²H NMR Studies of Simple Organic Groups Covalently Attached to Ordered Mesoporous Silica

Abstract

The rotational dynamics of several chemically simple organic functional groups covalently attached to ordered mesoporous silica (OMS) are reported using solid-state 2H NMR spectroscopy. Hexamethyldisilazane-treated OMS samples show that resulting trimethylsilyl groups rapidly rotate on the OMS surfaces, even at low temperatures. However, subtle differences are observed between TMS groups on MCM-41 versus SBA-15 surfaces, possibly due to the presence of micropores in SBA-15. Different secondary amines display different dynamic behaviors, which appear largely dependent on whether the amine can engage in hydrogen bonding with residual surface silanols. The N-methylamine groups exhibit dynamic behavior that is dependent upon loading. At low loadings a spectrum consistent with a methyl rotor is observed, and at higher loadings a fast rotation about the methyl group 3-fold axis and additional motion that leads to further narrowing of the line shape is observed. The N-benzylamine line shape is quite complex, qualitatively consistent with a 180° π-flip and an additional motion. In contrast, the piperazine groups are rigid on the NMR time scale. This is attributed to the inability of the secondary amine of the piperazine group to hydrogen bond with surface silanols due to conformational constraints.