Temperature and hydration dependence of proton MAS NMR spectra in MCM-41: model based on motion induced chemical shift averaging

Jaspreet Walia; Joshua Crone; Jianzhen Liang; Mohamad Niknam; Claude Lemaire; R Terry Thompson; Hartwig Peemoeller

doi:10.1016/j.ssnmr.2012.11.003

Temperature and hydration dependence of proton MAS NMR spectra in MCM-41: model based on motion induced chemical shift averaging

Solid State Nucl Magn Reson. 2013 Feb:49-50:26-32. doi: 10.1016/j.ssnmr.2012.11.003. Epub 2012 Nov 27.

Authors

Jaspreet Walia¹, Joshua Crone, Jianzhen Liang, Mohamad Niknam, Claude Lemaire, R Terry Thompson, Hartwig Peemoeller

Affiliation

¹ Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.

PMID: 23266020
DOI: 10.1016/j.ssnmr.2012.11.003

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.