Science Concentrates SPECTROSCOPY
NANOMATERIALS
Composed of a network of molecule-sized channels and pores, zeolites have been used for decades in catalysis, separations, ion exchange, and adsorption. Researchers would like to make chiral versions of these molecular sieves for separating mixtures of stereoisomers and carrying out chiral reactions. Yet making bulk samples of chiral zeolite crystals that are made entirely of one enantiomer has remained an elusive goal. A team of researchers led by Mark E. Davis of California Institute of Technology has now synthesized enantio-enriched batches of germanosilicate molecular sieves exhibiting R and S chirality. The team has used these materials to separate chiral molecules and as heterogeneous catalysts to mediate chiral reactions (Proc. Natl. Acad. Sci. USA 2017, DOI: 10.1073/ pnas.1704638114). To make the materials, Davis and coworkers used computational methods to
technique. That aspect of the work, which was described in a paper published at the same time as the PNAS study, was led by Osamu Terasaki, an electron microscopist who holds positions at ShanghaiTech University and Stockholm University. The technique involves recording a series of diffraction patterns from a single crystal in an electron microscope as the crystal is tilted about a characteristic crystallographic axis (Nat. Mater. 2017, DOI: 10.1038/nmat4890). To demonstrate the chiral zeolites’ potential usefulness, the researchers used them to conduct catalytic epoxide ring-opening reactions. The R and S enantiomers of the molecular sieve yielded corresponding enantiospecific diol products. Synthesizing enantiomerically pure or enriched zeolites is a key scientific issue that can have important industrial applica-
design and evaluate organic structure-directing agents (OSDAs), which are used to guide the three-dimensional assembly of zeolite components. They synthesized enantiopure samples of a chosen OSDA, a bisimidazolium salt, and reacted each of its enantiomers separately with silicon- and germanium-based precursors. Then they removed the OSDA, leaving behind a germanosilicate chiral framework (shown). Confirming that each OSDA enantiomer indeed resulted in a molecular sieve of the predicted chirality required developing a novel electron crystallography
tions, says Avelino Corma, a leading zeolite specialist at Polytechnic University of Valencia. “The authors unambiguously prove they have synthesized each enantiomeric form of the zeolite,” he says. Alexander Katz, a chirality specialist at the University of California, Berkeley, remarks that this is “a beautiful testament to the power of rational functional-materials design.” This type of chiral shape selectivity can, in principle, be combined with various forms of chirality to cause amplification in enantioselectivity of other adsorption and catalytic processes, he adds.—MITCH
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C&EN | CEN.ACS.ORG | MAY 8, 2017
JACOBY
Poor sensitivity is one of the drawbacks of nuclear magnetic resonance spectroscopy. A method called dynamic nuclear polarization (DNP) can improve the situation, particularly for solid-state NMR, by transferring spin polarization from unpaired electrons on paramagnetic agents to the nuclei of the molecules being analyzed. This aligns the nuclei and thus boosts the NMR signal. But the electron spins are such strong magnets that they interfere with the magnetization they just induced on the nuclei, causing the boosted signal to decay quickly and broadening the peaks in the resulting spectra. Alexander B. Barnes and coworkers at Washington University in St. Louis counteract the drawbacks of the electron spins but retain the sensitivity boosts from DNP by decoupling the electron spins from the nuclear spins (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b02714). To achieve that decoupling, Barnes and coworkers built a new device called a gyrotron that can rapidly switch between microwave frequencies. “We first transfer the sensitivity from the electron spins to the NMR spins with DNP,” Barnes says. “Then we change the microwave frequency so we’re continually rotating the electron spins very fast. We’re averaging out the effect of the electron spins.” The researchers tested their new strategy by analyzing 13C-labeled urea immobilized in a glassy matrix. For 13C spins, the combination of DNP and electron decoupling increased the signal intensity by 14% and decreased the spectral peak widths by 11% relative to DNP alone. Barnes predicts the method will be useful for studying drug binding to proteins and for characterizing surfaces in materials science.—CELIA ARNAUD
CREDIT: MARK DAVIS/CALTECH
Making chiral zeolites in bulk Boosting NMR Enantio-enriched molecular sieves separate chiral molecules and mediate chiral reactions sensitivity
