Heteronuclear Correlation Solid-state NMR Spectroscopy with Indirect Detection under Fast Magic-angle Spinning

TAKESHI KOBAYASHI, YUSUKE NISHIYAMA AND MAREK PRUSKI

1.1 Introduction

The solid-state (SS) NMR community has recently witnessed the development of probes capable of magic-angle spinning (MAS) at stunningly high rates, which have tripled from about 40 kHz to 120+ kHz over the last 15 years. Even a cursory review of the latest literature shows that fast MAS technology offers more than an incremental improvement of resolution and sensitivity, but constitutes a breakthrough in the field of SSNMR. In addition to the anticipated benefits, such as line narrowing, greater separation of the spinning sidebands (SSBs) in the spectra of spin-1/2 and quadrupolar nuclei, and the ability to generate very high RF magnetic fields, fast MAS has opened prospects for exploiting concepts and methodologies hitherto practiced exclusively in solutions, catalyzing the convergence of solid-state and solution NMR disciplines. The key capability facilitating these developments is the reduction of the homogeneous component of the 1H line width, enabling, for the first time, the effective use of 1H-detected (or indirectly detected) multidimensional heteronuclear correlation (HETCOR) schemes in SSNMR.

Whereas the theoretical principles of line narrowing by fast MAS and the background related to the resulting multidimensional methodology can be found in source articles and several recent reviews, the experimental strategies remain less known to practicing SSNMR spectroscopists interested in this emerging field. The main focus of this chapter is to address this gap by providing a hands-on guide to fast MAS experiments, with a particular focus on indirect detection. Although our experience is limited to the respective laboratories in Ames and Yokohama, we hope that our descriptions of experimental setups and optimization procedures are sufficiently general to be applicable to all modern instruments and a wide range of applications. The chapter is organized as follows: Section 1.2 briefly introduces the fast MAS technology and its main advantages. In Section 1.3, we describe the hardware associated with this remarkable technology and provide practical advices on its use, including procedures for loading and unloading the samples, maintaining the probe, reducing t1 noise, etc. In Section 1.4, we describe the principles and hands-on aspects of experiments involving the indirect detection of spin-1/2 and 14N nuclei.

1.2 Basic Aspects of Fast MAS

Ever since the discovery of MAS almost 60 years ago, a quest has been underway for higher and higher spinning rates (vR) to improve the resolution and sensitivity of SSNMR spectroscopy. Until quite recently, these efforts were primarily driven by the challenges associated with nuclei other than 1H, to overcome inhomogeneous line broadening due to chemical shift anisotropy (CSA) and quadrupolar interactions. Indeed, in powdered samples with wide CSA or quadrupolar powder patterns, sample spinning at a higher rate increases the spacing between the SSBs thereby decreasing their number, intensifies the centerbands and reduces the spectral overlap. For several decades, the homogeneous 1H–1H interactions had to be tackled by using suitable sequences of RF pulses, which were continuously improved for compatibility with MAS at higher rates. More recently, however, especially following the development of probes operated at vR = 40 kHz, the possibility of suppressing the 1H–1H homonuclear dipolar broadening by MAS alone has been the primary force driving further advances. The results of these endeavors are quite astounding: the first prototype probes capable of MAS at rates of 100 kHz were available in 2013, and by the end of 2016 commercial MAS probes operated at vR = 110 kHz were being offered by JEOL and Bruker. Concurrently with these technological advances, numerous studies have demonstrated the remarkable competencies of fast MAS probes, which produce high-quality spectra of organic and inorganic compounds, and have been used to investigate various classes of solid materials, including bio-related solids, surfaces and heterogeneous catalysts. In the following paragraphs, we briefly summarize some of the key features of fast MAS pertaining to sensitivity, resolution, and selectivity; more detailed information can be found in the cited literature and the most recent reviews.

1.2.1 Sensitivity

Small sample volume is an obvious concern in experiments performed under fast MAS. With the maximum MAS frequency being limited by the speed of the drive gas near the rotor surface, the only practical way to increase vR is to reduce the rotor's diameter. Indeed, rotors capable of MAS at 100 kHz have an outer diameter (OD) of 0.7–0.8 mm and a sample volume of less than 0.5 µL, whereas those designed for vR = 40 kHz have an OD of ~1.6 mm and a volume of almost 10 µL. By comparison, the standard 4 mm rotors can accommodate up to 50 µL of sample volume, but are typically operated at a much slower frequency (vR = 15 kHz).

The sensitivity penalty is not as severe as the loss of sample volume would suggest, however, for several reasons. First, the receptivity per unit volume increases in small coils, which in part compensates for the smaller rotor capacity. A crude analysis shows that for coils with the same length-to-diameter ratio (l/dcoil), the signal-to-noise ratio (SNR) per unit volume should scale as (dcoil)-1. We recently compared the relative sensitivity in experiments carried out with a 1.6 mm MAS probe (active sample volume Vsample = 6 µL) and a 0.75 mm MAS probe (Vsample = 290 nL) by measuring 1D 13C MAS spectra...