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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