Cross-polarization phenomena in the NMR of fast spinning solids subject to adiabatic sweeps

Abstract

Cross-polarization magic-angle spinning (CPMAS) experiments employing frequency-swept pulses are explored within the context of obtaining broadband signal enhancements for rare spin S = 1/2 nuclei at very high magnetic fields. These experiments employ adiabatic inversion pulses on the S-channel (13 C) to cover a wide frequency offset range, while simultaneously applying conventional spin-locking pulse on the I-channel (1 H). Conditions are explored where the adiabatic frequency sweep width, ∆ν, is changed from selectively irradiating a single magic-angle-spinning (MAS) spinning centerband or sideband, to sweeping over multiple sidebands. A number of new physical features emerge upon assessing the swept-CP method under these conditions, including multiple zero-and double-quantum CP transfers happening in unison with MAS-driven rotary resonance phenomena. These were examined using an average Hamiltonian theory specifically designed to tackle these experiments, with extensive numerical simulations, and with experiments on model compounds. Ultrawide CP profiles spanning frequency ranges of nearly 6 • γB s 1 were predicted and observed utilizing this new approach. Potential extensions and applications of this extremely broadband transfer conditions are briefly discussed.

Rapid acquisition of wideline MAS solid-state NMR spectra with fast MAS, proton detection, and dipolar HMQC pulse sequences

“…27,43 Conventional ramped CPMAS experiments are typically not applicable in such cases since they suffer from very poor bandwidth due to the narrowband spin lock pulse in the CP step. 50 Subsequently, direct excitation experiments, possibly incorporating frequency swept shaped pulses, are often used to acquire wideline MAS NMR spectra. [51][52][53] Similar to static wideline NMR experiments, if the MAS sideband manifold is not uniformly excited, frequency stepped acquisition of the MAS pattern may be performed.…”

87Sr, 119Sn, 127I Single and {1H/19F}‐Double Resonance Solid‐State NMR Experiments: Application to Inorganic Materials and Nanobuilding Blocks

“…We stress here that the above mentioned experiments were all implemented in the static mode. Very recently, the extension of BRAIN to MAS conditions was described theoretically by Wi et al . (without experimental applications).…”

Spin-locking and cross-polarization under magic-angle spinning of uniformly labeled solids

“…CP is usually mediated by spin flip-flop (I + S À + I À S + ), or the so-called zero-quantum (ZQ) dipolar Hamiltonian, but under MAS, flip-flip/flop-flop (I À S À + I + S + ) can also occur via the double-quantum (DQ) dipolar Hamiltonian mechanism at the conditions m 1I + m 1S = n Á m r , which can often be met under fast MAS and weak spin-locking rf fields [10][11][12][13]. In order to compensate mismatch of the CP conditions due to inhomogeneous rf fields, a sweep of the rf field amplitude [14][15][16] or frequency offset [17][18][19] across one of the matching conditions is usually applied to improve CP efficiency [14][15][16]. More sophisticated pulse schemes have been developed as well by taking the MAS modulation into consideration [20,21].…”