Modern Methods in Solid-state NMR: A Practitioner's Guide: Volume 15 (New Developments in NMR) - Hardcover

Hardcover

ISBN 10: 1782628541 ISBN 13: 9781782628545

Verlag: Royal Society of Chemistry, 2018

Inhaltsangabe

Solid-state NMR covers an enormous range of material types and experimental techniques. Although the basic instrumentation and techniques of solids NMR are readily accessible, there can be significant barriers, even for existing experts, to exploring the bewildering array of more sophisticated techniques. In this unique volume, a range of experts in different areas of modern solid-state NMR explain about their area of expertise, emphasising the “practical aspects” of implementing different techniques, and illustrating what questions can and cannot be addressed. Later chapters address complex materials, showing how different NMR techniques discussed in earlier chapters can be brought together to characterise important materials types. The volume as a whole focusses on topics relevant to the developing field of “NMR crystallography” – the use of solids NMR as a complement to diffraction crystallography.
This book is an ideal complement to existing introductory texts and reviews on solid-state NMR. New researchers wanting to understand new areas of solid-state NMR will find each chapter to be the equivalent to spending time in the laboratory of an internationally leading expert, learning the hints and tips that make the difference between knowing about a technique and being ready to put it into action. With no equivalent on the market, it will be of interest to every solid-state NMR researcher (academic and postgraduate) working in the chemical sciences.

Über diesen Titel

Über die Autorin bzw. den Autor

Dr Paul Hodgkinson is a Reader in Magnetic Resonance at Durham University, UK. His research combines interests in technique development and methodology in solid-state NMR as well as applications to chemical problems. Applications of NMR have been in the area of structural chemistry, particularly of pharmaceutical materials and systems with mobility, such as soft solids and solvates. A particular interest is in combining information from diffraction-based experiments, NMR and computation of NMR parameters (using DFT codes), and dynamics (molecular dynamics simulations).

Von der hinteren Coverseite

Solid-state NMR covers an enormous range of material types and experimental techniques. Although the basic instrumentation and techniques of solids NMR are readily accessible, there can be significant barriers, even for existing experts, to exploring the bewildering array of more sophisticated techniques. In this unique volume, a range of experts in different areas of modern solid-state NMR explain about their area of expertise, emphasising the “practical aspects” of implementing different techniques, and illustrating what questions can and cannot be addressed. Later chapters address complex materials, showing how different NMR techniques discussed in earlier chapters can be brought together to characterise important materials types. The volume as a whole focusses on topics relevant to the developing field of “NMR crystallography” – the use of solids NMR as a complement to diffraction crystallography.
This book is an ideal complement to existing introductory texts and reviews on solid-state NMR. New researchers wanting to understand new areas of solid-state NMR will find each chapter to be the equivalent to spending time in the laboratory of an internationally leading expert, learning the hints and tips that make the difference between knowing about a technique and being ready to put it into action. With no equivalent on the market, it will be of interest to every solid-state NMR researcher (academic and postgraduate) working in the chemical sciences.

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Modern Methods in Solid-state NMR

A Practitioner's Guide

By Paul Hodgkinson

The Royal Society of Chemistry

Copyright © 2018 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-78262-854-5

Contents

Methods for Spin-1/2 Nuclei, 
Chapter 1 Heteronuclear Correlation Solid-state NMR Spectroscopy with Indirect Detection under Fast Magic-angle Spinning Takeshi Kobayashi, Yusuke Nishiyama and Marek Pruski, 3, 
Chapter 2 High-resolution 1H 2D Magic-angle Spinning Techniques for Organic Solids Steven P. Brown, 39, 
Chapter 3 Isotropic vs. Anisotropic Chemical Shift Separation Ivan Hung and Zhehong Gan, 75, 
Chapter 4 Two-dimensional Methods for Half-integer Quadrupolar Nuclei F. Pourpoint, O. Lafon, R. Gauvin, J.-P. Amoureux and L. Delevoye, 99, 
Chapter 5 14N Solid-state NMR Luke A. O'Dell, 134, 
Chapter 6 CODEX-based Methods for Studying Slow Dynamics Detlef Reichert and Alexey Krushelnitsky, 163, 
Chapter 7 NMR Studies of Ionic Dynamics in Solids Roland Böhmer, Michael Storek and Michael Vogel, 193, 
Chapter 8 Low-temperature NMR: Techniques and Applications D. Arcon, I. Heinmaa and R. Stern, 233, 
Chapter 9 NMR at High Temperature Jonathan F. Stebbins, 262, 
Chapter 10 Isotopically Enriched Systems C. Beumer, A. König, D. Schölzel, B. Uluca, F. Weirich and H. Heise, 291, 
Chapter 11 NMR Studies of Electrochemical Storage Materials Pieter C. M. M. Magusin, Ieuan D. Seymour, Oliver Pecher and Clare P. Grey, 322, 
Chapter 12 Disordered Solids P. Florian and F. Fayon, 356, 
Chapter 13 Characterization of Liquid-crystalline Materials by Separated Local Field Methods S. V. Dvinskikh, 391, 
Subject Index, 424, 

CHAPTER 1

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 of hexamethylbenzene under direct polarization. The spectra acquired under equivalent conditions yielded SNRs that differed by a factor of ~12 in favor of the larger rotor, which agrees very well with the above prediction, rather than a factor of ~21 from a simple ratio of sample volumes. Secondly, in samples with large anisotropic interactions, faster MAS reduces the number of SSBs, and accordingly the signals became more intense and better resolved. A number of additional factors that influence the sensitivity are associated with the effect of MAS on 1H line width, as discussed below.

1.2.2 1H Resolution: Indirect Detection of Lower-? Nuclei

Most importantly for the scope of this chapter, fast MAS can help reduce the line broadening due to the strong homonuclear dipolar couplings between 1H nuclei. The total 1H MAS line width, D, in strongly coupled systems of protons can be written as a sum of the homogeneous contribution (?hom), which is non-refocusable by the spin-echo, and the inhomogeneous contribution (?inhom), which can be refocused. The ?inhom part is typically governed by the distribution of chemical shifts due to structural inhomogeneity and/or by broadening due to anisotropic bulk magnetic susceptibility (ABMS). These two broadening sources are independent of vR and the strength of the magnetic field B0; i.e., they are constant in ppm. In most solids, however, the overall 1H line width is dominated by strong homonuclear dipolar interaction between 1H nuclei, which in static organic solids results in homogeneous broadening, with ?hom approaching 100 kHz. In the absence of RF decoupling, the intrinsic value of ?hom scales down roughly in proportion to (vR)-1.

? = ?inhom + ?hom = ?inhom(1+k(vR).sup.-1] (1.1)

The effect of MAS on resolution is determined by the empirical proportionality factor k, defined as k = vR?hom/?inhom, which depends on the dimensionality, geometry and dynamics of the 1H network and the B0 field strength. Ultrafast MAS can attain the limit of inhomogeneous broadening (vR [much greater than] k), either in weakly coupled systems where the local molecular dynamics limits ?hom to a fraction of its 'static' value (e.g., in the case of catalytic moieties bound to a surface), or when the ?inhom value is significant (due to disorder or an ABMS effect). In many diamagnetic organic solids, however, the opposite condition holds (vR<k), even at the highest currently achievable MAS rates of 100+ kHz. Still, despite the incomplete suppression of 1H–1H dipolar interactions, MAS at such rates can provide remarkably improved 1H resolution, rivaling that achieved with state-of-the-art CRAMPS methods. This is demonstrated in Figure 1.1, which shows a series of 1H NMR spectra of L-histidine · HCl · H2O taken at 20 kHz = vR = 110 kHz. The line-narrowing effect of MAS can be further enhanced by using a stronger magnetic field, due to the linear increase of separation between the resonance lines and the consequent reduction of homogeneous line broadening ?hom.

The added, and arguably most important, consequence of this increased 1H resolution is that it enables, for the first time, the practical use of 2D HETCOR SSNMR schemes that take advantage of the indirect detection of insensitive lower-g nuclei, such as 13C or 15N, via the more sensitive 1H nuclei. The indirect methods, utilizing the general scheme X{H}-t1-H{X}-t2, have been in routine use in solution NMR for over three decades for the detection of correlations between different-type nuclei separated by single or multiple bonds. Until recently, SSNMR HETCOR spectroscopy has used direct (or X-detected) schemes, H-t1-X{H}-t2, simply because the inherently broad 1H line necessitated the use of RF pulse sequences to achieve 1H–1H homonuclear decoupling during the 1H evolution period, rendering proton detection useless. However, assuming that adequate resolution can be achieved by MAS alone, indirect detection offers a sensitivity gain of roughly:

[MATHEMATICAL EXPRESSION OMITTED], (1.2)

where DH/X and gH/X denote the effective line widths and gyromagnetic ratios of 1H and X nuclei, respectively, and f accounts for a number of experimental factors, including the efficiency of X [right arrow] H magnetization transfer, probe Q factors and receiver noise at both frequencies. In the case of 1H–15N HETCOR, the (?H/?X) factor equals ~31, potentially offering ~1000-fold time savings, although the experimentally observed gains are generally lower.

The enhancement of sensitivity by indirect detection in solids was first reported by Ishii et al., who subjected 15N-labeled peptides and naturally 13C-abundant polymers to MAS at ~30 kHz. Spiess and co-workers used a similar approach to acquire 15N{1H} spectra of 15N-enriched polymer precursors. Other studies used magnetic dilution by deuteration in combination with 20–30 kHz MAS to reduce ?H, thereby enabling indirect-detection 15N–1H experiments on 15N-enriched proteins and peptides. Subsequent progress was based on MAS probes operated at 40 kHz. Our laboratory reported indirect-detection HETCOR measurements involving a catalyst surface; using 40 kHz MAS, a 1H{13C} spectrum of MCM-41 type silica containing ~300 µg of naturally 13C-abundant covalently bound allyl groups was measured in just 15 min — a result that earlier would have been considered unthinkable. Rienstra's group used a series of 2D and 3D proton detection methods to achieve complete structure determination of fully protonated and uniformly 13C,15N-labeled proteins, as well as pharmaceutical samples with natural 13C abundance.

To demonstrate the sensitivity benefits of indirect detection, we show in Figure 1.2 two sets of 2D 1H{13C} and 1H{15N} HETCOR spectra of tripeptide (N-formyl-L-methionyl-L-leucyl-L-phenylalanine-OH; referred to as MLF), acquired at vR1/440 kHz and 80/100 kHz, using 1.6 mm, 1.0 mm and 0.75 mm MAS probes, with sample volumes of 6 mL, 0.8 mL and 290 nL, respectively. The 13C and 15N skyline projections of 2D spectra are accompanied by the corresponding 1D 13C and 15N CPMAS spectra obtained within the same experimental time-frame of 5 h. Note that not only did the indirectly detected HETCOR spectra provide valuable internuclear correlations, but their projections have better SNR than the 1D spectra, especially in the case of lower ?X (i.e.,15N, Figure 1.2d) and at faster MAS rates (due to narrower line width ?, Figures 1.2b,d). The peak assignments and the analysis of relative peak intensities in the spectra of MLF can be found in ref. 46 and 47.

The pulse sequences used in these earlier experiments were based on 1H [right arrow] X and X [right arrow] 1H cross-polarization (CP/CP) transfers. However, it has also been demonstrated that (ultra)fast MAS is surprisingly compatible with homonuclear 1H-1H decoupling, using either known or newly designed RF pulse sequences, yielding equivalent or better 1H resolution than that obtained in previous state-of-the-art CRAMPS experiments. Concomitant with the reduction of ?hom is an increased lifetime of transverse magnetizations of both 1H and nearby X nuclei (T'2H and T'2X), which in turn offers opportunities for through-bond heteronuclear SSNMR spectroscopy. The first such experiments based on a 1H-detected, CP/INEPT scheme were amongst an array of HETCOR methods utilizing single quantum (HSQC) and multiple quantum (HMQC) coherence transfer schemes with various combinations of magnetization transfers via dipolar and J-couplings (see below for further details). Fast MAS was instrumental in improving the efficiency of J-driven transfers because of its ability to slow down the decoherence of both 1H and X nuclei during the polarization transfers and/or the t1 evolution.

(Continues...)

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Details zur Ausgabe

• Verlag: Royal Society of Chemistry
• Erscheinungsdatum: 2018
• Sprache: Englisch
• ISBN 10: 1782628541
• ISBN 13: 9781782628545
• Einband: Tapa dura
• Anzahl der Seiten: 456
• Herausgeber: Hodgkinson Paul
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: Nicht verfügbar