Al's Update

Consolidated silica glass from nanoparticles
Journal of Solid State Chemistry 181 (2008) 2442– 2447
Thomas G. Mayerhöfer, Zhijian Shen, Ekaterina Leonova, Mattias Edén, Antje Kriltz, Jürgen Popp
Abstract:
A dense silica glass was prepared by consolidating a highly dispersed silicic acid powder (particle size less than 10 nm) with the Spark Plasma Sintering (SPS) technique. The glass was characterized by ellipsometry, transmission electron microscopy (TEM), infrared reflectance and transmittance spectroscopy, as well as by Raman, UV–Vis–NIR and solid-state nuclear magnetic resonance (NMR) spectroscopy. The prototypic sample showed a transmittance of about 63% compared to silica glass in the UV–Vis spectral range. Based on the results of infrared transmittance spectroscopy this lower transparency is due to the comparably high water content, which is about 40 times higher than that in silica glass. 1H magic-angle spinning (MAS) NMR confirmed an increase in hydroxyl groups in the sample prepared by SPS relative to that of the conventional SiO2 reference glass. Aside from the comparably high water content, we conclude from the similarity of the IR-reflectance and the 29Si MAS NMR spectra of the SPS sample and the corresponding spectra of the conventionally prepared silica glass, that the short- and medium-range order is virtually the same in both materials. Raman spectroscopy, however, suggests that the number of three- and four-membered rings is significantly smaller in the SPS sample compared to the conventionally prepared sample. Based on these results we conclude that it is possible to prepare glasses by compacting amorphous powders by the SPS process. The SPS process may therefore enable the preparation of glasses with compositions inaccessible by conventional methods.

The mechanism of paramagnetic NMR relaxation produced by Mn(II): Role of orthorhombic and fourth-order zero field splitting terms
J. Chem. Phys. 129, 144307 (2008)
Robert Sharp
Abstract:
Mn(II) is a spin-5/2 paramagnetic ion that mediates a characteristically large NMR paramagnetic relaxation enhancement (NMR-PRE) of nuclear spins in solution. In the range of high magnetic field strengths (above about 0.3 T), where the electronic Zeeman interaction provides the largest term of the electron spin Hamiltonian, NMR relaxation mechanism is well understood. In the lower field range, the physical picture is more complex because of the presence in the spin Hamiltonian of zero field splitting (ZFS) terms that are comparable to or greater than the Zeeman term. This work describes a systematic study of the relaxation mechanism in the low field range, particularly aspects involving the dependence of NMR-PRE on the orthorhombic (E) and fourth-order (a q(4)">, q=0,2,4) ZFS tensor components. It is shown that the fourfold (a4(4)"> ) and twofold (a2(4)"> ) fourth-order components exert large orientation-dependent influences on the NMR-PRE. Thus, fourth-order terms with magnitudes equal to only a few percent of the quadratic ZFS terms (D,E) produce large changes in the shape of the magnetic field profile of the PRE. Effects arising from the orthorhombic quadratic ZFS term (E) are much smaller than those of the fourth-order terms and can in most cases be neglected. However, effects due to a4(4)"> and a2(4)"> need to be included in simulations of low field data.

A magic-angle-spinning NMR method for 1H–1H distance measurement using coherent polarization transfer in 13C-labeled organic solids
J. Chem. Phys. 129, 154504 (2008)
Hiroki Takahashi, Hideo Akutsu, and Toshimichi Fujiwara
Abstract:
We have developed a theory for 1H–1H distance measurements from the direct polarization transfer in 13C-labeled solids under magic-angle spinning. The polarization transfer caused by the 1H–1H dipolar interactions was analyzed with zeroth-order average Hamiltonian for a 1H–13C–13C–1H spin system in the frame modulated by 13C–1H dipolar interactions and chemical shifts. Strong 13C–1H dipolar couplings primarily determine the recovery of the 1H–1H coupling as a function of sample spinning frequency. The effect of additional 1H spins on the polarization transfer was also taken into account. We have applied this method to the distance measurements for uniformly 13C-, 15N-labeled L-valine and adenosine. Experimental 1H polarization transfer was monitored through high-resolution 13C-NMR. The theoretical analysis provided the distances up to about 3 Å with an accuracy of about 0.2 Å and those of about 4 Å with 1 Å even from the transfer amplitudes at a few mixing times. The longer distances are partly affected by the relayed polarization transfer which makes apparent 1H–1H distances shorter. Our theory based on the coherent polarization transfer in the initial build-up regime was compared to the description by the rate equations with spin diffusion time constants.

Symmetry-based recoupling in double-rotation NMR spectroscopy
J. Chem. Phys. 129, 174507 (2008)
Andreas Brinkmann, Arno P. M. Kentgens, Tiit Anupõld, and Ago Samoson
Abstract:
In this contribution, we extend the theory of symmetry-based pulse sequences of types CN νn and RN νn in magic-angle-spinning nuclear resonance spectroscopy [M. H. Levitt, in Encyclopedia of Nuclear Magnetic Resonance, edited by D. M. Grant and R. K. Harris (Wiley, Chichester, 2002), Vol. 9]. to the case of rotating the sample simultaneously around two different angles with respect to the external magnetic field (double-rotation). We consider the case of spin-1/2 nuclei in general and the case of half-integer quadrupolar nuclei that are subjected to weak radio frequency pulses operating selectively on the central-transition polarizations. The transformation properties of the homonuclear dipolar interactions and J-couplings under central-transition-selective spin rotations are presented. We show that the pulse sequence R221R22-1 originally developed for homonuclear dipolar recoupling of half-integer quadrupolar nuclei under magic-angle-spinning conditions [M. Edén, D. Zhou, and J. Yu, Chem. Phys. Lett. 431, 397 (2006)] may be used for the same purpose in the case of double rotation, if the radio frequency pulses are synchronized with the outer rotation of the sample. We apply this sequence, sandwiched by central-transition selective 90° pulses, to excite double-quantum coherences in homonuclear spin systems consisting of 23Na and 27Al nuclei.

Three Polymorphic Forms of the Co-Crystal 4,4-Bipyridine/Pimelic Acid and their Structural, Thermal, and Spectroscopic Characterization
Chem. Eur. J., Volume 14 Issue 32, Pages 10149 - 10159
Dario Braga, Prof., Giuseppe Palladino, Dr., Marco Polito, Dr., Katia Rubini, Dr., Fabrizia Grepioni, Prof., Michele R. Chierotti, Dr., Roberto Gobetto, Prof.
Abstract:
Three crystal forms of the co-crystal 4,4-bipy/pimelic acid (bipy: bipyridine), [NH4C5-C5H4N][HOOC(CH2)5COOH], have been prepared and their relationship investigated by single-crystal X-ray diffraction, variable-temperature X-ray powder diffraction, differential scanning calorimetry and solid-state NMR spectroscopy. Both X-ray and NMR spectroscopic results indicate that no proton transfer takes place, that is, the three crystal forms are true co-crystals of neutral molecules. Forms I and II both convert into Form III at high temperature, Forms II and III being the thermodynamically stable forms at room and high temperature, respectively.