Probing surface hydrogen bonding and dynamics by natural abundance, multidimensional, 17O DNP-NMR spectroscopy

Dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy is increasingly being used as a tool for the atomic-level characterization of surface sites. DNP surface-enhanced SSNMR spectroscopy of materials has, however, been limited to studying relatively receptive nuclei, and the particularly rare ¹⁷O nuclide, which is of great interest for materials science, has not been utilized. We demonstrate that advanced ¹⁷O SSNMR experiments can be performed on surface species at natural isotopic abundance using DNP. We use ¹⁷O DNP surface-enhanced 2D SSNMR to measure ¹⁷O{¹H} HETCOR spectra as well as dipolar oscillations on a series of thermally treated mesoporous silica nanoparticle samples having different pore diameters. These experiments allow for a nonintrusive and unambiguous characterization of hydrogen bonding and dynamics at the surface of the material; no other single experiment can give such details about the interactions at the surface. Our data show that, upon drying, strongly hydrogen-bonded surface silanols, whose motions are greatly restricted by the interaction when compared to lone silanols, are selectively dehydroxylated.