Instrumentation
Bernard C. Gerstein Department of Chemistry Iowa State University and Ames Laboratory, U.S. DOE Ames, Iowa 50011
High-Resolution NMR Spectrometry of Solids Part I Nuclei reside in houses of marvelous and fascinating architecture. The design of these residences is limited only by the variety of nature and by the scientist's imagination. The details of the plumbing, electricity, and house plans are responsible for processes as diverse as the response of the human eye to light and the lubrication of an automobile engine.
Ever since engaging in the first effort at materials science, the production of pottery by dehydration of kaolinite, we have been attempting to put this molecular architecture to our own use. In doing so, we have continually attempted to understand the details of the static and dynamic behavior of ensembles of nuclei and electrons in a manner that would give us
some predictive power over that behavior. Prior to the mid-1950s, the chemist spent months and sometimes years of arduous synthetic activity validating the details of a particular molecular structure. The discovery of nuclear magnetic resonance (NMR) in 1946, and the subsequent realization that the transition frequency for a nucleus in a particular chemical environ-
Figure 1. NMR powder patterns of nuclei The following interactions are present: 1) dipolar coupling between two isolated spins (a), dipolar coupling between three isolated spins (b), and dipolar coupling among many spins (c); 2) shielding anisotropy, nonsymmetric (d) and axially symmetric (e) patterns: 3) quadrupolar splitting, the central V Î - V Î transition for a spin 3/2 system (f); 4) J coupling, same form as two-spin dipolar pattern (g); and 5) lifetime broadening (h). The total spectrum of nuclei experiencing all of the above interactions will be a convolution (®) of all of the above spectra
0003-2700/83/0351-781A$01.50/0 © 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983 · 781 A
shielding anisotropies, dipolar broadening, and the current state of the art of high-resolution solid-state NMR. The second section, to be published in the July issue, will cover applications to polymers, biopolymers, fossil fuels, and technical achievements such as magic angle spinning (MAS, see below) at low temperatures, slow spinning, recovery of shielding anisotropies in systems with many chemically distinct nuclei, high-resolution NMR in systems with quadrupolar nuclei, and multiple quantum NMR. Shielding Anisotropies
Figure 2. Static powder pattern and high-resolution NMR (under CPMAS) of 1,1,1-trimethyl-2,2,2-triphenyldisilane
29
Si in
Top curve through the experimental points is least squares fit to a superposition of axially symmetric powder patterns (broad peaks, center). The high-resolution spectrum is also shown
ment is an incredibly delicate probe of that environment, added a quantitative and qualitative analytical tool of enormous power to the chemist's workshop. (The resolution attainable today corresponds to being able to distinguish from Earth two cats sitting 30 cm apart on the moon.) The use of synthesis and infrared (IR) and ultraviolet (UV) spectroscopy, in addition to NMR, made a structure determination of a molecule in the liquid state almost routine. With the advent of large, fast digital computers in the 1960s, and appropriate algorithms for the solution of the phase problem, it became possible (but not routine) to determine molecular architecture in crystalline solids for systems as large as enzymes via X-ray methods (i). A rather large and important fraction of matter, however, is neither liquid, nor nicely crystalline solid, nor amenable to structural determination by standard liquid-state or solid-state probes. Such systems are "solids" that are not in thermodynamic equilibrium with respect to phase changes (e.g., glasses and glassy polymers) or in which the concept of "macroscopic phase" has no meaning (e.g., cell membranes, biopolymers, and coals). The NMR spectra of randomly oriented solids of such materials as well as of randomly oriented crystalline
solids are generally broad and featureless as a result of four interactions: dipolar coupling, shielding anisotropy, electric field gradients in nuclei with quadrupole moments, and scalar, or J-coupling, anisotropy. In addition to the above, there is lifetime broadening. The general forms of powder spectra associated with the above interactions are shown in Figure 1. For a nucleus experiencing all of the above broadening interactions, the total powder spectrum is a convolution (®) of all the above possible spectra. One of the real triumphs of high-resolution solid-state NMR is the deconvolution of such spectra so that the chemical and physical information available from each interaction is obtained. Dipolar interactions yield internuclear distances and molecular geometries. Shielding anisotropies give information on coordination symmetries and relative nearest neighbor distances. Quadrupole splittings yield the electric field gradient at the nucleus and thus are a probe of molecular wave functions. Scalar coupling is also a probe of molecular wave functions. Because of the explosive growth of applications of solid-state NMR to chemistry, it is not possible to adequately cover all developments in one article. For this reason, this feature will be published in two sections. The first will cover introductory concepts,
782 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
The additional information available from solid-state NMR spectra comes as a blessing and a curse. The blessing is that more information is available for the chemist to correlate with trends in reactivity etc., so that we have more predictive power over our environment. The curse is that we must learn some new and perhaps unfamiliar concepts to be able to use this information. One of the new concepts needed to use this information is that of a shielding "powder pattern." The basic idea is that a single chemical species, such as hydrogen, which exhibits one sharp NMR line in a liquid (ignoring scalar coupling), exhibits a spectrum in a powdered solid that can be as wide as 100 ppm (2), which is about 10 times the total range of isotropic chemical shifts of protons in organic liquids. It is important to understand at this point that the line broadening represented by shielding anisotropy is an inhomogeneous broadening associated with the fact that it is a superposition of spectra of randomly oriented individual nuclei. Each of these nuclei has an inherently sharp line for a particular orientation of nuclear environment with respect to the static field. This fact will be important later when we consider spectra obtained under MAS slower than the shielding anisotropy. For example, in the case of 29 Si in trimethyl triphenyldisilane, the isotropic and anisotropic values of the shielding tensors of both silicons in this molecule have been determined in our laboratory. Figure 2 shows the high-resolution spectrum of the solid, obtained using combined cross polarization (CP), strong proton decoupling, and MAS, and the powder pattern of a nonspinning (static) sample fitted to a superposition of two axial powder patterns. Both shielding environments are axially symmetric. A fascinating feature of the shielding symmetries is that they are inverted with respect to each other, i.e., one shielding tensor is oblate, the other prolate. The shielding anisotropy of the silicon attached to the methyl groups is 18 ppm. Similarly, the an-
The Annual ACS LabGuide— the Definitive Source for • Laboratory supply houses • Sources for chemicals and instrumentation • Meeting information • Research and analytical services • Trade names and new books
Figure 3 . High-resolution NMR spectrum of protons in maleic acid ( 17) obtained under conditions of combined sample rotation and multiple-pulse proton-decoupling spectroscopy
With the annual ACS LabGuide, the information is right where you need it—at your fingertips. It's most used because it's most useful! Use the coupon below to order your copy today. Distribution Center American Chemical Society 1155 Sixteenth Street, N.W Washmston, D.C. 20036 YES! Send me the annual ACS LabGuide. am enclosing S6 for each copy ordered. Quantity
Total
Price
California residents add 6% state use tax.
Name.
Address.
City.
State .
-Zip.
isotropy of the other silicon is a negative 31 ppm. Note that, in addition to an isotropic value of a shielding, the chemist now has a symmetry, a sign, and a magnitude with which to characterize a shielding environment. The amount of available information from a high-resolution solid-state NMR measurement of a powder has been multiplied by a factor of four relative to a measurement of a liquid! There are three important points in the above discussion. The first is that "nonstandard" (in general, low abundance) nuclei such as 29 Si are readily measured with sensitivity enhancement techniques available to the solid-state NMR spectroscopist. The second is that even with severely overlapping tensor powder patterns, individual shielding anisotropies may be obtained via a number of available techniques developed recently. Third, certain nuclei, such as u 3 C d , have isotropic and anisotropic shieldings that are relatively very sensitive to local environments. This fact promises to be of great use in studying active sites in enzymes in which metal ions may be replaced by Cd 2 + . A detailed list of 113 Cd chemical shifts and some anisotropies of Cd in solids has been published by the author (3) and by Mannitt et al. (4). In addition to the added fingerprint of shielding anisotropies, it also becomes necessary to learn how this information can be used to infer details of molecular behavior not readily available from high-resolution liquidstate NMR. We have seen that a nucleus in a particular liquid-state envi-
784 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
ronment has a characteristic isotropic shielding. In the solid, the anisotropy of the shielding is added information. However, in a solid in which there is nonisotropic motion, the above information becomes modified in a predictable manner. Examples of such solids are liquid crystals, membranes, and mobile surface-deposited species. A part of our subsequent discussion will deal with the effect of motion on the shielding powder pattern. Changes in the powder pattern characterizing the rigid lattice state of the system and the "motional" state can help to characterize both frequency and amplitude of motion in favorable cases. Dipolar Broadening
Thus far, we have discussed some of the features of solid-state NMR that make attainable results exciting for the chemist. We now discuss one of the reasons why standard liquidlike techniques applied to solids yield relatively little information. In addition to the shielding anisotropy discussed above, there are two major broadening interactions for the NMR of solid-state species. These are dipolar interactions between different nuclei, a many-body phenomenon, and, for non-spin-V2 nuclei, the quadrupolar interaction in a single spin of nuclei such as 14 N. The anisotropy of both interactions can be described in tensor form (5). We defer discussion of quadrupolar species until later and concentrate here on the utility and removal of dipolar interactions between spin-V2 nuclei, e.g., between XH and 13 C. The premiere fact to be under-
Baseball Shifts S Vkem£st~Vées Our 50% cotton/50% polyblend in 3 styles to unleash the chemist (and baseball player) in you! R e d a n d b l a c k print o n a gray shirt
1 Multicolored • ancient 'elements" on ΐ a sandy- t o n e °»«ηιι*«ΜΒ3·1 shirt
W h i t e shirt, navy s l e e v e s , navy a n d red print. T-shirt b l u e a n d red o n a light b l u e shirt
Sizes: Small (34), Medium (36 38). Large (40- 42). Extra-Large (44) Price: T-shirts 1 to ,'i $7 50 uach. 4 to 9 shirts S7 00 each. 10 or more So 95 each Baseball shirts • 1 to -i $9.00 each. 4 to 9 $tf 50 each. 10 or mora $7 95 each Postage and handling: Si 90 per order Foreign shipment surcharge:Canada SO. 75 per shin, other countries ^1 75 per shirt Payment: Use VISA or MasterCard or make checks payable to the American Chemical Society: orders must be prepaid Send to: Department of Educational Activities, American Chemical Society, 1155 Sixteenth Street, N.W., Washington, D.C. 20036, U.S.A. Style
S
M
L
XL
Unit Cost
Total
Chemistry Matters—T-shirt Experiment with a Chemist—T-shirt Chemists have Solutions—T-shirt Experiment with a Chemist—Baseball shirt Subtotal M a s t e r C a r d ™r.
In Calif. 6% state tax $1.90
Postage and handling Fvpiratinn Hate
Foreign shipment TOTAL
VISA n n
Expiration date . Name Address .
fStreetJ
(State) Please allow 6—8 weeks for delivery
fCity; (Zip)
786 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
(Country)
stood in this context is that a spin-1/^ nuclear moment, 1%, produces a mag netic field (the dipole field), Bo, which has the same symmetry and spatial dependence as the magnetic field of a current through a loop of wire or the familiar pattern seen in iron filings arranged about a bar mag net. This magnetic field adds, or sub tracts (since I2 can align with or against the dc field), from the static field seen by a neighboring nuclear magnet, Ιχ, producing the energy level scheme and spectrum for Ιχ that splits the single Zeeman line into a doublet (the Pake doublet). The dipolar splitting, Δ / D , is depen dent upon the length of the internuclear vector, r , and the angle between the applied field, BB, and the internuclear vector, but not upon the magni tude of B0. This means that a mea surement of Δ / Β as a function of Θ, ob tained by rotating a single crystal or from an NMR powder pattern of the spectrum of an isolated pair of spins, will yield the internuclear distance r. An example of such isolated pairs of spins is Ή in CaS0 4 -2H 2 0 (6). This means that for an isolated pair of spins, NMR in the solid state has the capability of probing internuclear dis tances with about the accuracy inher ent in an X-ray diffraction measure ment. Unlike X-ray diffraction, how ever, there need not be a minimum crystalline size present for such a mea surement. This means that structures of surface-adsorbed species can in principle be determined by NMR, even though they would be unattain able by X-ray diffraction. Extended X-ray absorption fine structure (EXAFS), of course, also offers this possibility. Another way of viewing this struc tural information attainable by NMR (which is not at all high resolution) is that for an isolated pair of spins, a sin gle spin, Ιχ, will precess in the dipolar field supplied by another spin, S, with a dipolar frequency that adds and subtracts from the Zeeman frequency, f0. A measurement of this additional dipolar frequency can yield the molec ular geometry. The broad NMR line associated with a powder containing isolated spin pairs is, like the shielding powder pattern, an inhomogeneously broadened line. Nature, however, seldom gives with out taking away. There are very few solids with isolated pairs of nuclear spins. This means that not only is the energy spectrum of spin Ιχ split into a doublet by the presence of the field of I2 or S, but it is also further split by the presence of 10 23 additional Iz's or S^s. The resulting energy spectrum then becomes a rather broad feature less band of what is essentially a con tinuum of levels. The lines of each
Rotations in Spin Space and in Real Space
Figure 4. 29 Si- 1 H CP spectrum of
29
Si in an amorphous silicon-hydrogen alloy
Top, static. Bottom, CPMAS. Comparison of this result in a glassy material with the result in a nicely crystalline material is an indication of the extremes of resolution expected for solid-state studies
spin have been homogeneously broadened. The NMR absorption spectrum is then a featureless broad band, about 40 kHz wide for protons in the presence of many other protons or for carbon in the presence of many protons. Indeed this width, in terms of the second moment (7), contains average structural information, but the nicety of detail in the case of isolated pairs of spins has been lost. It is to be noted, in addition, that even in solids with isolated pairs of spins, the presence of anisotropic shielding leads to confusion, such as in the simple spectrum of Figure 3. For 13 C, the shielding anisotropy can be comparable to the magnitude of dipolar broadening between isolated C-C spin pairs, so the confusion in this case is particularly severe. Yannoni and Kendrick (8) and Zilm and Grant (9) have recently shown means of removing the shielding anisotropy broadening from the response of pairs of 13C nuclei in solid organic molecules to specific rf pulse excitations, while maintaining information on the dipolar splitting. The resulting dipolar powder patterns yield the C-C internuclear distances to an accuracy of 0.005 Â and can be used to orient dipolar vectors with respect to shielding tensors. Thus, internuclear distances of surface-adsorbed species may be determined. One of the triumphs of recent advances in high-resolution solid-state NMR has been that in systems of
many (1023) coupled spins, the dipolar frequencies of individual hetero-spinV2 pairs (e.g., 13C coupled to 1 H in a system with many protons) may be measured, and molecular geometries can be determined. Among the major advances in solidstate NMR has been the invention of time-dependent techniques to selectively remove one or more of the many interactions that nuclei see in a solid. It is an interesting fact that in 1950, the same year as the Proctor-Yu discovery of chemical shifts, Erwin Hahn (10) laid the basis for what we now call high-resolution solid-state NMR with the discovery of the spin echo. This experiment was an example of the decoupling of a nuclear moment from external field inhomogeneities and from spins of different (heteronuclear) species. Eight years later Lowe (11) and Powles and Mansfield (12) realized that dipolar interactions of spins of the same type (e.g., l H in the presence of 1H) could be decoupled with the attainment of the solid echo. The theory and techniques of achieving high-resolution NMR in strongly dipolar-coupled spin-1/^ systems (e.g., 13 C coupled to Ή , or lH coupled to l H, but not 14 N coupled to Ή ) have been extensively developed, as recent ly reviewed (13), to achieve resolution of NMR signals in solids approaching that in liquids. Recent advances in homonuclear decoupling include im proved pulse sequences (14) and windowless sequences (15).
788 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
Homonuclear Dipolar Broadened Systems. A recent discussion of con siderations pertinent to attainment of high-resolution NMR for randomly ordered strongly homonuclear-coupled dipolar systems has been given (13)—both with regard to salient points of the theory and to experimen tal details—so no more than a brief summary will be presented here. The major broadening interactions consid ered in this case are shielding anisot ropy and homonuclear dipolar cou pling. Strong rf pulse decoupling (rotations in spin space) is used to at tenuate the dipolar broadening, leav ing shielding anisotropy as the major residual broadening interaction. Re moval of the shielding anisotropy is accomplished with a rotation in real space, known as magic angle spinning (16). Minimum requirements are rf powers of at least 400 W (1 kW is com fortable), with a broadband transmit ter being desirable, but not absolutely necessary, to obtain minimum inter ference from transients inherent in tuned circuits. The author's experi ence has been with protons at 56 MHz, but there is at least one com mercial spectrometer capable of per forming these experiments at 200 MHz, and the Jena group (17) has re ported combined homonuclear de coupling and magic angle spinning at 270 MHz for protons, with the resolu tion for protons in maleic acid shown in Figure 3. Easily attainable spinning speeds of 3 kHz are sufficient to avoid the artifact of sidebands, but in fact the problem of sideband interference may be avoided in other ways. How ever, sidebands contain useful infor mation that may dictate the use of spinning speeds that are small com pared to the shielding anisotropy. MAS at frequencies larger than the shielding anisotropies will avoid spin ning sidebands within the spectral width of interest. For protons, this means frequencies generally larger than 15 ppm or greater than about 3 kHz at 270 MHz. There are many descriptions of de vices for achieving MAS. Basically they are all variations of the early con ical designs of Beams (18) and the cy lindrical configuration used by Lowe (19). Designs for achieving the homo geneous rf field necessary for homonu clear decoupling experiments have been published by the author (20) and by the Jena group (17). One of the most important contri butions to the understanding of rotor stability is that of Doty and Ellis (21 ). The reader is referred to that paper for some earlier designs in the field. A major contribution of the Doty-Ellis
work is t h e recognition t h a t r o t o r s ex h i b i t r e s o n a n c e frequencies (conical a n d cylindrical) b e y o n d which it is h a r d t o d r i v e t h e m . In t h e p a s t , r o t o r s have b e e n d r i v e n t h r o u g h t h e s e reso n a n c e s by m e a n s of physical stabiliza tion w i t h a c o t t o n s w a b , a fingernail, or by s o m e o t h e r a r t i s t i c s u b t e r f u g e . W i t h t h e p u b l i c a t i o n of t h e D o t y - E l l i s work, m u c h of t h e magic h a s b e e n re m o v e d from t h e field. M a c h i n i n g tol e r a n c e s o n s t a b l e cylindrical r o t o r s sufficient t o raise t h e r e s o n a n c e frequencies as high as possible a r e very t i g h t , however, a n d r e q u i r e m a t e rials s u c h as m a c h i n a b l e c e r a m i c for s t a t o r s . I t is t h e a u t h o r ' s preference t o use t h e B e a m s design or a v a r i a n t for m o s t work involving p r o t o n s , b e c a u s e of t h e relatively forgiving n a t u r e of such systems to machining errors. M o s t c o m m e r c i a l designs p r e s e n t l y u s e a v a r i a t i o n of t h e conical B e a m s configuration, b u t D o t y is p r o d u c i n g cylindrical r o t o r s c o m m e r c i a l l y . Of particular interest among rotor-stator s y s t e m s for M A S a r e t h e conical d e signs of Zilm e t al. (22), with achiev able r o t o r frequencies of 10 k H z , a n d of Fyfe e t al. (23), which allows M A S a t t e m p e r a t u r e s as low as 20 K. T h e critical r e a d e r m i g h t ask a t t h i s p o i n t , " W h y r o t a t i o n s in s p i n s p a c e and in real s p a c e ? W h y c a n ' t o n e re m o v e shielding a n i s o t r o p y and d i p o l a r i n t e r a c t i o n s by j u s t s p i n - s p a c e pulsing or b y j u s t real-space s p i n n i n g ? " In fact, b o t h a r e possible! I t m i g h t h e l p t h e r e a d e r t o realize t h a t radiofrequency pulse sequences t h a t remove s h i e l d i n g anisotropics also r e m o v e s h i e l d i n g differences. T h e differences in isotropic values of s h i e l d i n g of all c h e m i c a l species of a p a r t i c u l a r nucle us a r e r e m o v e d , so m u c h chemically d e s i r a b l e i n f o r m a t i o n is lost. P u l s e se q u e n c e s t h a t r e m o v e b o t h d i p o l a r cou pling a n d shielding a n i s o t r o p i c s , how ever, (13) a r e used t o reveal r e s i d u a l b r o a d e n i n g d u e t o lifetimes a n d a r e an important probe. MAS, at a rotational frequency g r e a t e r t h a n e i t h e r t h e di p o l a r i n t e r a c t i o n or t h e shielding a n isotropy, will remove b o t h b r o a d e n i n g i n t e r a c t i o n s . However, easily achiev a b l e s p i n n i n g frequencies d o n o t ex ceed 4 k H z , a n d dipolar line w i d t h s c a n be as large as 40 k H z . H e n c e t h e n e e d for t h e c o m b i n a t i o n of r o t a t i o n s in s p i n a n d real space t o achieve re m o v a l of b o t h dipolar a n d shielding anisotropy broadening. R a r e N u c l e i : S p i n '/ 2 . High-resolu t i o n N M R of l o w - a b u n d a n c e nuclei s u c h a s 1 1 3 Cd or 1 3 C generally involves a c o m b i n a t i o n of r o t a t i o n s in spin a n d real s p a c e . T h e m o s t c o m m o n l y used c o m b i n a t i o n is d e t e c t i o n of t h e r a r e s p i n , u s i n g polarization transfer from t h e a b u n d a n t s p i n , a n d M A S (24, 25). S h i e l d i n g a n i s o t r o p i c s of r a r e s p i n s a r e generally m u c h higher t h a n t h o s e
of p r o t o n s , so if o n e wishes t o avoid dealing w i t h s p i n n i n g s i d e b a n d s it is necessary t o realize t h a t shielding a n isotropics scale as t h e dc field. T h i s m e a n s t h a t a I 3 C shielding a n i s o t r o p y t h a t is 5 k H z a t a c a r b o n N M R fre q u e n c y of 14 M H z will b e c o m e 21 k H z a t a c a r b o n frequency of 60 M H z ! In fact, as previously m e n t i o n e d , one d o e s n o t necessarily wish t o avoid spinning sidebands, and there are m e a n s of dealing with t h e m a t spin ning s p e e d s m u c h lower t h a n half t h e a n i s o t r o p y ; t h e s e will be discussed in P a r t II u n d e r " S l o w S p i n n i n g . " A general discussion of t h e limits of resolution of 1 3 C N M R signals in sol ids h a s b e e n p r o v i d e d by G a r r o w a y e t al. (26). D o m i n a n t l i n e - b r o a d e n i n g m e c h a n i s m s (ignoring, for t h e m o m e n t , t h e p r e s e n c e of q u a d r u p o l a r nuclei) a r e identified as a n i s o t r o p i c m a g n e t i c susceptibilities, m a g n e t i c inequivalencies p r e s e n t in solids b u t n o t in liquids, a n d m o t i o n . R e s o l u t i o n s of 0.1 p p m h a v e b e e n a t t a i n e d for 1 3 C in s o m e nicely crystalline solids, b u t res o l u t i o n s of 0.2-0.7 p p m a r e c h a r a c t e r istic of crystalline solids a n d b e c o m e worse by a factor of 10 for glassy poly m e r s . A p a r t i c u l a r l y i m p o r t a n t p a r t of t h i s discussion involves t h e influence of dc field s t r e n g t h on resolution. Some broadening interactions are found t o increase with m a g n e t i c field, so larger ( a n d m o r e expensive) fields a r e n o t d e s i r a b l e for all possible sys tems under study. A p r i m e e x a m p l e of t h e t w o ex t r e m e s of resolution for r a r e spin-V2 nuclei in t h e solid s t a t e is supplied by N M R s p e c t r a of ^ S i in crystalline 1,1,1 -trimethyl-2,2,2-triphenyldisilane ( F i g u r e 2), as c o m p a r e d w i t h 2 9 Si in an a m o r p h o u s s i l i c o n - h y d r o g e n alloy ( F i g u r e 4) (27). T h e high-resolution s p e c t r u m of silicon in t h e crystalline c o m p o u n d h a s a line w i d t h of a b o u t 0.2 p p m . O n t h e o t h e r h a n d , t h e highresolution s p e c t r u m of 2 9 Si in a m o r p h o u s silicon alloys is found t o have a line w i d t h of a b o u t 40 p p m , with s o m e d i s c e r n a b l e s t r u c t u r e p e r h a p s assign a b l e t o silicon, or t o S i H r g r o u p s . In o n e case, t h e r e is little dispersion of local g e o m e t r y a b o u t silicon, a n d t h e crystalline c o m p o u n d yields a s h a r p s p e c t r u m . In t h e case of t h e a m o r p h o u s glassy m a t e r i a l , t h e e n o r m o u s d i s p e r s i o n of g e o m e t r i e s a n d d i s o r d e r i n h e r e n t in t h e glassy s t a t e lead t o rel atively severe b r o a d e n i n g . S o m e w h e r e b e t w e e n t h e s e two limits will lie possi bilities for resolution of solid m a t e r i a l s of i n t e r e s t t o c h e m i s t s . T h e above dis cussion d o e s n o t t a k e i n t o a c c o u n t t h e case of h e t e r o n u c l e a r dipolar i n t e r a c tions with q u a d r u p o l a r nuclei, s u c h as 14
N and
35
C1.
References (1) Hackert, M. L.; Ford, G. C ; Rossman,
790 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
M. G. J. Mol. Biol. 1976, 78, 665-73. (2) Murphy, P. DuBois; Gerstein, B. C. J. Chem. Phys. 1979, 70, 4552-56. (3) Murphy, P. DuBois; Stevens, W. C ; Cheung, T.T.P.; Lacelle, S.; Gerstein, B. C.; Kurtz, D. M. J. Am. Chem. Soc. 1981,103,4400-4405. (4) Mannitt, G.; Shatlock, M. P.; Bartuska, V. J.; Maciel, G. E. J. Phys. Chem. 1981, 85, 2087-91. (5) Mehring, M. "High Resolution NMR Spectroscopy in Solids"; Springer Verlag, 1976. (6) McKnett, C. L.; Dybowski, C. R.; Vaughan, R. W. J. Chem. Phys. 1975, 63, 4578-81. (7) Van Vleck, J. H. Phys. Rev. 1948,14, 1168-83. (8) Yannoni, C. S.; Kendrick, R. D. J. Chem. Phys. 1981, 74, 747-J9. (9) Zilm, K. W.; Grant, D. M. J. Am. Chem. Soc. 1981,103, 2913-22. (10) Hahn, E. L. Phys. Rev. 1950,80, 580-94. (11) Lowe, I. J. Bull. Am. Phys. Soc. 1957, 2, 344. (12) Powles, J. G.; Mansfield, P. Phys. Lett. 1962,2, 58-59. (13) Gerstein, B. C. Phil. Trans. Roy. Soc. London 1981, Λ299, 521-46. (14) Burum, D. P.; Rhim, W. K. J. Chem. Phys. 1979,77,944-56. (15) Burum, D. P.; Linder, M.; Ernst, R. R. Bull. Magn. Res. 1980, 2, 413. (16) Andrew, E. R. Prog. Nuc. Magn. Res. Spectrosc. 1971,8,1-40. (17) Scheler, G.; Haubenreisser, V.; Rosenberger, H. J. Magn. Res. 1981, 44, 134-44. (18) Beams, J. W. Rev. Sci. Instr. 1930, 1, 667-71. (19) Lowe, I. J. Phys. Rev. Lett. 1959, 2, 285-87. (20) Pembleton, R. G.; Ryan, L. M.; Ger stein, B. C. Rev. Sci. Instr. 1977, 48, 1286-89. (21) Doty, F. David; Ellis, Paul D. Rev. Sci. Instr. 1981,52, 1869-75. (22) Zilm, K. W.; Alderman, D. W.; Grant, D. M. J. Magn. Res. 1978,30, 563-70. (23) Fyfe, C. Α.; Mossbrugger, H.; Yanno ni, C. S. J. Magn. Res. 1979,36, 61-68. (24) Pines, Α.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973,59, 569-95. (25) Schaefer, J.; Stejskal, E. O. J. Am. Chem. Soc. 1976, 98, 1031-32. (26) Garroway, A. N.; VanderHart, D. L.; Earl, W. L. Phil. Trans. Roy. Soc. Lon don 1981, A299, 609-28. (27) Reimer, J. Α.; Murphy, P. DuBois; Gerstein, B. C ; Knights, J. C. J. Chem. Phys. 1981, 74, 1501-3.
Bernard Gerstein is professor of chemistry at Iowa State University, where he earned his PhD in 1960. His research interests include applica tions of pulsed magnetic resonance to chemistry; heterogeneous catalysis on semiconductors and insulators; and chemical structures and anisotropic motion in molecular solids, polymers, and biopolymers.
