Tuesday, April 06, 2010

PCCP, vol. 12, Issues 10 - 15

Phys. Chem. Chem. Phys., 2010, 12, 2895 - 2914, DOI: 10.1039/b915587b

The conformation and orientational order of a 1,2-disubstituted ethane nematogenic molecule (I22) in liquid crystalline and isotropic phases studied by NMR spectroscopy

James W. Emsley, Philippe Lesot, Anne Lesage, Giuseppina De Luca, Denis Merlet and Giuseppe Pileio

The structure, conformation and orientational order of the mesogen I22 have been studied by proton, carbon-13 and deuterium 1D and 2D-NMR spectroscopies at natural abundance and at various magnetic fields when in the nematic phase, the isotropic phase close to the nematic-isotropic phase transition, and as a solute in the chiral nematic solution comprised of the polypeptide PBLG dissolved in chloroform. It is concluded that 95% of conformers have a trans arrangement about the central C–C bond of the ethane fragment in all phases

Phys. Chem. Chem. Phys., 2010, 12, 2989 - 2998, DOI: 10.1039/b924666e

Dynamics on the microsecond timescale in hydrous silicates studied by solid-state 2H NMR spectroscopy

John M. Griffin, Andrew J. Miller, Andrew J. Berry, Stephen Wimperis and Sharon E. Ashbrook

Solid-state 2H NMR spectroscopy has been used to probe the dynamic disorder of hydroxyl deuterons in a synthetic sample of deuterated hydroxyl-clinohumite (4Mg2SiO4·Mg(OD)2), a proposed model for the incorporation of water within the Earths mantle. Both static and magic angle spinning (MAS) NMR methods were used. Static 2H NMR appears to reveal little evidence of the dynamic process, yielding results similar to those obtained from deuterated brucite (Mg(OD)2), where no dynamics on the relevant timescale are expected to be present. However, in 2H MAS NMR spectra, considerable line broadening is observed for hydroxyl-clinohumite and a 2H double-quantum (DQ) MAS NMR spectrum confirms that this is due to motion on the microsecond timescale. Using a model for dynamic exchange of the hydroxyl deuterons between two sites identified in previous diffraction studies, first-principles density functional theory (DFT) calculations of 2H (spin I = 1) quadrupolar NMR parameters, and a simple analytical model for dynamic line broadening in MAS NMR experiments, we were able to reproduce the observed motional line broadening and use this to estimate a rate constant for the dynamic process. From analysis of the observed 2H linewidths in variable-temperature MAS experiments, an activation energy for the exchange process was also determined. A simulated static 2H NMR lineshape based on our dynamic model is consistent with the observed experimental static NMR spectrum, confirming that the motion present in this system is not easily detectable using a static NMR approach. Finally, a 2H DQMAS NMR spectrum of fluorine-substituted 2H-enriched hydroxyl-clinohumite shows how the dynamic exchange process is inhibited by O–DF- hydrogen-bonding interactions.

Phys. Chem. Chem. Phys., 2010, 12, 3254 - 3259, DOI: 10.1039/b925326b

14N NQR and proton NMR study of ferroelectric phase transition and proton exchange in organic ferroelectric (H2-TPPZ)(Hca)2

Janez Seliger, Veselko agar, Tetsuo Asaji and Yumi Hasegawa

The complete 14N nuclear quadrupole resonance spectrum has been measured in ferroelectric (H2-TPPZ)(Hca)2 using nuclear quadrupole double resonance. The quadrupole coupling tensors are assigned to various nitrogen positions in the crystal structure. Two types of asymmetric N–H+N hydrogen bonds are observed in the ferroelectric phase. A slow dynamics influencing the 14N NQR spectrum and relaxation has been observed in the paraelectric phase. The analysis of the 14N NQR spectra in the paraelectric phase shows that above Tc each hydrogen bond exchanges between the two types observed in the ferroelectric phase. The change of the type of hydrogen bond is associated with the transfer of protons within the bond.

Phys. Chem. Chem. Phys., 2010, 12, 3895 - 3903, DOI: 10.1039/b915401a

Extra-framework aluminium species in hydrated faujasite zeolite as investigated by two-dimensional solid-state NMR spectroscopy and theoretical calculations

Shenhui Li, Anmin Zheng, Yongchao Su, Hanjun Fang, Wanling Shen, Zhiwu Yu, Lei Chen and Feng Deng

Extra-framework aluminium (EFAL) species in hydrated dealuminated HY zeolite were thoroughly investigated by various two-dimensional solid-state NMR techniques as well as density functional theoretical calculations. 27Al MQ MAS NMR experiments demonstrated that five-coordinated and four-coordinated extra-framework aluminium subsequently disappeared with the increase of water loading, and the quadrupole interaction of each aluminium species decreased gradually during the hydration process. 1H double quantum MAS NMR revealed that the EFAL species in the hydrated zeolite consisted of three components: a hydroxyl AlOH group, and two types of water molecule (rigid and mobile water). 1H–27Al LG-CP HETCOR experiments indicated that both the extra-framework and the framework Al atoms were in close proximity to the rigid water in the fully rehydrated zeolite. The experimental results were further confirmed by DFT theoretical calculations. Moreover, theoretical calculation results further demonstrated that the EFAL species in the hydrated zeolite consisted of the three components and the calculated 1H NMR chemical shift for each component agreed well with our NMR observations. It is the rigid water that connects the extra-framework aluminium with the four-coordinated framework aluminium through strong hydrogen bonds

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