Broadband adiabatic inversion experiments for the measurement of longitudinal relaxation time constants

Abstract

Accurate measurements of longitudinal relaxation time constants (T1) in solid-state nuclear magnetic resonance (SSNMR) experiments are important for the study of molecular-level structure and dynamics. Such measurements are often made under magic-angle spinning conditions; however, there are numerous instances where they must be made on stationary samples, which often give rise to broad powder patterns arising from large anisotropic NMR interactions. In this work, we explore the use of wideband uniform-rate smooth-truncation pulses for the measurement of T1 constants. Two experiments are introduced: (i) BRAIN-CPT1, a modification of the BRAIN-CP (BRoadband Adiabatic-INversion-Cross Polarization) sequence, for broadband CP-based T1 measurements and (ii) WCPMG-IR, a modification of the WURST-CPMG sequence, for direct-excitation (DE) inversion-recovery experiments. A series of T1 constants are measured for spin-1/2 and quadrupolar nuclei with broad powder patterns, such as 119Sn (I = 1/2), 35Cl (I = 3/2), 2H (I = 1), and 195Pt (I = 1/2). High signal-to-noise spectra with uniform patterns can be obtained due to signal enhancements from T2eff-weighted echo trains, and in favorable cases, BRAIN-CPT1 allows for the rapid measurement of T1 in comparison to DE experiments. Protocols for spectral acquisition, processing, and analysis of relaxation data are discussed. In most cases, relaxation behavior can be modeled with either monoexponential or biexponential functions based upon measurements of integrated powder pattern intensity; however, it is also demonstrated that one must interpret such T1 values with caution, as demonstrated by measurements of T1 anisotropy in 119Sn, 2H, and 195Pt NMR spectra.

3D relaxation-assisted separation of wideline solid-state NMR patterns for achieving site resolution

“…Elastic-net (EN) is one form of regularization that we have adopted, and refers to the linear combination of additional l 2 and l 1 norm constraints. 35,47 The additional l 2 norm serves to minimize the condition number of the kernel by penalizing its small non-zero singular values, which attenuates the amplitude of the experimental noise in the NNLS fit. The l 1 norm imposes a sparsity constraint that can force some solutions to zero, which is often useful for multi-component relaxation or distributions of relaxation rates.…”

“…26 These precedents could further benefit from the application of WURST 28,29 pulses (for direct excitation) 30 or broadband adiabatic inversion-cross polarization (BRAIN-CP) (for CP-enhanced excitation), 31 combined with measurements of T 1 and T 2 (or T eff 2 ) relaxation time constants using inversion recovery (IR) 32 and CPMG 33,34 sequences, respectively. 35 In particular, all of these sequences are relatively simple, utilize low RF powers, and can be applied to a wide array of spin-1/2 and quadrupolar nuclei. 27,36,37 All RAS approaches involve an inversion of the NMR relaxation data, transforming it from the time domain to the relaxation-rate domain.…”

“…We extend these methods for improved pattern resolution by utilizing 3D frequency-R 1 -R 2 correlation spectra. Synthetic and experimental datasets, including 35 Cl (I = 3/2), 2 H (I = 1), and 14 N (I = 1) NMR of organic and biological compounds, are explored with both regularized 2D RAS and 3D RAS; comparison of these data reveal improved resolution in the latter case. These methods have great potential for separating overlapping powder patterns under both static and MAS conditions.…”

Proton-Detected Solid-State NMR Spectroscopy of Spin-1/2 Nuclei with Large Chemical Shift Anisotropy

“…13 Broadband adiabatic inversion cross polarization (BRAIN-CP) further enhances sensitivity by polarization transfer from nearby high-g spins such as 1 H; 14 adiabatic inversion sequences have also been used to measure longitudinal relaxation constants (T1). 15 BRAIN-CP WCPMG sequences can be implemented into the VOCS procedures to accelerate acquisition times and reduce the number of sub-spectra required. Dynamic nuclear polarization (DNP) has further improved the sensitivity of BRAIN-CP-WCPMG NMR experiments with heavy spin-1/2 nuclei [16][17] and enabled the characterization of low-concentration surface Pt sites in heterogenous catalysts.…”

“…[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. 26 Fast MAS has been combined with indirect detection to obtain SSNMR spectra of spin-1/2 nuclei such as 13 C, 15 N, and 29 Si using double cross polarization (CP) schemes. [27][28][29][30][31][32] However, CP schemes often fail when studying quadrupolar nuclei or spin-1/2 nuclei with large CSA.…”