Characterization of Nanostructured Organic-Inorganic Hybrid Materials Using Advanced Solid-State NMR Spectroscopy
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1184-HH07-01
Characterization of Nanostructured Organic-Inorganic Hybrid Materials Using Advanced Solid-State NMR Spectroscopy Kanmi Mao1,2, Jennifer L. Rapp1, Jerzy W. Wiench1 and Marek Pruski1,2 1 U.S. DOE Ames Laboratory, Iowa State University, Ames, IA 50011, USA 2 Department of Chemistry, Iowa State University, Ames, IA 50011, USA
ABSTRACT We demonstrate the applications of several novel techniques in solid-state nuclear magnetic resonance spectroscopy (SSNMR) to the structural studies of mesoporous organic-inorganic hybrid catalytic materials. Most of these latest capabilities of solid-state NMR were made possible by combining fast magic angle spinning (at ≥ 40 kHz) with new multiple RF pulse sequences. Remarkable gains in sensitivity have been achieved in heteronuclear correlation (HETCOR) spectroscopy through the detection of high-γ (1H) rather than low-γ (e.g., 13C, 15N) nuclei. This so-called indirect detection technique can yield through-space 2D 13C-1H HETCOR spectra of surface species under natural abundance within minutes, a result that earlier has been out of reach. The 15N-1H correlation spectra of species bound to a surface can now be acquired, also without isotope enrichment. The first indirectly detected through-bond 2D 13C-1H spectra of solid samples are shown, as well. In the case of 1D and 2D 29Si NMR, the possibility of generating multiple Carr-Purcell-Meiboom-Gill (CPMG) echoes during data acquisition offered time savings by a factor of ten to one hundred. Examples of the studied materials involve mesoporous silica and mixed oxide nanoparticles functionalized with various types of organic groups, where solid-state NMR provides the definitive characterization.
INTRODUCTION Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is becoming a very valuable technique for the characterization of nanomaterials, as the quest for achieving solution-like resolution has been further advanced by the development of fast magic angle spinning (MAS) at rates of up to 70 kHz [1-3]. Advantages of fast MAS include the possibility of using low-power radiofrequency (rf) decoupling schemes [4-6], minimization/elimination of spinning sidebands and increased frequency range in the indirect dimension of rotor-synchronized experiments. In many applications, the greatest advantage is the reduction, and in some cases practical elimination, of homonuclear dipolar couplings between high-γ nuclei, such as 1H and 19F. The ability to decouple 1H nuclei from each other by means of fast MAS has led to the development of 2D HETCOR experiments, which rely on the detection of high-γ (sensitive) 1H nuclei instead of the low-γ (insensitive) nuclei, such as 13C [7-13]. This technique, referred to as indirect detection, offers considerable enhancements in sensitivity, which can reduce the acquisition time by a factor of more than 10, in some cases [10]. Typically, the internuclear exchange of magnetization in HETCOR experiments occurs via cross polarization (CP), i.e., relies on dipolar
interaction between spins, thereby p
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