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

摘要

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