Spinning crystals leads to significant enhancement in 13C spectral resolution in MAS experiments on organic compounds: a new aid in studying phase transitions.
While Andrew [E.R. Andrew, Arch. Sci. (Geneva), 12 (1959) 103; E.R. Andrew, A. Bradbury and R.G. Eades, Nature, 183 (1959) 1802] utilized single crystals in his pioneering magic angle spinning (MAS) experiments, current studies use powders. Our recent MAS measurements on some organic compounds show that single crystals yield significantly narrower peaks than powders, especially for the 13C atoms that are not protonated. In our 13C CP (cross polarization)/MAS measurements on squaric acid (H2C4O4), for example, single crystals yielded a four-fold reduction in line widths as compared to its powder. This additional gain in resolution enabled us to differentiate between the order-disorder versus displasive contributions to the mechanism of the paraelectric-antiferroelectric phase transition of squaric acid at 373 K, which had not been possible with other techniques. Moreover, this gain in resolution appears to increase in proportion to the applied Zeeman field, while retaining the benefit of higher dispersion.
Klymachyov
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Department of Chemistry, Florida State University, Tallahassee 32306, USA.
Dalal
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D002247:Carbon Isotopes; D003460:Crystallization; D003503:Cyclobutanes / Q000737:chemistry*; D006801:Humans; D009682:Magnetic Resonance Spectroscopy / Q000379:methods
D002247:Carbon Isotopes; D003503:Cyclobutanes; C030042:squaric acid