Anal. Chem. 1988, 60, 1743-1750
pedance transformer. That is, the change required in the value of the much smaller lumped notch tuning resistance (R,) to reestablish the preexposed, optimally tuned state is comparable with the change observed in the notch depth and in the film’s resistance (R). When carefully thermostated and operated in a regulated air-flow environment, the sensor is stable and variations in the notch depth and frequency average less than 0.6% when continuously operated over a 3-day period. While a 0.1 ppm DIMP concentration can be resolved, measurements for concentrations greater than l ppm are highly repeatable. Although the sensor’s support electronics are more complicated compared to the conventional direct current impedance bridge technique, the required technology can be realized with current microelectronics capabilities. By fabricating the sensor using monolithic silicon technology, a diffused resistor (heat source) and diode (temperature sensor) combination could be configured to provide the critical thermostated environment. Therefore, this sensor concept offers significant promise as an alternative technology for detecting a host of gaseous contaminants when coupled with an appropriate chemically active film. An array of discrete sensors, each having a different chemically active film, could be fabricated and electronically multiplexed to measure different species. A dedicated microprocessor could be employed to process the results and enhance the specificity of a measurement. Finally, the incorporation of a common reference sensor that is shielded from DIMP exposure would permit differential measurements and result in improved signal-tonoise performance.
1743
Registry NO.DIMP, 1445-75-6;CU,7440-50-8; CUO,1317-39-1.
LITERATURE CITED Kolesar, E. S.,Jr.; Walser, R. M. Anal. Chem., preceding paper in this issue. Morris, J. E.; Coutts, T. J. Thin SolidFilms 1977, 4 7 , 3-47. Boella, M. Ana Freq. 1934, 3, 728-730. Howe, G. W. 0. Wireless Engineer 1940, 77, 471-473. Humphrey, J. N.; Lummis, F. L.; Scanlon, W. W. fhys. Rev. 1953, 90, 111-114. Hill, R. M. ffoc. R . Soc. London, A 1969, 309, 397-411. Chasmar, R. P. Nature (London) 1948, 161 281-282. Crowell, A. D.; Deshpande, S. M.; Juenker, D. W. fhysica (Amsterdam) 1969, 4 4 , 614-617. Joglekar, A. V. J. fhys. D 1974, 7 , 270-275. Offret, P. M.; Vodar. M. B. J. fhys. Radium 1955, 77, 237-240. Hlrsch, A. A.; Bazian, S. fhysica (Amsterdam) 1964, 30, 258-263. Maissel, L. I.“Electrical Properties of Metallic Thin Films”; Handbook of Thin Film Technology; Maissel, L. I., Giang, I?.,Eds.; McGraw-Hill: New York. 1970. Deshpande, S. M.; Croweii, A. D. J. Vac. Sci. Techno/. 1972, 9, ~
w -. - i n i
Springett, B. E. fhys. Rev. Lett. 1973, 37, 1463-1465. Springett, B. E. J. Appl. fhys. 1973, 4 4 , 2925-2930. Kaufman, W. M. R o c . IRE 1960, 4 8 , 1540-1548. Koiesar, E. S., Jr. Ph.D. Dissertatlon, May 1985, available from NTIS as AD A158181 and through University Microfilms International, Ann Arbor, MI. (18) Levenburg, K. Q. Appl. Math. 1944, 2 , 164-173. (19) Marquardt, D. W. J. SOC. Ind. Appl. Math. 1964, 7 7 , 431-437. (20) Nielson, K. L.; Goidstein, L. Math. fhys. 1947, 26, 120-124.
RECEIVED for review October 27, 1987. Accepted April 25, 1988. The authors wish to acknowledge the financial support for this work provided by the United States Air Force, Air Force Systems Command, Aerospace Medical Division, School of Aerospace Medicine, Brooks Air Force Base, TX, under Contract F33615-83-K-0610.
Combined Rotation and Multiple Pulse Spectroscopy as an Analytical Proton Nuclear Magnetic Resonance Technique for Solids Charles E. Bronnimann, Bruce L. Hawkins, Ming Zhang, and Gary E. Maciel* Department of Chemistry, Colorado State University, Ft. Collins, Colorado 80523
’H nuclear magnetlc resonance (NMR) experlments, using the CRAMPS (comblned rotation and muitlple pulse spectroscopy) technique, have been carried out on a wlde range of samples at 187 MHz. Optlmal performance Is made possible by modlflcatlonsand components in the radio frequency circuitry that are presented. Both qualitative and quantitative aspects of the technique are examined by using powdered crystalline compounds, polymers, mlxtures, hlgh-surfaceareasillcas, and organic geochemical samples. The utility and power of ‘H CRAMPS NMR for chemical anaiysls are demonstrated and assessed.
Nuclear magnetic resonance (NMR) has evolved into one of the most pervasive spectroscopic approaches for both qualitative and quantitative chemical analyses (1-6) and has in recent years emerged as the basis for important imaging techniques, especially in medicine (7). Most of the impact of NMR in chemical analysis has, until rather recently, been
based on NMR signals of species in liquid samples. During the past few years, the advent of such line-narrowing techniques as magic-angle spinning (MAS) has extended the analytical applicability of NMR to a wide range of solid samples (8-12). For cases of isotopically or spatially dilute spin-1/2 nuclides, e.g., 13C, 29Si,15N, and 31P, the main obstacles to obtaining high-resolution NMR spectra of solids have been inefficient spin-lattice relaxation, dipolar interactions between the dilute nuclei and protons, and chemical shift anisotropy (CSA). These influences are now routinely overcome by cross polarization, high-power proton decoupling, and magic-angle spinning, respectively (8-12). These same kinds of techniques have also proven to be useful in obtaining relatively sharp NMR peaks from the central transitions of nuclei with n / 2 spin, where n is an odd integer, e.g., 27Al(9, 13, 14). A major analytical deficiency of NMR, and a corresponding frustration throughout the past 3 decades of NMR developments, has been the absence of a technique capable of providing high-resolution ’H NMR spectra of solids. Unlike the
0003-2700/88/0360-1743$01.50/0 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
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case of isotopically dilute spin-1/2 nuclei, for protons the dominant line broadening mechanism in the 'H magnetic resonance of solids is normally the 'H-'H dipolar interaction. Consequently, to obtain narrow 'H resonances of solid samples, one must eliminate the line broadening effects of the homonuclear dipolar interactions while the spectral manifestation of the chemical shift interaction is retained. As one wishes to observe 'H NMR signals, straightforward 'H decoupling techniques, as used to decouple heteronuclear dipolar interactions, cannot be employed. The line-broadening effects of homonuclear 'H-'H dipolar interactions can in principle be removed by magic-angle spinning, if the spinning speed is high enough. If the protons in a sample are sufficiently dilute, then usable spectra can be obtained with even moderate MAS speeds (15). However, even the 20-23 kHz MAS speed that has recently been reported (16, 17) is not sufficient for typical solids. It has been known for several years that line narrowing of the homonuclear dipolar interaction can also be achieved by multiple-pulse techniques (18-21) and can be combined with MAS for removing the chemical shift anisotropy (CSA). This combination (22, 23) is referred to as CRAMPS (combined rotation and multiple pulse spectroscopy). The CRAMPS approach, shown pictorially in Figure 1,has been one of the most difficult and elusive of the high-resolution NMR techniques for solids that have been of widespread interest during the past few years. This elusiveness grew out of the technical difficulties associated with its strict requirements on the widths, shapes, phases, and intervals of the high-power radio frequency (rf) pulses employed. Recent technical advances and concentration on the problem have overcome these difficulties and resulted in a more-or-less
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high-resolution 'H CRAMPS capability in a few laboratories (22-27). This article explores the suitability of 'H CRAMPS NMR as an analytical technique for solids.
EXPERIMENTAL SECTION NMR Experiments. The 'H CRc\MF% NMR spectra reported here were obtained at 187 MHz on a severely modified Nicolet NT-200 spectrometer. A block diagram of the modified system is presented in Figure 2. In order to augment the pulse programming capabilities of the Nicolet 293A' and achieve the fast looping required for the multiple-pulsesequence, we have employed the AdNic Box (AdNic Products, LaFayette, NY), which controls the NMR experiment during the data acquisition period, sending pulse and phase control signals to the phase shifter and driving the analog to digital (A/D) converter. The initial phase-shifting and rf pulse-generating circuitry of the NT-200 was replaced with a unit, shown in Figure 3, that provides fine tuning of the four phases and pulse widths. A 187 MHz rf output from the phase shifter unit (at about 0.5 V peak to peak) is amplified to about 80 W by a broad-band EN1
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
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550L amplifier and then to 400-500 W by a modified E T 0 amplifier (Erhorn Technological Operations, Canon City, CO) and delivered to the probe via a matching network designed to reduce phase transients (28). A two-stage GaAs FET preamplifier provides 40-50 dB gain with fast response, approximately 1 ps recovery from pulse overload. The rf signal from the preamp is filtered, amplified, and mixed directly to audio in a single-conversion receiver, shown schematically in Figure 4, prior to digitization. Although other multiple-pulse sequences, most notably the MREV-8 sequence (20),have also been used successfully, the most reliably successful results have been obtained in our laboratory with the BR-24 sequence (29)and it is this sequence that produced the spectra shown in this paper. The multiple pulse cycle time ranged from 108 to 144 ps, corresponding to a pulse spacing, T of 3-4 ps. Optimum repetition delays are determined by probe heating and the appropriate 'H TIvalues. For the majority of samples the repetition delays are fixed at 2 5 s by rf heating of the probe. If relaxation is found to be slow over a 5-9 time period, the repetition rate is decreased until the signal is not saturated after a few (14) scans. In the unusual cases in which proton spin diffusion is inefficient in equalizing all proton T1values, such as in physical mixtures, then the proton T1values of the various components were measured by an analogue of the Freeman-Hill modification (30)of the standard inversion recavery experiment, using CRAMPS during the detection period. The repetition delay was then set to at least three times the largest Tlvalue. Chemical shifts were measured by substitution of samples containing tetrakis(trimethylsily1)methane( " M S M ) and are reported relative to tetramethylsilane (TMS); we estimate the accuracy of these values to be *0.3 ppm. Magic-angle spinning employed a spinner based on the design of Gay (31)and operated at speeds of 1.C-3.5 kHz. Samples, typically 1C-20 mg, were contained in thick-walled 5-mm 0.d. (2 mm i.d.) glass tubes, which can be sealed under vacuum if desired. Samples. Adipic acid, benzoic acid, malonic acid, and 4,4'dimethylbenzophenone were obtained as powdered solids from Eastman Kodak. Tartaric acid was obtained from Fisher Scientific. Durene and dimethyl terephthalate were obtained from Aldrich. The polystyrenesulfonicacid was obtained from Rohm and Haas (Amberlist No. 15). The phenolic resin was a resol resin obtained from the Forest Products Laboratory, Madison, WI. Other polymer samples were obtained from Polysciences, Inc. The silica gel was Fisher No. S-679. RESULTS AND DISCUSSION 1. General Features of C R A M P S . The multiple-pulse sequence at the heart of a CRAMPS experiment is preceded by a delay period during which spin-lattice relaxation of the nuclei to be observed is allowed to occur, ideally to a condition of thermal equilibrium. The relaxation period is followed by a single preparation pulse. Hence, the intensities obtained from 'H CRAMPS spectra should be proportional to the spin concentrations relevant to the corresponding species and
1745
should be capable of providing analytical quantitation. In this sense, 'H CRAMPS NMR can avoid some of the uncertainties that often plague MAS NMR data obtained via cross polarization (8-12 32). However, in order to achieve this quantitative behavior, in an absolute sense, it is necessary to employ sufficiently long delay periods between repetitions of the multiple-pulse sequence that essentially complete spin-lattice relaxation occurs during the delay. This is a cumbersome requirement for some types of samples, especially powdered crystalline materials (vide infra). However, if one is content with quantitative accuracy in relative intensities for peaks in the spectrum of a given substance, then this constraint on the repetition rate can be removed if spin diffusion is efficient. There are some features, or artifacts, of the CRAMPS experiment that can be identified with the multiple-pulse aspect of the technique and would be present even in the absence of MAS. These include chemical shift scaling and pedestal effects (11, 18, 22). During the multiple-pulse sequence the evolution of the nuclear spins in the magnetic field is not a "free precession". Under multiple-pulse decoupling, the proton signal is acquired while the multiple-pulse train drives the spin states. In addition to removing homonuclear dipolar interactions, the pulse train also alters the chemical shift interaction. The effect on the chemical shift can be viewed (18) as if the magnetic field is rotated away from the z axis and scaled in intensity. The first effect, tilting of the field, has the result that polarization that is not prepared orthogonal to the effective magnetic field, and is thus "locked" along the effective field, will have a nonzero component in the transverse plane. Because this component is locked along the effective field, there will be no chemical shift precession about this static field Be* Instead, the locked polarization will decay under ?",-type processes, which are often rather fast, resulting in a signal a t zero offset, sometimes called the pedestal. If the decay constant of the magnetization along the effective field is such that this pedestal has appreciable width, it can interfere with the remainder of the spectrum. One can take steps to minimize the pedestal by preparing the polarization orthogonal to the tilted field and by adjusting the receiver phase so that is is orthogonal to the signal from the pedestal. The second effect, scaling of the static field intensity, results in a compression of the chemical shift scale, the magnitude of which may be calculated from theory or measured directly, depending on the desired acuracy of the chemical shift measurement. For the experiments reported here the scaling factor was typically 0.402-0.406. Another characteristic of the CRAMPS approach arises from MAS alone, even in the absence of the multiple-pulse feature of the experiment. This characteristic is spinning sidebands, which are a direct consequence of the periodic modulation of anisotropic inhomogeneous interactions by sample spinning (33,34). Assuming that the sample is diamagnetic and that the homonuclear 'H-lH dipolar interactions are being effectively eliminated by the multiple-pulse sequence, than this modulation arises from the 'H chemical shift anisotropy (CSA) and heteronuclear 'H-X dipolar interactions. MAS averages the former to zero, leaving the desired isotropic chemical shift for observation, and the latter to zero if there are no complicating influences due to strong quadrupolar interactions of the X nucelus or strong X-X dipolar interactions, as for example could arise with X = 19Fin a solid containing a large concentration of fluorine. Effects of the quadrupolar interaction in this regard can be minimized by performing experiments in a strong static magnetic field, which minimizes deviations of the X-spin quantization axis from Bo and optimizes the effectiveness of MAS averaging of the 'H-X dipolar interaction.
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Flgure 5. 187-MHz 'H NMR spectra of CH3CH,0,CCH=CHC0,H: (a) result of a single-pulse experiment (many repetitions) on a static sample; (b) result of a single-pulse experiment on a sample undergoing magic-angle spinning; (c) result of a BR-24 multiple-pulse experiment on a static sample; (d) result of the 'H CRAMPS experiment. As the size of the CSA scales with Bo,the amplitudes of the modulation and the resulting sidebands increase as Bo is increased. Hence, just as in the cases of other nuclides, such as 13C,for which MAS is routinely performed, higher MAS speeds are required if one wishes to avoid large spinning sidebands at higher magnetic fields. Furthermore, although 'H CSAs are typically much smaller than the CSA's typical of other nuclides (if CSA values are expressed in parts per million), the much larger magnetogyric ratio of the proton renders the 'H CSA in hertz large enough to be a concern in many cases in a high-field experiment. The "brute force" approach of simply increasing the MAS speed must be approached with caution. One of the key features of the CRAMPS experiment that involves recognition of the simultaneous multiple-pulse averaging of homonuclear dipolar interactions and MAS averaging of inhomogeneous anisotropic interactions is that, in order for both types of averaging to be effective, the cycle times of these two periodic averaging processes should be much different. Although one might argue about which time period one should consider to be the pertinent cycle time for the multiple-pulse dipolar averaging in the CRAMPS experiment, we have found that resolution begins to degrade if the ratio of the spinner period to the BR-24 cycle time is less than about 5. Thus a MAS speed to 1.5-2.0 kHz seems optimal at the present time, as far as resolution is concerned. Another feature, or artifact, of the CRAMPS experiment that arises from the simultaneous use of multiple-pulse and MAS averaging is the appearance of rotor lines (36). Rotor frequency lines appear ubiquitous whenever multiple-pulse coherent averaging techniques, be they chemical shift scaling or multiple-pulse decoupling, are applied. These artifacts, which in the CRAMPS spectrum can appear as a sharp spike or as an extra line(s) in the spectrum or result in the disappearance of all or part of a spectrum, occur when the offset of a resonance from the transmitter is equal to a multiple of the spinning rate. In practice, the need to eliminate rotor lines, when coupled with restrictions on the MAS rate and the offset-dependent resolution typical of CRAMPS, often leads to a very narrow range of conditions under which optimum performance is achieved. This is particularly true in experiments on complex materials where the spectral intensity is spread continuously over a frequency width comparable in size to maximum allowed spinning rates and offsets. Figure 5 shows explicitly the effects of the multiple-pulse homonuclear dipolar line narrowing and MAS on the spectrum of monoethyl fumarate, CH3CH2O2CH=CHCO2H. One sees
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Flgure 6. 187-MHz 'H CRAMPS spectra of powdered, crystalline samples of (a)malonic acid, (b) adipic acid, (c)tartaric add, (d) durene, (e) benzoic acid, (f) dimethyl terephthalate, and (9) 4,4'-dimethylbenzophenone. that from a single-pulse experiment carried out without either line narrowing technique (Figure 5), there is little evidence of a spectrum on the scale displayed. Incorporation of MAS, without multiple-pulse line narrowing, provides only a broad featureless pattern that is dominated by the effects of strong 'H-'H dipolar interactions (Figure 5b). A multiple-pulse line narrowing experiment on a static sample provides a spectrum with some fine structure, but still retaining the CSA line broadening (Figure 5c). Only with the combination of multiple-pulse line narrowing and MAS, i.e., CRAMPS (Figure 5d), does one see the chemical shift fine structure that one can identify straightforwardly with molecular structure. In this spectrum one sees a COzH peak at about 11.4 ppm, a peak at about 6.5 ppm due to protons of the ethylenic framework, a peak a t about 3.5 ppm due to the CH2 protons, and a peak at about 1.0 ppm for the CH3 protons. 2. Qualitative Features and Analysis. The general features of lH CRAMPS NMR spectra of typical samples are seen in Figures 5d, 6, 7, and 8. Figure 6 deals with samples that are powdered, crystalline pure substances: malonic acid (Figure 6a), adipic acid (Figure 6b), tartaric acid (Figure 6c), durene (Figure 6d), benzoic acid (Figure 6e), dimethyl terephthalate (Figure 6f), and 4,4'-dimethylbenzophenone(Figure 6g). Figure 7 represents samples that are somewhat inhomogeneous chemically: poly(ethy1ene oxide) (Figure 7a), poly(methy1 methacrylate) (Figure 7b), polystyrene (Figure 7c), poly(butanedio1 terephthalate) (Figure 7d), silica gel (Figure 7e), and trimethylsilyl-derivatized silica gel (Figure 7f). Figure 8 focuses on samples that are chemically very inhomogeneous: a poly(styrenesu1fonic acid) (Figure 8a), a phenolic resin (Figure 8b), a humic acid (Figure 8c), a coal (Figure 8d), and a reductively alkylated coal (Figure 8e). Figure 6a shows the 'H CRAMPS spectrum of malonic acid, H02CCH2C02H.Only one CH2 peak is observed (line width < 0.3 ppm), and two carboxyl proton peaks are seen, which reflect two distinct hydrogen bonding environments. One sees from the spectrum of adipic acid, H02CCH2CH2CH2CH2CO2H(Figure 6b), that the a and (3 methylene groups (cy and p with respect to the carboxy position) are distinguished from each other a t about 2.5 and 1.5 ppm, re-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
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Flgure 8. 187-MHz 'H CRAMPS spectra of amorphous samples of (a) poly(styrenesu1fonicacid), (b) phenolic resin (c) humic acid (Minnesota peat) (38).(d) Turkish coal, and (e) reductively ethylated Turkish coal.
spectively, and that the line widths of the two methylene peaks are less than about 0.3 ppm. The spectrometer must be extremely well tuned to provide this level of resolution in the resonance(s) of relatively rigid CH2 groups in a proton-rich environment, for which 'H-'H dipolar interactions are very strong. This sample provides a good test case for the IH CRAMPS performance of a spectrometer. The 'H CRAMPS spectrum of tartaric acid, HO,CCH(OH)CH(OH)C02H(Figure 6c), shows a high level of structural detail. The peaks of two chemically nonequivalent carboxy protons, differing in hydrogen-bonding arrangements, are seen at about 11.0 and 12.3 ppm. Two nonequivalent CHOH hydroxyl proton peaks are expected and are tentatively assigned here as a clearly resolved peak at about 6.5 ppm and a resolved shoulder at about 4.6 ppm. The CH proton resonance is found as the peak a t 5.1 ppm.
For durene, 1,2,4,5-tetramethylbenzene(Figure 6d), one sees well-separated peaks for the aromatic and methyl protons. The relative intensities are close to the expected ratio of 1:6. The benzoic acid spectrum (Figure 6e) shows, as expected, three peaks in the aromatic proton region of the spectrum, one of those peaks appearing as a slight shoulder at about 7.0 ppm, and a peak for the hydrogen-bonded carboxyl proton at about 12.8 ppm. The relative intensities of the two regions of the spectrum are seen to be close to the expected 51. The spectrum of dimethyl terephthalate
(Figure 6f), shows a peak at 4.1 ppm due to the methyl protons and resonances of aromatic protons appearing as a peak at 7.2 ppm and a shoulder a t about 6.0 ppm. This aromatic proton nonequivalence, which is absent in the spectrum of a sample dissolved in a liquid, must be due to the conformational arrangement of the carbomethoxy groups in the crystal, which renders the unsubstituted aromatic sites nonequivalent. The spectrum of 4,4'-dimethylbenzophenone (Figure 6g) shows not only some expected fine structure (chemical shift discrimination) in the aromatic proton region but also some unexpected fine structure in the CH3 resonance. This fine structure presumably displays some measure of chemical nonequivalence that results from the packing pattern of molecules in the crystal lattice. I t should be noted that this compound has often been used as a kind of "standard" for assessing the 'H CRAMPS performance of a given spectrometer. However, this sample does not constitute a good choice, as there are no strongly coupled protons (e.g., no rigid CH2 groups) to provide a severe test for the homonuclear dipolar decoupling feature of the CRAMPS experiment. Furthermore, the resolution of the spectrum of 4,4'-dimethylbenzophenone is lower than that for adipic acid and is relatively insensitive to the quality of spectrometer tuning. It may be noteworthy that the line widths observed in Figure 6 for the compounds containing aromatic groups tend to be larger than for compounds not containing such groups. We have observed this trend in other powdered samples not represented in this paper. One possible explanation is the large anisotropies in bulk magnetic susceptibility characteristic of aromatic compounds and the fact that for powdered samples MAS does not completely average this kind of effect to yield sharp lines (37). If this explanation is correct, one should see narrower lines for a large single crystal, relative to what is observed in a powder. Figure 9, showing spectra of a single crystal and a powdered sample of durene, demonstrates the expected effect.
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
Among the less homogeneous samples represented in Figure 7, the spectrum of poly(ethy1ene oxide), Figure 7a, shows a broad peak due to a highly amorphous component superimposed on a sharp peak due to a more highly ordered (crystalline) component. For poly(methy1 methacrylate), (CH2C(CH3)C02CH3),,for which the spectrum is shown in Figure 7b, the 0-CH3 resonance at 4.8 ppm is distinguished from the CCH3 and CH, resonances, which partially overlap at about 2-3 ppm. For polystyrene (Figure 7c) the 'H CRAMPS technique clearly distinguishes resonances in the aliphatic and aromatic regions but does not distinguish among the three different hydrogen sites (ortho, meta, para) of the phenyl group, nor between the >CH- or >CH, aliphatic sites. This lack of resolution, in contrast, say, to the fine structure seen for the phenyl groups in benzoic acid, displays the fundamental difference between the order and homogeneity in a crystalline sample, relative to that in an amorphous polymer. In the latter, the numerous conformational/configurational differences present in the sample are manifested in a chemical shift dispersion, or inhomogeneous broadening, for each type of hydrogen site. A similar situation is well-known in 13C MAS NMR. It is also possible that some fraction of the broadening observed in amorphous materials might be due to having molecular motion with a characteristic time constant near the multiple pulse cycle time. The absence of discrimination within the aromatic proton region is also seen for the polymer, poly(butanedio1 terephthalate) (Figure 7d). In this case the a and /3 CH2 environments are distinguished, but not as sharply as for the polycrystalline adipic acid case. The broad aromatic peak (line width -2 pm) is substantially broader than those for individual aromatic proton peaks in Figure 6 and arises from the superposition of ortho and meta peaks corresponding to a wide range of conformations. The 'H CRAMPS spectrum of dry silica gel shown in Figure 7e can be demonstrated by heating and relaxation measurements (27) to consist primarily of a broad peak centered at about 3.5 ppm (due to hydrogen-bonded silanols) and a sharp peak at about 1.7 ppm (due to isolated silanols). Silylation of a dried silica gel sample yields a trimethylsilyl-derivatized surface with the 'H CRAMPS spectrum shown in Figure 7f, having an additional peak a t about 0 ppm due to (CH,),Si moieties. The samples that are chemically the most inhomogeneous ones examined in this study are represented in Figure 8. The spectrum of a polystyrenesulfonic acid (Figure 8a) shows a broad peak due to CH and CH, protons of the polystyrene framework, centered at 1.1ppm, and a broad aromatic proton peak centered a t about 9 ppm as a shoulder on an acidic proton resonance at about 9.5 ppm. The 'H CRAMPS spectrum of a phenolic resin (Figure 8b) shows essentially two partially overlapping peaks, with maxima at about 7.5 ppm (protons attached to aromatic rings) and about 4.5 ppm (CH2 protons in Ar-CH2-Ar arrangements (Ar = aromatic ring). The humic acid sample provides a spectrum (Figure 8c) showing three main maxima. The maximum at about 1.6 ppm can be identified with a peak due to protons on sp3 carbons. Partitioning of the intensity between 10 and 3 ppm is less straightforward, but probably involves contributions due to +C-OH protons of various types, protons attached to sp2 carbons and water in various types of hydrogen-bonding configurations. The 'H CRAMPS spectrum of a coal sample (Figure 8d) shows maxima a t about 7.2 ppm and about 1.8 ppm, corresponding to maxima in the chemical shift distributions of protons attached to aromatic moieties and sp3 carbons, respectively. Reductive ethylation of the coal yielded a sample with the 'H CRAMPS spectrum shown in Figure 8e, which
shows a much higher proportion of protons attached to sp3 carbons than in the original coal. There are two important kinds of sample-to-sample differences found in the 'H CRAMPS experiments represented in Figures 5d-8. One is the generally sharper and better resolved character of the peaks in the spectra of the pure crystalline, powdered substances of Figures 5d and 6, compared to the spectra of the less homogeneous samples represented in Figures 7 and 8, especially the latter. As indicated above, the main reason for the large linewidths in the less homogeneous samples is inhomogeneous broadening due to chemical shift variations. These variations are due mainly to conformational and related geometrical variations in the sample of Figure 7 and to more substantial variations in chemical structure in the samples of Figure 8. This is the same general kind of pattern observed in the high-resolution 13C NMR of solids. The second major feature that is found to vary from sample to sample in the systems represented by Figures 5d-8 is the proton Tl value. For some of the pure, powdered substances, especially those without methyl groups, the 'H Tl value can be extremely long, a minute or more. The presence of a rapidly rotating methyl group often greatly improves this situation, as does the generally larger degree of motion in the more disordered samples represented in Figures 7 and 8. Thus, the 'H T1value of polystyrene (just one value for all of the protons, because of efficient 'H spin diffusion) is about 2 s. For coal, in which unpaired electrons also provide a mechanism for 'H spin-lattice relaxation, the 'H Tl value is less than 1 s. This relaxation issue is an important one in determining the feasibility of 'H CRAMPS studies. Fortunately, most samples of the types for which analytical data are needed in practical applications fall into the categories represented by Figures 7 and 8, for which values of Tl are in a workable range. 3. Quantitative Analysis by 'H CRAMPS. In this section the issue of yuantitation is considered from the following three points of view: (a) relative intensities for substances of known empirical formula, (b) relative intensities in mixtures of known components, and (c) the requirements of an intensity standard for spin counting. Samples of the general type represented in Figures 5d and 6 were often found to be largely unsuitable for convenient quantitation for one or both of the following reasons: (1)very large (estimated) values, of T I ,which precluded the achievement of thermal equilibrium in a reasonable time; (2) peak "overshoot" below the base line, which we believe is caused by detuning of the probe and/or amplifier due to rf heating during data acquisition and consequent drift in the scaling factor over a single scan. This effect is visible only in lines with widths less than about 50 Hz and is minimized by increasing the cycle time. An exception to these features was 4,4'-dimethylbenzophenone, which yielded 1.32 and 1.35 for the ratio of aromatic to aliphatic protons from single determinations carried out in separate weeks, using simple integration. The theoretical value is 1.33. Similarly, for adipic acid, by using extremely long delays to optimize the signal-to-noise ratio, we obtained values of 0.249 and 0.256 for the carboxyl-to-methylene proton ratio (theoretical value, 0.250) in single determinations in separate weeks, using simple integration. One should note that these ratios should be independent of the delay time, as 'H spin diffusion is efficient in these samples. The reproducibility of spectra taken by using this technique was examined more extensively with polystyrene, for which the broad but largely separated peaks shown in Figure 7a do not suffer from the two characteristics indicated above. The ratio of aliphatic to aromatic 'H intensity obtained from seven determinations spanning several weeks was found to be 1.59
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
1749
A
Figure 10. Quantitative analysis of the 'H CRAMPS spectrum of polystyrene via deconvolution: (a) experimental 187-MHz spectrum; (b) computer-simulated spectrum; (c) deconvoluted contributions to b.
f 0.12 (standard deviations Indicated), compared to the theoretical value of 1.67. These values were obtained by a simple integration procedure, using a boundary at 4.0 ppm for assigning integrated intensity to aromatic and aliphatic hydrogen sites. When a deconvolution approach was used, employing the Nicolet computer routine with Gaussian components to fit the experimental spectra, as shown in Figure 10, the result was found to be 1.67 f 0.12. For most samples, especially mixtures with substantial peak overlaps in the 'H CRAMPS spectra, we have found the deconvolution approach to provide much better precision than a simple integration approach. The simple integration approach works well for the polystyrene case because of the rather good separation between the peaks identified with hydrogens attached to sp2 and sp3 carbons. In single determinations carried out in different weeks on poly(a-methylstyrene), which yields a spectrum that is qualitatively similar to that of polystyrene, the values 0.983 and 1.02 were obtained by the simple integration method, compared to the theoretical value, 1.00. Values of 0.982 and 1.11were obtained by deconvolution. For poly(butanedio1 terephthalate) (Figure 7d), single determinations of the aromatic to aliphatic proton ratio in separate weeks yielded 0.451 and 0.481 by the simple integration approach, compared to 0.455 and 0.510 by a deconvolution procedure. The theoretical value is 0.50 for this polymer, if it had an infinite chain length. In order to examine prospects for quantitative applications of lH CRAMPS to mixtures of materials, a mixture of poly(methylene oxide) (Delrin)) and polystyrene prepared in the weight ratio of 1:2.52 was examined. The spectrum of the mixture was deconvoluted in the manner shown in Figure 11, using the sum of Gaussian contributions to simulate the spectrum of each separate polymer and the mixture. The result obtained for the ratio in three experiments over several weeks was 2.62 f 0.18. Single determinations by an analogous procedure on a 1.074:l mixture of poly(butanedio1 terephthalate) and polystyrene (Figure 12) yielded the values 1.076 and 1.069. The positive results obtained with Delrin in the mixture with polystyrene essentially demonstrated the viability of "spin counting" in lH CRAMPS, Le., employing as an intensity standard a compound with a simple spectrum that does not severely overlap the spectral region of the sample with which it is to be mixed and on which a quantitative proton count is to be obtained. The quantitative analysis of mixtures of polymers or other complex samples by means of 'H CRAMPS NMR is less
Flgure 11. Quantitative analysis of the 'H CRAMPS spectrum of a mixture of polystyrene and poly(methy1ene oxide): (a) experimental 187-MHz spectrum; (b) computer-simulated spectrum; (c) deconvoluted contributions to b; (d) deconvolution of the polystyrene spectrum; (e) poly(methy1ene oxide) spectrum.
e
A C A
a Figure 12. Quantitative analysis of the 'H CRAMPS spectrum of a mixture of polystyrene and poly(butanedio1terephthalate): (a) experimental 187-MHz spectrum; (b) computer-simulated spectrum; (c) deconvoluted contributions to b; (d) deconvolution of the poly(butanedio1 terephthalate) spectrum; (e) deconvolution of the polystyrene spectrum.
straightforward if there are substantial peak overlaps. The example of Figure 12 shows that reasonable results can be obtained even in the case of peak overlaps if there is some region of the spectrum of one component for which there is no intensity in the spectrum of the other component (about 4 ppm in the case cited). In general, we believe that an analysis by a systematic method of pattern recognition would be advantageous for such cases. A computer simulation of spectra of arbitrary mixtures can be generated, as shown in Figure 13 for selected mixtures of polystyrene and poly(butanediol terephthalate), and pattern recognition can be used to match the experimental spectrum of an actual mixture to the best-fit computer simulation, i.e., the best analytical result. Such methods are being explored. The search for an "internal" intensity standard for 'H spin counting is concerned with finding a suitable solid reference that can be mixed in with the sample of interest. This search was based on the following criteria: (a) the standard should be reasonably inert chemically, permitting its mixing with
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988 POLYSTYRENE
POLYBUTANED~O L T E R E P H L A L A T E (WEIGHT RATIO)
0 : l
1
:
5,n
1 : 2,s
1 : 2.0
compatible with certain general requirements and the specific requirements (e.g., regarding peak overlaps) of individual analyses, characteristics that render it useful as an “internal” intensity standard. Registry No. CH,CHZO2CH=CHCOzH,2459-05-4;H02CCHZCOZH, 141-82-2; HO&CH2CH&H&HZC02H, 124-04-9; HOzCCH(OH)CH(OH)COzH,87-69-4; (CH&(CH3)C02CH3)n, 9011-14-7; durene, 95-93-2; benzoic acid, 65-85-0; dimethyl terephthalate, 120-61-6; 4,4’-dimethylbenzophone, 611-97-2; poly(styrenesulfonic acid), 50851-57-5; polystyrene, 9003-53-6; poly(butanediol terephthalate), 26062-94-2; poly(ethy1ene oxide), 25322-68-3; poly(methy1ene oxide), 30525-89-4.
LITERATURE CITED 1
:
1,s
Pople. J. A,; Schneider, W. G.; Bernstein, H. J. High-ResolutionNuclear Magnetic Resonance: McGraw-Hill: New York, 1959. Becker, E. D. High Resolution NMR Theory and Chemical Applica tions, 2nd ed.: Academic: New York, 1980. Wehrli, F. W.; Wirthlin, T. Interpretation of Carbon- 13 NMR Spectra; Heyden: Philadelphia, 1976. Farrar, T. C.; Becker, E. D. Puis8 and Fourier Transform NMR. Introduction to Theory and Methods; Academic: New York, 1971. Bax. A. Two-Dimensional Nuclear Magnetic Resonance in Liquids : Reidel: Boston, MA, 1982. NMR and the Periodic Table: Harris, R. K., Mann, E. A,, Eds.; Academic: New York, 1982. Mansfield, P.; Morris, P. C. Adw. Magn. Reson. Suppl. 2 ,NMR Imaging in Biomedicine; Academic: New York, 1982. Yannoni, C. S. Acc. Chem. Res. 1982 15, 201. Maciel, G. E. Science (Washington, D.C.) 1984. 226, 282. Fyfe, C. A. Solid-state NMR for Chemists; C.F.C. Press: Guelph, ON, 1983. Haeberlen, U. High Resolution NMR in Solids . Selective A weraging : Academic: New York, 1976. Mehring, M. Principles of High Resolution NMR in Solids, 2nd ed.; Springer-Verlag: New York, 1983. Ganapathy. S.;Schramm, S.; Oldfield, E. J. Chem. Phys. 1982, 77, 4360. Meadows, M. D.; Smith, K. A.; Kinsey, R. A.; Rothgeb, T. M.; Skarjune, R. P.; Oldfield, E. R o c . Natl. Aced. Sci. U.S.A. 1982, 79,135. Vega, A. J.; Luz, 2. J. Phys. Chem. 1987, 91, 365. Dec, S.F.: Wind, R. A.; Maciel G. E.; Anthonio, F. E. J. Magn. Reson. 1986, 70,355. Dec, S. F.; Wind, R. A.; Maciel, G. E. Macromolecules 1987, 20, 2754. Waugh, J. S.:Huber, L. M.; Haeberlen, U. Phys, Rev. Lett. 1986, 20, 180. Mansfield, P.; Orchard, M. J.; Stalker, D. C.; Richards, K. H. B. Phys. Rev. B : Solid State 1973, 7 ,90. Rhim, W. K.; Elieman, D. D.; Vaughan, R. W. J. Chem. Phys. 1973, 58, 1772. Rhim. W. K.; Elleman. D. D.; Vaughan, R. W. J. Chem. Phys. 1973, 59,3740. Gerstein, B. C.;Dybowski, C. R. Transient Techniques in NMR in Solids; Academic: New York, 1985. Gerstein, B. C.; Chou, C.; Pembleton. R. G.; Wilson, R. C. J. Phys. Chem. 1977, 81, 585. Rosenberger. H. Z.Chem. 1983, 23, 34. Schmiers, V. H.; Rosenberger, H.; Scheler, G. Forschungserebneiss 1982, 20, 1. Bronnimann, C. E.; Chuang, I. S.;Hawkins, B. L.; Maciel, G. E. J. Am. Chem. SOC.1987, 109, 1562. Bronnimann, C. E.: Zeigler, R. C.; Maciel G. E. J. Am. Chem. SOC. 1988, 110, 2023. Gerstein, B. C.; Dybowski, C. R. Transient Techniques in NMR in Solids; Academic: New York, 1985: Chapter 5. Burum. D. P.; Rhim, W. K. J. Chem. Phys. 1979, 71,944. Freeman, R.; Hill, H. D. W. J. Chem. Phys. 1971, 5 4 , 3367. Gay. I. D. J. Magn. Reson. 1984, 58, 413. Pines, A.; Gibby, M. G.; Waugh, J. S.J. Chem. Phys. 1973, 59,569. Ye, C.; Sun, B.; Maclel, G. E. J. Magn. Reson. 1986, 70, 241. Maricq, M.; Waugh, J. S. J . Chem. Phys. 1979, 70,3300. O’Donnell, D. J.; Greaves, R.; Maciel, G. E. Advances in Chemistry. Series 196; Alzea, E. C., Meek, D. W., Eds.; American Chemical Society: Washington, DC, 1982: p 389. Vega, S.;Olejniczak. E. T.: Griffin, R. G. J. Chem. Phys. 1984, 80, 4832. VanderHart, D. L.: Earl, W. L.; Garroway, A. N. J. Magn. Reson. 1981. 44, 361. Hatcher, P. G.; VanderHart, D. L.; Earl, W. L. Org. Geochem. 1980, ~
1 : l 1.5 : 1
2,’j :
1
5,3 ’ 1
10
J v 10
:
1
1 ’ 0
0
- 10
PPM
Flgure 13. Computer simulation of the ‘H CRAMPS spectra of various mixtures of poly(butanedio1terephthalate) and polystyrene.
other powdered solids; (b) the standard should not be hygroscopic, which would compromise the significance of weighings, possibly leading to relevant changes in NMR properties and possibly give rise to a violation of criterion a; (c) the standard should not have such a large number of equivalent hydrogens with a narrow resonance (e.g., most (CH3)3Si- species) that a serious dynamic range problem is encountered if more than 1 mg of standard is used; (d) the standard should have NMR characteristics that are convenient for the purpose, Le., a reasonably sharp spectrum (but not too sharp! vide supra) outside the normal chemical shift range of interest (say 11-0 ppm for most samples), with a value of T I not larger than about 4 s and without a major spinning sideband problem. Several possible species have been examined. For those cases in which its resonance does not interfere with the spectral region of interest, Delrin is useful, as discussed above. Oxalic acid, (C02H)z,shows considerable promise, yielding a spectrum with two peaks at about 11and 12 ppm. Experiments are currently under way to explore these and other possibilities further.
CONCLUSIONS ’H CRAMPS is capable of providing high-resolution NMR spectra with chemical shift fine structure that can be extremely useful for qualitative analysis. Furthermore, this approach is a quantitatively useful tool, which can provide relative intensities reliable to within 2-3% for the various peaks of a given substance or in a mixture, provide distortions associated with longitudinal relaxation and debilitating peak overlaps can be avoided. Spin counting can be accomplished if an intensity standard can be found with properties that are
2. 07.
RECEIVED for review November 7, 1987. Accepted March 21, 1988. The authors gratefully acknowledge partial support of this research by the Amoco Corp. and the use of the Colorado State University Regional NMR Center, funded by NSF Grant. No. CHE-8616437.
