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Cross-polarization (CP) is a technique commonly used for the signal enhancement of NMR spectra; however, applications to quadrupolar nuclei have heretofore been limited due to a number of problems, including poor spin-locking efficiency, inconvenient relaxation times, and reduced CP efficiencies over broad spectral bandwidths─this is unfortunate, since they constitute 73% of NMR-active nuclei in the periodic table. The Broadband Adiabatic Inversion CP (BRAIN-CP) pulse sequence has proven useful for the signal enhancement of wideline and ultra-wideline (i.e., 250 kHz to several MHz in breadth) powder patterns arising from stationary samples; however, a comprehensive investigation of its application to half-integer quadrupolar nuclei (HIQN) is currently lacking. Herein, we present theoretical and experimental considerations for applying BRAIN-CP to acquire central-transition (CT, +1/2 ↔ −1/2) powder patterns of HIQN. Consideration is given to parameters crucial to the success of the experiment, such as the Hartmann–Hahn (HH) matching conditions and the phase modulation of the contact pulse. Modifications to the BRAIN-CP sequence such as flip-back (FB) pulses and ramped contact pulses applied to the 1H spins are used for the reduction of experimental times and increased CP bandwidth capabilities, respectively. Spectra for a series of quadrupolar nuclei with broad CT powder patterns, including 35Cl (S = 3/2), 55Mn (S = 5/2), 59Co (S = 7/2), and 93Nb (S = 9/2), are acquired via direct excitation (CPMG and WCPMG) and indirect excitation (CP/CPMG and BRAIN-CP) methods. We demonstrate that proper implementation of the sequence can enable 1H-S broadband CP over a bandwidth of 1 MHz, which to the best of our knowledge is the largest CP bandwidth reported to date. Finally, we establish the basic principles necessary for simplified optimization and execution of the BRAIN-CP pulse sequence for a wide range of HIQNs.
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Broadband adiabatic inversion cross‐polarization to integer‐spin nuclei with application to deuterium NMR
Abstract Solid‐state NMR (SSNMR) spectroscopy of integer‐spin quadrupolar nuclei is important for the molecular‐level characterization of a variety of materials and biological solids; of the integer spins,2H (S = 1) is by far the most widely studied, due to its usefulness in probing dynamical motions. SSNMR spectra of integer‐spin nuclei often feature very broad powder patterns that arise largely from the effects of the first‐order quadrupolar interaction; as such, the acquisition of high‐quality spectra continues to remain a challenge. The broadband adiabatic inversion cross‐polarization (BRAIN‐CP) pulse sequence, which is capable of cross‐polarization (CP) enhancement over large bandwidths, has found success for the acquisition of SSNMR spectra of integer‐spin nuclei, including14N (S = 1), especially when coupled with Carr–Purcell/Meiboom–Gill pulse sequences featuring frequency‐swept WURST pulses (WURST‐CPMG) forT2‐based signal enhancement. However, to date, there has not been a systematic investigation of the spin dynamics underlying BRAIN‐CP, nor any concrete theoretical models to aid in its parameterization for applications to integer‐spin nuclei. In addition, the BRAIN‐CP/WURST‐CPMG scheme has not been demonstrated for generalized application to wideline or ultra‐wideline (UW)2H SSNMR. Herein, we provide a theoretical description of the BRAIN‐CP pulse sequence for spin‐1/2 → spin‐1 CP under static conditions, featuring a set of analytical equations describing Hartmann–Hahn matching conditions and numerical simulations that elucidate a CP mechanism involving polarization transfer, coherence exchange, and adiabatic inversion. Several experimental examples are presented for comparison with theoretical models and previously developed integer‐spin CP methods, demonstrating rapid acquisition of2H NMR spectra from efficient broadband CP.
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- Award ID(s):
- 2003854
- PAR ID:
- 10449545
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Magnetic Resonance in Chemistry
- Volume:
- 59
- Issue:
- 9-10
- ISSN:
- 0749-1581
- Format(s):
- Medium: X Size: p. 1009-1023
- Size(s):
- p. 1009-1023
- Sponsoring Org:
- National Science Foundation
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