SCIENCE & TECHNOLOGY
CRYSTALLOGRAPHY BY NMR Solid-state techniques help fill in gaps where X-RAY DIFFRACTION fails
FOR MOST CHEMISTS, the term crystalas higher-field magnets enabled higher lography is interchangeable with X-ray sensitivity and spectral resolution and as diffraction. But really crystallography is techniques advanced for data analysis and just the study of the forms and structures quantum mechanical modeling. The latter of crystals, and there are plenty of cryshelped the field develop in part because, talline materials for which diffraction although researchers can solve a crystal fails to reveal a structure. In recent years, structure by NMR crystallography alone, the advances in solid-state nuclear magnetic technique benefits from X-ray diffraction as resonance techniques have allowed NMR well as computational modeling of possible to emerge as an important addition to structures to aid interpretacrystallographers’ toolboxes. tion of experimental data WATERY Possible In some cases, researchers have (J. Am. Chem. Soc. 2013, structure used NMR to independently reproDOI: 10.1021/ja311649m). of hydrous duce structures determined by X-ray “Diffraction and NMR wadsleyite, a key diffraction. “There is deep symmetry provide different complecomponent of Earth’s mantle, in between X-ray diffraction and NMR,” mentary constraints” to which the addition help understand crystalline which makes the two methods comof protons to form plementary, says Manish A. Mehta, a structures, says Bradley F. Mg-O-H groups professor of chemistry at Oberlin Col- displaces some Chmelka, a professor of lege. X-ray diffraction yields electron chemical engineering at the Mg atoms. densities that allow scientists to then infer a molecule’s nuclear coordinates. “In solid-state NMR, it works the other way around,” Mehta says. “What you have are the nuclei that tell you something about what’s going on with the electrons around them.” But the real power of NMR crystallography lies in getting structural information when X-ray diffraction cannot, such as when crystals are too small to yield good diffraction data, or when they incorporate elements that can’t be distinguished by diffraction, contain defects or lack periodic order. Additionally, NMR allows scientists to study the motion of atoms in crystals, says Francis Taulelle, a research group director at the Lavoisier Institute of Versailles and a professor at the Centre for Surface Chemistry & Catalysis at KU Leuven. “Very often diffraction gives the impression that a crystal is a static object—that a crystal is solid and nothing moves. This is absolutely wrong.” The NMR crystallography field grew up over the past couple of decades, CEN.ACS.ORG
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CHEM. SCI.
JYLLIAN KEMSLEY, C&EN WEST COAST
University of California, Santa Barbara. Taulelle and Chmelka separately use NMR crystallography to study microporous silicate zeolites that may be used for catalysis or gas separation. Determining zeolite structures is challenging because they can be difficult to crystallize into large enough crystals to generate good diffraction data. Additionally, X-ray diffraction can’t distinguish between silicon and aluminum or boron atoms found in the zeolites. As a result, X-ray data limited the conclusions researchers could make about some zeolites’ structures. “For a long, long period, we all believed that the elements were homogeneously distributed, and this is absolutely wrong,” Taulelle says. “Every zeolite has a different element distribution, and the properties come from that distribution. We have to go beyond the crystal network to find why the aluminum is distributed in a particular way and correlate that distribution to a zeolite’s properties.” Chmelka sees similar characteristics in phosphors for solid-state lighting (Chem. Mater. 2013, DOI: 10.1021/cm401598n). “You take a white powder such as yttrium aluminum garnet or calcium scandate and put in a tiny amount of a rare-earth element, and it becomes a very bright phosphor,” Chmelka says. The rare-earth elements incorporate into the material seemingly at random, without long-range order, and with phosphor performance depending strongly on rare-earth element amount and distribution. Understanding why they have such a strong effect on the photoluminescence requires more structural information than diffraction can provide. STUDYING DISORDER in crystal-
line materials is also the specialty of Sharon Ashbrook, a chemistry professor at the University of St. Andrews. “Sometimes people skip over the idea of disorder and say it doesn’t matter,” Ashbrook says. “But disorder really changes the physical and chemical properties of the material.” Her research includes investigating hydration of anhydrous silicates such as wadsleyite, a form of Mg2SiO4 that exists at high pressures in Earth’s mantle (Chem. Sci. 2013, DOI: 10.1039/ c3sc21892a). “The amount of protons in that material has implications for plate tectonics, but no one knows where the protons sit,” Ashbrook says. A better understanding of water in wadsleyite
J. MAGN. RESON.
mineral structures onto changes that arise in osteoporosis,” she says. Oberlin’s Mehta and Lyndon Emsley, head of the Laboratory of Magnetic Resonance at Swiss Federal Institute of Technology, Lausanne, each also study mixtures of materials with an eye toward pharmaceutical applications. Mehta’s research focuses on the formation and structures of cocrystals, which are crystals that contain two or more molecular entities held together by noncovalent interactions. Pharmaceutical makers add so-called coformers such as malonic acid to the active drug in a formulation to generate a different crystal form. Tweaking these structures can affect the drug’s properties, such as its ability to be absorbed by the body. Mehta wants to understand the mechanisms behind how ingrediELASTIC Unit cell structure of a mineral analog of ents combine and cocrystallize. He’s bone showing citrate (see arrow) bridging the water been using NMR to study the combilayer between calcium phosphate platelets. This may create a viscoelastic layer to dissipate energy. nation of caffeine and malonic acid, which spontaneously cocrystallize when mixed (J. Phys. Chem. Lett. could also inform use of similar minerals for 2014, DOI: 10.1021/jz501699h). He hopes water and hydrogen storage applications. to identify intermediates and determine Researchers also use NMR crystalthe energetics of the process. lography to study composite and organic Emsley, for his part, would like to be materials. Bone, for example, is largely a able to “take a pill, subject it to solid-state composite of stacks NMR, and determine of crystalline platede novo what the lets of calcium phosstructure of the acphate that are sepative pharmaceutical rated by water layers ingredient is in the and housed within formulation and proteinaceous collahow it interacts with gen fibrils. NMR was excipients,” such as critical to identifying binders or preservathe presence and role tives, he says. To that of water and, more recently, citrate in end, he is experimenting with combining bone mineral. solid-state NMR with dynamic nuclear poStudying an octacalcium phosphate larization, which involves adding a stable model of bone with powder X-ray diffracradical compound to a sample to enhance tion can determine only that the mineral’s NMR signals (J. Am. Chem. Soc. 2014, DOI: unit cell size expands with the addition of 10.1021/ja4092038). citrate, says Melinda J. Duer, a chemistry Fifteen years ago, the term NMR crysprofessor at the University of Cambridge. tallography meant nothing to people, She and colleagues used NMR to reveal Emsley notes. Now, the International that citrate binds in multiple ways to Union of Crystallography has created a bridge the water between the crystalcommission for it in recognition of its line layers, perhaps helping to create a usefulness, and conferences are being viscoelastic layer that dissipates energy organized that aim to promote structure (J. Magn. Reson. 2015, DOI: 10.1016/j. elucidation using multiple techniques. jmr.2014.12.011). Duer is also investigating Says Emsley, “We’re seeing something other organic acids that affect bone strucemerging about using methods in converture. “We’re hoping to map those changed gent ways to solve structures.” ◾
“There is deep symmetry between X-ray diffraction and nuclear magnetic resonance.”
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