A method for temperature calibration of CP/MAS NMR probes and their temperature controllers is described that relies upon known phase changes in the compounds adamantane, d-camphor, pivalic acid and DABCO. The method appears to give a convenient and reliable calibration over the temperature range cu. 173-373 K. In a separate experiment, making use of a well defined lineshape coalescence, it was shown that the temperature effect of spinning in a 4 mm Bruker MAS probe with zirconia rotors could be fitted by a squared relationship but that if treated as approximately linear the variation was cu. 1 K kHz-' KEY WORDS temperature measurement; 13C CP/MAS NMR ~~ INTRODUCTTONThe establishment of sample temperatures in magic angle spinning (MAS) NMR experiments is an important but difficult problem. It is important because MAS NMR measurements are increasingly being used for the study of molecular dynamics such as molecular motions and exchange processes,'-12 kinetics of chemical reactions'3-' and phase changes"-" in the solid state, and the correct extraction of activation parameters from experimental data depends upon an accurate knowledge of sample temperatures. It is difficult because it has been demonstrated by several that the MAS process causes the sample to heat, possibly owing to friction and to high-power decoupling, and that the temperature effect of spinning varies with spinning speed. It has also recently been elegantly demonstrated that temperature gradients exist across samples with the regions around the bearings being several degrees higher than the centre of the sample.2 A wide variety of samples have been suggested for the measurement of temperatures, including samarium acetate (13C),22,23 isotopically labelled ammonium nitrate" and tetraazaannulene (15N),24*25 P 4 S 3( 31 p),26 lanthanide stannates (1'9Sn),27 lithium manganese oxides (6Li and 7Li)30 and, most recently, lead(I1) nitrate (207Pb).29 All of these samples are inorganic and many of them rely heavily upon paramagnetically * Author to whom correspondence should be addressed.induced temperature dependences of chemical shifts. All of these methods have relied upon some external method or unspun sample to establish sample temperatures, e.g. unspun methanol for the 'methanol thermometer.' Such 'thermometer samples' are again dependent on an even earlier calibration in highresolution, low-power NMR. All of the standards except one rely upon a nuclear species other than 13C, necessitating probe retuning to run standard organic samples. In our experience, the samarium acetate sample which does rely upon I3C measurements is a poor means of calibration with small frequency changes with temperature relative to the linewidth, and we believe that it possibly gives incorrect temperatures. It seemed to us, therefore, that a better 13C-based method for calibration of MAS sample temperatures was needed, which involves no 'thermometer samples' and gives the temperature reading directly, enabling the temperature controller probe combination to be recalibrated re...
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