A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials

Abstract

A new nitroxide-based biradical having a long electron spin-lattice relaxation time (T(1e)) has been developed as an exogenous polarization source for DNP solid-state NMR experiments. The performance of this new biradical is demonstrated on hybrid silica-based mesostructured materials impregnated with 1,1,2,2-tetrachloroethane radical containing solutions, as well as in frozen bulk solutions, yielding DNP enhancement factors (ε) of over 100 at a magnetic field of 9.4 T and sample temperatures of ~100 K. The effects of radical concentration on the DNP enhancement factors and on the overall sensitivity enhancements (Σ(†)) are reported. The relatively high DNP efficiency of the biradical is attributed to an increased T(1e), which enables more effective saturation of the electron resonance. This new biradical is shown to outperform the polarizing agents used so far in DNP surface-enhanced NMR spectroscopy of materials, yielding a 113-fold increase in overall sensitivity for silicon-29 CPMAS spectra as compared to conventional NMR experiments at room temperature. This results in a reduction in experimental times by a factor >12,700, making the acquisition of (13)C and (15)N one- and two-dimensional NMR spectra at natural isotopic abundance rapid (hours). It has been used here to monitor a series of chemical reactions carried out on the surface functionalities of a hybrid organic-silica material.

Dynamic Nuclear Polarization Solid-State NMR in Heterogeneous Catalysis Research

“…For example, 15 N DNP SSNMR measurements made it possible to monitor the chemical reactions on functionalized mesoporous silica surface. 34 In another study, DNP-enhanced 1 H− 15 N HETCOR spectra of Pt 2+ -loaded metal−organic frameworks (UiO-66-NH 2 ) provided direct evidence of host−guest interactions between Pt 2+ and −NH 2 groups in UiO-66-NH 2 ( Figure 4D). 46 Within this virtual special issue, we also report that commonly-used DNP conditions are compatible with the studies of surface-grafted organotransition metal complexes, such as catalytically active zirconium dimethylamide groups.…”

Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3

“…4 The BZY samples for the DNP experiments were prepared by wet impregnation using d 6 This mixture was quickly stirred, centrifuged into a sapphire rotor, capped with a polyfluoroethylene plug and a zirconia drive tip, and inserted into the DNP NMR probe. 4 One sample of BZY30 was prepared similarly using 30 µL of 10 mM of bCTbK 5,6 biradical solution in 1,1,2,2-tetrabromoethane.…”

Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy

“…29 Si solid-state NMR is useful to confirm molecules are covalently grafted to the support and obtain information about the conformation of molecules on the surface. 26,27 Notably, Emsley and co-workers recently showed that by combining DNP-enhanced 2D HETCOR experiments and REDOR-based dipolar coupling measurements it was possible to determine the three-dimensional structure of molecules grafted onto silica surfaces ( Figure 3A). 27 DNP-enhanced 2D 29 Si- 29 Si correlation experiments were used to observe the bonding/connectivity of silicon atoms at the silica surface ( Figure 3B) 28 and probe the spatial distribution of organic fragments on functionalized silica surfaces.…”