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

Abstract

Magic Angle Spinning (MAS) (19)F 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, Deltadelta(gas)(surface), is described by the empirical relationship: Deltadelta(gas)(surface) = delta(s) + (alpha(s)+pi(s))/alpha(r) (Deltadelta(gas)(reference) - delta(r)) + delta(HBA) + delta(HBD), where delta(s) and delta(r) are constants that describe the chemical shift induced by the electromagnetic field 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 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.

Metrologically traceable quantification of trifluoroacetic acid content in peptide reference materials by19F solid-state NMR

“…Furthermore, the use of an internal calibrant allowed to quantify the F 2 content [64]. 19 F solid-state qNMR employing external calibrants has been used to study the degradation of Nafion fuel cell membranes [65], to quantify accessible hydroxyl sites on fibreglass surfaces [66], to quantify organo fluorines absorbed on inorganic surfaces [67,68], to determine the reactive surface area of clay minerals [69], to quantify the amount of surface functional groups of coumarin-stained beads [70], and to determine the fluorine content in 2,2,2-trifluoroethylamine labelled polymer particles [71]. In the latter case, metrological traceability was achieved by using a certified reference material (CRM) as external calibrant.…”

Determining adsorbate configuration on alumina surfaces with 13C nuclear magnetic resonance relaxation time analysis

“…7 Nevertheless, this method has been used to study the chemisorption of hydrogen on Pt and Pd surfaces using 1 H NMR, 7 and of fluorine-containing adsorbates on a variety of surfaces, including aluminas, using 19 F NMR. 8 13 C NMR has been used to investigate oligomers grafted onto silica surfaces, 9 ionic surfactants adsorbed on silica, 5 and in applications to catalysis; for example, hydrocarbon adsorption on zeolites. 6 The use of 13 C NMR has practical advantages over 1 H NMR.…”