Maximizing the sensitivity in 13C cross-polarization magic-angle-spinning solid-state NMR measurements with flip-back pulses

Using Abundant 1H Polarization to Enhance the Sensitivity of Solid-State NMR Spectroscopy

“…However, even in the simple 1 H → 13 C CP experiment where all 13 C signals are enhanced, only around 1% of 1 H polarization is transferred to 13 C in natural abundant samples, while 99% of 1 H polarization is destroyed by T 1ρ relaxation during CP and later by heteronuclear decoupling. In simple circumstances, the residual 1 H polarization can be recycled for enhancing spin–lattice relaxation rate and thus shortening recycle delay, leading to sensitivity enhancement per unit time. ,− Such an idea was actually incorporated into a 2D 1 H-detected HETCOR experiment at fast MAS by preserving unused 1 H polarization during or after 13 C t 1 evolution time . Unfortunately, this method is only suitable for isotopically 13 C labeled samples .…”

Elucidations of Structure and Molecular Dynamics of Complex Polymers by State-of-the-Art Solid-State NMR Spectroscopy

“…In this case, the low-γ nuclei chemical shift dimension has to be involved for further enhancing the spectral resolution, which significantly reduces the S/N ratio even with proton detection. To address this issue, the initial proton equilibrium polarization can be fully exploited for heteronuclear 1 H–X polarization transfer via multiple steps of cross-polarization (CP) in each transient scan. , In addition to that, the residual proton polarization after CP can be even recycled to enhance proton T 1 relaxation or further utilized to enable the simultaneous acquisition of the 2D/3D proton multiple quantum correlation NMR spectrum and 2D 1 H–X HETCOR spectra from a single experiment, , leading to significant reduction of experimental time compared to separated acquisition of different spectra. An example is given in Figure , where a single SQ-SQ-HETCOR experiment enables the simultaneous acquisition of the 2D 1 H SQ/SQ spectrum and 2D 13 C/ 1 H HETCOR spectra of the PNIPAm sample, leading to fast resonance assignments .…”

Rapid Structural Analysis of Minute Quantities of Organic Solids by Exhausting 1H Polarization in Solid-State NMR Spectroscopy Under Fast Magic Angle Spinning

“…Indeed, numerous approaches have been proposed to enhance the sensitivity of solid-state NMR spectroscopy per unit time, enabling extraction of rich structural/dynamic information in a single experiment. On one hand, the experimental time of multidimensional solid-state NMR experiments can be substantially reduced by accelerating the 1 H spin–lattice relaxation ( T 1 ) time, via recycling residual 1 H polarization after low-γ nuclei signal acquisition, − or selective excitation to accelerate the repolarization of 1 H that is correlated to the heteronuclear spins of interest. , On the other hand, multiple acquisition can be implemented in a single scan via multiple detector/receivers − or pulse sequence development by utilizing different magnetization reservoirs and creating multiple polarization transfer pathways. − Such a strategy is also named parallel NMR spectroscopy, , which enables obtaining multiple multidimensional solid-state NMR spectra in a single experiment. Notably, because of the typically long T 1 relaxation time, 13 C or 15 N reservoirs are typically manipulated in parallel to obtain multiple 2D data sets from a single experiment .…”