Restricted Dynamics of a Deuterated Linker Grafted on SBA-15 Revealed by Deuterium MAS NMR

Abstract

Deuterium (2H) magic angle spinning (MAS) NMR has been employed to monitor the restricted mobility of a molecule grafted at the inner surface of mesoporous SBA-15. The grafted linear moiety (O3Si–CH2–CH2–C(O)–N(H)–CD2–CD2–NH2) has two deuterated methylene groups toward the end terminus of the chain. A series of 2H-MAS (at 6 and 9 kHz) spectra of the grafted molecule at room temperature were recorded and decomposed into spectra with different quadrupolar tensor components {CQ, η} to illustrate the mobility of the chain as function of the humidity. At very low water contents the MAS spectra resemble a static CD2 spectrum. At high water concentrations the spectrum with the large symmetric quadrupolar tensor of CD2 decomposes into two spectra, one with an asymmetric tensor and the other with a significantly smaller tensor depicting substantial mobility. Molecular dynamic (MD) simulations were employed to support the experimental data. The results from these MD studies show the possible binding of the molecule with silanol groups on the silica surface of SBA-15 through C(O)···HO, N(H)···OH, or NH2···OH hydrogen bonding. Insights obtained from these MD calculations toward molecular mobility induced by single water molecules are discussed.