Vautable-Tcmpeuatuue SolidState
h
James F. Haw
Department of Chemistry Texas A8M University College Station. Tex. 77843
Many important chemical problems involve solid materials such as catalysts. polymers, or ceramics. In addition. many problems involving discrete inorganic or organic compounds concern their nature or behavior in various solid phases. Properties of interest in solidstate research projects can include structure; morphology; reactivity; motion (of molecules, ions, chains, defects. or functional groups); and magnetic and electronic behaviur. 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 promise for the study of solids. As materials research grow3 into an important srientific and technological objective, solidstate NMR will increase in importance as an analytical technique. and many of the applications will invulve variabletemperature (VTJstudies. Several general articles on solid-state NMH have been published elsewhere (1-4). Applications of the V T experiment prior to 1982 have also been reviewed (5).
Magic-angle splmhg Currently most solid-state NMR experiments are performed in high-reso. lution modes that employ one or more line-narrowing techniques, the most important of which is magic-angle
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 techniques is cross-polarization (CP) (6). The emphasis of this article is on MAS experiments, and it reflects the chemical utility and instrumental challenges of such experiments, especially when spinning must be performed at extreme temperatures. The MAS ex-
lNS7RUMEN7ATION spinning (MAS). In MAS NMR, the sample is rapidly rotated about an angle that is inclined a t 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 0 - 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/$O1.50/0 @ 1988 American Chemical Society
periment is technically difficult because spinning speeds range from several kHz to several tens of kHz, depending 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 corresponds to 0.54 million rpm. With a cylindrical sample diameter of several millimeters, the outer edge of the rotor travels a t an appreciable fraction of the speed of sound. Mechanical linkages
cannot be used to generate such impressive 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 contraction, or if the gas flow is disrupted, the rotor can “crash,” sometimes damaging 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 operating temperature range; otherwise, the rotor or the spinning system could be damaged. To complicate matters further, the choice of material usually is restricted by spectral background problems. For example, many engineering polymers are unsuitable for 13C experiments, and many ceramics have appreciable %i and/or *‘AI backgrounds. In spite of these and other problems (see below), VT MAS NMR experiments over moderate temperature ranges have become more or less routine in some academic and industrial laboratories. The remainder of this article discusses how the experiment is
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9. MAY 1, 1988
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I=-"
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Figure 1. Conceptual drawings of three types of magic-angle spinning systems used in variable-temperature studies. deslgn. (b) A design fw spinning sealee g ! a s tubes. (c) The deslgn used by Yannoni. Fyfe and co-wwkem Iu V l MAS experlmnta In
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performed and illustrates some of the chemical problem areas that are amenable to study. Conceptual drawings of three representative V T 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 a t 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, although smaller samples are sometimes used in high-speed experiments. Rotors for double-bearing spinning systems can be fabricated from plastics, ceramics, or a combination of the two materials. The spinning system in Figure l h 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 operation is achieved hy 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 extreme temperatures, and that air- or moisture-sensitive samples can be examined in sealed tubes. The disadvantages 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 he balanced. The spinning system in Figure IC was developed by Colin Fyfe and coworkers (8) for VT MAS experiments in an electromagnet. The sample volume of this system is only 0.07 cm3, hut good-quality '3c spectra can be obtained through the use of enriched samples and as a result of favorable signal-to-noise ratios (S/N) a t very low temperatures.
somewhat arbitrary, but that of Table I is supported (in part) hy the boiling noints of crvoeenic fluids. John Waueh bf MIT is &icting chemical physicsoriented, wide-line experiments at temperatures as low as 0.01 K (9).The instrumentation used in these experiments differs in a number of ways from conventional spectrometers. For example, a cable-and-pulley system is used to hoist the magnet out of the basement and onto the probe, which is submerged in the liquid helium reservoir of the superconducting solenoid. The sample is cooled to millikelvin temperatures by aSHel4Hedilution refrigerator, and nuclear spin polarization is transferred to the sample surface by direct contact with a SHe bath. Several physics reaearcb groups are carrying out NMR experiments in the microkel-
Table 1. Reposed classlRcation of temperatue ranges In varlabletemperature solid-state NMR. Temperature range C4K
ANALYTICAL CHEMISTRY. VOL. 60, NO.
CIasslRcatlon
Ultra-low-temperature subcategories. Miilikelvin
Mkmkelvln Nanokelvin Very low temperature
Low temperature
High temperature Very high temperature
Below mom temperature It is convenient to claasify VT MAS experiments according to temperature. Any such classification is bound to be 560A
vin and nanokelvin ranges. Such incredibly low temperatures are reached by adiabatic demametization of nucle& spins in a bloik of copper metal, which is in thermal contact with the sample. MAS experiments a t temperatures significantly below 4 K are not practical because of insulation problems and the need for a spinning gas. It is reasonable to expect that ultra-low-temperature NMR will remain the province of nonspinning experiments, which are also common in the very-low-temperature range ( P 7 7 K).For example, David Grant's group a t Utah is using matrix isolation NMR techniques in a fundamental study of the relationship between chemical shift and structure (IO). These studies are done a t approximately 20 K to prevent motional aver-
9, MAY 1, 1988
Status
Magic-angiespinning (MAS) is not leasible Wide-line experiments are used to probe ultra-low-tefnperature physical phenomena Three repats of MAS experiments at temperahues as low as 8 K.
Nonspinning experiments require standard cryogenic teohniques. MAS experiments at temperaMes below 100 K are routine in several labaatories. Many labs have 170 K capabilities. MAS experiments at 500 K are currently possible in a few labs; 400 K is far more common. MAS developmentwork is underway in several locations. Severe materlai constraints exlst abave
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aging of chemical-shift powder patterns. (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, although the expense would he manageable for laboratories that are connected to a helium recovery and liquefaction system. There are also technical problems. Helium gas undergoes an electrical breakdown when it is in contact with radio frequency (rf) coils, necessitating the isolation of gas from coil. The design shown in Figure ICisolates the gas from the coil, albeit with a loss of filling factor and hence SIN. Yannoni and co-workers have published two reports of MAS NMR experiments a t very low temperatures (11,12).Spectra were obtained a t temperatures as low as 15 K, hut spinning problems were severe for some samples. To correct spinning problems, one must he able to move the probe into and out of the magnet without interrupting 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 problem 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 coworkers performed MAS NMR experiments a t 8 K using a prohe incorporating the Ian Gay spinning system design (1.3). Their experiment was constructed around a very-wide-bore (125-mm) 1.4-T superconducting magnet. Fortbately, a temperature of 77 K is sufficient to freeze out the reactivity and dynamics of a great many chemically interesting solids, and liquid nitrogen 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 CPLMAS probe and accessories used a t 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 resistance temperature devices (RTDs) are mounted in the probe to provide temperature measurements a t key points near the sample: in the drive gas channel, approximately 5 em from the spinning system; in the bearing gas channel, a t a similar distance from the spinning system; and in the exit stack, approximately 2 cm above the spinning 562A
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Figure 2. Schematic of a double-bearing MAS probe and accessories used for lowtemperature studies. Locations of Me resistance temperaturedevices (RTDs) are shown for I b drive (D). bearing (E), 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 SIN because of rf pick-up. This sensor is lowered into position for temperature measurement immediately after data acquisition is completed. The bearing and drive sensors are well shielded from rf and remain in position a t all times. Completely independent gas streams (usually Nz) are used for hearing and drive. A series of needle valves is used to either direct the gas streams through cooling coils immersed in liquid nitrogen or bypass the cooling coils. Intermediate temveratures are most efficiently obtained by proportioning the flow between the cooline coils and bvpass loops. These valves and bypa& loops are also useful for rapid temperature increases, and they allow air to be used for ambient temperature experiments by bypassing the cooling coils. The 50-L cooling Dewar allows 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, responding to BRTD and DRTD, respectively. Ideally, the three sensors in Figure 2 should report the same temperature after the system has equilibrated for a few minutes. In practice, differences of i 1 K are typical. Failure to carefully regulate the temperature of both the bear-
ANALYTICAL CHEMISTRY, VOL. 60, NO.
9, MAY 1. 1988
ing and drive gas streams can lead to very large temperature errors (&lo K) andlor 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 SIN ratio. To a first approximation, the temperature dependence of SIN 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 temperature can be estimated with the following equation
in which K is a collection of (avvroxi. .. mately) temperatureindependent terms ( 1 5 ) . T. is the samole temoerature. T, is the tkmperaturLof the receiver coil; and p ( T J is the resistivity of the coil wire (usually made from copper) a t the temperature T,.Equation 1should he 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 tuning the probe a t extreme operating temperatures. Aside from these complications, Equation 1 does predict some interesting results. For example, the SIN a t 77 K could be as much as 13 times the value a t 298 K. Improvements in SIN are often seen in MAS
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experiments a t 77 K, but they are much less than a factor of 13 (for the abovementioned reasons). The nonmagnetic, high-power rf capacitors used in solid-state NMR probes have large temperature coefficients. If the temperature of the matching 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 variable capacitors can “freeze up” because of changing mechanical tolerances or frost formation. For these reasons, most V T MAS probes use insulation and/or purge gas streams to maintain the probe electronics at or near room temperature. Although thii practice minimizes the aggravation of retuning the probe at each temperature, keeping most of the probe electronics a t room temperature may also preclude a substantial improvement in S/N for many samples a t cryogenic temperatures.
Above room temperature Several of the projects in our laboratory involve the study of surface species a t high temperatures, and it is important to optimize the S/N in such experiments. Equation 1 predicts that the S/N will be one-third the room temperature value when both the sample and coil are a t 500 K.A simplistic application of Equation 1predicts that the loss of S/N a t 500 K would be significantly less if the coil could he kept near room temperature, but there is no obvious way to insulate the sample from the coil without reducing the filling factor and hence the S/N. With a combination of careful probe design and the use of isotopic enrichment (when necessary), the S/N a t high temperatures is usually satisfactory. At 800 K, however, the S/N will he much lower than a t 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 machined from ceramics such as alumina, zirconia, Macor, or boron nitride. Some of these materials have excellent thermal stability, but other problems complicate their use in MAS NMR experiments. First among these is the unfortunate fact that when ceramics fail, they fail catastrophically. Rotor “explosions” can occur with excessive spinning speeds, unbalanced samples, or without apparent reason. Boron nitride loses much of its strength upon prolonged exposure to atmospheric moisture, so it is prudent to keep rotors machined from this material in a desiccator when not in use. Rotors are fairly easy to replace, but rotor explosions 564A
Figure 3. ‘% CPIMAS NMR spectre
1.4-diazabicycio- [2,2,2]-octane (DABCO) at several temperatures in the vicinity of the solid-solid phase transition temperature (351 K). (Adapted wlth permissicm horn Reference 16 Copyri@ 1986 by Academic keas, Inc )
sometimes damage the coil and occasionally destroy the entire spinning system. Some ceramics are notoriously difficult to machine, so much so that vendors of solid-state NMR probes contract out their ceramics fabrication work to high-technologyceramics companies. Fortunately, boron nitride and Macor are fairly easy to work with, and some ceramic spinning system development work has been possible in academic laboratories. A further disadvantage of ceramic materials is that they sometimes exhibit unexpected background signals because of impurities or additives intentionally introduced 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 spinning systems currently in use a t Texas A&M, three are ceramic, and several of the others can accommodate hybrid rotors made from ceramics and plastics. One of our MAS spinning systems is made from Torlon, a high-temperature
ANALYTICAL CHEMISTRY. VOL. 60, NO. 9. MAY 1, 1988
engineering polymer. Although this material has unacceptable I3C, ‘5N, and ‘H background signals, it is completely 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 system to obtain high-quality 3IP MAS spectra a t temperatures as high as 523 Kwithaspinningapeedof4.5 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 advantage of that design is that it provides some thermal isolation between the sample region and the rest of the spinning 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 requirements for MAS NMR a t elevated tempera ture8. It is appropriate to discuss one final instrumental consideration in V T MAS NMR. The vacuum seals in superconducting magnets can leak a t elevated or reduced temperatures. Seal leakage will cause dramatic increases in the Nz 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 temperatures is, therefore, an important consideration when setting up a spectrometer for VT MAS studies. Wide-bore magnets provide adequate space for Dewars, insulation, and amhient-temperature 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 a t temperatures as low as 77 K or as high as 550 K before the boil-off rates become excessive. A new type of probe incorporating Dewars is expected to eliminate this problem entirely. ApplkaUOns Many of the applications of VT MAS NMR involve the study of dynamic processes such as chemical exchange or molecular motion. A conceptually simple 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 transition a t 351 K to a high-temperature plastic crystalline phase in which rapid reorientation is possible about all axes. Figure 3 shows ‘SC CP/MAS spectra of DABCO obtained a t several temperatures in the vicinity of the transition. In the low-temperature phase, the line
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1. 1988
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shape is dominated by an incompletely averaged dipolar coupling to the quadrupolar 14N nucleus (17). In the hightemperature phase, molecular reorientation is sufficiently rapid to average the “N-I3C dipolar coupling to zero, and the I3C resonance is a single sharp line. Compounds exhibiting solid-solid phase transitions are useful test Samples for exercising new VT MAS probe designs (18). More sophisticated applications of VT MAS NMR to problems of molecular motioncan heillustrated by some of our studies of polyphosphazenes, which are inorganic polymers based on chains of altemating phosphorus and nitrogen atoms. These polymers are of considerable interest because of their mechanical properties at extreme temperatures, thermal stability, solvent resistance, and potential for providing hiocompatihle materials. We are using multinuclear VT MAS NMR to study the synthesis, morphology, reactivity, and molecular dynamics of polyphosphazenes (19). Figure 4 shows variable-temperature I3C CP/MAS and 3IP MAS spectra of poly[his(ethoxy)phosphazene] (PBEP) at various temperatures. This polymer has a glasa transition (TJa t 189 K. The line shapes and intensities of the VT spectra, in conjunction with relaxation measurements (not shown), provide information about molecular motion and the morphology of the solid polymer. The I3C spectrum a t 298 K has poor
S/N.The polymer is so mobile a t this temperature that the IH-W dipolar interaction responsible for cross-polarization 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 decrease in molecular motion. Inspecting the I3C spectra in Figure 4, one sees that the line width of the CH2 resonance is strongly temperature dependent; it is sharp a t both high and low temperatures, but hroad a t intermediate temperatures. At 203 K, for example, the CH2 carbon signal is so hroad that it is not observed. Rothwell and Waugh have shown how molecular motion a t the frequency of the ‘Hdecoupling field (yB1,48 kHz in this case) accounts for this type of behavior (20). T o further investigate molecular motion in solid PBEP, 3IP MAS spectra, which are sensitive to main-chain motions in phosphazene polymers, were acquired over the &e temperature range used for the I3C data. Reducing the sample temperature produces a gradual increase in the 31P 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 31P with these experimental conditions) would he completely averaged. Upon dropping below Tg, there is a significant further increase in line width. These data indicate that the main chains of PBEP
are undergoing large-amplitude anisotropic motions above the glass-transition temperature. This main-chain motion 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 morphology of several phosphazene polymers including PBEP are being explored in experiments currently underway. The development and characterization of materials with interesting magnetic properties is an important area of solid-state chemistry. Furthermore, many inorganic compounds (including transition-metal dimers and clusters) display paramagnetism or more complex temperature-dependent susceptibilities, and their spectral properties a t 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 characterization of such nondiamagnetic solids (2123).Figure 5 presents VT CP/MAS
Figure 5. Variable-temperature ‘% CP MAS NMR spectra of the carbonyl region of samarium acetate tetrahydrate. Flgure 4. Variable-temperature (a)I3C C P l M A S and (b) 3’P MAS spectra of the inorganic polymer poly [bis(ethoxy)phosphazene] (PBEP). (Adapted ham Relaenat I9 I 566A
ANALYTICAL CHEMISTRY. VOL. 60. NO 9, MAY 1. 1988
a paramagnetic solid compound. Spinning sidebands are denoted by ssb. (A6apted with permission tmm Reference 22. Copyright 1986 by Academic RBss. 1°C.)
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Flgure 6. Variabletemperature 13C CP/MAS NMR spectra of anhydrous copper(l1) +butyrate, a solid compound exhibiting antiferromagnetic exchange coupling. C1 ref- to carboxylate carbons,C, and C,. refer la methyl carbons, and C. and C3 are interme dele methylene carbons.
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spectra ofa paramagnetic solid, samarium acetate tetrahydrate, the chemical shifts of which obey the Curie law (i.e., they are linear with 1/77 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 thermom. eter. Determining temperature is one of the most basic physical memurements. In MAS NMR, hower,er,it is not a trivial operation. The sample temperature (T.) can differ from that of the SUIrounding gas streams (especially if the sample issubject to rf heating), but the sample itself is inaccessible to thermocouplesor other sensingdevices. Determining the chemical shift of the chelating-only carbonyl resonance of samarium acetate tetrahydrate allows the sample temperature to be calculated using Equation 2
(2)
This equation is useful from I7 K to approximately 270 K, a t higher temperatures, the change in chemical shift with temperature is too small for accurate measurements. We have applied the samarium acetate tetrahydrate thermometer in a detailed study (24) of rf heating effects, thermal equilibration 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 magnetic phenomenon, antiferromagnetic exchange coupling, is illustrated in Figure 6, which shows I3C data for anhydrous Cu(I1) rr-butyrate. Solid copper (11) carboxylates have interacting pairs of metal centers. The electronic ground state has all electron spins paired and is, therefore, a singlet. There is, however, a triplet state a few hundred cm-I above the ground state. Thermal population of the upper state balanced with thermal randomization of electronspin orientation gives rise to a maximum in the susceptibility curve near room temperature. Starting with basic magnetic theory and assuming a contact shift mechanism, it can be shown that the resonance frequencies of nuclear spins in d 9 U 9 dimers should have the following temperature dependence
where -W 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 predicts the temperature dependence of the '3C chemical shifts of all of the anhydrous Cu(I1) carboxylates that we have examined t o date. Statistical analysis of 6 vs. T data allows values of -ZJ, A, and lidia to be determined for these complexes. This information sheds light on the mechanism of antiferromagnetic 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 reactions occurring in the NMR tube, and it is reasonable to expect that solid-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 l-diazo-2-propanone ( I ) to methyl ketene CH3-C-C-H
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side 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 CPIMAS spectrum of an HY zeolite sample onto which propene-2-'3C has been adsorbed (25). The NMR spectrum is consistent with extensive oligomerization of the adsorbed propene a t room temperature. More interesting, however, is the resonance a t 250 ppm, which is assigned to a carhocation 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 l3C VT MAS. In one example (261,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 '3C signals were obtained by single-pulse excitation (Bloch decay) with MAS. Cross-polarization was used to generate a l3C 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 a t high temperatures. Future developments Further refinements in VT MAS NMR will include more reliable spinning systems capable of routine operation a t 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 a t elevated temperatures. Some of the
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CiRCLE 150 ON READER SERVICE CARD
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
569A
leferemes 1) Yann0ni.C. 20148.
S.Aee. Chem.Res.1982,15,
(2) Maciel, G. E. Science 1984,226,282-88.
(3) Gerstein, B. C. Anal. Chem. 1983, 55, 781 A-790 A. (4) Gerstein, B. C . Anal. Chem. 1983, 55, 899 A-907 A. (5) Lyerla, J. R.; Yannoni, C. S.; Fyfe, C. A. Acc. Chem. Res. 1982,15,20??-16.
(6) Pines, A,; Gibby, M. G.; Waugh, J. S. J.
Chem. Phys. 1973,59,569-90. ( I ).. Gay,I. D. J . Magn.Reson. 1984,58,41320.
8) Fyfe, C. A,; Mossbrugger, H.; Yannoni,
Flgure 7. 13C CPlMAS spectrum of a n HY zeolite sample onto which propene2-% had been adsorbed a t room tem-
perature. A resonance amibutable 10 carbocatim Centers is visible at 250 ppm. Spinning sidebands are d e noted by srb. (Adapted from Reference 25.1
more interesting applications of t h e V T MAS NMR technique will probe catalytic materials a n d reactive surface species. This future work, like that which has already been done, will result from a harmonious cornhination of chemistry a n d chemical instrumentation. This article WBS made possible by support from the National Science Foundation (grant CHE8700667).
6181. (25) Zardkoohi, M.; Haw, J. F.; Lunsford,J. H. J . Am. Chem. Sac. 1987,109,527&80. (26) Haw, J. F.; Johnson, N. A. Anal. Chem. 1986,58,325&56.
C. S. J. Magn.Reson. 1979,36,6148. Kuhns, P.; Gone", 0.;Waugh, J. J . Mngn. Reson. 1987,72,548-50. (10) Facelli, J. C.; Grant, D. M.; Miehl, J. Acc. Chem. Res. 1987,20,152-58. (11) Macho, V.; Kendrick, P.; Yannoni, C. S. J. Magn. Reson. 1983,52,450-56. (12) Yannoni, C. S.; Clarke, T. C.; Kendrick, R. D.; Macho, V.; Miller, R. D.; Myhre, P. C. Mol. Cryst.Liq. Cryst. 1983, (9)
96,305-11. (13) Hackman", A,;
Seidel, H.; Kendrick, R. D.; Myhre, P. C.; Yannoni, C. S., suhmitted for publication in J . Magn. Reson. (14) Haw, J. F.; Campbell, G. C.; Crosby, R. C. Anal. Chem. 1986,58,3172-71. (15) Hoult, D. I.; Richards, R. E. J. Mngn. Reson. 1976.24,71-85. (16) Crosby, R. C: Hay, J. F.; Lewis, D.. submitted for ublicatmn in Anal. Chem. (17) Hexam, J. &Frey, M. H.; Opella, S.J. J. Chem. Phys. 1982,77,3841. (18) Haw, J. F.; Crook, R. A,; Croshy, R. C. J . Magn. Reson. 1986,66551-54. (19) Crosby, R. C.; Haw, J. F. Macromole-
James F. Haw received his Ph.D. f r o m Virginia Tech i n 1982. He is currently a n assistant professor i n the Department of Chemistry a t Texas A&M University. His research interests include the development o f solid-state NMR instrumentation and its application t o problems in heterogeneous catalysis, p o l y m e r chemistry, a n d magnetic materials research.
Ordered Media in
Chemical
14
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CIRCLE 67 ON READER SERVICE CARD ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1. 1988
T
his bwk is the
firstto offer the scientist a concise lempbon of the different roles and uses of ordered media in separabon science The past decade has Seen rapid advances in the ubiimtion of surfactant normal and reversal micelles. surfactant vesicles. and cyciodextnns in separation xience This volume will intraduce you to the differenttypes of ordered media. and familiarize you wth their use in vanous separatlon science techniques Just a few of the areas covered include 0 micellar liquid chromatography0 micellar-enhanced ultrafiltration 0 micellar electrokinetic capillary chromatography 0 extraction of biopmducts with reversed micelles 0 utilization of cyclodextrines as stationary andlor mobil-pha%components in chromatography. if you're involved in chromatography. separabon science. analytical or micellar chemistty.or bioseparations. you'll find this book the one source of information you must have to keep current in this rapidly expanding area of science Willie A. Him. and Daniel W. Armstrong. Editors 279 pager (1987) Clothbound ACS Symposium Sener No. W US 8 Canada $54.95 Expart $65.95 ic 87 13563 ISBN 0 8412 1402-6
