Three-Dimensional Structure Determination of Surface Species by DNP Enhanced Solid-state NMR

International audienceThree-dimensional molecular structures determined from single crystals by diffraction methods transformed Chemistry in the twentieth century, leading to today’s structure based understanding of the field. However, if the system under investigation is located at a surface, as in many of the most interesting functional materials today, the problem of structure determination is largely unsolved. Such samples are encountered with increasing frequency, particularly in the area of energy and catalysis, so that there is a critical need to provide new methods for structural characterization of surfaces. Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy would be the method of choice for surface characterization if it were not that the detection limit of NMR is too low to allow the study of many modern materials. The sensitivity of NMR is thus the major limitation to surface structure determination. We have recently introduced an approach using Dynamic Nuclear Polarization (DNP) under Magic Angle Spinning (MAS) to greatly enhance NMR signals of surfaces. With the recent introduction of polarizing agents of high molecular weight like TEKPOL signal enhancement factors of two orders of magnitude (between 100 and 200 in mesoporous materials) are routinely obtained at magnetic fields of 5−9.4 T and sample temperatures of ca. 80−105 K.Here we show that the gain provided by DNP surface enhanced NMR spectroscopy (SENS) is sufficient to enable the implementation with high sensitivity of a series of multi-dimensional correlation experiments (including HETCOR, 29Si-29Si INADEQUATE, 13C-{15N} and 29Si-{15N} REDOR) that allow us to solvethe three-dimensional structure of organic fragments incorporated on a silica surface. Using a total of 8 internuclear constraints, we could determine the structure ofa Pt complex, used asprototypical metal site for moleculary-defined supported catalyst