Wednesday, February 09, 2011

Phys. Chem. Chem. Phys., 2011

Determination of coordination modes and estimation of the 31P–31P distances in heterogeneous catalyst by solid state double quantum filtered 31P NMR spectroscopy
Si-Yong Zhang, Mei-Tao Wang, Qing-Hua Liu, Bing-Wen Hu, Qun Chen, He-Xing Li and Jean-Paul Amoureux

Phys. Chem. Chem. Phys., 2011, Advance Article

To overcome the separation difficulty of the palladium-based homogeneous catalyst, the palladium complex can be anchored on various supports such as silica. However, it is difficult to determine the amounts of the two coordination modes of the Pd nucleus, that is, Pd coordinates with one phosphorus atom and Pd coordinates with two phosphorus atoms. Here a 31P double-quantum filtered (DQ-filtered) method in solid-state NMR is introduced for the palladium-based heterogenous catalyst system. With the DQ-filtered method, we can not only determine the amounts of the two different kinds of palladium coordination modes, we can also estimate the interatomic distance of two 31P nuclei bonded to a palladium nucleus. With the help of this method, we can quickly estimate interatomic distances in our designed system and accurately re-design the palladium system to accommodate either one 31P or two 31P.

A highly ordered mesostructured material containing regularly distributed phenols: preparation and characterization at a molecular level through ultra-fast magic angle spinning proton NMR spectroscopy
Arthur Roussey, David Gajan, Tarun K. Maishal, Anhurada Mukerjee, Laurent Veyre, Anne Lesage, Lyndon Emsley, Christophe Copéret and Chloé Thieuleux

Phys. Chem. Chem. Phys., 2011, Advance Article

Highly ordered organic–inorganic mesostructured material containing regularly distributed phenols is synthesized by combining a direct synthesis of the functional material and a protection–deprotection strategy and characterized at a molecular level through ultra-fast magic angle spinning proton NMR spectroscopy.

Influence of particle size on solid solution formation and phase interfaces in Li0.5FePO4 revealed by 31P and 7Li solid state NMR spectroscopy
L. J. M. Davis, I. Heinmaa, B. L. Ellis, L. F. Nazar and G. R. Goward

Phys. Chem. Chem. Phys., 2011, Advance Article

Here we report the observation of electron delocalization in nano-dimension xLiFePO4:(1 − x)FePO4 (x = 0.5) using high temperature, static, 31P solid state NMR. The 31P paramagnetic shift in this material shows extreme sensitivity to the oxidation state of the Fe center. At room temperature two distinct 31P resonances arising from FePO4 and LiFePO4 are observed at 5800 ppm and 3800 ppm, respectively. At temperatures near 400 °C these resonances coalesce into a single narrowed peak centered around 3200 ppm caused by the averaging of the electronic environments at the phosphate centers, resulting from the delocalization of the electrons among the iron centers. 7Li MAS NMR spectra of nanometre sized xLiFePO4:(1 − x)FePO4 (x = 0.5) particles at ambient temperature reveal evidence of Li residing at the phase interface between the LiFePO4 and FePO4 domains. Moreover, a new broad resonance is resolved at 65 ppm, and is attributed to Li adjacent to the anti-site Fe defect. This information is considered in light of the 7Li MAS spectrum of LiMnPO4, which despite being iso-structural with LiFePO4 yields a remarkably different 7Li MAS spectrum due to the different electronic states of the paramagnetic centers. For LiMnPO4 the higher 7Li MAS paramagnetic shift (65 ppm) and narrowed isotropic resonance (FWHM ≈ 500 Hz) is attributed to an additional unpaired electron in the t2g orbital as compared to LiFePO4 which has δiso = −11 ppm and a FWHM = 9500 Hz. Only the delithiated phase FePO4 is iso-electronic and iso-structural with LiMnPO4. This similarity is readily observed in the 7Li MAS spectrum of xLiFePO4:(1 − x)FePO4 (x = 0.5) where Li sitting near Fe in the 3+ oxidation state takes on spectral features reminiscent of LiMnPO4. Overall, these spectral features allow for better understanding of the chemical and electrochemical (de)lithiation mechanisms of LiFePO4 and the Li-environments generated upon cycling.

Phase evolution in lithium disilicate glass–ceramics based on non-stoichiometric compositions of a multi-component system: structural studies by 29Si single and double resonance solid state NMR
Christine Bischoff, Hellmut Eckert, Elke Apel, Volker M. Rheinberger and Wolfram Höland

Phys. Chem. Chem. Phys., 2011, Advance Article

The crystallization mechanism of a high-strength lithium disilicate glass–ceramic in the SiO2–Li2O–P2O5–Al2O3–K2O–(ZrO2) system, used as restorative dentistry material, has been examined on the basis of quantitative 29Si magic angle spinning (MAS) and 29Si{7Li} rotational echo double resonance (REDOR) NMR spectroscopy. Crystallization occurs in two stages: near 650 °C a significant fraction of the Q(3) units disproportionates into crystalline Li2SiO3 and Q(4) units. Upon further annealing of this glass–ceramic to 850 °C the crystalline Li2SiO3 phase reacts with the Q(4) units of the softened residual glass matrix, resulting in the crystallization of Li2Si2O5. The NMR experiments provide detailed insight into the spatial distribution of the lithium ions suggesting the absence of lithium ion clustering in the residual glassy component of the final glass–ceramic. 31P MAS-NMR spectra indicate that phosphate acts as a lithium ion scavenger, resulting in the predominant formation of orthophosphate (P(0)) and some pyrophosphate (P(1)) groups. Crystallization of Li2SiO3 occurs concomitantly with the formation of a highly disordered Li3PO4 phase as evidenced from strong linebroadening effects in the 31P MAS-NMR spectra. Well-crystallized Li3PO4 is only formed at annealing conditions resulting in the formation of crystalline lithium disilicate. These results argue against an epitaxial nucleation process previously proposed in the literature and rather suggest that the nucleation of both lithium metasilicate and lithium disilicate starts at the phase boundary between the disordered lithium phosphate phase and the glass matrix.

Longer-range distances by spinning-angle-encoding solid-state NMR spectroscopy
Johanna Becker-Baldus, Thomas F. Kemp, Jaan Past, Andres Reinhold, Ago Samoson and Steven P. Brown

Phys. Chem. Chem. Phys., 2011, Advance Article

A new spinning-angle-encoding spin-echo solid-state NMR approach is used to accurately determine the dipolar coupling corresponding to a C–C distance over 4 Å in a fully labelled dipeptide. The dipolar coupling dependent spin-echo modulation was recorded off magic angle, switching back to the magic angle for the acquisition of the free-induction decay, so as to obtain optimum sensitivity. The retention of both ideal resolution and long-range distance sensitivity was achieved by redesigning a 600 MHz HX MAS NMR probe to provide fast angle switching during the NMR experiment: for 1.8 mm rotors, angle changes of up to [similar]5° in [similar]10 ms were achieved at 12 kHz MAS. A new experimental design that combines a reference and a dipolar-modulated experiment and a master-curve approach to data interpretation is presented.

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