Simultaneous Temperature Measurement during Acquisition of Pulsed Fourier-Transform Carbon- 13 Magnetic Resonance Spectra Drouet Warren Vidrine and Paul E. Peterson* Chemistry Department, University of South Carolina, Columbia, S.C. 29208
The 13C NMR shifts of CHsI, CH212, and cyclooctane, relatlve to neat TMS, are given over a range of temperatures. Solutlons of these suitable for use as NMR thermometers are calibrated for temperature measurement in the range -60 to +lo0 OC, to an accuracy of f 0 . 1 5 OC. The absolute temperature can be calculated directly from the shift difference between the two nuclei of each thermometric solution, A&
1 / T = 0.0161165 I / T = 0.0220027
- 0.000570057
-
A6(TMS:CH31, 1:3, v:v)
-60 to +20 OC 0.000223362 A6(cycoc:c~212,1:5, v:v) +20 to +loo OC
T h e uncertainty of t e m p e r a t u r e measurement i n nuclear magnetic resonance spectroscopy ( N M R ) has long been an i m p e d i m e n t i n the s t u d y of kinetics and equilibria. M e t h a nol ( I ) and ethylene glycol ( I , 2) have been used as lH NMR thermometers although their resonances sometimes obscure i m p o r t a n t p a r t s of t h e observed proton spectrum. To avoid t h i s obscuration, these substances, or a thermocouple, are often used b y t h e replacement method, which has intrinsic difficulties: i t requires t w o separate operations, is blind t o t e m p e r a t u r e drift during spectral acquisition, and is susceptible t o systematic errors resulting from t u b e non-uniformity (2). W h e n observing other nuclei, e.g., 13C, use of the proton thermometers requires the capability to observe t w o nuclei within seconds, a feat of which few ins t r u m e n t s a r e capable. T h r e e recent non-lH methods have appeared: a 19F m e t h o d using fluorocarbons ( 3 ) ,and two 13C methods, using carbonium-halonium ion equilibria ( 4 ) and a lanthanide pseudocontact shift ( 5 ) .T h e s e have absol u t e calibration accuracies of f3,42,a n d f1.5 "C, respectively. W e present h e r e an NMR thermometer for 13C spectroscopy which makes possible simultaneous measurement of temperature, a free choice of solvent composition, and a higher degree of accuracy t h e n previous methods. Starting with the observation of t e m p e r a t u r e dependence of 13C shifts i n iodomethane (6, 7), we investigated the temperat u r e sensitivity of several haloalkanes, noticing t h e relationship I > Br > C1. Iodomethane mixed with t e t r a m e t h ylsilane ( T M S ) 3:l (v:v) was selected a s a thermometric solution suitable for t e m p e r a t u r e measurement in the range -65 t o +30 "C, and diiodomethane mixed with cyclooctane 5:l (v:v) as suitable for the range +20 t o +175 "C. All these 13C nuclei, except those of - cyclooctane, have chemical shifts at 6's below t h e range of m o s t organic compounds, and should not interfere in most applications. Calibration has been a problem with most t e m p e r a t u r e measurement methods heretofore, especially with those using replacement methods. We report here a calibration m e t h o d requiring only a calibrated thermocouple, which eliminates t h e objections connected with replacement techniques (2),and t h e line broadening associated with simultaneous use of a thermocouple or thermistor ( I , 8).
EXPERIMENTAL Calibration Apparatus. 13C NMR measurements were made on a Varian CFT-20 20 MHz 13Cfast-Four-ier-transform spectrometer with variable temperature capability. The parameters used for calibration of thermometric solutions were: 100 pulses (averaged), acquisition time (AT) = 0.5 s (per pulse), no pulse delay, apodization number = 0.127, and resolution enhancement (SE) = 0.4. Other parameters used were optimized, and 'H decoupling was used throughout. Temperature measurements were performed with a polyperfluoroethylene-insulated copper-constantin thermocouple, 0.5-mm wire diameter, calibrated to f0.02 "C vs. a National Bureau of Standards certified (Myers and Knapp, No. S-147) platinum resistance thermometer. Temperatures were converted to the current International Practical Temperature Scale of 1968, (9) and expressed as T (absolute Kelvin temperature), or t "C (T- 273.15). The calibration tube (Figure 1) consisted of a 5-mm thin-wall NMR tube mounted coaxially within an 8-mm (outside diameter) tube, with polyperfluoroethylene spacers. The inside tube was filled to a depth of 15 mm with perdeuterioacetone (low temperature) or DzO (high temperature), which served both as the heat transfer medium to the thermocouple junction, and as the lock substance during an acquisition. The space between the inner and outer tube was filled to 15-mm depth with thermometric solution. Reagents. Thermometric solutions were made up with distilled reagents in exact volume ratios, a t approximately 0 "C (tetramethylsilane:CHaI, 1:3), or at 25 "C (cyclooctane:CHnIz,1:5). Calibration Procedure. With the inner tube of Figure 1 filled with lock substance, and the outer space filled with the thermometric solution, and the thermocouple placed a t correct depth in the inner tube, the ensemble was placed in the probe and allowed to equilibrate until the thermocouple temperature changed less than 0.04 "C in a 5-min period (40-60 min for equilibration of probe and tube). The thermocouple was quickly removed, whereupon the tube commenced spinning spontaneously. The spectrometer was locked and a 50-s acquisition begun, within 15 s. The averaged free induction decay was stored on magnetic tape, and another spectrum acquired beginning 90 s after the removal of the thermocouple. Only when data from the two acquisitions agreed within 0.1 Hz (0.005 ppm) was the data set retained. The half-time for thermal equilibration of the NMR tube was measured at 0.9 min, giving detection limits for systematic errors due to thermal drift after thermocouple removal of 0.09 "C (TMS:CH3I) and 0.07 "C (cyc0c:CHzIz) for each individual data set, and 0.03 "C for both of the respective averaged drifts. All thermal drifts measured were zero. The causes of systematic error include thermal leakage through the thermocouple wire, temperature changes caused by spinning, and heating of the wire or liquid by radiofrequency energy, especially by the l H decoupler. Vertical thermal gradients in the tube were checked by changing the thermocouple position. No significant gradients (
