Molecular motion of tethered molecules in bulk and surface-functionalized materials: a comparative study of confinement

Journal of the American Chemical Society
Jessica L DefreeseAlexander Katz

Abstract

Achieving high degrees of molecular confinement in materials is a difficult synthetic challenge that is critical for understanding supramolecular chemistry on solid surfaces and control of host-guest complexation for selective adsorption and heterogeneous catalysis. In this Article, using 2H MAS NMR spectroscopy of tethered carbamates as a molecular probe, we systematically investigate the degree of steric confinement within three types of materials: two-dimensional silica surface, bulk amorphous microporous silica, and bulk amorphous mesoporous silica. The resulting NMR spectra are described with a simple two-site hopping model for motion and prove that the bulk silica network severely limits the molecular mobility of the immobilized carbamate at room temperature to the same degree as surface-functionalized materials at low-temperatures (approximately 210 K). Raising the temperature of the bulk materials to 413 K still demonstrates the effect of confinement, as manifested in significantly longer characteristic times for the immobilized carbamate relative to surface-functionalized materials at room temperature. The environment surrounding the carbonyl functionality of the immobilized carbamate is investigated using FT-IR spectr...Continue Reading

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