NMR Crystallography

by Oliver Pecher

This article is the first in a series of entries to the „Young Crystallographers'“ blog that focus on an overview of the diversity and application of crystallography in different scientific fields, like geology, materials science, chemistry, physics, etc. The authors will try to give a general introduction to the topic but of course the overall tenor may be slightly subjective, since our authors mainly pick out information and examples of their current scientific work fields. Furthermore, we give references and links for further reading – for all those of you, who would like to dive deeper into some of the topics.

The Short Version

Text adopted from: http://www.senker.uni-bayreuth.de/en/research/projects/15/index.html

„Nuclear Magnetic Resonance (NMR) crystallography combines the complementary techniques of solid-state NMR, powder diffraction and computational chemistry. While diffraction experiments reveal topological data, solid-state NMR unravels connections, distances and orientation relations on local and intermediate length scales. Molecular modelling and quantum chemical simulations help to create meaningful model structures.“

SMARTER Crystallography

In the context of a combined application of NMR spectroscopy, diffraction and quantum mechanical calculations also the term of SMARTER crystallography should be mentioned.

Text adopted from: http://www.smarter3.uvsq.fr/

„SMARTER stands for Structure elucidation by coMbining mAgnetic Resonance, compuTation modEling and diffRactions. The aim of SMARTER is to bring together specialists from the different areas of material sciences, such as chemists, processing engineers, diffractionists, spectroscopists, and computational structuralists that contribute to the development of a common language for SMARTER Crystallography, i.e. for solving structures by using geometrical crystallography, diffraction, modelling and NMR crystallographies.“

Adopted from: http://smarter.web.ua.pt/

In the Encyclopedia of Magnetic Resonance (eMagRes)

Content description and part of the back cover of the Encylopedia of Magnetic Resonance handbook (abbreviation EMR up to 2012 and eMagRes since 2013 and onward): Robin K. Harris, Roderick E. Wasylishen, Melinda J. Duer (Editors) NMR Crystallography, John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, Sussex, PO19 8SQ, UK, 2009.

„The term ‚NMR Crystallography‘ has only recently come into common usage, and even now causes raised eyebrows within some parts of the diffraction community. The power of solid-state NMR to give crystallographic information has considerably increased since the CPMAS suite of techniques was introduced in 1976. In the first years of the 21^st century, the ability of NMR to provide information to support and facilitate the analysis of single-crystal and powder diffraction patterns has become widely accepted. Indeed, NMR can now be used to refine diffraction results and, in favorable cases, to solve crystal structures with minimal (or even no) diffraction data. The increasing ability to relate chemical shifts (including the tensor components) to the crystallographic location of relevant atoms in the unit cell via computational methods has added significantly to the practice of NMR crystallography. Diffraction experts will increasingly welcome NMR as an allied technique in their structural analyses. Indeed, it may be that in the future crystal structures will be determined by simultaneously fitting diffraction patterns and NMR spectra.“

In the International Union of Crystallography (IUCr)

Robin K. Harris (University of Durham, UK) on NMR crystallography; adopted from the IUCr newsletter volume 13, number 1: http://www.iucr.org/news/newsletter/volume-13/number-1/nmr-crystallography

„When the President of the IUCr gave the introduction to the 35th International School on Crystallography at Erice, Sicily, in June 2004, he listed particular divisions of the subject as X-ray Crystallography, Neutron Crystallography and Electron Crystallography. This list, whilst probably not intended to be exhaustive, is now rather obviously defective in not containing the increasingly important area of NMR crystallography.
NMR is an exceedingly important tool for chemical structure determination and also for the study of molecular-level mobility. It is accepted as by far the most powerful technique for such investigations for chemical compounds in the solution state. This situation arises from the fact that NMR spectra show exceedingly high resolution, thus distinguishing between atoms in closely similar chemical sites.
The situation is very different for solids, which, without the use of special techniques, generally give rise to broad unresolved resonances. Therefore, in the period 1955-1975 chemists largely ignored NMR of solids. However, the situation was revolutionised in 1976 by the development of the suite of techniques for ¹³C NMR (and subsequently for other nuclei) comprising cross-polarisation (CP), magic angle spinning (MAS) and high-power proton decoupling (HPPD). Since that date these techniques have become widely used and many refinements and developments have been produced. The result is that much crystallographic information can now be derived from NMR and the technique is increasingly important in this area.
NMR methods are sufficiently advanced that complete crystal structures have now been determined for the first time without other information. There is now truly a subject which may be called “NMR Crystallography”. This is generally complementary to diffraction crystallography, as demonstrated by a number of the above points. Unfortunately, NMR efforts in the investigation of crystallography have been very slow to be integrated with those of the rest of the crystallographic community, so that many “diffraction-based” crystallographers do not consider solid-state NMR as a natural part of their suite of techniques. Whilst it is mildly encouraging to see that the program for the Florence Congress this August contains sub-sections under two of the Congress main topics (though both, for some inexplicable reason, appear to be limited to macromolecules), these are the only mentions of NMR and there is nothing under the main topic number 01 “Instrumentation and Experimental Techniques”.
Perhaps now is the time to consider constituting a division of NMR crystallography within IUCr. Certainly a formal or semiformal link with the solid-state NMR community is desirable.

A few points about NMR and its potential applications to crystallography are as follows:

• NMR responds to the short-range environment of relevant atoms and is not directly influenced by long-range order.
• It can therefore be applied to amorphous materials as well as crystalline ones, though with broader lines for the former to encompass the variations in nuclear environments.
• It can be readily used to determine the chemical nature of a solid compound, including crystallographically important information such as conformation and tautomeric form.
• Chemical shifts give information about intermolecular interactions.
• Inter- and intra-molecular hydrogen-bond linkages can be identified.
• Information on crystallographic asymmetric units is especially readily available, usually merely by counting lines.
• Polymorphs are usually easily distinguished.
• Phase transitions can be monitored.
• Crystallographic disorder is detectable, and distinctions between spatial and temporal disorder can be made.
• Motions such as internal rotation and ring inversion can be detected and their rates obtained, even in cases of mutual exchange (e.g. 180° ringflips of phenyl groups).
• More general information about molecular-level mobility can be obtained by measurements of relaxation times.
• Measurement of dipolar coupling constants yields through-space inter-atomic (i.e. internuclear) distances, though these will be modulated by local mobility.
• NMR data can be used as restraints in carrying out full structure determination from powder diffraction data.
• Heterogeneous materials can be studied and selective spectra for particular domains obtained by the use of special pulse sequences.
• Intensities can be made quantitative (e.g. for polymorph ratios or crystallinity proportions).
• NMR is a multinuclear technique, each relevant isotope and element having its own specific frequency range.
• NMR experiments can be tailored to produce particular results by suitable choice of pulse sequences.“

In the Laboratory: One Example for an Application – Aluminophosphates / MOFs

NMR crystallography has provided structural models for a huge variety of materials [1]. For nanoporous aluminophosphates the TECTOSPIN group (F. Taulelle et al., Institute Lavoisier, Université de Versailles, France) has recently developed a very quick and 100 % successful structure topology determination by adding constraints concerning the coordination and number of atoms based on ²⁷Al and ³¹P chemical shifts as well as the connectivity between the polyhedra determined from two-dimensional (2D) NMR experiments [2]. The results also indicated that optimum efficiency is reached if information about both – the nodes and vertices of the network – is known: like inorganic clusters and organic linkers in metal organic frameworks (MOFs).
From this, various applications of SMARTER crystallography in the field of structure solution and determination of polycrystalline MOFs are envisioned. For environmentally significant Zr/Ti based MOFs the ¹⁷O NMR spectroscopic investigations of different oxygen environments are a promising way to gain information about the inorganic clusters, since the high-valence metal centres are hardly accessible by NMR spectroscopy.

[1] C. Martineau, A. Cadiau, B. Bouchevreau, J. Senker, F. Taulelle, K. Adil, Dalton Trans. 2012, 41, 6232.
[2] B. Bouchevreau, C. Martineau, C. Mellot-Draznieks, A. Tuel, J. Trébosc, O. Lafon, J. P. Amoureux, F. Taulelle, Chem. Eur. J. 2013, 19, 5009.

Further Reading

F. Taulelle, Fundamental Principles of NMR Crystallography. eMagRes 2009. http://onlinelibrary.wiley.com/doi/10.1002/9780470034590.emrstm1003/abstract