title: Temperature and hydration dependence of proton MAS NMR spectra in MCM-41: model based on motion induced chemical shift averaging.
abstract: The proton MAS NMR spectra in MCM-41 at low hydration levels (less than hydration amounting to one water molecule per surface hydroxyl group) show complex proton resonance peak structures, with hydroxyl proton resonances seen in dry MCM-41 disappearing as water is introduced into the pores and new peaks appearing, representing water and hydrated silanol groups. Surface hydroxyl group-water molecule chemical exchange and chemical shift averaging brought about by a water molecule visiting different surface hydrogen bonding sites have been proposed as possible causes for the observed spectral changes. In this report a simple model based on chemical shift averaging, due to the making and breaking of hydrogen bonds as water molecules move on the MCM-41 surface, is shown to fully reproduce the NMR spectra, both as a function of hydration and temperature. Surface proton-water proton chemical exchange is not required in this model at low hydration levels.
authors.affiliation: Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
authors.forename: Jaspreet
authors.initials: J
authors.lastname: Walia
authors.forename: Joshua
authors.initials: J
authors.lastname: Crone
authors.forename: Jianzhen
authors.initials: J
authors.lastname: Liang
authors.forename: Mohamad
authors.initials: M
authors.lastname: Niknam
authors.forename: Claude
authors.initials: C
authors.lastname: Lemaire
authors.forename: R
authors.initials: R
authors.lastname: Terry Thompson
authors.forename: Hartwig
authors.initials: H
authors.lastname: Peemoeller