Practical Aspects of Recording Ultra-Wideline NMR Patterns under Magic-Angle Spinning Conditions

Abstract

Many NMR-active nuclei give rise to solid-state NMR spectra that span extremely large frequency regions due to the effects of large anisotropic NMR interactions; such spectra, which can range from 250 kHz to several MHz in breadth, have been termed ultrawideline (UW) NMR spectra. UWNMR spectra are often too broad to be uniformly excited by conventional pulse sequences that implement rectangular radiofrequency (RF) pulses. Therefore, they are typically acquired with specialized pulse sequences and frequencyswept (FS) pulses; however, such experiments are conducted predominantly upon stationary samples (i.e., static NMR with no magic-angle spinning, MAS). Herein, we demonstrate how to implement Carr−Purcell Meiboom−Gill (CPMG) type pulse sequences that utilize rectangular pulses to acquire high-quality wideline and UWNMR spectra under MAS conditions, which are useful for providing uniformly excited patterns with substantial signal enhancements in comparison to conventional MAS NMR spectra and identifying peaks and/or patterns corresponding to magnetically nonequivalent sites. We discuss the pulse timings, delays, and the duration of windowed acquisition periods that are necessary for using CPMG-type pulse sequences for T 2 -dependent enhancement under MAS conditions while allowing for chemical shift resolution and maintaining a conventional spinning-sideband (SSB) manifold, as well as protocols for processing these spectra. Careful consideration is given to pulse lengths and RF amplitudes used in these pulse sequences. Using several spin-1 / 2 (i.e., 119 Sn, 207 Pb, 195 Pt) nuclei and one integer-spin quadrupolar nucleus (i.e., 2 H), we show how sensitivity-enhancing protocols, including CPMG and cross-polarization (CP), can be used to deliver high-quality MAS NMR spectra, which feature high signal-to-noise (S/N) ratios and uniformly excited SSB manifolds. The methods outlined herein are facile to implement and offer the potential to open up MAS NMR experiments to a wide variety of elements from across the periodic table.

Bidentate Disilicate Framework for Bis‐Grafted Surface Species

“…Instead, in our study, a CP‐CPMG sequence (Figure 7) was used with a high RF field strength. The conditions required for a CP‐CPMG sequence to acquire UW spectra have recently been discussed by the Schurko group [24] …”

“…The conditions required for a CP-CPMG sequence to acquire UW spectra have recently been discussed by the Schurko group. [24] Based on this study, the 195 Pt RF field strength used for spinlock and CPMG refocusing π/2 pulse was set to 100 kHz. This high RF field strength maintained during the CP-CPMG sequence can excite across a broad band and permits a 75-kHz RF transmitter frequency step for variable offset cumulative spectrum (VOCS) acquisition.…”

The Molecular and Electronic Structure of Isolated Platinum Sites Enabled by Expedient Measurement of 195Pt Chemical Shift Anisotropy

“…[55][56][57][58][59][60] Towards this goal, various schemes have been proposed for the acquisition of wideline solid-state NMR spectra. 53,[61][62][63] DNP has been used to enhance further the sensitivity of wideline solid-state NMR experiments under static 46,[64][65] and moderate magic angle spinning (MAS) conditions. 49 However, these methods offer poor 195 Pt NMR resolution.…”

Proton-detected solid-state NMR spectroscopy of spin-1/2 nuclei with large chemical shift anisotropy

“…21 Recently, Altenhof et al demonstrated the use of MAS CPMG experiments for the acquisition wideline spectra. 22 Fast MAS (MAS with frequencies > 30 kHz) is a powerful approach to improve the sensitivity and resolution of SSNMR spectra. [23][24][25] Notably, Lange, Nishiyama and coworkers have used fast MAS frequencies of 75 kHz to improve 119 Sn SSNMR sensitivity by focusing the intensity of 119 Sn CSA patterns into fewer sidebands.…”