Anal. Chem. 1986. 58. 3172-3177
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paper be fully exploited. The modifications we have done to the Harrick diffuse reflectance accessory permit us to position the sample at the optimum height so that changes in the sample height due to thermal expansion will not result in unexpected changes in the intensity of the bands and thus complicate the interpretation of the DRIFT spectra. Similar studies on the effect of sample height must be done prior to using the diffuse reflectance accessory from other vendors for obtaining quantitative VT-DRIFT data. We believe that the possible applications of VT-DRIFT spectrometry for rapid, quantitative, and in situ thermal decomposition studies are numerous. Registry No. TMMS, 1185-55-3.
(6) Hamadeh, I.M.; King, D.; Griffiths, P. R. J. Catal. 1984, 8 8 , 264-272. (7) Murthy, R. S.S.; Caravajal, G. S.; Leyden, D. E. Chemically Modified Surfaces; Leyden, D. E., Ed.; Gordon and Breach: New York, 1986; Vol 1. pp 141-155. (8) Anderson, D. R. I n Analysis of Silicones, Smith, A. L., Ed.; Wiley: New York, 1974; Chapter 10; p 275. (9) Waddell, T. G.; Leyden, D. E.; DeBello. M. T. J . Am. Chem. SOC. 1981, 103, 5303-5307. 10) Caravajal, G. S.; Leyden, D. E.; Maciel, G. E. Chemically Modified Surfaces: Leyden, D. E., Gordon and Breach: New York, 1986; pp 283-303. 11) McKenzie, M. T.: Culler, S. R.; Koenig, J. L. Appl. Specfrosc. 1984, 38, 786-790. 12) Graf. R. T.; Koenig, J. L.: Ishida, H. Anal. Chem. 1984, 5 6 , 773-778. 13) Perry's Chemical Engineers' Handbook, 6th ed.; Green, D. W., Ed.; McGraw-Hill: New York, 1984; pp 23-29. 14) Field, R. S.Ph.D. Dissertation, Colorado State University, Fort Collins, CO, Sept 1985. 15) Wendlandt, W. W.; Hecht, H. G. Reflectance Spectroscopy; Interscience: New York, 1966; p 52. 16) Kortum, G. Reflectance Spectroscopy; Springer Verlag: New York, 1969: pp 58-71.
LITERATURE C I T E D (1) Blitz, J. P.; Murthy, R. S.S.; Leyden, D. E. Appl. Spectrosc. 1986, 4 0 , 829-831. (2) Conroy, C. M.; Griffiths, P. R.; Jinno. K. Anal. Chem. 1985, 5 7 , 822-625. (3) Shafer, K. H.; Pentoney, S. L., Jr.; Griffiths, P. R. Anal. Chem. 1986, 58,58-64. (4) Murthy. R. S. S.; Leyden, D. E. Anal. Chem. 1986, 58, 1228-1233. (5) Smyrl, N. R.; Fuller, E. L., Jr.; Powell, G. L. Appl. Spectrosc. 1983, 37, 38-44.
RECEIVED for review May 27,1986. Accepted August 11,1986. This work was supported in part from a grant from the National Science Foundation (CHE 85-13247). The Nicolet 60SX FTIR spectrometer was purchased in part from a grant from the National Science Foundation (CHE 83-17079).
Experimental Considerations in Variable-Temperature Solid-state Nuclear Magnetic Resonance with Cross Polarization and Magic-Angle Spinning J a m e s F. Haw,* Gordon C. Campbell, a n d Richard C. Crosby
Department of Chemistry, Texas A&M University, College Station, Texas 77843
The problem of temperature measurement and control is considered in detail for solid-state nuclear magnetic resonance (NMR) wlth cross polarizationand magic-angle spinning (CP/MAS) NMR. By use of the recently developed technique of CP/MAS NMR chemical shift thermometry, the various factors contrlbuting to the uncertainty in sample temperature are identifled and assessed. The Important factors demonstrated in this study include radio frequency sample heating, mismatches in the drlve- and bearing-gas channel temperatures, sample temperature equilibration times, and Joule-Thompson cooling and heating. Technlques for measuring these contributlons and the resulting temperature gradients are demonstrated. Technlques are reported that allow the uncertalnty In sample temperature to be reduced to approximately flK, an Improvement of an order of magnltude. Although general in scope, this study emphasizes the temperature range from 77 K to ambient, a range encompasslng most of the variable-temperature CP/MAS studies in the chemical literature. The design of an Improved variable-temperature accessory, whlch provides greater reliability and convenlence, Is also reported.
Variable-temperature NMR spectroscopy with cross polarization and magic-angle spinning (VT CP/MAS) is an important technique for the study of structure, dynamics, reactivity ( I ) , and magnetic properties ( 2 ) in the solid state.
Unfortunately, this technique has had an important limitation; it has been impossible to accurately measure and control the sample temperature in this experiment, and when the estimated uncertainty in sample temperature is reported, it ranges from A10 K ( 3 )to several times larger, especially for double bearing spinning systems ( 4 ) . This degree of uncertainty is at least 2 orders of magnitude higher than that typical for solution-state NMR studies but can be appreciated by considering that the gas flow rates, radio frequency (rf) power levels, and sample spinning rates are typically 1 or 2 orders of magnitude higher in CP/MAS experiments than for the corresponding experiments in solution and that the temperature range in CP/MAS studies typically has a much lower limit. An uncertainty in temperature this large has important implications for chemical investigations of solids. Without an accurate method of measuring sample temperature, chemical parameters determined by VT CP/MAS NMR (e.g., rate constants, activation energies, etc.) could rightly be viewed with some suspicion. By use of the recently developed technique of CP/MAS NMR chemical shift thermometry (5),the problem of temperature measurement has been studied in detail. This contribution reports the results of these investigations and demonstrates that the sources of temperature uncertainty in CPJMAS experiments can be characterized and minimized. The sample temperature can now be measured with an uncertainty of approximately A1 K over a temperature range of 77 K to approximately 270 K, the range of most of the VT studies in the chemical literature. Bearing and drive
0003-2700/86/0358-3172$01.50/0 0 1986 American Chemical Society
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gas temperature mismatches, rf heating of conductive samples, and the Joule-Thompson effect can each produce significant sample temperature errors. Chemical shift thermometry experiments have also shown that it is prudent to wait a minimum of 8-10 min for thermal equilibration following a change in the spinning gas temperature and prior to initiating data collection. Several modifications in instrumentation and procedures which, in the experience of this laboratory, increase the reliability and convenience of VT CP/MAS investigations are also reported. EXPERIMENTAL SECTION All experiments were performed on a Chemagnetics M-100s solid-state NMR spectrometer equipped with a 2.35-T superconducting solenoid magnet with an air bore diameter of 70 mm and a variable-temperature I3C CP/MAS probe. The diameter of this probe is 60 mm; since the magnet is not equipped with room temperature shims, there is a 5-mm air space between the probe and the bore surface of the magnet. This air space is an important, if unplanned, safety feature for low-temperature NMR studies on the instrument. Several other laboratories have reported seal failures and resulting magnetic field quenches while attempting to operate at temperatures well below the manufacturers’ specifications. The CP/MAS NMR probe has two independent spinning gas channels: the drive gas, which is used to propel the sample rotor; and the bearing gas, which is used to stabilize the rotor. The drive gas impacts the bottom of the rotor, while the bearing gas is directed at the top of the rotor. Several modifications of the VT CP/MAS probe and the variable-temperature accessories are described under Results and Discussion. Samarium acetate tetrahydrate (1)was obtained from Strem Chemical Co. and used as a CP/MAS NMR chemical shift thermometer without further purification. The development of this material as the first chemical shift thermometer for CP/MAS NMR is described in ref 5. The three 13C signals in the CP/MAS spectrum of 1have chemical shifts that are functions of !P(Curie law dependence). The chemical shifts as a function of temperature for these signals are
chelating-only carbonyl: dppm = -4867(1/Ts)
+ 209
chelating-and-bridging carbonyl: dppm = -2670(1/Ts)
+ 195
methyl: bppm = 2672(1/Ts)
+ 14
(3)
T s is the sample temperature, and 6 is the chemical shift of the indicated resonance measured in parts per million from tetramethylsilane. The temperature dependence of the two carbonyl signals can be appreciated in Figure 1 (reproduced from ref 5 ) which shows the low shielding region of the 13CCP/MAS spectrum of 1 at five representative temperatures. The chelating-only carbonyl signal (the more-intense signal in Figure 1) is the most temperature dependent of the three resonances and is, therefore, best suited to temperature measurement. The chemical shift of 1 is hereafter taken to refer to the chelating-only carbonyl signal. Equation 1 is rewritten in the form of eq 4 for temperature measurement. Equation 4 was used to determine the sample temperature, Ts, for all of the experiments. Ts = 4867/(209 - b,,)
(4)
This laboratory has occasionally observed small systematic deviations in the sample temperatures measured by using the chemical shift of samarium acetate tetrahydrate and eq 4. These deviations are attributed to small variations in magnetic SUSceptibility due to sample packing, and the following procedure appears to eliminate the problem: The sample is equilibrated at a suitably low temperature such that the chemical shift of 1
“p s ?
77 K
,
i,
\+-f
J“
A
-
.”r
ppm
220
m
120
Figure 1. Variable-temperature 13C CPIMAS NMR spectra of the carbonyl region of samarium acetate tetrahydrate (1); ssb denotes spinning sidebands. Reprinted with permission from ref 5. Copyright 1986
Academic Press.
is strongly temperature dependent (e.g., 120 K), and the chemical shift reference position is reset, if necessary, t o force the chelating-only carbonyl chemical shift of 1 to that required by eq 4. This procedure compensates for small variations in susceptibility from sample to sample and, in our experience, provides accurate sample temperature measurements for a wide range of materials. Conventional temperature sensors (vide infra) are used to measure the one temperature required in this calibration step. Cross polarization contact times of 2 ms were used, and the repetition delay was generally 1 s. A series of dummy pulses was used prior to actual data acquisition to ensure thermal equilibration of rf heating effects (when present). In the experiment designed to measure rf heating effects, high rf duty cycles were achieved by increasing the decoupling period, decreasing the repetition delay, and adding a carbon prepulse prior to the cross polarization sequence. RESULTS AND DISCUSSION The factors that cause sample temperature uncertainty in VT CP/MAS NMR include temperature measurement and control when two or more spinning gas streams are used (double-bearing design), the necessity of placing the temperature sensor(s) several centimeters from the sample, Joule-Thompson cooling or heating, rf heating, temperature equilibration times, frictional heating and other problems associated with marginal spinning, and other less common problems. The relative importance of these factors has been assessed, and each will be treated in turn. The three or four commercial VT CP/MAS probe designs in use in most laboratories share a common feature; the spinning system uses two gas flows (double-bearing design), one to propel the rotor (drive gas) and one to stabilize the rotor (bearing gas). It is generally assumed that the gas stream that flows over the bulk of the sample (the drive on some designs, the bearing on others) is the only stream that influences the sample temperature and is, therefore, the only temperature that need be monitored and regulated. This assumption can lead to large errors in temperature measurement (vide infra). Figure 2 is a schematic of the VT CP/MAS probe and accessories, with emphasis upon the modifications developed for the present study. Three resistance temperature devices
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14(
L 1
P 12c LIQUID N 2
PROBE
1oc
Figure 2. Schematic of the variable-temperature CP/MAS probe and
accessories. D.H. and B.H. denote drive and bearing gas heaters, respectively. Locations of temperature sensors (RTDs) are shown for the drive (D), bearing (B), and exit (E) gas streams. (RTDs, Omega TFD) were mounted in the probe to provide temperature measurements at key points near the sample: in the drive gas channel, approximately 5 cm from the spinning system; in the bearing gas channel, at a similar distance from the spinning system; and in the exit stack approximately 2 cm above the spinning system and positioned so as to be in the stream of gas exiting from around the sample. These sensors are designated in Figure 2 as D.R.T.D., B.R.T.D., and E.R.T.D., respectively. The temperatures reported by these devices are symbolized by TD (drive temperature), TB (bearing temperature), and TE (exit temperature), respectively. The exit RTD can be retracted away from the spinning system during data acquisition; failure to do so can result in loss of signal-to-noise ratio due to rf pick-up. This sensor is lowered into position for temperature measurement immediately after data acquisition is completed. Dummy pulses can be applied to the probe during this measurement to more closely re-create the conditions present during spectral acquisition. The bearing and drive sensors are well shielded from rf and remain in position a t all times. Independent gas streams (usually N,) are used for bearing and drive. A series of needle valves is used to direct the gas streams either through cooling coils immersed in liquid nitrogen or to bypass the cooling coils. Intermediate temperatures can be obtained by proportioning the flow between the cooling coils and bypass loops. The valves on both sides of the cooling coils must be closed when the flow is to be directed exclusively through the bypass loops, otherwise severe pressure fluctuations will occur. These valves-and the bypass loops are also useful for rapid temperature increases and allow humid air to be used for ambient temperature experiments by bypassing the cooling coils. The cooling Dewar has a capacity of 55 L, which provides for stable VT operation for up to 2.5 h without refilling. Temperature control is achieved with two heaters, one on the bearing gas channel and the other on the drive gas channel; these are driven by independent controllers, responding to TB and TD, respectively. Stable pressure regulation for both bearing and drive channels is essential for reliable temperature control, as can be seen in Figure 3 which shows drive temperature (TD)as a function of drive pressure for three conditions of the bearing gas: off, 12 psi through bypass, and 12 psi through the cooling coils. In practice, small pressure fluctuations usually do not result in noticeable temperature fluctuations, because of the high thermal mass of the probe. Figure 3 also shows that cooling the bearing gas makes it possible to approach 77 K with lower drive pressures, an expected result. Given the general lack of Dewaring and minimal insulation in the VT system (Figure 2), it may seem surprising that temperatures near 77 K are so easily reached. Magic-angle spinning systems use small gas orifices on both the bearing and stator (drive)
80 DRIVE GAS PRESSURE ( p s i )
Figure 3. Drive gas temperature (T,) vs. drive gas pressure for three conditions of the bearing gas flow: (a) 0 psi, X ; (b) 12 psi through bypass, 0; (c) 12 psi through liquid N, cooling coils, A . In each experiment, the drive gas is cooled by liquid N,, and both heaters are
off. surfaces. I t is not surprising to expect such a system to act like a Joule-Thompson expansion valve; with a reasonable gas pressure, the upper part of the probe is efficiently cooled to the boiling point of nitrogen by this process. Evidence for the role of the Joule-Thompson effect in VT CP/MAS temperature control is afforded by the following observation. When experiments similar to those in Figure 3 were repeated with He gas instead of N,gas (liquid N2 was still used as the cryogenic fluid), the lowest temperature that could be achieved was 130 K. He has a maximum Joule-Thompson inversion temperature of 43 K (6). For temperatures above this value, He becomes hotter upon expansion in a Joule experiment, whereas N,(with a maximum inversion temperature of 607 K) becomes cooler. The Joule-Thompson effect is, therefore, an important factor in temperature control in VT CP/MAS NMR and accounts for the ease with which 77 K is reached without extensive Dewaring or insulation. The role of the Joule-Thompson effect in cooling the sample can be best appreciated through the results in Figure 4, which also address the question of sample temperature gradients with double-bearing designs. Figure 4a is a spectrum of samarium acetate tetrahydrate obtained with TD = 83 K, TB = 77 K, and T E= 77 K. The chemical shift of the chelating-only carbonyl signal (146 ppm) corresponds to a sample temperature, Ts, of 77 K. If the sample temperature were given by a weighted average of TDand TB,it would be several degrees higher than the value reported by the chemical shift thermometer. When operating conditions similar those in Figure 4a are used with the top of the probe open for inspection, a liquid nitrogen mist is observed to condense as the gas exits the spinning system. Figure 4b shows a spectrum of 1 obtained with conditions similar to those discussed previously, with the exception that the drive temperature, T,,was set at 111K, 34 K higher than TB. In this case, the sample temperature, Ts, is 79 K, indicating that Joule-Thompson cooling can greatly reduce the sample temperature below the value reported by a drive sensor several centimeters upstream. This problem is particularly acute at temperatures near the boiling point of nitrogen, and spectra in the sample temperature range of 80-90 K are sometimes more difficult to obtain than spectra at 77 K. The spectral line widths in Figure 4b are broader than those in Figure 4a, indicating that there is a temperature gradient across the sample and consistent with the fact that TDand
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To-173 K T8-254 K TE-211 K Ts-208K
A A I\
ToTSTETs-
b
172 K 172 K 172 K 172 K
A
T
a
T0'83 Ts-77 TE-77 Ts-77
K K K K
ppm
200
175
150
125
Flgure 4. I3C CP/MAS NMR spectra of the carbonyl region of 1 demonstrating Joule-Thompson sample cooling and the effects of setting the drive and bearing gas streams to different temperatures, including large errors in mean sample temperature and temperature gradients across the sample; see text for discussion. T,, T,, T E ,and T , denote drive, bearing, exit, and sample temperature, respectively.
TBwere intentionally set to differ by 34 K. From the excess line width in Figure 4b, a sample temperature gradient, AT,, of 2 K is estimated. This is smaller than TD - TB because of Joule-Thompson cooling and because some thermal equilibration of the two gas streams is achieved downstream of the drive and bearing sensors in a plastic block on which the spinning system is mounted. Figure 4c,d shows that larger sample temperature gradients are possible at higher temperatures, where Joule-Thompson cooling is less efficient at equalizing temperature differences. Figure 4c serves as a control experiment and illustrates that very uniform probe and sample temperatures can be achieved if both bearing and drive temperatures are regulated and if the Joule-Thompson effect is not too severe. In Figure 4d, the bearing temperature was intentionally set 81 K higher than the drive temperature, which was kept a t the same temperature used for both TB and TDin Figure 4c. By use of the excess line width in Figure 4d relative to Figure 4c to provide a rough estimate of ATs, a gradient of 9 K was calculated. Although the large differences in TBand TDreported in these experiments were created intentionally, differences of comparable magnitude were observed routinely after the bearing temperature sensor (B.R.T.D. in Figure 2) was installed but prior to adding cooling coils, a heater, and a controller to the bearing gas channel. Failure to monitor and regulate both gas channels is an important source of error in VT CP/MAS experiments, as is the Joule-Thompson effect, especially at temperatures for which the Joule-Thompson coefficient is large. For nitrogen, the Joule-Thompson effect will become larger as the temperature is decreased to 77 K, but for helium the extent of Joule-Thompson heating will decrease as the temperature is lowered until the inversion temperature is reached.
ppm I
I
I
200
180
160
Flgure 5. 13C CP/MAS NMR spectra of the carbonyl region of 1 for an equal mixture of 1 and graphite, demonstrating rf heating of the sample. The I3Cand 'H irradiation duty cycles were significantly higher (see text) for (b) relative to (a) while ail other conditions were kept constant.
Radio frequency (rf) heating is another potentially severe source of temperature uncertainty in VT CP/MAS experiments. Rf heating is a well-known problem in solution-state NMR studies, especially with conductive aqueous electrolyte solutions (7). Recently, highly efficient scalar decoupling pulse sequences have made it possible to achieve adequate decoupling in solution-state NMR with modest decoupling power levels (Le., 1-2 W), and this contribution to sample heating has been minimized (8). Since hundreds of watts of rf power is used for cross polarization and dipolar decoupling, the potential for rf heating in CP/MAS studies of conductive solids is great. Figure 5 illustrates the use of compound 1 as a chemical shift thermometer to measure rf heating in a CP/MAS NMR experiment with graphite; the sample was 50% (v/v) graphite with the remainder 1. The spectrum in Figure 5a was acquired with TB = 144 K and TD = 146 K; the 'H and 13C irradiation duty cycles were 2.6% and 0.2%, respectively, typical values for a basic CP/MAS experiment, and with these conditions no rf heating was observed (Ts = 145 K). NMR relaxation time measurements such as TlpHand TlpC require that spectra be obtained as a function of an extended irradiation period. The spectrum in Figure 5b was obtained in a fashion identical with that in Figure 5a, with the exception of the 'H and I3C irradiation duty cycles, which were 10.2% and 2.1%, respectively. These increased duty cycles simulate those that would be encountered in relaxation measurements. As can be seen in Figure 5b, the increased irradiation duty cycles result in 28 K of rf sample heating ( Ts = 173 K); this increase is not reported by any of the three temperature sensors in the probe and would have gone undetected without the chemical shift thermometer. These data also demonstrate the importance of equalizing rf duty cycles in relaxation measurements by the use of dummy pulses. The line width in Figure 5b is slightly greater than in Figure 5a, indicating the presence of a temperature gradient. By use of the method outlined previously, ATs was estimated to be 19 K. Unlike the case in which sample temperature gradients existed across the length of the sample due to mismatches of TBand TD,the temperature gradient produced by rf heating is probably across the sample radius, reflecting the competing effects of rf heating and thermal conduction.
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29C
27C
-
.. . ... ... .. x’
e
25C
-
*
.....
e
w (I
3
+ 4
6 230 n
2c
TS To +TB
2
e
210
19c
170
2
4
6
0
10
12
14
TIME (rnin)
Figure 6. Direct measurement of sample temperature (T,) changes following a rapid increase in the drive and bearing gas temperatures (T, and T,, respectively). Sample temperature was measured by using 1. The sample is seen to require 8-10 rnin to equilibrate once the final
temperature is reached. The same graphite sample was also used to measure rf heating at other temperatures. No rf heating was observed at 77 K, and the rf heating was so severe a t 300 K that the sample rotor was destroyed upon application of the first pulse. These observations illustrate that the extent of rf heating in CP/MAS NMR experiments can be very temperature dependent. Another potential source of temperature uncertainty in VT CP/MAS NMR studies is slow equilibration of the sample and gas temperatures following a change in the temperature of the latter. For example, if the temperature of the rotor is homogeneous and 173 K, and the temperature of the surface of the rotor is suddenly increased to 303 K, how long will it take for the center of the rotor to reach a temperature of 302 K? The problem is a classical one in heat flow and can be modeled by an infinite rod of Kel-F with the diameter of the MAS rotor (3/s in.). By use of the manufacturer’s values for the thermal conductivity, specific heat and density of Kel-F, and standard heat flow tables (9),an equilibration time of 3.4 min was calculated. This problem was also studied experimentally using chemical shift thermometry and a tightly compacted sample of 1 in a Kel-F rotor. The experiment was as follows: The bearing and drive temperatures were slowly reduced to 173 K and equilibrated at that temperature for an extended period. The sample temperature was verified by collecting a spectrum and making use of eq 4, as usual. At t = 0 rnin (Figure 6) the bearing and drive heaters were activated at maximum current, and both gas flows were switched to bypass the cooling coils. These operations were carried out to achieve, as closely as possible, a step increase in the bearing and drive gas temperatures to 303 K. The average of these quantities is plotted in Figure 6, and it can be seen that the target gas temperature was essentially attained within 4 min. The sample temperature was followed by collecting and storing the spectrum of 1 every 2 min. In the first 2 min, the sample temperature did not measurably respond to the increase in the gas stream temperature (Figure 6). Between 2 and 8 min, however, it changed rapidly, and the spectra obtained at 4 and 6 rnin were very broad, reflecting changes in T s over the data collection
periods as well as a radial temperature gradient. Figure 6 suggests that approximately 8-10 min are required for the sample temperature to equilibrate to the gas temperature (i.e., once the gas temperature has the average of TB and TD), stabilized. The experimental value of the equilibration time is nearly 3 times the calculated value, but this difference can be easily reconciled. The simple model used for the calculation did not account for heat loss between the temperature sensors and the rotor; in particular, the relatively massive spinning system and its support cradle were neglected. Since these objects have very complicated shapes, a more exact calculation of the thermal equilibration time would be very difficult. The actual sample also differed from that used in the model, which did not include the heat flow properties of 1, which were not available. Given the simplifications used in the calculation, the agreement with experiment is satisfying. Several other factors can lead to uncertainties in the temperature of VT CP/MAS experiments and merit discussion. Unstable spinning disrupts the gas flow about the rotor and also produces frictional heating, as can be judged by the melt-polished appearance of plastic rotors that have rubbed against the bearing surface of the spinning system. It would be difficult to design a chemical shift thermometry experiment to measure the extent of sample heating due to unstable spinning, but the experience of this laboratory is that experiments performed with marginal spinning must be repeated, especially when the sample temperature needs to be accurately known. Although the potential use of VT CP/MAS NMR in the study of chemical reactions in the crystalline solid state remains largely unexplored, the first experiments of this type have recently been performed ( I O ) , and the question arises whether or not the sample temperature might be affected by an exo- or endothermic chemical reaction. Since 13C NMR is a comparably slow technique for the study of chemical reaction kinetics (exchange phenomena excepted), it is likely that such experiments will be performed at low enough temperatures to ensure reaction half lives on the order 10 min or more. With such slow reactions, it will usually be reasonable to assume that the sample temperature is equilibrated with the gas stream temperature. If this condition is not met, it should be possible to use chemical shift thermometry to monitor the sample temperature, although it might be necessary to isolate the thermometer substance from the reactive solid using, for example, a concentric capillary. Miscalibration of the temperature sensors could cause a determinant error in VT CP/MAS experiments, especially if chemical shift thermometry or other phenomena such as solid-solid phase transitions ( I I ) have not been used to characterize the VT system. The errors associated with the use of properly calibrated temperature sensors (which must be remote from the sample) have already been discussed. The calibration of temperature sensors must be checked at several points over the operating range, especially at the extrema. The calibration should be checked both in and out of the magnetic field, because the phenomenon of magnetoresistance (if present for the sensor device) could invalidate a calibration performed in zero field. This laboratory has not observed such problems with the RTDs used in this work, but it would be prudent to check for this effect with other sensors or at higher field strengths. The various factors affecting sample temperature measurement in VT CP/MAS NMR are compared in Table I; included in the table is this laboratory’s assessment of the error in the sample temperature that can be induced by each factor, the likelihood of temperature gradients due to each factor, and an estimate of the sample dependence for each factor. The various contributions in Table I easily account for re-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
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Table I. Factors Contributing to Errors in Sample Temperature Measurement in VT CP/MAS NMR approx error (TD- Ts)
source TB f TD
Joule-Thompson rf heating equilibration time unstable spinning chemical reaction sensor calibration
=t20 K" 8 Kb*' large and negative'
+
=t5 K unknown, negative small to negligible determinant, easily measd
temp gradients
sample dependence
510 K 13 K 520 K 15 K possible
none none very strong weak noned
"The magnitude and sign of this error can depend on where the measuring sensor is placed. Figure 4d is an extreme case in point. bErrors of the opposite sign can be obtained with a He gas stream. CThiseffect is known to be temperature dependent. dSome samples tend to spin better than others, but for equal spinning this effect is approximately sample independent.
ported uncertainties in sample temperature in VT CP/MAS experiments. The question of how sample temperature should be measured in VT CP/MAS NMR is now addressed. If a double bearing spinning system is used (as in most laboratories) the bearing and drive gases must be independently measured and controlled. For the temperature range considered in this contribution (77 K to ambient), chemical shift thermometry using I or some future, superior substance in conjunction with drive, bearing, and exit sensors is the surest way to know the sample temperature. Failing this, if bearing, drive, and exit sensors are used, it will generally be possible to arrive at a good estimate of the sample temperature. TDand TB should be brought within 2 K of each other, and the sample should be equilibrated with the gas stream for at least 8 min after the set point has been reached. Dummy pulses should be used for several minutes to equilibrate rf heating effects, if any, prior to data acquisition. This latter precaution is probably necessary only for conductive samples. If rf heating effects are present, an accurate measure of the sample temperature will be obtained only with chemical shift thermometry. If the desired temperature is
