species.

J. Phys. Chem. C, 112 (32), 12515–12523, 2008. 10.1021/jp801985h
Structure of Tetrakis(melaminium) Bis(dihydrogenphosphate) Monohydrogenphosphate Trihydrate from X-ray Powder Diffraction and Solid-State NMR Spectroscopy
Vladimir Brodski, René Peschar,* and Henk Schenk
Andreas Brinkmann, Ernst R. H. van Eck, and Arno P. M. Kentgens
Abstract: The crystal structure of the melamine phosphate salt tetrakis(melaminium) bis(dihydrogenphosphate) monohydrogenphosphate trihydrate with as much as ten independent moieties in the unit cell was determined by a direct-space global optimization technique from X-ray powder diffraction data using additional geometry constraints obtained by 31P double-quantum solid-state NMR spectroscopy. The structure analysis of the compound and its comparison with other melamine phosphates reveals the packing and bonding characteristics that are important for melamine-phosphate salts with a melamine-to-phosphor ratio larger than one, which are promising environmental-friendly flame retardants.

J. Phys. Chem. C, 112 (32), 12530–12539, 2008. 10.1021/jp8035549
The Mixed-Network Former Effect in Phosphate Glasses: NMR and XPS Studies of the Connectivity Distribution in the Glass System (NaPO3)1−x(B2O3)x
Devidas Raskar, Matthias T. Rinke, and Hellmut Eckert*
Abstract: The structural organization of sodium borophosphate glasses with composition (NaPO3)1−x(B2O3)x (0.0 ≤ x ≤ 0.3) has been investigated by X-ray photoelectron spectroscopy (XPS), as well as single- and double-resonance 11B and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. O-1s XPS data provides a quantitative distinction between B−O−B, B−O−P, and P−O−P linkages as well as nonbridging oxygen atoms. 11B and 31P MAS NMR data indicate that within the compositional region 0 ≤ x ≤ 0.20 the entire boron inventory is present in the form of anionic BO4− units, resulting in the repolymerization of an equivalent fraction of the phosphate units (conversion of anionic metaphosphate (P(2)) into neutral branching groups (P(3)) species. Both XPS as well as 31P{11B} and 11B{31P} rotational echo double resonance (REDOR) NMR results reveal strong interactions between the two network formers boron oxide and phosphorus oxide, resulting in the dominant formation of B−O−P linkages. In addition, the shape of the REDOR curve reveals a certain tendency of these linkages to cluster, consistent with a preference of P(3) units to form more than one P−O−B linkage, even at low boron contents. The enhanced degree of network polymerization correlates with a significant increase of the glass transition temperature as a function of boron content

J. Phys. Chem. C, 112 (33), 12853–12860, 2008. 10.1021/jp801223c
Water Dynamics in Bulk and Dispersed in Silica CaCl2 Hydrates Studied by 2H NMR
Daniil I. Kolokolov,†‡ Ivan S. Glaznev,† Yurii I. Aristov,† Alexander G. Stepanov,*† and Hervé Jobic‡
Abstract: The mobility of water in deuterated analogues of CaCl2·nH2O (n = 2, 4, 6) hydrates has been studied by solid-state 2H NMR spectroscopy. Dynamics of water molecules in hydrates dispersed in the mesopores of silica are compared with those in the bulk state. Analysis of the 2H NMR line shape and T1 and T2 relaxation times allowed us to characterize the water mobility in a wide temperature range (120−493 K). In both crystalline and melted hydrates, the mobility of water molecules is governed by O−D···Cl hydrogen bonding. Both bulk and dispersed hydrates have been found to exhibit three types of molecular motion. Two of these represent fast internal and local motions performed on a time scale of 10−10−10−11 s. The third, slow isotropic reorientation occurs on a time of 10−6−10−7 s. Dispersed hydrates become involved in the slow isotropic motion at temperatures 50−130 K lower than the corresponding bulk hydrates. The temperature TNMR at which dispersed hydrates are involved in isotropic motion represents the melting point of the hydrates located in the silica pores. The decrease of the melting point for the dispersed hydrates is in good accordance with the Gibbs−Thompson effect for dispersed materials. In dispersed hydrates, water molecules reorient isotropically 1 order of magnitude faster in the temperature range 230−490 K; that is, water is more mobile in the dispersed hydrates. The slow isotropic reorientation of water molecules is influenced by both the quantity of water in the hydrate and the dispersibility of the hydrates. In the case of the hydrate with n = 4, the activation energy of this motion decreases by ca. 3 times when the hydrate becomes dispersed in the silica pores.

A Potential Novel Rapid Screening NMR Approach to Boundary Film Formation at Solid Interfaces in Contact with Ionic Liquids

Maria Forsyth*†, Thomas F. Kemp‡, Patrick C. Howlett†, Jiazeng Sun† and Mark E. Smith‡
J. Phys. Chem. C, 2008, 112 (36), pp 13801–13804

Abstract: The boundary films generated on a series of inorganic compounds, typical of native films on metal and ceramic surfaces, when exposed to various ionic liquids (ILs) based on the trihexyl(tetradecyl)phosphonium cation have been characterized using multinuclear solid-state NMR. The NMR results indicate that SiO2 and Mg(OH)2 interact strongly with the anion and cation of each IL through a mechanism of adsorption of the anion and subsequent close proximity of the cation in a surface double layer (as observed through 1H−29Si cross polarization experiments). In contrast, Al2O3, MgO, ZnO, and ZrO2 appear less active, strongly suggesting the necessity of hydroxylated surface groups in order to enhance the generation of these interfacial films. Using solid-state NMR to characterize such interfaces not only has the potential to elucidate mechanisms of wear resistance and corrosion protection via ILs, but is also likely to allow their rapid screening for such durability applications.

Studies of Inclusion Complexes of Dichloromethane in Cryptophanes by Exchange Kinetics and 13C NMR in Solution and the Solid State
S. Nikkhou Aski†, A. Y. H. Lo†, T. Brotin‡, J. P. Dutasta‡, M. Edén† and J. Kowalewski*†

J. Phys. Chem. C, 2008, 112 (36), pp 13873–13881

Abstract: Cryptophanes are molecular hosts having high binding affinities for small, neutral molecules in weakly polar solvents. We have previously studied dichloromethane as a guest inside the cavity of cryptophane-E (cryptophane-333), in solution as well as in the solid state. Here, we present a solution 13C relaxation study of dichloromethane as the guest molecule complexed in the cavity of two smaller hosts: cryptophane-A (cryptophane-222) and cryptophane-223. 13C relaxation measurements were performed for both guest and host molecules. Exchange kinetics have to be explicitly taken into consideration in order to derive 13C relaxation properties of the bound guest. Rate coefficients were measured using proton 1D-exchange experiments. We have also investigated dichloromethane complexed in the cavity of cryptophane-233 in the solid state, by estimating the motionally averaged 13C−1H dipolar interactions from recoupling experiments under magic angle spinning (MAS) conditions. The measurements for the three cryptophanes provide a consistent set of results for the extent of rotational freedom of the guest inside the host cavities.

A Solid-State NMR Spectroscopic Study of the Adsorption of Toluene in Zeolite LiK-L
Jianfeng Zhu and Yining Huang*
J. Phys. Chem. C, 2008, 112 (37), pp 14241–14246

Abstract: The adsorption of toluene in partially Li+-exchanged zeolite K-L (LiK-L) was examined by solid-state NMR spectroscopy. The environment of the Li+ ions at different sites in the zeolite before and after adsorption was characterized by 7Li magic-angle spinning (MAS) NMR. 1H → 7Li cross polarization (CP) and 7Li{1H} rotational-echo double-resonance (REDOR) experiments were performed to probe the cation−sorbate interactions. The data were also used for spectral assignment. The 7Li NMR results indicate that most of the Li+ ions occupy the sites A (39%) and B (43%) and that there are fewer Li+ ions at sites C (17%) and D (1%). The four 7Li NMR signals corresponding to the Li+ ions at four sites are not resolved in the spectrum of the dehydrated zeolite, but are well-separated after the adsorption of toluene. The NMR results show that a toluene molecule is directly coordinated to the Li+ ion at site D near the wall of the main channel via formation of a π-complex, resulting in a significant change in 7Li chemical shift toward a more shielded direction. Although not directly accessible by the toluene molecules, the Li+ ions at sites A and C also exhibit an observable shift upon adsorption. Only the Li+ ions at site B do not experience a significant change in chemical shift. The effect of the adsorption on 7Li chemical shift is discussed. Dynamic behavior of toluene molecules inside the channels was also investigated by wide-line 2H NMR.

Probing the Spatial Proximities among Acid Sites in Dealuminated H-Y Zeolite by Solid-State NMR Spectroscopy
Shenhui Li†, Shing-Jong Huang‡§, Wanling Shen†, Hailu Zhang†, Hanjun Fang†, Anmin Zheng†, Shang-Bin Liu*‡ and Feng Deng*†
J. Phys. Chem. C, 2008, 112 (37), pp 14486–14494

Abstract: A comprehensive study has been made to probe the spatial proximities among different acid sites in dealuminated H-Y zeolites modified with various degrees of calcination, steam, and acid treatments by using a variety of different solid-state NMR techniques, including multinuclear MAS NMR and two-dimensional 1H double-quantum (DQ) MAS NMR spectroscopy. The effects of dealumination treatments on the nature, concentration, and location of extraframework Al species in H-Y zeolites were followed by 1H DQ MAS NMR of hydroxyl protons in conjunction with 1H, 27Al, and 29Si MAS NMR results. It was found that the extraframework AlOH species (Lewis acid sites) are always in close proximity to the bridging AlOHSi hydroxyls (Brønsted acid sites) on the framework of dealuminated H-Y zeolites prepared by thermal and hydrothermal treatments, indicating the presence of a Brønsted/Lewis acid synergy effect. However, such an effect is absent in acid-treated H-Y zeolites, as also confirmed by 13C CP/MAS NMR of adsorbed 2-13C-acetone.

Site-Dependent 13C Chemical Shifts of CO Adsorbed on Pt Electrocatalysts
Patrick McGrath, Aurora Marie Fojas, Jeffrey A. Reimer* and Elton J. Cairns
J. Phys. Chem. C, 2008, 112 (38), pp 14702–14705

Abstract: 13C NMR and hydrogen-region cyclic voltammetry are used to parse the distribution of adsorbed CO on Pt electrocatalysts into two different types of sites. Trends in the NMR shift data show that 13CO adsorbed on so-called weakly bound H sites show larger Knight shifts as compared to 13CO adsorbed onto strongly bound H sites, and thus experience greater back-bonding from the Pt conduction band. These results are discussed in the context of local electron densities of states and the varying oxidation reactivities associated with these sites on the Pt surface.

Size Dependent Coordination Behavior and Cation Distribution in MgAl2O4 Nanoparticles from 27Al Solid State NMR Studies
V. Sreeja†, T. S. Smitha‡, Deepak Nand‡, T. G. Ajithkumar*‡ and P. A. Joy*†
J. Phys. Chem. C, 2008, 112 (38), pp 14737–14744

Abstract Nanoparticles of spinel-type oxides such as ferrites offer great advantages and applications in many important areas. Decreasing the size of the particles to nanometer size will increase the surface-to-volume ratio and this will strongly influence the physical and chemical properties of these materials. For magnetic nanoparticles, the exchange interactions at the surface of a particle will be different from those inside due to changes in the coordination behavior at the surface. Therefore, studying and understanding the coordination and distribution behavior of the different metal ions in the nanoparticles of spinel-type oxides is very important. Solid state NMR is a useful and important technique to obtain information on local structural variations. The degree of the distribution of the Al3+ ions in the tetrahedral and octahedral sites in the nanoparticles of the nonmagnetic spinel MgAl2O4 having different particle sizes has been determined by 27Al magic-angle spinning (MAS) NMR spectroscopy. It has been observed that the inversion parameter decreases with increasing particle size. Apart from the usual tetrahedral and octahedral coordinations present in the bulk material, the presence of five- and three-coordinated Al has been observed in nanoparticles with sizes less than 18 nm and a second octahedral coordination is observed for nanoparticles of larger sizes.

Direct Observation of the Mesopores in ZSM-5 Zeolites with Hierarchical Porous Structures by Laser-Hyperpolarized 129Xe NMR
Yong Liu†, Weiping Zhang*†, Zhicheng Liu‡, Shutao Xu†, Yangdong Wang‡, Zaiku Xie‡, Xiuwen Han† and Xinhe Bao*†
J. Phys. Chem. C, 2008, 112 (39), pp 15375–15381

Abstract: Mesopore-modified ZSM-5 zeolites with hierarchical porous structures (Meso-ZSM-5) have been synthesized by using tetrapropylammonium hydroxide and starch as cotemplates. One- and two-dimensional 129Xe NMR spectroscopy has been employed to study the porosity under the continuous flow of laser-hyperpolarized xenon gas. Besides the micropores, the mesoporous domains in Meso-ZSM-5 zeolites are directly observed by variable-temperature experiments. Combining with nitrogen adsorption, the influence of the Si/Al ratios on the mesopores in Meso-ZSM-5 is also investigated. The exchange of Xe atoms in different types of pores is very fast at ambient temperature. Two-dimensional exchange spectroscopy (EXSY) is used for the first time to monitor such an exchange process, and the results indicate that even at very low temperature Xe atoms still undergo much faster exchange between mesopores and micropores in Meso-ZSM-5 than in the mechanical mixture of conventional ZSM-5 and mesoporous silica. The results demonstrate that these hierarchical pores may have good connected networks that facilitate xenon diffusion and exchange. Also, this may give some indications for other molecules adsorption and diffusion in mesoporous zeolites especially in the process of catalysis.

Crystalline Aluminum Hydroxy Fluorides: Structural Insights Obtained by High Field Solid State NMR and Trend Analyses
R. König†, G. Scholz†, A. Pawlik‡, C. Jäger‡, B. van Rossum§, H. Oschkinat§ and E. Kemnitz*†
J. Phys. Chem. C, 2008, 112 (40), pp 15708–15720

Abstract: A series of crystalline aluminum hydroxy fluorides in cubic pyrochlore structure AlFx(OH)3−x·H2O with variable F-content x were investigated by solid-state NMR by applying different magnetic fields up to 21.1 T. Distinguishable octahedral species AlFx(OH)6−x (x = 1−6) were identified in the crystalline aluminum hydroxy fluorides. The subsequent analysis of the highfield 27Al MAS NMR data allows the derivation of the trend analysis graphs giving correlations between the 27Al chemical shifts and the quadrupolar frequencies and the F-content x in AlFx(OH)6−x. Clear trends were obtained for both, which are, along with the 19F MAS chemical-shift trend analysis presented earlier, valuable tools for the interpretation of MAS NMR spectra of amorphous AlFx(OX)3−x compounds (X = H, alkyl). Following the dehydration of the pyrochlores by solid-state NMR eventually reveals a remarkable influence of the incorporated solvent molecules (H2O) on the 19F chemical shift. On that basis, a new chemical-shift trend analysis for 19F chemical shifts in correlation with x in AlFx(OH)6−x units for proton-poor substances (in the Al, F, O, H system) was determined. By using this correlation, high-surface AlF3 has a mean bulk Al:F ratio similar to that found for ACF, namely, AlF2.8(O/OH)0.2.

Formation, Location, and Photocatalytic Reactivity of Methoxy Species on Keggin 12-H3PW12O40: A Joint Solid-State NMR Spectroscopy and DFT Calculation Study
Hailu Zhang†‡, Anmin Zheng†, Huaguang Yu†‡, Shenhui Li†‡, Xin Lu§ and Feng Deng*†
J. Phys. Chem. C, 2008, 112 (40), pp 15765–15770

Abstract: Solid-state NMR experiments and quantum chemical Density Functional Theory (DFT) calculations were employed to investigate the formation, location, and photocatalytic reactivity of methoxy species on anhydrous 12-H3PW12O40. Two different types of methoxy species were identified on the methanol-adsorbed 12-H3PW12O40 catalyst. Rotational Echo DOuble Resonance (REDOR) NMR experiments combined with quantum chemical DFT calculations demonstrated that the two corresponding methyl groups reside on the Oc and Od atoms of the Keggin anion, forming the surface OcCH3 and OdCH3 species. Photocatalytic experiments further indicated that the two methoxy species are both photochemically reactive species with the OdCH3 species being much more reactive, and the methoxy species are preferentially mineralized to the final product CO2 directly.

First-Principles Calculation of 13C NMR Chemical Shifts of Infinite Single-Walled Carbon Nanotubes: New Data for Large-Diameter and Four-Helical Nanotubes
Lin Lai†, Jing Lu*†‡, Wei Song†, Ming Ni†, Lu Wang†, Guangfu Luo†, Jing Zhou†, Wai Ning Mei‡, Zhengxiang Gao*† and Dapeng Yu†
J. Phys. Chem. C, 2008, 112 (42), pp 16417–16421

Abstract: By using the density functional theory method, we calculate the 13C NMR isotropic chemical shifts of the semiconducting and semimetallic infinite single-walled carbon nanotubes (SWNTs). We find that the 13C chemical shifts of SWNTs with the diameter smaller than 1.4 Å can be classified into two distinct groups according to their electronic structures: the semiconducting group and the semimetallic group. The chemical shifts of the semiconducting group decrease monotonously with the increasing nanotube diameter, and are 0−12 ppm strikingly larger than those of their semimetallic counterparts in the typical diameter range (1.05 ± 0.2 nm) of SWNTs produced by the common high-pressure CO decomposition method (HiPCO). The chemical shifts of the two groups overlap around the diameter of 1.4 Å. Then the chemical shift of the semimetallic group becomes larger than that of the similar-sized semiconducting group as the diameter is larger than 1.4 Å. The chemical shifts of the four examined helical SWNTs are very close to those of the zigzag SWNTs with similar diameters and electronic structures.

On the Use of CHClF2 as a Probe of Basic Sites in Zeolites: The Host−Guest Interactions Investigated by Multinuclear NMR
Manuel Sánchez-Sánchez†‡, Teresa Blasco*† and Avelino Corma†
Instituto de Tecnología Química (UPV-CSIC), Avda. Los Naranjos, s/n, 46022 Valencia, Spain, and Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan Carlos, C/ Tulipán s/n, 28933 Móstoles, Madrid, Spain
J. Phys. Chem. C, 2008, 112 (43), pp 16961–16967

Abstract: The use of chlorodifluoromethane (CHClF2) as a probe molecule of zeolites basicity has been investigated by using infrared and multinuclear NMR spectroscopies and a series of alkali-exchanged faujasite with different Si/Al ratios (X and Y) and compensating cations of different nature. The 1H NMR peak of adsorbed CHClF2 shifts to low fields and the CH stretching frequency (νCH) shifts to low wavenumbers as the zeolite basicity determined by the Sanderson method increases. Poorer linear correlation is observed for the νCH band suggesting the occurrence of extra interactions of the fluorine atoms of the adsorbed chlorodifluoromethane with the nonframework cations. This interaction is evidenced for the first time by the spectroscopic modification of the probe molecule itself; 19F NMR chemical shifts and 1J(C,F) spin−spin coupling constants are determined by the nature of the extraframework atoms and not by the framework basicity. The occurrence of the interactions between the fluoride atoms and the compensating cation does not allow quantifying the number of sites of similar basicity.

Nitrogen-Doped Titanium Dioxide Active in Photocatalytic Reactions with Visible Light: A Multi-Technique Characterization of Differently Prepared Materials
S. Livraghi†, M. R. Chierotti†, E. Giamello*†, G. Magnacca†, M. C. Paganini†, G. Cappelletti‡ and C. L. Bianchi‡
Dipartimento di Chimica IFM, NIS Centre of Excellence, and CNISM, via Giuria 7, 10125 Torino, Italy, and Dipartimento di Chimica Fisica ed Elettrochimica, via Golgi 19, 20133 Milano, Italy
J. Phys. Chem. C, 2008, 112 (44), pp 17244–17252

Abstract: Nitrogen-doped TiO2 materials were successfully prepared following three different preparation routes (sol−gel, mechanochemistry, and oxidation of TiN) and characterized by X-ray diffraction, electron microscopy, and various spectroscopic techniques. All samples absorb visible light, and the one obtained via sol−gel, showing the anatase structure, is the most active in the decomposition of organic compounds under visible light. Various nitrogen-containing species have been observed in the materials, whose presence and abundances depends on the preparative route. Ammonium NH4+ ions are residual of the synthesis using ammonium salts (sol−gel, mechanochemistry) and are quite easily eliminated, as shown by the parallel behavior of both NMR and XPS spectra. Cyanide CN− ions form at high temperature in parallel with the phase transition of the solid to rutile. Molecular nitric oxide forms in the case of materials exhibiting close porosity. The already reported bulk radical species, Nb•, is the only paramagnetic center observed in all types of samples, and is responsible for the visible light sensitization of TiO2. A mechanism for the formation of such a species in chemically prepared N-doped TiO2 materials is for the first time proposed based on the reduction of Nitric Oxide (NO) at oxygen vacancies

Molecules Immobilization in Titania Nanotubes: A Solid-State NMR and Computational Chemistry Study
Qiang Chen†, Yuanyuan Jia†, Shubin Liu‡, Gregory Mogilevsky†, Alfred Kleinhammes† and Yue Wu*†
J. Phys. Chem. C, 2008, 112 (44), pp 17331–17335

Abstract: Molecule immobilization in hydrothermally synthesized titania nanotubes is investigated by 13C solid-state NMR aided density functional theory calculations. The changes of 13C NMR spectra before and after immobilization indicate that phenols are chemisorbed and that aliphatic acid is possibly physisorbed in titania nanotubes. Hydroquinone exhibits monodentate bonds, and catechol exhibits bidentate bonds to accessible surface Ti sites in titania nanotubes. Immobilization of anthrarobin confirms that bidentate bonding is favored over monodentate bonding. Density functional calculations at the B3LYP/6-311+G(2d,p) level suggest that catechol is immobilized dissociatively via bridge bidentate bonding to neighboring surface Ti sites rather than chelate bidentate bonding to single surface Ti sites

Local Effects of the Electrochemical Reaction of Lithium with Sn2ClPO4 and SnHPO4: A Combined 31P, 7Li MAS NMR and 119Sn Mossbauer Spectroscopy Study
J. I. Corredor, B. León*, C. Pérez Vicente and J. L. Tirado

J. Phys. Chem. C, 2008, 112 (44), pp 17436–17442
Abstract: In recent years, metallic tin and tin compounds have proven to be interesting electrode materials for lithium batteries. However, detailed information about the mechanism of reaction in phosphate compounds is needed to improve the performance of some previously examined and future electrode materials containing these species. In the present work, powerful techniques for the study of the local environments of the atoms, such as 119Sn Mössbauer spectrometry and 7Li and 31P MAS NMR, are used to obtain basic information on the processes involved during the discharge/reduction of the electrode material. The results allow extracting valuable information about the possible interactions between the participating atoms and the surrounding framework.

Field-Cycling NMR Relaxometry Study of Dynamic Processes in Conducting Polyaniline
Eoin Murray†, Darren Carty†, Peter C. Innis‡, Gordon G. Wallace‡ and Dermot F. Brougham*†§
J. Phys. Chem. C, 2008, 112 (45), pp 17688–17693
Abstract: Fast-field cycling NMR relaxometry been applied to investigate dynamic processes in the conducting polymer, polyaniline. For a group of samples with different concentrations of the dopant trifluoromethanesulfonic acid, the 1H spin−lattice relaxation rates exhibit power law dependence on the Larmor frequency. The powers obtained are found to increase above a percolation threshold in dopant concentration and to show similar concentration and temperature dependence as is observed for the macroscopic polymer conductivity. These observations are discussed in terms of the accepted models for both the fast polaron dynamics and the slower, low-frequency, polymer dynamics.

133Cs NMR and ESR Studies of Cesium-Loaded LiX and LiA Zeolites
Catherine J. Reinhold†, Paul A. Anderson*†, Peter P. Edwards*†§, Victor V. Terskikh‡, Christopher I. Ratcliffe*‡ and John A. Ripmeester‡

J. Phys. Chem. C, 2008, 112 (46), pp 17796–17803

Abstract: The species generated when cesium metal was loaded into zeolites LiA, LiX, and LiLSX by vapor deposition were systematically investigated by 133Cs NMR and ESR as a function of loading level. The primary 133Cs NMR signal in Cs-loaded LiA at low loading was assigned to Cs+ in the eight-ring SII sites and showed axial anisotropy of the chemical shift. The primary 133Cs NMR signal in Cs-loaded LiX and LiLSX was isotropic and was assigned to Cs+ dynamically exchanging among the SIII sites. This dynamics was found to be frozen out at 173 K. At higher loadings, additional broader 133Cs NMR signals, some with large shifts, indicated the presence of diamagnetic species influenced by neighboring paramagnetic species. The ESR results were characteristic of interacting paramagnetic species, and the spin counts showed that a significant fraction of the introduced spins became spin-paired, consistent with the presence of both paramagnetic and diamagnetic clusters. The existence of caeside Cs− ion in any of these materials is still an open question.

Stability and Reversibility of Lithium Borohydrides Doped by Metal Halides and Hydrides
Ming Au*†, Arthur R. Jurgensen†, William A. Spencer†, Donald L. Anton†, Frederick E. Pinkerton‡, Son-Jong Hwang§, Chul Kim§ and Robert C. Bowman, Jr.∥
J. Phys. Chem. C, 2008, 112 (47), pp 18661–18671

Abstract: In an effort to develop reversible metal borohydrides with high hydrogen storage capacities and low dehydriding temperature, doping LiBH4 with various metal halides and hydrides has been conducted. Several metal halides such as TiCl3, TiF3, and ZnF2 effectively reduced the dehydriding temperature through a cation exchange interaction. Some of the halide doped LiBH4 are partially reversible. The LiBH4 + 0.1TiF3 desorbed 3.5 wt % and 8.5 wt % hydrogen at 150 and 450 °C, respectively, with subsequent reabsorption of 6 wt % hydrogen at 500 °C and 70 bar observed. XRD and NMR analysis of the rehydrided samples confirmed the reformation of LiBH4. The existence of the (B12H12)−2 species in dehydrided and rehydrided samples gives insight into the resultant partial reversibility. A number of other halides, MgF2, MgCl2, CaCl2, SrCl2, and FeCl3, did not reduce the dehydriding temperature of LiBH4 significantly. XRD and TGA-RGA analyses indicated that an increasing proportion of halides such as TiCl3, TiF3, and ZnCl2 from 0.1 to 0.5 mol makes lithium borohydrides less stable and volatile. Although the less stable borohydrides such as LiBH4 + 0.5TiCl3, LiBH4 + 0.5TiF3, and LiBH4 + 0.5ZnCl2 release hydrogen at room temperature, they are not reversible due to unrecoverable boron loss caused by diborane emission. In most cases, doping that produced less stable borohydrides also reduced the reversible hydrogen uptake. It was also observed that halide doping changed the melting points and reduced air sensitivity of lithium borohydrides.

Nuclear Magnetic Resonance Study of the Rotational Motion and the Phase Transition in LiBH4
Alexander V. Skripov*†, Alexei V. Soloninin†, Yaroslav Filinchuk‡ and Dmitry Chernyshov‡
J. Phys. Chem. C, 2008, 112 (47), pp 18701–18705

Abstract: To study the rotational motion of BH4 tetrahedra in LiBH4, we have measured the 1H and 11B nuclear magnetic resonance spectra and spin−lattice relaxation rates in this compound over wide ranges of temperature (92−424 K) and resonance frequency (14−90 MHz for 1H and 14−28 MHz for 11B). In the low-temperature (orthorhombic) phase of LiBH4, our spin−lattice relaxation results are consistent with a coexistence of two types of the rotational motion of BH4 tetrahedra with the activation energies of 0.182 ± 0.003 eV and 0.251 ± 0.004 eV. For both types of motions, the jump rates of the reorientations reach the values of the order of 1011 s−1 near the upper limit of the temperature range of the orthorhombic phase stability (T0 ≈ 381 K). In the high-temperature (hexagonal) phase, both the 1H and 11B spin−lattice relaxation rates are governed by an additional low-frequency fluctuation process (with the characteristic rate of the order of 107 s−1 just above T0) due to the translational diffusion of Li ions.

Robert L. Corey†‡, David T. Shane†, Robert C. Bowman, Jr.§ and Mark S. Conradi*†
J. Phys. Chem. C, 2008, 112 (47), pp 18706–18710

Abstract:1H, 7Li, and 11B NMR measurements were used to understand atomic translational motions in both the low- and high-temperature phases (LT, HT) of LiBH4. In the HT phase 7Li spectra, spin−echo T2, and T1 all indicate very rapid lithium ion diffusion. Just above the phase transition, the hydrogen resonance is broad, about 22 kHz fwhm (full width at half of maximum), showing that H translations remain slow. From 120 to 170 °C, a rapidly decreasing T1D (relaxation time of dipolar spin-order) shows that the hydrogens diffuse increasingly rapidly. This motion eventually results in marked hydrogen line-narrowing centered near 190 °C; the hydrogen diffusion is likely relevant to the kinetics of dehydriding. The extent of 11B line-narrowing demonstrates that the boron atoms also diffuse rapidly at temperatures above 200 °C. In the LT phase, the hydrogen T1D decreases rapidly with increasing temperature, here due to 7Li diffusion which is too slow for line-narrowing.

Prediction of Chemical Anisotropy on Sidewall of Boron Nitride Nanotubes: A New Application of Directional Curvature Theory
Yong Chen, Jun-Qian Li* and Chun-Li Hu
J. Phys. Chem. C, 2008, 112 (48), pp 18787–18792

Abstract: The bond curvature (K), derived from the Directional-Curvature Theory, is developed as a simple and efficient criterion for structures and chemical anisotropy of sidewall [2+1] cycloadditions on single-walled boron nitride nanotubes (SWBNNTs). The origin of the relationship between the chemical anisotropy of SWBNNTs and the bond curvature is explained based on the viewpoint of hybrid orbital theory. The first-principle calculations for the additions on various types of the SWBNNTs show that not those single-parameter criteria, but the K, in which the two parameters R and θ are involved, can solely determine the structure types of the cycloadditions on the B−N bonds and predict the chemical anisotropy of the SWBNNTs. The larger the K is, the more easily the B−N bond is broken, and the binding energies of the opened structures change linearly with K. For the cycloaddition on SWBNNTs with moderate diameter, the boundary of K for determining whether the B−N bond is broken or not is about 1.45 nm−1.

Characterization of Organic Molecules Attached to Gold Nanoparticle Surface Using High Resolution Magic Angle Spinning 1H NMR
Hongyu Zhou†‡, Fenfang Du‡, Xi Li†, Bin Zhang‡, Wei Li*§ and Bing Yan*†‡
J. Phys. Chem. C, 2008, 112 (49), pp 19360–19366

Abstract: Structural elucidation of molecules attached to nanoparticle surface holds key to the successful chemical modifications of nanomaterial surface. In this investigation, we effectively optimized 1H HRMAS NMR conditions and applied one- and two-dimensional techniques to fully characterize ligand structures on surfaces of gold nanoparticles (GNPs). We found that there are significant differences in detection sensitivity depending on the distance between the surface of GNP and protons in the ligand molecule, with the loss of sensitivity for protons closer to the nanoparticles. Furthermore, NMR spectra of aromatic protons in ligands attached to GNP seem to have a broad base compared with aliphatic ligands, indicating some degree of potential π−π stacking effects. Our results demonstrate that 1H HRMAS NMR is an irreplaceable method for fully characterizing nanoparticle surface-bound molecules.

Hydrogen Motion in Magnesium Hydride by NMR
Robert L. Corey†‡, Timothy M. Ivancic†, David T. Shane†, Erik A. Carl†, Robert C. Bowman, Jr.§, José M. Bellosta von Colbe∥, Martin Dornheim∥, Rüdiger Bormann∥, Jaques Huot⊥, Ragaiy Zidan#, Ashley C. Stowe#∇ and Mark S. Conradi*†

J. Phys. Chem. C, 2008, 112 (49), pp 19784–19790
Abstract: In coarse-grained MgH2, the diffusive motion of hydrogen remains too slow (<105 hops s−1) to narrow the H NMR line up to 400 °C. Slow-motion dipolar relaxation time T1D measurements reveal the motion, with hopping rate ωH from 0.1 to 430 s−1over the range of 260 to 400 °C, the first direct measurement of H hopping in MgH2. The ωH data are described by an activation energy of 1.72 eV (166 kJ/mol) and attempt frequency of 2.5 × 1015 s−1. In ball-milled MgH2 with 0.5 mol % added Nb2O5 catalyst, line-narrowing is evident already at 50 °C. The line shape shows distinct broad and narrow components corresponding to immobile and mobile H, respectively. The fraction of mobile H grows continuously with temperature, reaching ∼30% at 400 °C. This demonstrates that this material’s superior reaction kinetics are due to an increased rate of H motion, in addition to the shorter diffusion paths from ball-milling. In ball-milled MgH2 without additives, the line-narrowed component is weaker and is due, at least in part, to trapped H2 gas. The spin−lattice relaxation rates T1−1 of all materials are compared, with ball-milling markedly increasing T1−1. The weak temperature dependence of T1−1 suggests a mechanism with paramagnetic relaxation centers arising from the mechanical milling.

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