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

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.

NMR and IR study of fluorobenzene and hexafluorobenzene adsorbed on alumina.

The adsorption of fluorobenzene (C6H5F) and hexafluorobenzene (C6F6) onto the surface of neutral alumina is investigated by reflectance IR spectroscopy, near-IR spectroscopy, and measurement of 19F NMR chemical shift values. Chemical shifts are dependent on surface coverage and reveal multiple peaks where different adsorption environments occur. C6H5F appears to be adsorbed through polar interactions with surface hydroxyls, whereas C6F6 shows separate resonances for the first layer of coverage and outer layers. Available surface areas are estimated, and reorientation of adsorbed hexafluorobenzene is proposed to account for chemical shift behavior and differences between calculated and measured monolayer coverage.

The application of modified mesoporous silicas in liquid phase catalysis.

This review focuses on recent developments in the preparation, properties and catalytic applications of chemically modified mesoporous silicas. Over the last few years, this group of materials has been the subject of intense activity in the materials community, and many applications have been found for these fascinating materials. This non-exhaustive review aims to highlight the key features of the materials, which are relevant and important to catalysis, and illustrates their utility with a series of recent examples.

Novel mesoporous silica-perfluorosulfonic acid hybrids as strong heterogeneous Brønsted catalysts.

Perfluorinated sulfonic acids have been immobilised into mesoporous silica frameworks by a one-step process, representing the first example of the successful incorporation of charged silanes using this route, and have been shown to be excellent catalysts for Brønsted acid catalysed transformations.

Chemical reactions of double bonds in activated carbon: microwave and bromination methods.

The reaction of localised C[double bond, length as m-dash]C bonds on the surface of activated carbons has been shown to be an effective method of chemical modification especially using microwave-assisted reactions.

Surface energy and surface area measurements by 19F MAS NMR of adsorbed trifluoroacetic acid.

Trifluoroacetic acid, when adsorbed on the surface of inorganic materials, is a useful (19)F NMR probe molecule for studying surface properties including surface energy and surface area.