Wednesday, June 06, 2007

Joel's Journal Update (March '07 - Present)

A Solid-State 17O NMR Study of Local Order and Crystallinity in Amine-Templated Mesoporous Nb Oxide
Boris O. Skadtchenko, Yuxiang Rao, Tom F. Kemp, Prodipta Bhattacharya, Pamela A. Thomas, Michel Trudeau, Mark E. Smith, Dave M. Antonelli
Angewandte (2007)46, 2635
Order locally: 17O magic-angle-spinning NMR spectroscopy of bulk sol-gel-prepared Nb2O5 and mesoporous, template-free Nb2O5 (see picture) shows very high local order in the mesoporous sample. The oxygen atoms are coordinated only as ONb2 in contrast with bulk phases in which the oxygen atoms are always present in a mixture of ONb2 and ONb3 coordination environments.

First-principles calcualtions of solid-state 17O and 29Si NMR spectra of Mg2SiO4 polymorphs
S.E. Ashbrook, L. Le Polles, C.J. Pickard, A.J. Berry, S. Wimperis, I Farnan.
The nuclear magnetic resonance (NMR) shielding and electric field gradient (EFG) tensors of three polymorphs of Mg2SiO4, forsterite (-Mg2SiO4), wadsleyite (-Mg2SiO4) and ringwoodite (-Mg2SiO4), have been calculated using a density functional theory (DFT) approach with a planewave basis set and pseudopotential approximation. These Mg2SiO4 polymorphs are the principal components of the Earth down to depths of 660 km and have been proposed as the hosts of water in the Earths upper mantle and transition zone. A comparison of our calculations with single-crystal spectroscopic data in the literature for the -polymorph, forsterite, shows that both the magnitude and orientation of the shielding and EFG tensors for O and Si can be obtained with sufficient accuracy to distinguish subtle differences in atomic positions between published structures. We compare calculated 17O MAS NMR quadrupolar powder lineshapes directly with experimental lineshapes and show that we are able to reproduce them within the precision with which the NMR parameters may be determined from multi-parameter fitting. The relatively small amounts of sample available for the - and -polymorphs, arising from the high pressures required for synthesis, has hindered the extraction of NMR parameters in previous work. The application of DFT calculations to these high-pressure polymorphs confirms previous spectral assignments, and provides deeper insight into the empirical correlations and observations reported in the literature. These first-principles methods are highly promising for the determination of local bonding in more complex materials, such as the hydrated forms of Mg2SiO4, by aiding analysis of their multinuclear NMR spectra.

2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials
A. Vyalikh, Th. Emmler, I. Shenderovich, Y. Zeng, G. H. Findenegg and G. Buntkowsky
Solid state deuterium NMR has been used to study the molecular motion of d6-isobutyric acid (d6-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I–solid II transition temperature is not affected by confinement, a significant lowering of the liquid–solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I–liquid) of d6-iBA in the pores occurs at a temperature 20–45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.

19F Magic angle spinning NMR reporter molecules: empirical measures of surface shielding, polarisability and H-bonding
Vitaliy L. Budarin, James H. Clark, Fabien E. I. Deswarte, Karl T. Mueller and Stewart J. Tavener
Magic Angle Spinning (MAS) 19F NMR spectra have been obtained and chemical shifts measured for 37 molecules in the gas phase and adsorbed on the surfaces of six common materials: octadecyl- and octyl-functionalised chromatography silicas, Kieselgel 100 silica, Brockmann neutral alumina, Norit activated charcoal and 3-(1-piperidino)propyl functionalised silica. From these six surfaces, octadecyl-silica is selected as a non-polar reference to which the others are compared. The change in chemical shift of a fluorine nucleus within a molecule on adsorption to a surface from the gas phase, surfacegas, is described by the empirical relationship: , where s and r are constants that describe the chemical shift induced by the electromagnetic field of the surface under investigation and reference surface, s and r are the relative surface polarisability for the surface and reference, s is an additional contribution to the surface polarisabilities due to its ability to interact with aromatic molecules, and HBA and HBD are measurements of the hydrogen acceptor and donor properties of the surface. These empirical parameters are measured for the surfaces under study. Silica and alumina are found to undergo specific interactions with aromatic reporter molecules and both accept and donate H-bonds. Activated charcoal was found to have an extreme effect on shielding but no specific interactions with the adsorbed molecules. 3-(1-Piperidino)propyl functionalised silica exhibits H-bond acceptor ability, but does not donate H-bonds.

Combining insights from solid-state NMR and first principles calculation: applications to the 19F NMR of octafluoronaphthalene
Andrew J. Robbins, William T. K. Ng, Dominik Jochym, Thomas W. Keal, Stewart J. Clark, David J. Tozer and Paul Hodgkinson
Advances in solid-state NMR methodology and computational chemistry are applied to the 19F NMR of solid octafluoronaphthalene. It is demonstrated experimentally, and confirmed by density functional theory (DFT) calculations, that the spectral resolution in the magic-angle spinning spectrum is limited by the anisotropy of the bulk magnetic susceptibility (ABMS). This leads to the unusual observation that the resolution improves as the sample is diluted. DFT calculations provide assignments of each of the peaks in the 19F spectrum, but the predictions are close to the limits of accuracy and correlation information from 2-D NMR is invaluable in confirming the assignments. The effects of non-Gaussian lineshapes on the use of 2-D NMR for mapping correlations of spectral frequencies (e.g. due to the ABMS) are also discussed.

Double Quantum 1H MAS NMR Studies of Hydrogen-Bonded Protons and Water Dynamics in Materials
Todd M. Alam,* May Nyman, and Sarah K. McIntyre
Two-dimensional double quantum (DQ) 1H MAS NMR was used to investigate different proton environments in a series of alkali (Na, K, Rb, Cs) [Nb6O19]8- Lindqvist salts, with the water and hydrogen-bound intercluster protons being clearly resolved. Through the analysis of the DQ 1H NMR spinning sideband pattern, it is possible to extract both the mean and distribution of the motionally averaged intramolecular homonuclear 1H-1H dipolar coupling for the different water environments and the intercluster protons. Motional order parameters for the water environments were then calculated from the averaged dipolar couplings. The influence of additional intermolecular dipolar couplings due to multispin interactions were simulated and discussed.

Ring Current Effects in Crystals. Evidence from 13C Chemical Shift Tensors for Intermolecular Shielding in 4,7-Di-t-butylacenaphthene versus 4,7-Di-t-butylacenaphthylene
Zhiru Ma, Merrill D. Halling, Mark S. Solum, James K. Harper, Anita M. Orendt, Julio C. Facelli, Ronald J. Pugmire, David M. Grant,* Aaron W. Amick, and Lawrence T. Scott
13C chemical shift tensor data from 2D FIREMAT spectra are reported for 4,7-di-t-butylacenaphthene and 4,7-di-t-butylacenaphthylene. In addition, calculations of the chemical shielding tensors were completed at the B3LYP/6-311G** level of theory. While the experimental tensor data on 4,7-di-t-butylacenaphthylene are in agreement with theory and with previous data on polycyclic aromatic hydrocarbons, the experimental and theoretical data on 4,7-di-t-butylacenaphthene lack agreement. Instead, larger than usual differences are observed between the experimental chemical shift components and the chemical shielding tensor components calculated on a single molecule of 4,7-di-t-butylacenaphthene, with a root mean square (rms) error of ±7.0 ppm. The greatest deviation is concentrated in the component perpendicular to the aromatic plane, with the largest value being a 23 ppm difference between experiment and theory for the 13CH2 carbon 11 component. These differences are attributed to an intermolecular chemical shift that arises from the graphitelike, stacked arrangement of molecules found in the crystal structure of 4,7-di-t-butylacenaphthene. This conclusion is supported by a calculation on a trimer of molecules, which improves the agreement between experiment and theory for this component by 14 ppm and reduces the overall rms error between experiment and theory to 4.0 ppm. This intermolecular effect may be modeled with the use of nuclei independent chemical shieldings (NICS) calculations and is also observed in the isotropic 1H chemical shift of the CH2 protons as a 4.2 ppm difference between the solution value and the solid-state chemical shift measured via a 13C-1H heteronuclear correlation experiment.

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