- Email: [email protected]
Apparatus for low-temperature magic-angle spinning NMR
JOURNAL
OF MAGNETIC
RESONANCE
92,6
14-6 17 ( I99 1)
Apparatus for Low-Temperature P.J. ALLEN,F.CREUZET,* Francis
Buter
National
Magnet
Laboratory
Technology.
Received
September
Magic-Angle
Spinning NMR
H.J.M.DEGRooT,~-ANDR.G.GRIFFIN and Department Cambridge.
of‘Chemi.vry,
Massachusetts
27. 1990; revised
~t-lassac~llr~.~rrt~~ Imtiture
of’
02139
November
12. 1990
Low-temperature magic-angle spinning (MAS) NMR experiments are of considerable interest since they permit the study of a variety of phenomena such as chemical exchange, phase transitions, and Knight shifts. In addition, the recent development of techniques for determining distances and distinguishing among molecular conformations in solids provides opportunities to examine the structure of reactions in intermediates (i.e., photolysis products and enzyme/inhibitor or substrate complexes) trapped at low temperatures. In this Note we describe a system that we have developed and successfully employed for performing MAS experiments for extended periods of continuous operation in the temperature range of 150-300 K. The desirable features of a low-temperature MAS system are severalfold. First, it should provide setable and stable spinning speeds for extended periods. For low temperatures, this dictates that at least the drive-bearing gas must be at room temperature and supplied to a spinner-speed controller before cooling in order to maintain constant speeds. Second, the spinning speed must be maintained when the temperature is changed. Third, facilities for replenishing the coolants and the drive and bearing gases must be provided to accommodate operation for extended periods. The first requirement above was initially addressed simply by circulating the drive and bearing gases through copper coils immersed in liquid nitrogen, the temperature being determined by adjusting the size of the coils and the flow rate. This is the method used at present in most laboratories and in most commercial equipment. However, with this arrangement the temperature and spinning speed cannot be adjusted independently. In addition, at the high pressures which are necessary to achieve high spinning speeds, condensation of the nitrogen occurs, resulting in pulsations of the spinning. Kendrick et al. (2) demonstrated that this problem could be circumvented by pressurizing the liquid nitrogen dewar above the pressure of the gas flow. However, this procedure has the disadvantages that it involves a large pressurized vessel (3) subject to strict safety rules in most operating environments, and that it is difficult to replenish the coolant. It is therefore unsatisfactory for samples requiring extended signal averaging periods. * Present Aubervilliers 7 Present erlands.
address: Cedex, address:
0022-2364/9
I $3.00
Unit6 Mixte de Recherche. CNRS/Sainte France. Gorlaeus Laboratorium, Rijksuniversiteit
Copyright 0 1991 by Academic Press. Inc. 411rights of reproduction in any form reserved.
614
Gobain.
39. Quai
Lucien
Lefranc.
te Leiden,
NL-2300
RA Leiden,
The
93303 Neth-
615
NOTES
The apparatus we have employed to solve these problems is illustrated schematically in Fig. 1. Nitrogen gas is provided by two 160 liter gas-delivery dewars which permit continuous gas flow, since one dewar is in use while the second is being filled. Following pressure regulation, the N2 is warmed to room temperature by passing it through a copper coil immersed in water. It then flows through a mass-flow valve which is regulated by a spinner-speed controller described elsewhere (4) and which ensures a constant gas flow. The flow is then divided into separate vacuum-jacketed transfer lines for drive and bearing gas before entering the heat exchanger, consisting of two coils residing in a copper container and immersed in a liquid nitrogen bath (Fig. 2 ).
/ I
GiS
FIG. priately.
I. Schematic Not shown
@ELI':iW
diagram of the low-temperature system for MAS with each component labeled is the MAS spinning-speed controller. which is described in detail in Ref. (4)
appro-
FIG. 2. Schematic diagram of the heat-exchange container. A finite pressure of Nz gas is placed in the container which condenses, since the heat exchanger is immersed in LN2. Passage of drive and bearing gaseous Nz through the coils vaporize this liquid, which then recondenses on the surfaces of the container. Temperature control is achieved by adjusting the level of the LN2 in the heat-exchange can.
The feature which distinguished our apparatus from those described previously is that the heat-exchange loops shown in Fig. 2 are not in direct contact with the liquid N2 bath. Cooling and temperature control are achieved by adjusting the pressure of N2 in the small container, which then condenses since the walls are maintained at 77 K. The warm gas which enters the heat-exchange loops vaporizes this liquid and is cooled in the process. Thus, the level of liquid N2 in the small heat exchanger determines the temperature of both the spinning and the drive gas, and it is possible to adjust the liquid level and the temperature without significantly altering the spinning speed. Moreover, since the heat exchanger is a closed vessel, the temperature will remain constant as long as the liquid N2 level is maintained in the cooling dewar. Since this dewar is operated at ambient pressure, it may be refilled continuously and therefore operated for extended periods. Note that in our apparatus the drive gas circulates through the higher loop and is therefore maintained at a higher temperature than the bearing, a feature which promotes more efficient spinning. It would be possible to
617
NOTES
remove this coil entirely from the system, as is the practice in some MAS probes. The cold gas is transferred to the probe via a set of flexible transfer lines and from the bottom of the probe to the spinner assembly via homebuilt stainless steel, vacuumjacketed transfer lines. We have employed this apparatus in conjunction with homebuilt. low-temperature MAS probes which use either 5 mm high-speed or 7 mm stators and rotors from Doty Scientific (Columbia, South Carolina). In the 5 mm system we have routinely performed spinning experiments at 10 kHz speeds, with t3 Hz stability and at temperatures as low as 200 K and with +l K temperature stability. With the 7 mm system. which utilizes speeds of up to 5 kHz, we have achieved *2 Hz stability in the spinning speed for temperatures in the 150-300 K regime. With more careful attention to insulation of the probe, it should be possible to achieve lower temperatures in both cases. Finally, with both stator and rotor systems we have performed experiments requiring several days of signal averaging at low temperatures, with both stable temperatures and stable spinning speeds (3, S-7). Drawings suitable for fabrication of the apparatus are available upon request. ACKNOWLEDGMENTS The research and RR-00995).
was supported by the National Institutes F.C. held an M.I.T. Bantrell Fellowship.
of Health
(GM-23289.
GM-23403.
GM-75505.
REFERENCES I 2 3 4. 5
6 7
V. MACHO, R. D. KENDRICK, AND C. S. YANNONI, J. Mugn. Rrson. 52,450 ( 1983). R. D. KENDRICK. S. FRIEDRICH. B. WEHRLE, H. H. LIMBACH. AND C. S. Y.ANNONI. J. Mu,cg~. Rcwn 65, I59 (1985). F. CREUZET. A. E. MCDERMOTT, R. GEBHARD, I. VAN DER HOEF, M. B. SPIJKER-ASSINK, J. HERZI;EL.D. J. LUGTENBLJRG. M. H. LEVITT, AND R. G. GRIFFIN, S&ncc~, submitted. H. J. M. DE GROOT, V. COP& S. 0. SMITH. J. PARDOEN. J. HERZFELD, J. LIJGTENBIJRG. AND R. G. GRIFFIN, J. .kfup~ Ret-on. 77, 251 (1988). J. HERZFELD. S. K. DAS GUPTA, M. R. FARRAR, G. S. HARBISON, A. E. MCDERMOTT, S. L. PELI.EI IEK, D. P. RALEIGH, S. 0. SMITH. C. WINKEL, J. LUGTENBURG. AND R. G. GRIFFIN. Broclrcmi.ctr~~ 29. 5567 (1990). V. COPI~. A. C. KOLBERT, D. H. DREWRY. P. A. BARTLE~. T. G. OAS, AND R. G. GRIFFIN, Bio&~rnrstr~ 29,9176 (1990). H. J. M. DE GROOI.. S. 0. SMITH. J. COURTIN, E. VAN DEN BERG. C. WINKEI.. J. LU~,TENBL~RG. R. G. GRIFFIN. AND J. HERZFELD, Bicxhcwisrr~~ 29, 6873 ( 1990).
OF MAGNETIC
RESONANCE
92,6
14-6 17 ( I99 1)
Apparatus for Low-Temperature P.J. ALLEN,F.CREUZET,* Francis
Buter
National
Magnet
Laboratory
Technology.
Received
September
Magic-Angle
Spinning NMR
H.J.M.DEGRooT,~-ANDR.G.GRIFFIN and Department Cambridge.
of‘Chemi.vry,
Massachusetts
27. 1990; revised
~t-lassac~llr~.~rrt~~ Imtiture
of’
02139
November
12. 1990
Low-temperature magic-angle spinning (MAS) NMR experiments are of considerable interest since they permit the study of a variety of phenomena such as chemical exchange, phase transitions, and Knight shifts. In addition, the recent development of techniques for determining distances and distinguishing among molecular conformations in solids provides opportunities to examine the structure of reactions in intermediates (i.e., photolysis products and enzyme/inhibitor or substrate complexes) trapped at low temperatures. In this Note we describe a system that we have developed and successfully employed for performing MAS experiments for extended periods of continuous operation in the temperature range of 150-300 K. The desirable features of a low-temperature MAS system are severalfold. First, it should provide setable and stable spinning speeds for extended periods. For low temperatures, this dictates that at least the drive-bearing gas must be at room temperature and supplied to a spinner-speed controller before cooling in order to maintain constant speeds. Second, the spinning speed must be maintained when the temperature is changed. Third, facilities for replenishing the coolants and the drive and bearing gases must be provided to accommodate operation for extended periods. The first requirement above was initially addressed simply by circulating the drive and bearing gases through copper coils immersed in liquid nitrogen, the temperature being determined by adjusting the size of the coils and the flow rate. This is the method used at present in most laboratories and in most commercial equipment. However, with this arrangement the temperature and spinning speed cannot be adjusted independently. In addition, at the high pressures which are necessary to achieve high spinning speeds, condensation of the nitrogen occurs, resulting in pulsations of the spinning. Kendrick et al. (2) demonstrated that this problem could be circumvented by pressurizing the liquid nitrogen dewar above the pressure of the gas flow. However, this procedure has the disadvantages that it involves a large pressurized vessel (3) subject to strict safety rules in most operating environments, and that it is difficult to replenish the coolant. It is therefore unsatisfactory for samples requiring extended signal averaging periods. * Present Aubervilliers 7 Present erlands.
address: Cedex, address:
0022-2364/9
I $3.00
Unit6 Mixte de Recherche. CNRS/Sainte France. Gorlaeus Laboratorium, Rijksuniversiteit
Copyright 0 1991 by Academic Press. Inc. 411rights of reproduction in any form reserved.
614
Gobain.
39. Quai
Lucien
Lefranc.
te Leiden,
NL-2300
RA Leiden,
The
93303 Neth-
615
NOTES
The apparatus we have employed to solve these problems is illustrated schematically in Fig. 1. Nitrogen gas is provided by two 160 liter gas-delivery dewars which permit continuous gas flow, since one dewar is in use while the second is being filled. Following pressure regulation, the N2 is warmed to room temperature by passing it through a copper coil immersed in water. It then flows through a mass-flow valve which is regulated by a spinner-speed controller described elsewhere (4) and which ensures a constant gas flow. The flow is then divided into separate vacuum-jacketed transfer lines for drive and bearing gas before entering the heat exchanger, consisting of two coils residing in a copper container and immersed in a liquid nitrogen bath (Fig. 2 ).
/ I
GiS
FIG. priately.
I. Schematic Not shown
@ELI':iW
diagram of the low-temperature system for MAS with each component labeled is the MAS spinning-speed controller. which is described in detail in Ref. (4)
appro-
FIG. 2. Schematic diagram of the heat-exchange container. A finite pressure of Nz gas is placed in the container which condenses, since the heat exchanger is immersed in LN2. Passage of drive and bearing gaseous Nz through the coils vaporize this liquid, which then recondenses on the surfaces of the container. Temperature control is achieved by adjusting the level of the LN2 in the heat-exchange can.
The feature which distinguished our apparatus from those described previously is that the heat-exchange loops shown in Fig. 2 are not in direct contact with the liquid N2 bath. Cooling and temperature control are achieved by adjusting the pressure of N2 in the small container, which then condenses since the walls are maintained at 77 K. The warm gas which enters the heat-exchange loops vaporizes this liquid and is cooled in the process. Thus, the level of liquid N2 in the small heat exchanger determines the temperature of both the spinning and the drive gas, and it is possible to adjust the liquid level and the temperature without significantly altering the spinning speed. Moreover, since the heat exchanger is a closed vessel, the temperature will remain constant as long as the liquid N2 level is maintained in the cooling dewar. Since this dewar is operated at ambient pressure, it may be refilled continuously and therefore operated for extended periods. Note that in our apparatus the drive gas circulates through the higher loop and is therefore maintained at a higher temperature than the bearing, a feature which promotes more efficient spinning. It would be possible to
617
NOTES
remove this coil entirely from the system, as is the practice in some MAS probes. The cold gas is transferred to the probe via a set of flexible transfer lines and from the bottom of the probe to the spinner assembly via homebuilt stainless steel, vacuumjacketed transfer lines. We have employed this apparatus in conjunction with homebuilt. low-temperature MAS probes which use either 5 mm high-speed or 7 mm stators and rotors from Doty Scientific (Columbia, South Carolina). In the 5 mm system we have routinely performed spinning experiments at 10 kHz speeds, with t3 Hz stability and at temperatures as low as 200 K and with +l K temperature stability. With the 7 mm system. which utilizes speeds of up to 5 kHz, we have achieved *2 Hz stability in the spinning speed for temperatures in the 150-300 K regime. With more careful attention to insulation of the probe, it should be possible to achieve lower temperatures in both cases. Finally, with both stator and rotor systems we have performed experiments requiring several days of signal averaging at low temperatures, with both stable temperatures and stable spinning speeds (3, S-7). Drawings suitable for fabrication of the apparatus are available upon request. ACKNOWLEDGMENTS The research and RR-00995).
was supported by the National Institutes F.C. held an M.I.T. Bantrell Fellowship.
of Health
(GM-23289.
GM-23403.
GM-75505.
REFERENCES I 2 3 4. 5
6 7
V. MACHO, R. D. KENDRICK, AND C. S. YANNONI, J. Mugn. Rrson. 52,450 ( 1983). R. D. KENDRICK. S. FRIEDRICH. B. WEHRLE, H. H. LIMBACH. AND C. S. Y.ANNONI. J. Mu,cg~. Rcwn 65, I59 (1985). F. CREUZET. A. E. MCDERMOTT, R. GEBHARD, I. VAN DER HOEF, M. B. SPIJKER-ASSINK, J. HERZI;EL.D. J. LUGTENBLJRG. M. H. LEVITT, AND R. G. GRIFFIN, S&ncc~, submitted. H. J. M. DE GROOT, V. COP& S. 0. SMITH. J. PARDOEN. J. HERZFELD, J. LIJGTENBIJRG. AND R. G. GRIFFIN, J. .kfup~ Ret-on. 77, 251 (1988). J. HERZFELD. S. K. DAS GUPTA, M. R. FARRAR, G. S. HARBISON, A. E. MCDERMOTT, S. L. PELI.EI IEK, D. P. RALEIGH, S. 0. SMITH. C. WINKEL, J. LUGTENBURG. AND R. G. GRIFFIN. Broclrcmi.ctr~~ 29. 5567 (1990). V. COPI~. A. C. KOLBERT, D. H. DREWRY. P. A. BARTLE~. T. G. OAS, AND R. G. GRIFFIN, Bio&~rnrstr~ 29,9176 (1990). H. J. M. DE GROOI.. S. 0. SMITH. J. COURTIN, E. VAN DEN BERG. C. WINKEI.. J. LU~,TENBL~RG. R. G. GRIFFIN. AND J. HERZFELD, Bicxhcwisrr~~ 29, 6873 ( 1990).