Variable-Temperature SolidState NMR James F. Haw
Department of Chemistry Texas A&M University College Station, Tex. 77843
Many important chemical problems in volve solid materials such as catalysts, polymers, or ceramics. In addition, many problems involving discrete inor ganic or organic compounds concern their nature or behavior in various solid phases. Properties of interest in solidstate research projects can include structure; morphology; reactivity; mo tion (of molecules, ions, chains, defects, or functional groups); and magnetic and electronic behavior. In interesting and useful solids, many or all of these properties are temperature dependent. Variable-temperature solid-state NMR is a relatively new analytical method that holds considerable prom ise for the study of solids. As materials research grows into an important scien tific and technological objective, solidstate NMR will increase in importance as an analytical technique, and many of the applications will involve variabletemperature (VT) studies. Several gen eral articles on solid-state NMR have been published elsewhere (1-4). Appli cations of the VT experiment prior to 1982 have also been reviewed (5).
Magic-angle spinning Currently most solid-state NMR ex periments are performed in high-reso lution modes that employ one or more line-narrowing techniques, the most important of which is magic-angle
Spectroscopy pling and multiple-pulse averaging. Some solid-state NMR experiments also use special techniques to greatly improve the signal strength over that obtained through conventional singlepulse excitation. The most important of these sensitivity enhancement tech niques is cross-polarization (CP) (6). The emphasis of this article is on MAS experiments, and it reflects the chemical utility and instrumental chal lenges of such experiments, especially when spinning must be performed at extreme temperatures. The MAS ex-
INSTRUMENTATION spinning (MAS). In MAS NMR, the sample is rapidly rotated about an an gle that is inclined at 54.7° relative to the static magnetic field. This angle is "magic" because rapid rotation at this angle averages to zero the geometric term 3 cos2 θ — 1 (which is found in the mathematical descriptions of several line-broadening interactions such as chemical-shift anisotropy). Other linenarrowing techniques that are used in various solid-state NMR experiments include high-power (dipolar) decou0003-2700/88/0360-559A/$01.50/0 © 1988 American Chemical Society
periment is technically difficult be cause spinning speeds range from sev eral kHz to several tens of kHz, de pending on the size of the interactions that must be averaged. A now-typical spinning speed of 9 kHz is difficult to appreciate until one converts it into revolutions per minute; 9 kHz corre sponds to 0.54 million rpm. With a cy lindrical sample diameter of several millimeters, the outer edge of the rotor travels at an appreciable fraction of the speed of sound. Mechanical linkages
cannot be used to generate such im pressive rotation rates. Instead, the sample must float upon, and be driven by, jets of compressed gas. Mechanical tolerances in MAS spinning systems are generally on the order of 0.05 mm. If these tolerances change significantly because of thermal expansion or con traction, or if the gas flow is disrupted, the rotor can "crash," sometimes dam aging the probe. Some of the instrumental problems inherent in VT MAS NMR can now be appreciated. The sample rotor and spinning system must be machined from materials that retain excellent mechanical properties over the operat ing temperature range; otherwise, the rotor or the spinning system could be damaged. To complicate matters fur ther, the choice of material usually is restricted by spectral background problems. For example, many engi neering polymers are unsuitable for 13C experiments, and many ceramics have appreciable 29 Si and/or 27A1 back grounds. In spite of these and other problems (see below), VT MAS NMR experiments over moderate tempera ture ranges have become more or less routine in some academic and industri al laboratories. The remainder of this article discusses how the experiment is
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988 · 559 A
(a)
(b)
Bearing gas—•
Bearing
I*—gas Sample rotor
Sealed glass, sample tube
Coil region
Drive - gas
Stator
Coil region
(c) Rotor skirt
Rotor
Drive -gas
Heat shield
Coil region
Stator Gas in
VTgas in
Stator
Figure 1. Conceptual drawings of three types of magic-angle spinning systems used in variable-temperature studies. (a) A typical double-bearing design, (b) A design for spinning sealed glass tubes, (c) The design used by Yannoni, Fyfe and co-workers for VT MAS experiments in an electromagnet.
performed and illustrates some of the chemical problem areas that are ame nable to study. Conceptual drawings of three repre sentative VT MAS systems are shown in Figure 1. A double-bearing design is illustrated in Figure la. The drive gas is used to propel the rotor at high speeds while the bearing gas provides an air cushion for stability. The sample payloads used in double-bearing spinning systems range from 0.1 to 0.6 cm3, al though smaller samples are sometimes used in high-speed experiments. Ro tors for double-bearing spinning sys tems can be fabricated from plastics, ceramics, or a combination of the two materials. The spinning system in Figure l b is based on a design developed by Ian Gay (7). The sample is sealed in a glass tube that is inserted into a plastic rotor skirt to form the complete rotor. VT opera tion is achieved by directing a gas stream onto the bottom of the sample tube. The advantages of this type of spinning system are that the stator and rotor skirt need not be exposed to ex treme temperatures, and that air- or moisture-sensitive samples can be ex amined in sealed tubes. The disadvan tages of this system are that spinning speeds are limited to 4 kHz or less and that careful attention must be paid to sealing the tube symmetrically so that the rotor will be balanced. The spinning system in Figure lc was developed by Colin Fyfe and co workers (8) for VT MAS experiments in an electromagnet. The sample vol ume of this system is only 0.07 cm 3 , but good-quality 13C spectra can be ob tained through the use of enriched samples and as a result of favorable signal-to-noise ratios (S/N) at very low temperatures.
somewhat arbitrary, but that of Table I is supported (in part) by the boiling points of cryogenic fluids. John Waugh of MIT is directing chemical physicsoriented, wide-line experiments at temperatures as low as 0.01 Κ (9). The instrumentation used in these experi ments differs in a number of ways from conventional spectrometers. For exam ple, a cable-and-pulley system is used to hoist the magnet out of the base ment and onto the probe, which is sub merged in the liquid helium reservoir of the superconducting solenoid. The sample is cooled to millikelvin tem peratures by a 3 He/ 4 He dilution refrig erator, and nuclear spin polarization is transferred to the sample surface by direct contact with a 3 He bath. Several physics research groups are carrying out NMR experiments in the microkel-
vin and nanokelvin ranges. Such in credibly low temperatures are reached by adiabatic demagnetization of nucle ar spins in a block of copper metal, which is in thermal contact with the sample. MAS experiments at temperatures significantly below 4 Κ are not practi cal because of insulation problems and the need for a spinning gas. It is reason able to expect that ultra-low-tempera ture NMR will remain the province of nonspinning experiments, which are also common in the very-low-tempera ture range (4-77 K). For example, Da vid Grant's group at Utah is using ma trix isolation NMR techniques in a fun damental study of the relationship between chemical shift and structure (10). These studies are done at approx imately 20 Κ to prevent motional aver-
Table 1. Proposed classification of temperature ranges in variabletemperature solid-state NMR. Temperature range 500K
Very high temperature
Below room temperature
It is convenient to classify VT MAS experiments according to temperature. Any such classification is bound to be 560 A · ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
Status Magic-angle spinning (MAS) is not feasible. Wide-line experiments are used to probe ultra-low-tempera ture physical phenomena. Three reports of MAS experiments at temperatures as low as 8 K. Nonspinning experiments require standard cryogenic techniques. MAS experiments at temperatures below 100 Κ are routine in several laboratories. Many labs have 170 Κ capabilities. MAS experiments at 500 Κ are cur rently possible in a few labs; 400 Κ is far more common. MAS development work is under way in several locations. Severe material constraints exist above 700 K.
aging of chemical-shift powder pat terns. (The principal components of the chemical-shift tensor are much more useful in theoretical correlations than the isotropic shift alone.) The principal obstacle to very-lowtemperature MAS NMR experiments is the high cost of liquid helium, al though the expense would be manage able for laboratories that are connected to a helium recovery and liquefaction system. There are also technical prob lems. Helium gas undergoes an electri cal breakdown when it is in contact with radio frequency (rf) coils, necessi tating the isolation of gas from coil. The design shown in Figure lc isolates the gas from the coil, albeit with a loss of filling factor and hence S/N. Yannoni and co-workers have pub lished two reports of MAS NMR ex periments at very low temperatures (11,12). Spectra were obtained at tem peratures as low as 15 K, but spinning problems were severe for some sam ples. To correct spinning problems, one must be able to move the probe into and out of the magnet without inter rupting the flow of the spinning gas, but helium transfer lines are not very flexible, which makes it difficult to couple an external helium reservoir to a probe mounted in the vertical air bore of a conventional superconducting NMR magnet. Yannoni and co-workers solved this plumbing geometry prob lem by designing their system around an electromagnet. Alternatively, one could use a horizontal-bore magnet and mount the probe and helium Dewar on a rail. Recently Hackmann and co workers performed MAS NMR experi ments at 8 Κ using a probe incorporat ing the Ian Gay spinning system design (13). Their experiment was construct ed around a very-wide-bore (125-mm) 1.4-T superconducting magnet. Fortunately, a temperature of 77 Κ is sufficient to freeze out the reactivity and dynamics of a great many chemi cally interesting solids, and liquid ni trogen is an inexpensive resource. On a busy day of low-temperature MAS NMR experiments (14), we go through 100 L or 200 L of liquid nitrogen, in roughly equal portions for spinning gas and temperature regulation. Figure 2 is a schematic of a double-bearing VT CP/MAS probe and accessories used at Texas A&M. Because both gas flows impinge upon the rotor, it is necessary to measure and regulate both bearing and drive gas temperatures. Three re sistance temperature devices (RTDs) are 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
Bypass
ERTD.
Bearing gas heater
,BRTD
DRTD.
Drive gas heater
Liquid N2
Probe Figure 2. Schematic of a double-bearing MAS probe and accessories used for lowtemperature studies. Locations of the resistance temperature devices (RTDs) are shown for the drive (D), bearing (B), and exit (E) gas streams.
system positioned so as to be in the stream of gas exiting from around the sample. These sensors are designated in Figure 2 as DRTD, BRTD, and ERTD, respectively. The ERTD can be retracted from the spinning system during data acquisition; failure to do so can result in loss of S/N because of rf pick-up. This sensor is lowered into po sition for temperature measurement immediately after data acquisition is completed. The bearing and drive sen sors are well shielded from rf and re main in position at all times. Completely independent gas streams (usually N2) are used for bearing and drive. A series of needle valves is used to either direct the gas streams through cooling coils immersed in liquid nitro gen or bypass the cooling coils. Inter mediate temperatures are most effi ciently obtained by proportioning the flow between the cooling coils and by pass loops. These valves and bypass loops are also useful for rapid tempera ture increases, and they allow air to be used for ambient temperature experi ments by bypassing the cooling coils. The 50-L cooling Dewar allows for sta ble VT operation for up to 2.5 h with out refilling. Temperature control is achieved with two heaters, one on the bearing gas channel and the other on the drive gas channel, responding to BRTD and DRTD, respectively. Ideal ly, the three sensors in Figure 2 should report the same temperature after the system has equilibrated for a few min utes. In practice, differences of ±1 Κ are typical. Failure to carefully regu late the temperature of both the bear
562 A · ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
ing and drive gas streams can lead to very large temperature errors (±10 K) and/or temperature gradients across the sample (14). Unfortunately, most commercial VT MAS systems regulate only one gas stream. An important consideration in the design of any NMR experiment is the S/N ratio. To a first approximation, the temperature dependence of S/N in an NMR experiment is determined by the difference in population between the ground state and the excited state (Boltzmann distribution) and thermal noise in the coil. The effect of tempera ture can be estimated with the follow ing equation S/N = Κ Τ-1 T-1/2[P(Tc)]-m
(1)
in which Κ is a collection of (approxi mately) temperature-independent terms (15), Ts is the sample temperature, Tc is the temperature of the receiver coil, and p{Tc) is the resistivity of the coil wire (usually made from copper) at the temperature Tc. Equation 1 should be used with some caution as it does not account for changes in NMR relaxation rates or line widths (both of which can be strongly temperature dependent), the equivalent noise temperature of the preamplifier, noise sources in the probe other than the coil, or problems in tun ing the probe at extreme operating temperatures. Aside from these com plications, Equation 1 does predict some interesting results. For example, the S/N at 77 Κ could be as much as 13 times the value at 298 K. Improve ments in S/N are often seen in MAS
experiments at 77 Κ, but they are much less than a factor of 13 (for the abovementioned reasons). The nonmagnetic, high-power rf ca pacitors used in solid-state NMR probes have large temperature coeffi cients. If the temperature of the match ing and tuning networks is allowed to vary, it becomes difficult to keep the probe tuned in the course of a VT MAS study. Additionally, some types of vari able capacitors can "freeze u p " because of changing mechanical tolerances or frost formation. For these reasons, most VT MAS probes use insulation and/or purge gas streams to maintain the probe electronics at or near room temperature. Although this practice minimizes the aggravation of retuning the probe at each temperature, keeping most of the probe electronics at room temperature may also preclude a sub stantial improvement in S/N for many samples at cryogenic temperatures.
355 Κ
352 Κ
344K
301 Κ
Above room temperature
Several of the projects in our laborato ry involve the study of surface species at high temperatures, and it is impor tant to optimize the S/N in such ex periments. Equation 1 predicts that the S/N will be one-third the room temperature value when both the sam ple and coil are at 500 K. A simplistic application of Equation 1 predicts that the loss of S/N at 500 Κ would be sig nificantly less if the coil could be kept near room temperature, but there is no obvious way to insulate the sample from the coil without reducing the fill ing factor and hence the S/N. With a combination of careful probe design and the use of isotopic enrichment (when necessary), the S/N at high tem peratures is usually satisfactory. At 800 K, however, the S/N will be much lower than at room temperature, so NMR experiments at that temperature will be restricted to favorable cases. A more detailed discussion of S/N in the NMR experiment has been published by Hoult and Richards (15). Most spinning systems and rotors used in high-temperature and veryhigh-temperature MAS NMR are ma chined from ceramics such as alumina, zirconia, Macor, or boron nitride. Some of these materials have excellent ther mal stability, but other problems com plicate their use in MAS NMR experi ments. First among these is the unfor tunate fact that when ceramics fail, they fail catastrophically. Rotor "ex plosions" can occur with excessive spinning speeds, unbalanced samples, or without apparent reason. Boron ni tride loses much of its strength upon prolonged exposure to atmospheric moisture, so it is prudent to keep rotors machined from this material in a desic cator when not in use. Rotors are fairly easy to replace, but rotor explosions
60
45
30
ppm
Figure 3. 1 3 C CP/MAS NMR spectra of 1,4-diazabicyclo-[2,2,2]-octane (DABCO) at several temperatures in the vicinity of the solid-solid phase transi tion temperature (351 K). (Adapted with permission from Reference 18. Copyright 1986 by Academic Press, Inc.)
sometimes damage the coil and occa sionally destroy the entire spinning system. Some ceramics are notoriously diffi cult to machine, so much so that ven dors of solid-state NMR probes con tract out their ceramics fabrication work to high-technology ceramics com panies. Fortunately, boron nitride and Macor are fairly easy to work with, and some ceramic spinning system devel opment work has been possible in aca demic laboratories. A further disad vantage of ceramic materials is that they sometimes exhibit unexpected background signals because of impuri ties or additives intentionally intro duced in the manufacturing process. The problem appears to be most severe for silicon and aluminum, which are commonly used elements in ceramics technology. In spite of these problems, ceramic spinning systems and rotors generally perform well and ultimately will make very-high-temperature MAS NMR a routine experiment. Of the nine spin ning systems currently in use at Texas A&M, three are ceramic, and several of the others can accommodate hybrid ro tors made from ceramics and plastics. One of our MAS spinning systems is made from Torlon, a high-temperature
564 A · ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
engineering polymer. Although this material has unacceptable 13 C, 15 N, and Ή background signals, it is com pletely free from other resonances. This spinning system was much easier to machine than analogous ceramicbased systems and is forgiving of rotor crashes. We have used the Torlon sys tem to obtain high-quality 31 P MAS spectra at temperatures as high as 523 Κ with a spinning speed of 4.5 kHz (16). An alternative approach to veryhigh-temperature MAS probe design (which we are also developing) makes use of a spinning system based on the Ian Gay design (Figure lb). The advan tage of that design is that it provides some thermal isolation between the sample region and the rest of the spin ning system. In addition, the stator and rotor skirt are well outside of the coil region, minimizing background signal problems. These two advantages will reduce the stringent materials require ments for MAS NMR at elevated tem peratures. It is appropriate to discuss one final instrumental consideration in VT MAS NMR. The vacuum seals in superconducting magnets can leak at elevated or reduced temperatures. Seal leakage will cause dramatic increases in the N2 and He boil-off rates. These rates will usually return to normal within an hour of the suspension of VT operation, but some laboratories have experienced total seal failures resulting in magnet quenches. Protecting the magnet bore from extreme tempera tures is, therefore, an important con sideration when setting up a spectrom eter for VT MAS studies. Wide-bore magnets provide adequate space for Dewars, insulation, and ambient-tem perature purge gas streams, and they are greatly preferred over narrow-bore magnets for VT MAS experiments. Currently we can spin for at least two hours at temperatures as low as 77 Κ or as high as 550 Κ before the boil-off rates become excessive. A new type of probe incorporating Dewars is expect ed to eliminate this problem entirely. Applications
Many of the applications of VT MAS NMR involve the study of dynamic processes such as chemical exchange or molecular motion. A conceptually sim ple example of such an investigation is shown in Figure 3. Dramatic changes in spectral parameters are often observed during solid-solid phase transitions. For example, 1,4-diazabicyclo-[2,2,2]octane (DABCO) undergoes a transi tion at 351 Κ to a high-temperature plastic crystalline phase in which rapid reorientation is possible about all axes. Figure 3 shows 13C CP/MAS spectra of DABCO obtained at several tempera tures in the vicinity of the transition. In the low-temperature phase, the line
shape is dominated by an incompletely averaged dipolar coupling to the quadrupolar U N nucleus (77). In the hightemperature phase, molecular reorien tation is sufficiently rapid to average the 14 N- 13 C dipolar coupling to zero, and the 13C resonance is a single sharp line. Compounds exhibiting solid-solid phase transitions are useful test sam ples for exercising new VT MAS probe designs (18). More sophisticated applications of VT MAS NMR to problems of molecu lar motion can be illustrated by some of our studies of polyphosphazenes, which are inorganic polymers based on chains of alternating phosphorus and nitrogen atoms. These polymers are of considerable interest because of their mechanical properties at extreme tem peratures, thermal stability, solvent re sistance, and potential for providing biocompatible materials. We are using multinuclear VT MAS NMR to study the synthesis, morphology, reactivity, and molecular dynamics of polyphos phazenes (19). Figure 4 shows variable-temperature 13 C CP/MAS and 31 P MAS spectra of poly[bis(ethoxy)phosphazene] (PBEP) at various temperatures. This polymer has a glass transition (Tg) at 189 K. The line shapes and intensities of the VT spectra, in conjunction with relaxation measurements (not shown), provide in formation about molecular motion and the morphology of the solid polymer. The 13C spectrum at 298 Κ has poor
S/N. The polymer is so mobile at this temperature that the Ή - 1 3 0 dipolar interaction responsible for cross-polar ization is greatly attenuated. At 173 K, a temperature well below Tg, the S/N ratio of the 13C CP/MAS spectrum is greatly increased because of the de crease in molecular motion. Inspecting the 13C spectra in Figure 4, one sees that the line width of the CH 2 resonance is strongly temperature dependent; it is sharp at both high and low temperatures, but broad at inter mediate temperatures. At 203 K, for example, the CH 2 carbon signal is so broad that it is not observed. Rothwell and Waugh have shown how molecular motion at the frequency of the ! H de coupling field (yBi, 48 kHz in this case) accounts for this type of behavior (20). To further investigate molecular mo tion in solid PBEP, 31 P MAS spectra, which are sensitive to main-chain mo tions in phosphazene polymers, were acquired over the same temperature range used for the 13C data. Reducing the sample temperature produces a gradual increase in the 31 P MAS line width. The main-chain motion is not isotropic and rapid in the temperature range studied; otherwise, the dipolar coupling to 14N (the dominant linebroadening interaction for 31 P with these experimental conditions) would be completely averaged. Upon drop ping below Tg, there is a significant fur ther increase in line width. These data indicate that the main chains of PBEP
(a)
are undergoing large-amplitude aniso tropic motions above the glass-transi tion temperature. This main-chain mo tion could account for the relaxation behavior observed for the CH2 carbon resonance because the side groups will share some of the main-chain motions. Details of the dynamics and morpholo gy of several phosphazene polymers in cluding PBEP are being explored in ex periments currently underway. The development and characteriza tion of materials with interesting mag netic properties is an important area of solid-state chemistry. Furthermore, many inorganic compounds (including transition-metal dimers and clusters) display paramagnetism or more com plex temperature-dependent suscepti bilities, and their spectral properties at room temperature may be unfavorable for NMR study. One of the projects in our laboratory is to explore the use of VT MAS NMR for the characteriza tion of such nondiamagnetic solids (2123). Figure 5 presents VT 13C CP/MAS
333 Κ
299Κ
(b) 174 Κ -
OEt
OEt_
\ /
-N=P 298 Κ
98 Κ
223 Κ
ssb
77K
ssb
203 Κ
73 Κ
220
170
120
ppm 100
-25 ppm
32
0
-32
ppm
Figure 4. Variable-temperature (a) 13C CP/MAS and (b) 31P MAS spectra of the inor ganic polymer poly[bis(ethoxy)phosphazene] (PBEP). (Adapted from Reference 19.) 566 A · ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
Figure 5. Variable-temperature 13C CP/ MAS NMR spectra of the carbonyl re gion of samarium acetate tetrahydrate, a paramagnetic solid compound. Spinning sidebands are denoted by ssb. (Adapted with permission from Reference 22. Copyright 1986 by Academic Press, Inc.)
T, = 4867/(209 - Ôppm) CI C3 C2 85K
For H PLC GC TLC andJgnd.,
c3
C(,C2
c 4 ,c 4 · 98 Κ
c4 C· 128 Κ
C3
c,
C
c4 C
CHROMAboNcf
141 Κ
c3 C
300
200
100
c4-
0
298 Κ
-100
ppm
Figure 6. Variable-temperature 13C CP/MAS NMR spectra of anhydrous copper(ll) n-butyrate, a solid compound exhibiting antiferromagnetic exchange coupling. Ci refers to carboxylate carbons, C4 and C4' refer to methyl carbons, and C2 and C3 are the intermediate methylene carbons.
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This equation is useful from 77 Κ to approximately 270 K; at higher tem peratures, the change in chemical shift with temperature is too small for accu rate measurements. We have applied the samarium acetate tetrahydrate thermometer in a detailed study (14) of rf heating effects, thermal equilibra tion times, temperature gradients, and other factors affecting temperature measurement and control in VT MAS experiments. These studies have led to the development of the temperaturesensing arrangement in Figure 2, which works well provided that rf heating is minimal. Discovering improved solidstate chemical-shift thermometers that either replace or supplement samarium acetate tetrahydrate would be a useful contribution to the field. The application of VT MAS NMR to a somewhat more complicated magnet ic phenomenon, antiferromagnetic ex change coupling, is illustrated in Fig ure 6, which shows 13C data for anhy drous Cu(II) n-butyrate. Solid copper (II) carboxylates have interacting pairs of metal centers. The electronic ground state has all electron spins paired and is, therefore, a singlet. There is, howev er, a triplet state a few hundred c m - 1 above the ground state. Thermal popu lation of the upper state balanced with thermal randomization of electronspin orientation gives rise to a maxi mum in the susceptibility curve near room temperature. Starting with basic magnetic theory and assuming a con tact shift mechanism, it can be shown that the resonance frequencies of nu clear spins in d9-d9 dimers should have the following temperature dependence
(7 C /27r)/eT
spectra of a paramagnetic solid, samarium acetate tetrahydrate, the chemical shifts of which obey the Curie law (i.e., they are linear with 1/T) over a wide temperature range (22). The chelatingonly carbonyl signal (the more intense signal in Figure 5) is the more temperature-dependent resonance, and it has been applied by our group as the first MAS NMR chemical-shift thermometer. Determining temperature is one of the most basic physical measurements. In MAS NMR, however, it is not a trivial operation. The sample temperature (Ts) can differ from that of the surrounding gas streams (especially if the sample is subject to rf heating), but the sample itself is inaccessible to thermocouples or other sensing devices. Determining the chemical shift of the chelating-only carbonyl resonance of samarium acetate tetrahydrate allows the sample temperature to be calculated using Equation 2:
(3) where — 2J is the singlet-triplet energy separation, A is the hyperfine coupling constant for a given nucleus, and the other terms have their usual meanings in magnetic resonance theory. We have found that Equation 3 accurately pre dicts the temperature dependence of the 13C chemical shifts of all of the an hydrous Cu(II) carboxylates that we have examined to date. Statistical analysis of δ vs. Τ data allows values of —2J, A, and 5dia to be determined for these complexes. This information sheds light on the mechanism of anti ferromagnetic exchange coupling and may be of use in the development of magneto-structural correlations. Synthetic chemists commonly use solution-state NMR to obtain kinetic and mechanistic data for chemical re actions occurring in the NMR tube, and it is reasonable to expect that sol id-state and surface reactions can be studied with VT MAS NMR. In one
particularly enticing example of the potential of VT MAS NMR to study reactivity in the solid state, Yannoni used matrix isolation methods to study the photochemical conversion of 1-diazo-2-propanone (1) to methyl ketene CH3-C-C-H
II 0
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/ H 2 (2). That reaction was carried out inside of an MAS rotor equipped with a transparent cap (24). We are performing in situ catalysis experiments using VT MAS NMR as the spectroscopic probe. As one example of the potential of this method, consider Figure 7, which shows the roomtemperature 13C CP/MAS spectrum of an HY zeolite sample onto which propene-2- 13 C has been adsorbed (25). The NMR spectrum is consistent with extensive oligomerization of the adsorbed propene at room temperature. More interesting, however, is the resonance at 250 ppm, which is assigned to a carbocation species. VT experiments are in progress to monitor the formation and subsequent chemistry of carbocations and other reactive intermediates on catalyst surfaces. We are also following polymerization reactions using 13C VT MAS. In one example (26), a liquid epoxy and curing agent were mixed and promptly loaded into a rotor. In the early stages of polymerization, the sample was liquid-like, and 13C signals were obtained by single-pulse excitation (Bloch decay) with MAS. Cross-polarization was used to generate a 13C signal after the reaction had proceeded to the point where the polymer became more like a solid than a liquid. Our current work in this area focuses on the synthesis and reactivity of phosphazene polymers at high temperatures.
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Future developments Further refinements in VT MAS NMR will include more reliable spinning systems capable of routine operation at increasingly higher temperatures. The number of solid-state NMR spectrometers in academic and industrial laboratories is rapidly increasing, as is the reliability of VT MAS probes. We can therefore expect to see an increase in the number of VT MAS studies appearing in the literature. Many of these studies will be fundamental investigations of chemical structure or reactivity. Many others, no doubt, will probe the synthesis or processing of materials at elevated temperatures. Some of the
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