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

Solid-state NMR spectroscopy has been playing a significant role in elucidating the structures and dynamics of materials and proteins at the atomic level for decades. As an extremely abundant nucleus with a very high gyromagnetic ratio, protons are widely present in most organic/inorganic materials. Thus, this Perspective highlights the advantages of proton detection at fast magic-angle spinning (MAS) and presents strategies to utilize and exhaust 1H polarization to achieve signal sensitivity enhancement of solid-state NMR spectroscopy, enabling substantial time savings and extraction of more structural and dynamics information per unit time. Those strategies include developing sensitivity-enhanced single-channel 1H multidimensional NMR spectroscopy, implementing multiple polarization transfer steps in each scan to enhance low-γ nuclei signals, and making full use of 1H polarization to obtain homonuclear and heteronuclear chemical shift correlation spectra in a single experiment. Finally, outlooks and perspectives are provided regarding the challenges and future for the further development of sensitivity-enhanced proton-based solid-state NMR spectroscopy.

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

Solid-state nuclear magnetic resonance (NMR) often suffers from significant limitations due to the inherent low signal sensitivity when low-γ nuclei are involved. Herein, we report an elegant solid-state NMR approach for rapid structural analysis of minute amounts of organic solids. By encoding staggered chemical shift evolution in the indirect dimension and staggered acquisition in the ¹H dimension, a proton-detected homonuclear ¹H/¹H and heteronuclear ¹³C/¹H chemical shift correlation (HETCOR) spectrum can be obtained simultaneously in a single experiment at a fast magic-angle-spinning (MAS) condition with barely increasing the experimental time. We further show that during the conventional ¹H-detected HETCOR experimental time, multiple homonuclear ¹H/¹H correlation spectra can be recorded in addition to the HETCOR spectrum, enabling the determination of ¹H-¹H distances. We establish that abundant ¹H polarization can be efficiently manipulated and fully utilized in proton-detected solid-state NMR spectroscopy for extraction of more critical structural information and thus reduction of the total experimental time.

Sensitivity enhancement in 2D Double Cross Polarization experiments under fast MAS by recycling unused protons

We demonstrate sensitivity enhancement via recycling of proton magnetization in 2D Double Cross Polarization (Double CP) experiments performed on fully protonated and uniformly labeled (¹³C, ¹⁵N) samples at a magic angle spinning rate of 60 kHz. Unused proton magnetization is preserved during t₁ evolution either by locking it with CW irradiation or by employing rotor-synchronized pi pulses. A flip-back pulse together with a modified second CP block preserves unused proton magnetization resulting in enhanced sensitivity. We have achieved sensitivity enhancements of 15-20% and 25-28% in ¹H-¹³C and ¹H-¹⁵N 2D Double CP experiments respectively. At shorter recycle delays (∼0.25T₁), relative sensitivity enhancements of 40-45% and 55% were obtained in ¹H-¹³C and ¹H-¹⁵N 2D Double CP experiments respectively. An analysis of the sensitivity enhancements and theoretical estimation of lineshapes in indirect dimension in the presence of proton recycling is provided.

Sensitivity enhancement via multiple contacts in the {1H–29Si}–1H cross polarization experiment: a case study of modified silica nanoparticle surfaces

{¹H–²⁹Si}–¹H double cross polarization inverse detection (DCPi) solid-state NMR, has recently been shown to be a powerful tool for studying molecules adsorbed on the silica surface. In this contribution, we develop an improved version (MCPi) which incorporates a block of multiple contact pulses, and quantitatively compare the sensitivities of MCPi and DCPi over a typical range of experimental parameters. The MCPi pulse sequence aims at higher sensitivity and robustness for studying samples with various relaxation characteristics. In the case of dimethyl sulfoxide (DMSO) molecules adsorbed on the silica surface, MCPi performs equally well or up to 2.5 times better than DCPi over a wide range of parameters. The applicability to and performance of MCPi on composite materials was demonstrated using a sample of polymer–silica composite, where significantly higher sensitivity could be achieved at very long total mixing times. The results also showed that both techniques are surface specific in the sense that only the groups close to the surface can be detected.

In this paper we demonstrate {¹H–²⁹Si}–¹H multiple cross polarization inverse detection (MCPi) solid state NMR as a robust technique for studying modified silica nanoparticle surfaces.

Reduction of the 13C cross-polarization experimental time for pharmaceutical samples with long T1 by ball milling in solid-state NMR

Many pharmaceutical samples have notably long ¹H T₁ (proton spin-lattice relaxation time), leading to lengthy experiments lasting several days in solid-state NMR studies. In this work, we propose the use of ball milling on the pharmaceutical samples to reduce the ¹H T₁, which also leads to enhanced sensitivity in {¹H}-¹³C Cross-Polarization (CP) experiments due to reduced particle sizes and increased surface areas of the samples. Experimentally, we determined that depending on the substrates and milling time, the signal-to-noise ratio (S/N) of a 1D ¹³C CP spectrum can be increased by a factor of 3-6, which means that the experimental time can be shortened by a factor of 9-36. Furthermore, the application of simple ball-milling within a short time avoids the amorphization of the studied samples such that no signal due to amorphous state is observed in the ¹³C CP spectrum. This simple ball milling method used for sensitivity enhancement can be further applied in the SS-NMR studies of pharmaceutical samples.