SPINNING CRYSTALS LEADS TO SIGNIFICANT ENHANCEMENT IN C-13 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. An drew, A. Bradbury and R.G. fades, Nature, 183 (1959) 1802] utilized si ngle crystals in his pioneering magic angle spinning (MAS) experiments , current studies use powders. Our recent MAS measurements on some org anic compounds show that single crystals yield significantly narrower peaks than powders, especially for the C-13 atoms that are not protona ted. In our C-13 CP (cross polarization)/MAS measurements on squaric a cid (H2C4O4), for example, single crystals yielded a four-fold reducti on in line widths as compared to its powder. This additional gain in r esolution enabled us to differentiate between the order-disorder versu s displacive contributions to the mechanism of the paraelectric-antife rroelectric phase transition of squaric acid at 373 K, which had not b een possible with other techniques. Moreover, this gain in resolution appears to increase in proportion to the applied Zeeman field, while r etaining the benefit of higher dispersion. (C) 1997 Elsevier Science B .V.