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F-19 Magic Angle Spinning NMR reporter molecules: empirical measures of surface shielding, polarisability and H-bonding

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JournalPhysical Chemistry Chemical Physics
DatePublished - 2007
Issue number18
Volume9
Number of pages10
Pages (from-to)2274-2283
Original languageEnglish

Abstract

Magic Angle Spinning (MAS) (FNMR)-F-19 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. uorine nucleus within a molecule on adsorption to a surface from the gas phase, Delta delta(surface)(gas), is described by the empirical relationship: Delta delta(surface)(gas) = delta s + alpha(s)+pi(s/alpha r) (Delta delta(reference)(gas) - delta(r)) + delta(HBA) + delta(HBD), where delta s and delta r are constants that describe the chemical shift induced by the electromagnetic. eld of the surface under investigation and reference surface, alpha(s) and alpha(r) are the relative surface polarisability for the surface and reference, pi(s) is an additional contribution to the surface polarisabilities due to its ability to interact with aromatic molecules, and delta(HBA) and delta(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 e. ect 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.

    Research areas

  • SOLID-STATE NMR, MAS NMR, PROBE MOLECULE, ACID SITES, TRIMETHYLPHOSPHINE OXIDE, ACTIVATED-CHARCOAL, REICHARDTS DYE, GAMMA-ALUMINA, P-31 NMR, ADSORPTION

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