(35)Cl profiling using centric scan SPRITE with variable flip angle excitation.

An efficient MRI technique for quantitative density profiling of samples with fast spin-lattice relaxation (T(1)<5ms) is introduced. The pulse scheme is based on the 1D centric scan SPRITE technique. Strong excitation of the sample at the k-space origin improves the sensitivity with respect to the original centric scan SPRITE technique. Radio frequency pulse durations are defined so as to provide uniform excitation of the sample at every k-space point. For a particular k-space point the pulse duration is required to be less than the inverse sample bandwidth. Simulations permit one to examine distortions from ideal profile geometry due to flip angle and spin-lattice relaxation effects. The proposed technique is especially suitable for the observation of low sensitivity samples, in particular, low-gamma nuclei like (35)Cl. In some cases, this strategy permits one to reduce the number of scans, i.e. the experiment time, by a factor of 100, depending on hardware, sample length and tolerable resolution loss. The designed pulse scheme is tested on cylindrical agar gel and type 1 Portland cement paste phantoms prepared to provide (1)H and (35)Cl signals, respectively.

129Xe nuclear magnetic resonance study of pitch-based activated carbon modified by air oxidation/pyrolysis cycles: a new approach to probe the micropore size.

(129)Xe NMR has been used to study a series of homologous activated carbons obtained from a KOH-activated pitch-based carbon molecular sieve modified by air oxidation/pyrolysis cycles. A clear correlation between the pore size of microporous carbons and the (129)Xe NMR of adsorbed xenon is proposed for the first time. The virial coefficient delta(Xe)(-)(Xe) arising from binary xenon collisions varied linearly with the micropore size and appeared to be a better probe of the microporosity than the chemical shift extrapolated to zero pressure. This correlation was explained by the fact that the xenon collision frequency increases with increasing micropore size. The chemical shift has been shown to vary very little with temperature (less than 9 ppm) for xenon trapped inside narrow and wide micropores. This is indicative of a smooth xenon-surface interaction potential.

93Nb NMR chemical shift scale for niobia systems.

93Nb solid-state NMR spectra of a series of inorganic niobates with Nb in different oxygen coordination environments were measured. For all studied compounds the chemical shielding and quadrupole tensor parameters were determined using conventional and ultrahigh field NMR facilities, ultrahigh speed MAS, DQ STMAS, solid-echo and computer modeling. It has been demonstrated that the 93Nb isotropic chemical shift is sensitive to the coordination number of Nb sites. For the first time the 93Nb NMR chemical shift scale for NbOx polyhedra in solid materials has been proposed: for four-coordinated Nb sites, the isotropic shifts occur from -650 to -950 ppm; five-coordinated Nb sites have the isotropic shifts in the range of -900 to -980 ppm; for six-coordinated Nb sites the isotropic shifts vary from -900 to -1360 ppm; the shifts from -1200 to -1600 ppm are typical for seven-coordinated Nb sites; for eight-coordinated Nb sites the shifts are higher than -1400 ppm. The possible correlation between the value of the isotropic chemical shift and the ionic character of the NbOx-MOy polyhedra association has been suggested. The magnitude of the 93Nb quadrupole coupling constant depends on the local symmetry of Nb sites and may vary from hundreds of kHz to hundreds of MHz.

95Mo magic angle spinning NMR at high field: improved measurements and structural analysis of the quadrupole interaction in monomolybdates and isopolymolybdates.

In this study, 95Mo quadrupole couplings in various molydbates were measured easily and accurately with magic angle spinning (MAS) NMR under a directing field of 19.6 T. The resonance frequency of 54 MHz was sufficiently high to remove acoustic ringing artifacts, and the spectra could be analyzed in the usual terms of chemical shift and quadrupolar line shapes. For monomolybdates and molybdite, the quadrupole coupling dominated the NMR response, and the quadrupole parameters could be measured with better accuracy than in previous lower field studies. Moreover, despite the low symmetry of the molybdenum coordination, the usefulness of such measurements to probe molybdenum environments was established by ab initio density functional theory (DFT) calculations of the electric field gradient from known structures. The experimental NMR data correlated perfectly with the refined structures. In isopolymolybdates, the resonances were shapeless and DFT calculations were impossible because of the large and low symmetry unit cells. Nevertheless, empirical but clear NMR signatures were obtained from the spinning sidebands analysis or the MQMAS spectra. This was possible for the first time thanks to the improved baseline and sensitivity at high fields. With the generalization of NMR spectrometers operating above 17 T, it was predicted that 95Mo MAS NMR could evolve as a routine characterization tool for ill-defined structures such as supported molybdates in catalysis.