Variable-temperature magic-angle spring

JOURNAL

OF MAGNETIC

RESONANCE

57, 49 1-493 ( 1984)

Variable-Temperature

Magic-Angle

Spinning

ALAN D. ENGLISH Central

Research and Development Department, E. I. du Pont de Nemours and Company. Wilmington, Delaware 19898 Received October 26, 1983

High-resolution solid-state NMR methods utilizing magic-angle spinning (I) are frequently being used to study the temperature dependence of dynamic processes (29). Variable-temperature magieangle-spinning studies have demonstrated in fluxional organometallics (3), identification of reactive intermediates native studies of macromolecular dynamics (5), inorganic structural determinations (7), and spin dynamics studies (8, 9). Our continuing interest in widely variable temperature studies of macromolecular dynamics has caused us to endeavor to aeq&e extensively variable-temperature magic-angle-spinning capabilities, In pursuit of this goal, we have been able to achieve temperature regulation with good precision ( 1 K), but considerable difficulty has been encountered in accurately defining the average temperature and its gradient across the sample. Previous investigations have reported that the exit gas temperature may be used as an indicator of sample temperature (2) with the thermocouple placed l-2 cm ( 7, 8) from the rotor. These investigations have all been carried out with rotors af the Beams-Andrew geometry (10, I I) where a copious amount of gas is exhausted from the bearing surface and hence envelopes that part Our experience with this rotor/stator design has is usually sufficiently close to the inlet-gas temperature that one believes that the temperature of the sample is known with an accuracy of easily better than 10OC. Our more recent experience with double-gas-bearing magic-angle-spinning stators is quite different and is illustrated in Fig. 1. Figure 1 illustrates the temperature variation observed between the inletgas temperature and either the exit gas temperature or the sample temperature for a doublegas-bearing magic-angle-spinning stator. The sample temperature was calibrated using a 4 mm spherical sample of either ethylene glycol or methanol, which was packed with KBr into a rotor; the standard method (12) of measuring the temperature dependence of the relative chemical shifts of dissimilar protons was used and then converting this splitting into a temperature value using those literature (IS, II) and instrument manufacturer’s conversion formulas that formed a consistent set. The stator reported upon here is manufactured by Doty scientific, Inc., and is housed in a probe capable of variable-temperature magic-angle-spinning experiments from 100 491

0022-2364/84 53.00 Copyristtt 8 1984 by Aadanic

Rcsa, Inc.

All rights of reproduction in any fWm resmed.

NOTES

492

0

m 0 A

EXIT GAS ETHYLENE GLYCOL METHANOL

INLET GAS TEMPERATURE PK)

FIG. 1. Temperature of exit gas and sample (as deduced from the temperature dependence of the relative chemical shifts of either ethylene glycol or methanol) versus inlet gas temperature for a double-gas-bearing magbangbspinning (1.5 kHz) stator. Sample temperatures were deduced from the conversion formulas of T = 460-98.286 and T = 468-l 14.1A6 for ethylene glycol and methanol, respectively.

to 500 K. As is illustrated in the figure, we have observed that the exit gas temperature is not a reliable method of temperature measurement with this stator design even if the exit gas is sampled quite close (4 mm) to the sample (e.g., within 0.4 mm of the rotor); the inlet gas temperature seems to be a more accurate measure. Despite the large variation in inlet and outlet gas temperature, the temperature gradient across the calibrating liquid was less than 2 K at all temperatures. The apparent discrepancy in the accuracy of calibrating the sample temperature via the exit gas temperature for a double-gas-bearing stator and a Beams-Andrew design are resolved by noting that the latter design exhausts a much larger, higher velocity volume of gas which is, therefore, relatively less aIkcted by ambient temperature heat sinks. It would appear to appropriate for investigators utilizing qualitatively similar stator designs in variable-temperature magic-angle-spinning experiments to consider, as a minimal effort, the measurement of both the entrance and exit gas temperature to estimate the possible temperature gradient across the sample; where the gradient is large compared to acceptable experimental uncertainty, more direct methods of temperature measurement should then be employed. REFERENCES 1. J. SCHAEFER AND E. 0. STF..ISKAL, J. Am. Chem. Sot. !M, 1031 (1976). 2. C. A. &FE, J. R. LYERLA, AND C. S. YANNONI, J. Am. Chem. Sot. NO,5635 (1978). 3. J. R. LYERLA, C. A. FkFE, AND C. S. YANNONI, J. Am. Chem. Sot. 101, 1351 (1979). 4. J. R. LYERLA, C. S. YANNONI, D. BRUCK, AND C. A. FYFE, J. Am. Chem. Sot. IO&4770 (1979). 5. W. W. FLEMING, C. A. FVFE, J. R. LYERLA, H. VANNI, AND C. S. YANNONI, Macromolecules 13, 460 (1980).

NOTES 6. 7. 8. 9.

10. II. 12. 13. 14. 15. 16.

493

C. S. YANNONI, V. MACHO, AND P. C. MYHRE, J. Am. Chem. Sot. 184,907 (1982). W. P. ROTHWELL, J. S. WAUGH, AND J. P. YESINOWSKI, J. Am. Chem. Sot. 102,2637 (1980). W. P. ROTHWELL AND J. S. WAUGH, J. Chem. Phys. 74,272l (1981). M. J. SULLIVAN AND G. E. MACIEL, Anal. Chem. 54, 16 15 (1982). J. W. BEAMS, Rev. Sri. Instrum. 1, 667 (1930). E. R. ANDREW, Progr. NMR Spectrosc. 8, 1 (1972). Varian Associates, Palo Alto, Calif. 94303, Publication Number 148 1. A. L. VAN GREET, Anal. Chem. 40, 2227 (1968). M. L. MARTIN, G. J. MARTIN, AND J. J. DELPUECH, “Practical NMR Spectroscopy,” pp. 336-337, Heyden, Philadelphia, 1980. Bruker Instruments, Inc., Billerica, Mass. 01821, B-VT lOOO-ER 4111 VT variable temperature unit operational manual. F. D. DoTY AND P. D. ELLIS, Rev. Sci. Znstrum. 52, 1868 (1981).