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Line Narrowing in Solid-State Proton NMR with Acquisition Delay
JOURNAL OF MAGNETIC RESONANCE, ARTICLE NO.
Series A 123, 56–63 (1996)
0213
Line Narrowing in Solid-State Proton NMR with Acquisition Delay B. M. FUNG, TAT-HUNG TONG, THILO DOLLASE,
AND
MATTHEW L. MAGNUSON
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019-0370 Received April 12, 1996; revised July 26, 1996
Organic solids have extensive proton–proton dipolar interactions, and their 1H NMR linewidths are very large even with magicangle spinning at moderate speeds. Recently it has been shown that substantial narrowing of the proton linewidths of organic solids can be achieved by using single-pulse excitation with acquisition delay or spin echo [S. Ding and C. A. McDowell, J. Magn. Reson. A 111, 212 (1994); 115, 141 (1995); 117, 171 (1995)]. This interesting line-narrowing phenomenon has been further examined through the study of several amino acids, their deuterated analogs, and some aromatic compounds. The results confirm that narrow proton peaks are observed with long acquisition delay, and the peaks appear in the appropriate chemical-shift ranges for organic protons (0–10 ppm with respect to tetramethylsilane). However, except for some special cases, the observed peaks cannot be assigned to individual types of protons based on chemical-shift considerations only. To explore the reason for the line narrowing, the effect of acquisition delay on the 19F linewidth of CaF2 was also studied and compared with that on the 1H linewidths of organic solids. It is suggested that the broad proton peak in an organic solid is a superposition of numerous transitions. These transitions have different linewidths, and the narrow peaks in the spectrum remain observable with long acquisition delays. q 1996 Academic
tation method can be used to complement the CRAMPS technique, which combines MAS with multiple-pulse sequences (6–8). Subsequently, Gerstein et al. (9) confirmed that single-pulse excitation with acquisition delay or spin echo allows the observation of narrow peaks, and suggested that the peaks may be due to a small fraction of molecules moving nearly isotropically. However, they pointed out that the intensities of the observed peaks do not always yield quantitative information. We have further examined the interesting proton NMR line-narrowing phenomenon of organic solids through the study of several amino acids, their N-deuterated analogs, and some aromatic compounds. The effect of acquisition delay on the 19F NMR of CaF2 and that on the 1H NMR of organic solids are also compared. Our results and interpretation are reported here. EXPERIMENTAL
Dimethyl terephthalate was synthesized by refluxing terephthalic acid in methanol and was recrystalized from methanol. 1,2,4,5-Tetramethylbenzene (durene), fluorene, 9-fluorenone, and naphthalene were purified through sublimation. The amino acids and 4-n-pentoxy-4 *-cyanobiphenyl (5OCB) were used without further purification. For N-deuteration of amino acids, each compound was equilibrated with D2O and dried in air three times. 9,9-Dideuterofluorene was synthesized by Clemenson reduction in DCl/D2O, separated from side-products by column chromatography, and purified by sublimation. To prepare samples for the MAS experiments, the amino acids and dimethyl terephthalate were ground with mortar and pestle and dried at 1107C in vacuum for 36–48 hours. Then, each compound was packed into a rotor in a nitrogen atmosphere. Better results were obtained by evacuating the packed rotors again for at least 24 hours at 1107C. Because the other aromatic compounds studied have rather high vapor pressures, they could not be dried in vacuum; after sublimation, each compound was transferred into the rotor and heated at 1107C to form a melt, and then cooled down slowly. 5OCB was treated similarly, except without sublimation. All experiments were carried out on a Varian VXR-500
Press, Inc.
For NMR spectra which contain both broad and narrow components, the broad peaks can be filtered out by discarding the initial portion of the free induction decay before carrying out Fourier transformation. This method was first introduced by Chan and co-workers (1), and is sometimes called ‘‘acquisition delay.’’ The filtering of broad peaks can also be achieved by the use of a spin-echo sequence, and we have shown that the spin-echo method can be useful for observing the 13C peaks of solute molecules dissolved in liquid-crystalline solvents (2). Recently, Ding and McDowell showed that the methods of acquisition delay (3) and spin echo (4, 5) can lead to a dramatic reduction of linewidths in the proton NMR spectra of solids, especially under the condition of magic-angle spinning (MAS). The most interesting result is that the observed peaks appeared in the appropriate chemical-shift ranges for organic protons. The relatively narrow peaks were attributed to protons in chemically distinct groups with Zeeman interactions only, and it was suggested that the single-pulse exci1064-1858/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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NMR spectrometer. The MAS probe was manufactured by Doty Scientific, Columbia, South Carolina. To observe narrow peaks in the proton spectra of solids and liquid crystals, an acquisition delay of 100 ms was used in most cases, so that the receiver could be set at a high gain without being overloaded by the strong signal of the initial part of the FID. To process the free-induction-decay data, a fixed number of initial points was discarded to obtain a desired length of delay time. The data were usually Fourier transformed with 20 Hz line broadening unless otherwise specified. Most spectra with long relaxation delays are presented in the ‘‘absolute value’’ mode because they could not be phased properly. For the amino acids, the chemicalshift scale was established by setting the methyl peak of a 1-hexanol sample sealed in a macor rotor to 0.88 ppm; for the aromatic compounds, the single proton peak of a sealed benzene sample was set to 7.24 ppm. This procedure was used to minimize the uncertainties due to the magnetic susceptibility effect, and the calibration was made before each set of experiments. RESULTS
To examine the effect of acquisition delay on the line narrowing of organic solids, we have studied several amino acids, aromatic compounds, and some of their partially deuterated analogs. It is hoped that the results will help us understand the origin of this interesting phenomenon. The proton spectrum of L-alanine was studied previously by the use of CRAMPS (10, 11) and single-pulse excitation (3, 9); the latter was carried out at 200 MHz ( 3) and 400 MHz (9). In each method, the spectrum contained three peaks, and they were assigned to the NH 3/ , CH, and CH3 groups. We have repeated the single-pulse experiment on Lalanine and N-deuterated L-alanine at 500 MHz. The spectrum of L-alanine shows three main peaks at 7.1, 4.7, and 1.5 ppm, respectively (Fig, 1, I). If these peaks are assigned to the NH 3/ , CH, and CH3 groups, the intensities do not correspond to the appropriate number of protons in each group, as was observed previously at 400 MHz (9). To ascertain that the intensities of the small peaks were not reduced due to saturation caused by long T 1 and insufficient pulse delay, we repeated the experiment with 10 and 100 s pulse intervals, and the spectra did not change. The large peak at 7.1 ppm has a fine structure which is not clear when the spectra were processed in an absolute value mode with 20 Hz line broadening. To observe the fine structure, the FID was reprocessed with a phased mode in a narrower spectral range with no line broadening, and the spectrum shows a triplet with equal splittings of 63 { 5 Hz (Fig. 1, Ic, inset). This value is comparable to the 14N– 1H scalar coupling of the NH 3/ group of the glycinium ion in water [J Å 55 Hz, calculated from the 15N– 1H coupling constant (12)]. This interesting feature for L-alanine was not reported
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FIG. 1. Solid-state proton NMR spectra of (I) L-alanine and (II) its Ndeuterated analog at 227C and 500 MHz, with MAS at 5.0 kHz. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (a) 200 ms, (b) 400 ms, and (c) 800 ms. The inset for Ic was processed without line broadening and is displayed in the phased mode with a width of 4 ppm. The peaks labeled SS are spinning sidebands.
earlier (3, 9), possibly because of smaller resolution at lower fields. The shape of the proton peak of the NH 3/ group would also be affected by the 14N– 1H dipolar coupling (11); although there is a hint of such an effect in the spectrum, it cannot be clearly identified. To examine the validity of the assignment of the proton peaks, we have studied the spectrum of N-deuterated L-alanine. Upon N-deuteration, the broad component of the spectrum persists longer (Fig. 1, II), and the linewidths of the sharp peaks are less than those of the protonated analog. The peak at 7.1 ppm disappears almost completely, and the relative intensity of the peak at 1.5 ppm increases substantially. In addition, there are two other peaks at 2.1 and 3.8 ppm, respectively. The former is not present in the spectra of protonated L-alanine, but the latter is quite close to the position of a very small peak at 3.3 ppm in those spectra (this peak is indicated by parentheses in Table 1). The disappearance of the peak at 7.1 ppm upon N-deuteration, together with its characteristic triplet splitting, lends credence to the assignment of this peak to the NH 3/ group. However, the four peaks at 1.5, 2.1, 3.8, and 4.7 ppm in the spectra cannot be simply assigned to the two chemically distinct groups, CH3 and CH, in the molecule. The spectra of three other amino acids and their N-deuterated analogs were studied, and are presented in Figs. 2– 4, respectively. For the protonated compounds, the broad background is barely present when the acquisition delay was set at 200 ms, and the essential features of the spectra do
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TABLE 1 The Proton Peak Positions of Four Amino Acids (Units, ppm) L-Alanine
b-Alanine
L-Valine
L-Isoleucine
3.45 (d); 1.77 (m); 1.26 (m); 1.07 (m); 0.79 (d); 0.72 (t) 6.1; 1.1; 00.1 7.3; 1.9; 00.1
In D2O solution
3.65 (q); 1.26 (d)
2.94 (t); 2.33 (t)
3.30 (d); 1.96 (m); 0.71 (dd)
In solid state, NH/3 In solid state, ND/3
7.1; 4.7; (3.3); 1.5 4.7; 3.8; 2.1; 1.5
7.7; 7.2; 7.0; 3.1 8.0; 5.8; 2.9; 2.0
6.0; 4.9; 2.2 8.2; 2.4; 1.1
Note. The notations q, d, t, and m indicate quartet, doublet, triplet, and multiplet, respectively. The –NH/3 peak was not observed in D2O.
not change with increased acquisition delay. For the N-deuterated compounds, the broad components are very prominent with 200 ms acquisition delay, but they disappear when the delay time was set at 400 ms or longer. For b-alanine, there is a major peak at 3.1 ppm and a group of smaller peaks from 7.0 to 7.7 ppm (Fig. 2, I). The positions of this group of peaks are similar to that assigned to the NH 3/ group for L-alanine, but they are not symmetrical, with separations of 215 and 144 Hz, respectively. In Ref. (5), two peaks were observed in the corresponding part of the spectrum, and they were assigned to the two different CH2 groups in b-alanine. However, there is a problem with this assignment: it is unlikely that the CH2 groups are less shielded than the NH 3/ group by several ppm. When the NH 3/ group is deuterated, the major peak at about 3.0 ppm remains unchanged, but new peaks appear at 5.8 and 2.0 ppm (Fig. 2, IIb). These features and the spectra of L-valine (Fig. 3) and L-isoleucine
(Fig. 4), both protonated and N-deuterated, cannot be readily explained based on chemical shifts only (Table 1). The data in Table 1 include the isotropic shifts observed in D2O solutions, which need not be exactly the same as those in the solid state because of different charge distributions, but the trends should be similar if the peaks in the solids arise from Zeeman interactions only. In addition to amino acids, we also studied the proton spectra of several aromatic compounds. The single-pulse proton NMR spectrum of solid 1,2,4,5-tetramethylbenzene (durene) with 400 ms acquisition delay is shown in Fig. 5a. The sample was prepared from sublimed crystals, which were packed into a rotor and heated up to 1207C to form a melt, and then cooled down slowly (about 0.47C per minute)
FIG. 2. Solid-state proton NMR spectra of (I) b-alanine and (II) its N-deuterated analog at 227C and 500 MHz, with MAS at 5.0 kHz. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (a) 200 ms, (b) 400 ms, and (c) 800 ms. The peaks labeled SS are spinning sidebands.
FIG. 3. Solid-state proton NMR spectra of (I) L-valine and (II) its Ndeuterated analog at 227C and 500 MHz, with MAS at 5.0 kHz. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (a) 200 ms, (b) 400 ms, and (c) 800 ms. The peaks labeled SS are spinning sidebands.
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FIG. 4. Solid-state proton NMR spectra of (I) L-isoleucine and (II) its N-deuterated analog at 227C and 500 MHz, with MAS at 5.0 kHz. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (a) 200 ms, (b) 400 ms, and (c) 800 ms. The peaks labeled SS are spinning sidebands.
to room temperature. There are two major peaks in the spectrum, and their positions are very close to the corresponding chemical shifts in an isotropic solution (Table 2). We have found that, in many cases, the presence of small amounts of adsorbed water is a serious problem in studying the proton spectra of organic solids with long acquisition delay. For example, without the melting/cooling process, when a sublimed sample of durene was ground and packed into the rotor in a N2 atmosphere, there were usually one or two extra peaks at about 1 ppm, and the positions and intensities of these peaks are dependent on the condition of the sample preparation. We suspected that these peaks were due to residual water adsorbed on the surfaces of the powder sample. Although the chemical shift of bulk water is at 4.63 ppm, a decrease in hydrogen bonding can cause OH protons to be much more shielded (13), and it is well known that in CDCl3 and CD2Cl2 there is always a residual water peak at about 1.5 ppm. Two experiments were carried out to ascertain this point. In the first experiment, the durene sample which gave rise to the spectrum in Fig. 5a was remelted in the rotor and cooled rapidly (about 157C per minute) in air. Then, two new peaks appeared in the proton spectrum at 1.6 and 1.3 ppm (Fig. 5b), just like the unmelted samples. We reasoned that the sample might have developed cracks when it was cooled down rapidly from the melt and adsorbed water on the surface. In a second experiment, some sulfur powder was evacuated at 1107C for 48 hours and packed into a rotor under N2 , and two sharp proton peaks were observed at 1.3
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and 1.8 ppm for the sample. Since there are no organic protons in the sample, these peaks are most likely due to residual water adsorbed on the surface of the sulfur powder. Because of these results, we tried to reduce residual water as much as possible in our samples by using the procedures described under Experimental. For naphthalene, the proton spectrum with 400 ms acquisition delay showed a large peak at 7.4 ppm and two small peaks at 1.7 and 1.1 ppm, respectively. The large peak is quite close to the positions of the isotropic peaks (Table 2), but it is not clear whether or not the small peaks are due to residual water. For dimethyl terephthalate, there are two peaks at 11.4 and 5.6 ppm, respectively (Fig. 5d). In contrast to durene and naphthalene, these peaks appear ‘‘downfield’’ from the corresponding isotropic peaks (Table 2). Furthermore, the difference (5.8 ppm) in the positions of the peaks obtained with acquisition delay is considerably larger than the difference (4.15 ppm) in an isotropic solution. In the deuterium MAS spectrum of the corresponding perdeuterated compound, there is a large peak due to the methyl group and two smaller peaks 1.6 ppm apart due to the aromatic deuterons cis and trans with respect to the C|O bonds (14). On the average, the aromatic peaks are 4.8 ppm, not 5.8 ppm, ‘‘downfield’’ from the methyl peak. Because the proton chemical shifts are expected to be similar to the deuteron
FIG. 5. Solid-state proton NMR spectra at 500 MHz and 227C with MAS at 5.0 kHz and an acquisition delays of 400 ms. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. (a) 1,2,4,5-Tetramethylbenzene (durene), prepared by cooling slowly from a melt; (b) durene, prepared by cooling rapidly from a melt; (3) naphthalene; (4) dimethyl terephthalate. The peaks labeled SS are spinning sidebands.
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TABLE 2 The Proton Peak Positions of Four Aromatic Compounds (Units, ppm) 1,2,4,5Tetramethylbenzene
Naphthalene
Dimethyl terephthalate
In CDCl3 solution
7.06; 2.36
8.00 (m); 7.63 (m)
8.09; 3.94
In solid state
6.7; 2.2
7.4; 1.7; 1.1
11.4; 5.6
Fluorene 7.79 (d); 7.55 (d); 7.38 (t); 7.30 (t); 3.90 9.0; 7.1; 0.6
Note. The notations d, t, and m indicate doublet, triplet, and multiplet, respectively.
chemical shifts, the two peaks in the proton spectrum of dimethyl terephthalate (Fig. 5d) cannot be readily assigned to the methyl protons and aromatic protons, respectively. In Ref. (5), two narrow peaks were observed for fluorene at 200 MHz, and they were assigned to the methylene and aromatic protons. At 500 MHz, we have also observed two major peaks for solid fluorene at 9.0 and 7.1 ppm, respectively, a shoulder at about 4.0 ppm, and a small peak at about 0.6 ppm (Fig. 6b). Although the small peak could be due to residual adsorbed water, the positions of the two large peaks (Table 2) differ substantially from the isotropic peaks of the two types of protons (Fig. 6a). When the protons in the CH2 group were replaced by deuterons, the two peaks at 9.0 and 7.1 ppm merged into one peak at 8.0 ppm, and a smaller peak appears at 2.0 ppm (Fig. 6c). For comparison,
9-fluorenone shows one major peak at 7.1 ppm, a shoulder at 3.2 ppm, and a small peak at 0.3 ppm (Fig. 6d). Finally, we investigated a compound with a much more complicated structure, 4-n-alkoxy-4 * -cyanobiphenyl (5OCB). The solid-state proton spectra of 5OCB are shown in Figs. 7b–7d, and the spectrum of its isotropic melt is shown in Fig. 7a for comparison. With an acquisition delay of 200 ms, the solid-state spectrum displays some resemblance, but not good correspondence, to the isotropic spectrum. With longer acquisition delays, the broad peak at 8.5 ppm disappears, and the solid-state spectra bear little resemblance to the isotropic spectrum. DISCUSSION
The results presented above confirm that the single-pulse proton spectra of organic solids with acquisition delay do show narrow peaks which appear in the proper chemicalshift ranges for organic compounds. In some cases, such as
FIG. 6. Proton NMR spectra of (a and b) fluorene, (c) 9,9-dideuterofluorene, and (d) 9-fluorenone at 500 MHz and 227C. (a) Isotropic spectrum of fluorene in CDCl3 solution. (b)–(d) Solid-state spectra, with MAS at 5.0 kHz and an acquisition delay of 200 ms; each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening.
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FIG. 7. Proton NMR spectra of 4-n-pentoxy-4 *-cyanobiphenyl (5OCB) at 500 MHz with MAS. (a) Isotropic melt at 707C, with a spinning rate of 2.0 kHz. (b)–(d) Solid-state spectra at 227C, with a spinning rate of 5.0 kHz; the spectra were obtained with 2000 scans having 4 s pulse intervals, and are displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (b) 200 ms, (c) 400 ms, and (d) 800 ms.
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FIG. 8. Solid-state NMR spectra at 227C with MAS at 5.0 kHz. (I) 19F resonance of CaF2 at 470 MHz; (II) 1H resonance of L-alanine at 500 MHz. The acquisition delays are: (a) 10 ms, (b) 100 ms, (c) 200 ms, (d) 300 ms, and (e) 400 ms. (a) and (b) were obtained with 200 scans, and (c)–(e) were obtained with 2000 scans, having 4 s pulse intervals in each case. The spectra are displayed in the phased mode with 20 Hz line broadening, and the magnification factor of each spectrum with respect to the corresponding spectrum in (a) is shown. Spectrum Ia is rather asymmetrical due to background 19F signal from the rotor caps and housing material.
the NH 3/ protons in L-alanine and the two kinds of protons in durene, the peaks can probably be assigned to individual groups with distinct chemical shifts. In other cases, it is difficult to assign the peaks based on chemical-shift considerations only. The presence of small amounts of adsorbed water often makes the situation more complicated. However, in spite of these problems, the phenomenon of line narrowing in solids with acquisition delay is a very interesting one, and deserves proper attention. Before proceeding to discuss the interpretation of the 1H line-narrowing phenomenon, we would like to examine the effect of acquisition delay on the 19F spectra of CaF2 with MAS. The study was made because the 19F– 19F dipolar interactions in CaF2 are homogeneous, whereas the 1H– 1H dipolar couplings in organic solids are not completely homogeneous because of the presence of both intramolecular and intermolecular interactions. With a 10 ms delay time (for receiver ‘‘ring-down’’), the spectrum of CaF2 is composed of a central peak with its spinning sidebands, plus some broad components due to the rotor housing and caps (Fig. 8, I). With an acquisition delay of 100 ms, the central peak has a linewidth of about 3000 Hz, and its shape and width do not change with further increase in acquisition delay. In contrast, the effect of line narrowing with acquisition delay is very
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pronounced for L-alanine (Fig. 8, II), and the width of the central peak is narrowed down to about 360 Hz with a delay time of 200 ms or more, with much less reduction in signal intensity than CaF2 . Besides L-alanine, the spectra of balanine (Figs. 9a and 9b), other amino acids (spectra not shown), and durene (Figs. 9c and 9d) also show the presence of narrow peaks without acquisition delay (except for 10 ms of receiver ‘‘ring-down’’ time). For systems with weak dipolar interactions, such as adamantane and hexamethylbenzene, a delay time as long as several milliseconds is required for high resolution (5). In the general expression of the free-induction-decay function (15), the total spin Hamiltonian and the time-dependent relaxation of the entire spin system are considered to be coupled, and the frequency-domain spectrum is obtained by the complex Fourier transformation of the FID. Recently, a different approach has been used to explain the presence of narrow proton peaks in the appropriate chemical-shift ranges for organic solids with acquisition delay (3–5). In this approach, the lineshape of each chemically inequivalent site is expressed as a convolution product of two terms (5). The first term is a lineshape function depending on the chemicalshift tensor, and the second term is the Fourier transform of a ‘‘memory function’’ (16–18), which is related to the second and higher moments of dipolar interactions. The observed line-narrowing effect was simulated using this assumption (5). This approach can also be used to explain
FIG. 9. Solid-state proton NMR spectra at 500 MHz and 227C with a 10 ms acquisition delay. (a) b-Alanine with MAS at 5.0 kHz; (b) N,N,Ntrideutero-b-alanine with MAS at 5.0 kHz; (c) durene with MAS at 5.0 kHz; (d) durene, nonspinning. Each spectrum was obtained with 200 scans having 4 s pulse intervals, and is displayed in the phased mode with 20 Hz line broadening.
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part of the results of our experiments on N-deuteration of the amino acids (Figs. 1–4): because partial deuteration reduces the second and higher moments of the proton dipolar interactions, the observed broad component in the proton spectra would persist longer. However, the observed results can also be explained without treating the Zeeman and dipolar contributions to the spin Hamiltonian separately. In an organic solid containing many spins with different chemical-shift tensors and dipolar couplings, there are numerous allowed transitions over a large frequency range, resulting in a very large number of proton peaks which are superimposed on each other to show a very broad spectrum. Because of complex static interactions and relaxation mechanisms, the many individual peaks can have very different linewidths. With acquisition delay or spin echo, the broad peaks are filtered out, leaving the narrow peaks. Although the detailed analysis is difficult for a complex spin system including intramolecular and intermolecular interactions, this explanation is quite plausible because differential line broadening for solids has been directly observed for protoncoupled 13C NMR (21). The use of MAS and the presence of rapid motions in the molecule, such as the rotation of adamantane and the internal rotation of CH3 and NH 3/ groups, would reduce static dipolar interactions in a solid and cause the proton linewidths to decrease. At the limit of very fast MAS, the static dipolar interactions are completely averaged out, and the proton peaks observed can be assigned to the chemical shifts of individual groups (20, 21). However, for samples with moderate spinning speed ( õ10 kHz), all the transitions are determined by a combination of Zeeman, dipolar, and scalar interactions, but the dipolar interactions would not become negligible. As a result, the peak positions and their linewidths would be field-dependent, which may account for some of the different observations reported here and in earlier work (3–5, 9). To further elucidate the view that the narrow peaks observed do not necessarily result from Zeeman interactions only, we offer the following consideration. Many organic solids showing narrow proton peaks that correspond to proper isotropic chemical shifts are characterized by either a rapid rotation of the whole molecule, or a combination of fast internal rotation and nonextensive dipolar couplings of the static protons. For example, the two static aromatic protons in durene are far apart from each other, and are separated by four rapidly rotating methyl groups; the positions of the two proton peaks (Fig. 5a and Figs. 9c and 9d) show a fairly good correspondence to the isotropic chemical shifts (Table 2). In L-alanine, the static CH group has a single proton and is attached to a CH3 group and a NH 3/ group, both undergoing rapid internal rotation at room temperature. The strong peak at 7.1 ppm (Fig. 1, I) is mostly likely due to the NH 3/ group, which seems to be ‘‘decoupled’’ from other protons and shows only Zeeman interaction and N–H scalar coupling. The reason for this unusual behavior is not
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clear to us, but other amino acids do not behave in the same way (Figs. 2–4). With N-deuteration, the dipolar interactions in the spin system change, which would result in different transition frequencies and linewidths. This was generally observed (Figs. 2–4 and 6) except for L-alanine, for which the positions of the peaks at 1.5 and 4.7 ppm do not change upon N-deuteration, but their intensities do. Another possibility for the narrow peaks in the proton spectra of organic solids is the presence of a subset of molecules with large thermally activated motions that are nearly isotropic (9). Alternatively, in view of a recent suggestion that double-quantum transitions in solids are encoded by the crystalline angle (20, 21), the narrow peaks may be due to a small fraction of molecules having special crystal orientations in the powder samples. However, it is difficult to use these interpretations to explain the spectral changes upon deuteration (Figs. 1–4 and 6), which would not change the crystallographic and thermal motion properties of the compounds substantially. Therefore, we believe that the narrow peaks are not due to a small fraction of extremely mobile or specially oriented molecules in the solid. Instead, they arise from the slowly decaying part of the FID of the bulk sample. CONCLUSIONS
In summary, we have confirmed the interesting observation that relatively narrow 1H peaks can be observed in solids with MAS and acquisition delay (3–5). However, the narrow peaks obtained cannot always be assigned to groups with different chemical shifts, and the intensity ratios of the peaks are often difficult to interpret. Therefore, it appears that the application of the single-pulse excitation method with acquisition delay or spin echo to identify individual groups in the proton NMR spectra of organic solids would be limited to some special cases. Furthermore, because the narrow peaks observed with long acquisition delays consist of only a small fraction of the original intensity, one must be very careful to make sure that these peaks are not due to small amounts of residual water or other mobile species. To explain the line-narrowing effect, we consider that the broad proton peaks observed for organic solids without acquisition delay are due to the superposition of numerous peaks with a wide range of transition frequencies. Because these transitions have different widths, the broader peaks can be filtered out with long acquisition delay, leaving the narrower peaks to be observed. Rapid internal motions and MAS reduce static dipolar interactions and make the effect of line narrowing more favorable. However, this qualitative interpretation does not explain why the narrow peaks are always located in the proper chemical-shift ranges for organic protons, even though those ranges are not necessarily in the central parts of the whole spectrum (Fig. 8, II, and Figs. 9a and 9b). Further theoretical analysis is needed to
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understand the origin of differential proton linewidths in solids. ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant DMR-9321114. We are grateful to Professors B. C. Gerstein and David M. Grant for communicating their work to us prior to its publication. Thilo Dollase was an exchange student from Technische Universita¨t Berlin and acknowledges financial support from this institution.
REFERENCES 1. C. H. A. Seiter, G. W. Feigenson, S. I. Chan, and M. Hsu, J. Am. Chem. Soc. 94, 2535 (1972). 2. B. M. Fung, J. Am. Chem. Soc. 105, 5713 (1983); J. Magn. Reson. 55, 475 (1983). 3. S. Ding and C. A. McDowell, J. Magn. Reson. A 111, 212 (1994). 4. S. Ding and C. A. McDowell, J. Magn. Reson. A 115, 141 (1995). 5. S. Ding and C. A. McDowell, J. Magn. Reson. A 117, 171 (1995). 6. R. G. Pembleton, L. M. Ryan, and B. D. Gerstein, Rev. Sci. Instrum. 48, 1286 (1977). 7. L. M. Ryan, R. E. Taylor, A. J. Paff, and B. C. Gerstein, J. Chem. Phys. 72, 508 (1980).
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8. G. E. Maciel, C. E. Bronmimann, and B. L. Hawkins, Adv. Magn. Reson. 14, 125 (1990). 9. B. C. Gerstein, J. Z. Hu, C. Ye, M. Solum, R. Pugmire, and D. M. Grant, Solid State NMR 6, 63 (1996). 10. C. G. Scheler, U. Haubenreisser, and H. Rosenberger, J. Magn. Reson. 44, 134 (1981). 11. A. Naito, A. Root, and C. A. McDowell, J. Phys. Chem. 95, 3578 (1991). 12. G. Binsch, J. B. Lambert, B. W. Roberts, and J. D. Roberts, J. Am. Chem. Soc. 86, 5564 (1964). 13. R. K. Harris, ‘‘Nuclear Magnetic Resonance Spectroscopy,’’ Longman Scientific & Technical, Essex, 1986. 14. R. Eckman, M. Alla, and A. Pines, J. Magn. Reson. 41, 440 (1980). 15. R. R. Ernst, G. Bodenhausen, and A. Wokaun, ‘‘Principles of Nuclear Magnetic Resonance in One and Two Dimensions,’’ Claredon, Oxford, 1986. 16. H. Mori, Prog. Theor. Phys. Jpn. 34, 399 (1965). 17. F. Lado, J. D. Memory, and G. W. Parker, Phys. Rev. B 4, 1406 (1971). 18. G. W. Parker and F. Lado, Phys. Rev. B 8, 3081 (1973). 19. E. Oldfiled, J. Chung, H. Le, T. Bowers, and J. Patterson, Macromolecules 25, 3027 (1992). 20. W. Sommer, J. Gottwald, D. E. Demco, and H. W. Spiess, J. Magn. Reson. A 113, 131 (1995). 21. H. Geen, J. J. Totman, J. Gottwald, and H. W. Spiess, J. Magn. Reson. A 114, 264 (1995).
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Line Narrowing in Solid-State Proton NMR with Acquisition Delay B. M. FUNG, TAT-HUNG TONG, THILO DOLLASE,
AND
MATTHEW L. MAGNUSON
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019-0370 Received April 12, 1996; revised July 26, 1996
Organic solids have extensive proton–proton dipolar interactions, and their 1H NMR linewidths are very large even with magicangle spinning at moderate speeds. Recently it has been shown that substantial narrowing of the proton linewidths of organic solids can be achieved by using single-pulse excitation with acquisition delay or spin echo [S. Ding and C. A. McDowell, J. Magn. Reson. A 111, 212 (1994); 115, 141 (1995); 117, 171 (1995)]. This interesting line-narrowing phenomenon has been further examined through the study of several amino acids, their deuterated analogs, and some aromatic compounds. The results confirm that narrow proton peaks are observed with long acquisition delay, and the peaks appear in the appropriate chemical-shift ranges for organic protons (0–10 ppm with respect to tetramethylsilane). However, except for some special cases, the observed peaks cannot be assigned to individual types of protons based on chemical-shift considerations only. To explore the reason for the line narrowing, the effect of acquisition delay on the 19F linewidth of CaF2 was also studied and compared with that on the 1H linewidths of organic solids. It is suggested that the broad proton peak in an organic solid is a superposition of numerous transitions. These transitions have different linewidths, and the narrow peaks in the spectrum remain observable with long acquisition delays. q 1996 Academic
tation method can be used to complement the CRAMPS technique, which combines MAS with multiple-pulse sequences (6–8). Subsequently, Gerstein et al. (9) confirmed that single-pulse excitation with acquisition delay or spin echo allows the observation of narrow peaks, and suggested that the peaks may be due to a small fraction of molecules moving nearly isotropically. However, they pointed out that the intensities of the observed peaks do not always yield quantitative information. We have further examined the interesting proton NMR line-narrowing phenomenon of organic solids through the study of several amino acids, their N-deuterated analogs, and some aromatic compounds. The effect of acquisition delay on the 19F NMR of CaF2 and that on the 1H NMR of organic solids are also compared. Our results and interpretation are reported here. EXPERIMENTAL
Dimethyl terephthalate was synthesized by refluxing terephthalic acid in methanol and was recrystalized from methanol. 1,2,4,5-Tetramethylbenzene (durene), fluorene, 9-fluorenone, and naphthalene were purified through sublimation. The amino acids and 4-n-pentoxy-4 *-cyanobiphenyl (5OCB) were used without further purification. For N-deuteration of amino acids, each compound was equilibrated with D2O and dried in air three times. 9,9-Dideuterofluorene was synthesized by Clemenson reduction in DCl/D2O, separated from side-products by column chromatography, and purified by sublimation. To prepare samples for the MAS experiments, the amino acids and dimethyl terephthalate were ground with mortar and pestle and dried at 1107C in vacuum for 36–48 hours. Then, each compound was packed into a rotor in a nitrogen atmosphere. Better results were obtained by evacuating the packed rotors again for at least 24 hours at 1107C. Because the other aromatic compounds studied have rather high vapor pressures, they could not be dried in vacuum; after sublimation, each compound was transferred into the rotor and heated at 1107C to form a melt, and then cooled down slowly. 5OCB was treated similarly, except without sublimation. All experiments were carried out on a Varian VXR-500
Press, Inc.
For NMR spectra which contain both broad and narrow components, the broad peaks can be filtered out by discarding the initial portion of the free induction decay before carrying out Fourier transformation. This method was first introduced by Chan and co-workers (1), and is sometimes called ‘‘acquisition delay.’’ The filtering of broad peaks can also be achieved by the use of a spin-echo sequence, and we have shown that the spin-echo method can be useful for observing the 13C peaks of solute molecules dissolved in liquid-crystalline solvents (2). Recently, Ding and McDowell showed that the methods of acquisition delay (3) and spin echo (4, 5) can lead to a dramatic reduction of linewidths in the proton NMR spectra of solids, especially under the condition of magic-angle spinning (MAS). The most interesting result is that the observed peaks appeared in the appropriate chemical-shift ranges for organic protons. The relatively narrow peaks were attributed to protons in chemically distinct groups with Zeeman interactions only, and it was suggested that the single-pulse exci1064-1858/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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NMR spectrometer. The MAS probe was manufactured by Doty Scientific, Columbia, South Carolina. To observe narrow peaks in the proton spectra of solids and liquid crystals, an acquisition delay of 100 ms was used in most cases, so that the receiver could be set at a high gain without being overloaded by the strong signal of the initial part of the FID. To process the free-induction-decay data, a fixed number of initial points was discarded to obtain a desired length of delay time. The data were usually Fourier transformed with 20 Hz line broadening unless otherwise specified. Most spectra with long relaxation delays are presented in the ‘‘absolute value’’ mode because they could not be phased properly. For the amino acids, the chemicalshift scale was established by setting the methyl peak of a 1-hexanol sample sealed in a macor rotor to 0.88 ppm; for the aromatic compounds, the single proton peak of a sealed benzene sample was set to 7.24 ppm. This procedure was used to minimize the uncertainties due to the magnetic susceptibility effect, and the calibration was made before each set of experiments. RESULTS
To examine the effect of acquisition delay on the line narrowing of organic solids, we have studied several amino acids, aromatic compounds, and some of their partially deuterated analogs. It is hoped that the results will help us understand the origin of this interesting phenomenon. The proton spectrum of L-alanine was studied previously by the use of CRAMPS (10, 11) and single-pulse excitation (3, 9); the latter was carried out at 200 MHz ( 3) and 400 MHz (9). In each method, the spectrum contained three peaks, and they were assigned to the NH 3/ , CH, and CH3 groups. We have repeated the single-pulse experiment on Lalanine and N-deuterated L-alanine at 500 MHz. The spectrum of L-alanine shows three main peaks at 7.1, 4.7, and 1.5 ppm, respectively (Fig, 1, I). If these peaks are assigned to the NH 3/ , CH, and CH3 groups, the intensities do not correspond to the appropriate number of protons in each group, as was observed previously at 400 MHz (9). To ascertain that the intensities of the small peaks were not reduced due to saturation caused by long T 1 and insufficient pulse delay, we repeated the experiment with 10 and 100 s pulse intervals, and the spectra did not change. The large peak at 7.1 ppm has a fine structure which is not clear when the spectra were processed in an absolute value mode with 20 Hz line broadening. To observe the fine structure, the FID was reprocessed with a phased mode in a narrower spectral range with no line broadening, and the spectrum shows a triplet with equal splittings of 63 { 5 Hz (Fig. 1, Ic, inset). This value is comparable to the 14N– 1H scalar coupling of the NH 3/ group of the glycinium ion in water [J Å 55 Hz, calculated from the 15N– 1H coupling constant (12)]. This interesting feature for L-alanine was not reported
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FIG. 1. Solid-state proton NMR spectra of (I) L-alanine and (II) its Ndeuterated analog at 227C and 500 MHz, with MAS at 5.0 kHz. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (a) 200 ms, (b) 400 ms, and (c) 800 ms. The inset for Ic was processed without line broadening and is displayed in the phased mode with a width of 4 ppm. The peaks labeled SS are spinning sidebands.
earlier (3, 9), possibly because of smaller resolution at lower fields. The shape of the proton peak of the NH 3/ group would also be affected by the 14N– 1H dipolar coupling (11); although there is a hint of such an effect in the spectrum, it cannot be clearly identified. To examine the validity of the assignment of the proton peaks, we have studied the spectrum of N-deuterated L-alanine. Upon N-deuteration, the broad component of the spectrum persists longer (Fig. 1, II), and the linewidths of the sharp peaks are less than those of the protonated analog. The peak at 7.1 ppm disappears almost completely, and the relative intensity of the peak at 1.5 ppm increases substantially. In addition, there are two other peaks at 2.1 and 3.8 ppm, respectively. The former is not present in the spectra of protonated L-alanine, but the latter is quite close to the position of a very small peak at 3.3 ppm in those spectra (this peak is indicated by parentheses in Table 1). The disappearance of the peak at 7.1 ppm upon N-deuteration, together with its characteristic triplet splitting, lends credence to the assignment of this peak to the NH 3/ group. However, the four peaks at 1.5, 2.1, 3.8, and 4.7 ppm in the spectra cannot be simply assigned to the two chemically distinct groups, CH3 and CH, in the molecule. The spectra of three other amino acids and their N-deuterated analogs were studied, and are presented in Figs. 2– 4, respectively. For the protonated compounds, the broad background is barely present when the acquisition delay was set at 200 ms, and the essential features of the spectra do
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TABLE 1 The Proton Peak Positions of Four Amino Acids (Units, ppm) L-Alanine
b-Alanine
L-Valine
L-Isoleucine
3.45 (d); 1.77 (m); 1.26 (m); 1.07 (m); 0.79 (d); 0.72 (t) 6.1; 1.1; 00.1 7.3; 1.9; 00.1
In D2O solution
3.65 (q); 1.26 (d)
2.94 (t); 2.33 (t)
3.30 (d); 1.96 (m); 0.71 (dd)
In solid state, NH/3 In solid state, ND/3
7.1; 4.7; (3.3); 1.5 4.7; 3.8; 2.1; 1.5
7.7; 7.2; 7.0; 3.1 8.0; 5.8; 2.9; 2.0
6.0; 4.9; 2.2 8.2; 2.4; 1.1
Note. The notations q, d, t, and m indicate quartet, doublet, triplet, and multiplet, respectively. The –NH/3 peak was not observed in D2O.
not change with increased acquisition delay. For the N-deuterated compounds, the broad components are very prominent with 200 ms acquisition delay, but they disappear when the delay time was set at 400 ms or longer. For b-alanine, there is a major peak at 3.1 ppm and a group of smaller peaks from 7.0 to 7.7 ppm (Fig. 2, I). The positions of this group of peaks are similar to that assigned to the NH 3/ group for L-alanine, but they are not symmetrical, with separations of 215 and 144 Hz, respectively. In Ref. (5), two peaks were observed in the corresponding part of the spectrum, and they were assigned to the two different CH2 groups in b-alanine. However, there is a problem with this assignment: it is unlikely that the CH2 groups are less shielded than the NH 3/ group by several ppm. When the NH 3/ group is deuterated, the major peak at about 3.0 ppm remains unchanged, but new peaks appear at 5.8 and 2.0 ppm (Fig. 2, IIb). These features and the spectra of L-valine (Fig. 3) and L-isoleucine
(Fig. 4), both protonated and N-deuterated, cannot be readily explained based on chemical shifts only (Table 1). The data in Table 1 include the isotropic shifts observed in D2O solutions, which need not be exactly the same as those in the solid state because of different charge distributions, but the trends should be similar if the peaks in the solids arise from Zeeman interactions only. In addition to amino acids, we also studied the proton spectra of several aromatic compounds. The single-pulse proton NMR spectrum of solid 1,2,4,5-tetramethylbenzene (durene) with 400 ms acquisition delay is shown in Fig. 5a. The sample was prepared from sublimed crystals, which were packed into a rotor and heated up to 1207C to form a melt, and then cooled down slowly (about 0.47C per minute)
FIG. 2. Solid-state proton NMR spectra of (I) b-alanine and (II) its N-deuterated analog at 227C and 500 MHz, with MAS at 5.0 kHz. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (a) 200 ms, (b) 400 ms, and (c) 800 ms. The peaks labeled SS are spinning sidebands.
FIG. 3. Solid-state proton NMR spectra of (I) L-valine and (II) its Ndeuterated analog at 227C and 500 MHz, with MAS at 5.0 kHz. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (a) 200 ms, (b) 400 ms, and (c) 800 ms. The peaks labeled SS are spinning sidebands.
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FIG. 4. Solid-state proton NMR spectra of (I) L-isoleucine and (II) its N-deuterated analog at 227C and 500 MHz, with MAS at 5.0 kHz. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (a) 200 ms, (b) 400 ms, and (c) 800 ms. The peaks labeled SS are spinning sidebands.
to room temperature. There are two major peaks in the spectrum, and their positions are very close to the corresponding chemical shifts in an isotropic solution (Table 2). We have found that, in many cases, the presence of small amounts of adsorbed water is a serious problem in studying the proton spectra of organic solids with long acquisition delay. For example, without the melting/cooling process, when a sublimed sample of durene was ground and packed into the rotor in a N2 atmosphere, there were usually one or two extra peaks at about 1 ppm, and the positions and intensities of these peaks are dependent on the condition of the sample preparation. We suspected that these peaks were due to residual water adsorbed on the surfaces of the powder sample. Although the chemical shift of bulk water is at 4.63 ppm, a decrease in hydrogen bonding can cause OH protons to be much more shielded (13), and it is well known that in CDCl3 and CD2Cl2 there is always a residual water peak at about 1.5 ppm. Two experiments were carried out to ascertain this point. In the first experiment, the durene sample which gave rise to the spectrum in Fig. 5a was remelted in the rotor and cooled rapidly (about 157C per minute) in air. Then, two new peaks appeared in the proton spectrum at 1.6 and 1.3 ppm (Fig. 5b), just like the unmelted samples. We reasoned that the sample might have developed cracks when it was cooled down rapidly from the melt and adsorbed water on the surface. In a second experiment, some sulfur powder was evacuated at 1107C for 48 hours and packed into a rotor under N2 , and two sharp proton peaks were observed at 1.3
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and 1.8 ppm for the sample. Since there are no organic protons in the sample, these peaks are most likely due to residual water adsorbed on the surface of the sulfur powder. Because of these results, we tried to reduce residual water as much as possible in our samples by using the procedures described under Experimental. For naphthalene, the proton spectrum with 400 ms acquisition delay showed a large peak at 7.4 ppm and two small peaks at 1.7 and 1.1 ppm, respectively. The large peak is quite close to the positions of the isotropic peaks (Table 2), but it is not clear whether or not the small peaks are due to residual water. For dimethyl terephthalate, there are two peaks at 11.4 and 5.6 ppm, respectively (Fig. 5d). In contrast to durene and naphthalene, these peaks appear ‘‘downfield’’ from the corresponding isotropic peaks (Table 2). Furthermore, the difference (5.8 ppm) in the positions of the peaks obtained with acquisition delay is considerably larger than the difference (4.15 ppm) in an isotropic solution. In the deuterium MAS spectrum of the corresponding perdeuterated compound, there is a large peak due to the methyl group and two smaller peaks 1.6 ppm apart due to the aromatic deuterons cis and trans with respect to the C|O bonds (14). On the average, the aromatic peaks are 4.8 ppm, not 5.8 ppm, ‘‘downfield’’ from the methyl peak. Because the proton chemical shifts are expected to be similar to the deuteron
FIG. 5. Solid-state proton NMR spectra at 500 MHz and 227C with MAS at 5.0 kHz and an acquisition delays of 400 ms. Each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening. (a) 1,2,4,5-Tetramethylbenzene (durene), prepared by cooling slowly from a melt; (b) durene, prepared by cooling rapidly from a melt; (3) naphthalene; (4) dimethyl terephthalate. The peaks labeled SS are spinning sidebands.
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TABLE 2 The Proton Peak Positions of Four Aromatic Compounds (Units, ppm) 1,2,4,5Tetramethylbenzene
Naphthalene
Dimethyl terephthalate
In CDCl3 solution
7.06; 2.36
8.00 (m); 7.63 (m)
8.09; 3.94
In solid state
6.7; 2.2
7.4; 1.7; 1.1
11.4; 5.6
Fluorene 7.79 (d); 7.55 (d); 7.38 (t); 7.30 (t); 3.90 9.0; 7.1; 0.6
Note. The notations d, t, and m indicate doublet, triplet, and multiplet, respectively.
chemical shifts, the two peaks in the proton spectrum of dimethyl terephthalate (Fig. 5d) cannot be readily assigned to the methyl protons and aromatic protons, respectively. In Ref. (5), two narrow peaks were observed for fluorene at 200 MHz, and they were assigned to the methylene and aromatic protons. At 500 MHz, we have also observed two major peaks for solid fluorene at 9.0 and 7.1 ppm, respectively, a shoulder at about 4.0 ppm, and a small peak at about 0.6 ppm (Fig. 6b). Although the small peak could be due to residual adsorbed water, the positions of the two large peaks (Table 2) differ substantially from the isotropic peaks of the two types of protons (Fig. 6a). When the protons in the CH2 group were replaced by deuterons, the two peaks at 9.0 and 7.1 ppm merged into one peak at 8.0 ppm, and a smaller peak appears at 2.0 ppm (Fig. 6c). For comparison,
9-fluorenone shows one major peak at 7.1 ppm, a shoulder at 3.2 ppm, and a small peak at 0.3 ppm (Fig. 6d). Finally, we investigated a compound with a much more complicated structure, 4-n-alkoxy-4 * -cyanobiphenyl (5OCB). The solid-state proton spectra of 5OCB are shown in Figs. 7b–7d, and the spectrum of its isotropic melt is shown in Fig. 7a for comparison. With an acquisition delay of 200 ms, the solid-state spectrum displays some resemblance, but not good correspondence, to the isotropic spectrum. With longer acquisition delays, the broad peak at 8.5 ppm disappears, and the solid-state spectra bear little resemblance to the isotropic spectrum. DISCUSSION
The results presented above confirm that the single-pulse proton spectra of organic solids with acquisition delay do show narrow peaks which appear in the proper chemicalshift ranges for organic compounds. In some cases, such as
FIG. 6. Proton NMR spectra of (a and b) fluorene, (c) 9,9-dideuterofluorene, and (d) 9-fluorenone at 500 MHz and 227C. (a) Isotropic spectrum of fluorene in CDCl3 solution. (b)–(d) Solid-state spectra, with MAS at 5.0 kHz and an acquisition delay of 200 ms; each spectrum was obtained with 2000 scans having 4 s pulse intervals, and is displayed in the absolute value mode with 20 Hz line broadening.
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FIG. 7. Proton NMR spectra of 4-n-pentoxy-4 *-cyanobiphenyl (5OCB) at 500 MHz with MAS. (a) Isotropic melt at 707C, with a spinning rate of 2.0 kHz. (b)–(d) Solid-state spectra at 227C, with a spinning rate of 5.0 kHz; the spectra were obtained with 2000 scans having 4 s pulse intervals, and are displayed in the absolute value mode with 20 Hz line broadening. The acquisition delays are: (b) 200 ms, (c) 400 ms, and (d) 800 ms.
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FIG. 8. Solid-state NMR spectra at 227C with MAS at 5.0 kHz. (I) 19F resonance of CaF2 at 470 MHz; (II) 1H resonance of L-alanine at 500 MHz. The acquisition delays are: (a) 10 ms, (b) 100 ms, (c) 200 ms, (d) 300 ms, and (e) 400 ms. (a) and (b) were obtained with 200 scans, and (c)–(e) were obtained with 2000 scans, having 4 s pulse intervals in each case. The spectra are displayed in the phased mode with 20 Hz line broadening, and the magnification factor of each spectrum with respect to the corresponding spectrum in (a) is shown. Spectrum Ia is rather asymmetrical due to background 19F signal from the rotor caps and housing material.
the NH 3/ protons in L-alanine and the two kinds of protons in durene, the peaks can probably be assigned to individual groups with distinct chemical shifts. In other cases, it is difficult to assign the peaks based on chemical-shift considerations only. The presence of small amounts of adsorbed water often makes the situation more complicated. However, in spite of these problems, the phenomenon of line narrowing in solids with acquisition delay is a very interesting one, and deserves proper attention. Before proceeding to discuss the interpretation of the 1H line-narrowing phenomenon, we would like to examine the effect of acquisition delay on the 19F spectra of CaF2 with MAS. The study was made because the 19F– 19F dipolar interactions in CaF2 are homogeneous, whereas the 1H– 1H dipolar couplings in organic solids are not completely homogeneous because of the presence of both intramolecular and intermolecular interactions. With a 10 ms delay time (for receiver ‘‘ring-down’’), the spectrum of CaF2 is composed of a central peak with its spinning sidebands, plus some broad components due to the rotor housing and caps (Fig. 8, I). With an acquisition delay of 100 ms, the central peak has a linewidth of about 3000 Hz, and its shape and width do not change with further increase in acquisition delay. In contrast, the effect of line narrowing with acquisition delay is very
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pronounced for L-alanine (Fig. 8, II), and the width of the central peak is narrowed down to about 360 Hz with a delay time of 200 ms or more, with much less reduction in signal intensity than CaF2 . Besides L-alanine, the spectra of balanine (Figs. 9a and 9b), other amino acids (spectra not shown), and durene (Figs. 9c and 9d) also show the presence of narrow peaks without acquisition delay (except for 10 ms of receiver ‘‘ring-down’’ time). For systems with weak dipolar interactions, such as adamantane and hexamethylbenzene, a delay time as long as several milliseconds is required for high resolution (5). In the general expression of the free-induction-decay function (15), the total spin Hamiltonian and the time-dependent relaxation of the entire spin system are considered to be coupled, and the frequency-domain spectrum is obtained by the complex Fourier transformation of the FID. Recently, a different approach has been used to explain the presence of narrow proton peaks in the appropriate chemical-shift ranges for organic solids with acquisition delay (3–5). In this approach, the lineshape of each chemically inequivalent site is expressed as a convolution product of two terms (5). The first term is a lineshape function depending on the chemicalshift tensor, and the second term is the Fourier transform of a ‘‘memory function’’ (16–18), which is related to the second and higher moments of dipolar interactions. The observed line-narrowing effect was simulated using this assumption (5). This approach can also be used to explain
FIG. 9. Solid-state proton NMR spectra at 500 MHz and 227C with a 10 ms acquisition delay. (a) b-Alanine with MAS at 5.0 kHz; (b) N,N,Ntrideutero-b-alanine with MAS at 5.0 kHz; (c) durene with MAS at 5.0 kHz; (d) durene, nonspinning. Each spectrum was obtained with 200 scans having 4 s pulse intervals, and is displayed in the phased mode with 20 Hz line broadening.
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part of the results of our experiments on N-deuteration of the amino acids (Figs. 1–4): because partial deuteration reduces the second and higher moments of the proton dipolar interactions, the observed broad component in the proton spectra would persist longer. However, the observed results can also be explained without treating the Zeeman and dipolar contributions to the spin Hamiltonian separately. In an organic solid containing many spins with different chemical-shift tensors and dipolar couplings, there are numerous allowed transitions over a large frequency range, resulting in a very large number of proton peaks which are superimposed on each other to show a very broad spectrum. Because of complex static interactions and relaxation mechanisms, the many individual peaks can have very different linewidths. With acquisition delay or spin echo, the broad peaks are filtered out, leaving the narrow peaks. Although the detailed analysis is difficult for a complex spin system including intramolecular and intermolecular interactions, this explanation is quite plausible because differential line broadening for solids has been directly observed for protoncoupled 13C NMR (21). The use of MAS and the presence of rapid motions in the molecule, such as the rotation of adamantane and the internal rotation of CH3 and NH 3/ groups, would reduce static dipolar interactions in a solid and cause the proton linewidths to decrease. At the limit of very fast MAS, the static dipolar interactions are completely averaged out, and the proton peaks observed can be assigned to the chemical shifts of individual groups (20, 21). However, for samples with moderate spinning speed ( õ10 kHz), all the transitions are determined by a combination of Zeeman, dipolar, and scalar interactions, but the dipolar interactions would not become negligible. As a result, the peak positions and their linewidths would be field-dependent, which may account for some of the different observations reported here and in earlier work (3–5, 9). To further elucidate the view that the narrow peaks observed do not necessarily result from Zeeman interactions only, we offer the following consideration. Many organic solids showing narrow proton peaks that correspond to proper isotropic chemical shifts are characterized by either a rapid rotation of the whole molecule, or a combination of fast internal rotation and nonextensive dipolar couplings of the static protons. For example, the two static aromatic protons in durene are far apart from each other, and are separated by four rapidly rotating methyl groups; the positions of the two proton peaks (Fig. 5a and Figs. 9c and 9d) show a fairly good correspondence to the isotropic chemical shifts (Table 2). In L-alanine, the static CH group has a single proton and is attached to a CH3 group and a NH 3/ group, both undergoing rapid internal rotation at room temperature. The strong peak at 7.1 ppm (Fig. 1, I) is mostly likely due to the NH 3/ group, which seems to be ‘‘decoupled’’ from other protons and shows only Zeeman interaction and N–H scalar coupling. The reason for this unusual behavior is not
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clear to us, but other amino acids do not behave in the same way (Figs. 2–4). With N-deuteration, the dipolar interactions in the spin system change, which would result in different transition frequencies and linewidths. This was generally observed (Figs. 2–4 and 6) except for L-alanine, for which the positions of the peaks at 1.5 and 4.7 ppm do not change upon N-deuteration, but their intensities do. Another possibility for the narrow peaks in the proton spectra of organic solids is the presence of a subset of molecules with large thermally activated motions that are nearly isotropic (9). Alternatively, in view of a recent suggestion that double-quantum transitions in solids are encoded by the crystalline angle (20, 21), the narrow peaks may be due to a small fraction of molecules having special crystal orientations in the powder samples. However, it is difficult to use these interpretations to explain the spectral changes upon deuteration (Figs. 1–4 and 6), which would not change the crystallographic and thermal motion properties of the compounds substantially. Therefore, we believe that the narrow peaks are not due to a small fraction of extremely mobile or specially oriented molecules in the solid. Instead, they arise from the slowly decaying part of the FID of the bulk sample. CONCLUSIONS
In summary, we have confirmed the interesting observation that relatively narrow 1H peaks can be observed in solids with MAS and acquisition delay (3–5). However, the narrow peaks obtained cannot always be assigned to groups with different chemical shifts, and the intensity ratios of the peaks are often difficult to interpret. Therefore, it appears that the application of the single-pulse excitation method with acquisition delay or spin echo to identify individual groups in the proton NMR spectra of organic solids would be limited to some special cases. Furthermore, because the narrow peaks observed with long acquisition delays consist of only a small fraction of the original intensity, one must be very careful to make sure that these peaks are not due to small amounts of residual water or other mobile species. To explain the line-narrowing effect, we consider that the broad proton peaks observed for organic solids without acquisition delay are due to the superposition of numerous peaks with a wide range of transition frequencies. Because these transitions have different widths, the broader peaks can be filtered out with long acquisition delay, leaving the narrower peaks to be observed. Rapid internal motions and MAS reduce static dipolar interactions and make the effect of line narrowing more favorable. However, this qualitative interpretation does not explain why the narrow peaks are always located in the proper chemical-shift ranges for organic protons, even though those ranges are not necessarily in the central parts of the whole spectrum (Fig. 8, II, and Figs. 9a and 9b). Further theoretical analysis is needed to
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understand the origin of differential proton linewidths in solids. ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant DMR-9321114. We are grateful to Professors B. C. Gerstein and David M. Grant for communicating their work to us prior to its publication. Thilo Dollase was an exchange student from Technische Universita¨t Berlin and acknowledges financial support from this institution.
REFERENCES 1. C. H. A. Seiter, G. W. Feigenson, S. I. Chan, and M. Hsu, J. Am. Chem. Soc. 94, 2535 (1972). 2. B. M. Fung, J. Am. Chem. Soc. 105, 5713 (1983); J. Magn. Reson. 55, 475 (1983). 3. S. Ding and C. A. McDowell, J. Magn. Reson. A 111, 212 (1994). 4. S. Ding and C. A. McDowell, J. Magn. Reson. A 115, 141 (1995). 5. S. Ding and C. A. McDowell, J. Magn. Reson. A 117, 171 (1995). 6. R. G. Pembleton, L. M. Ryan, and B. D. Gerstein, Rev. Sci. Instrum. 48, 1286 (1977). 7. L. M. Ryan, R. E. Taylor, A. J. Paff, and B. C. Gerstein, J. Chem. Phys. 72, 508 (1980).
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8. G. E. Maciel, C. E. Bronmimann, and B. L. Hawkins, Adv. Magn. Reson. 14, 125 (1990). 9. B. C. Gerstein, J. Z. Hu, C. Ye, M. Solum, R. Pugmire, and D. M. Grant, Solid State NMR 6, 63 (1996). 10. C. G. Scheler, U. Haubenreisser, and H. Rosenberger, J. Magn. Reson. 44, 134 (1981). 11. A. Naito, A. Root, and C. A. McDowell, J. Phys. Chem. 95, 3578 (1991). 12. G. Binsch, J. B. Lambert, B. W. Roberts, and J. D. Roberts, J. Am. Chem. Soc. 86, 5564 (1964). 13. R. K. Harris, ‘‘Nuclear Magnetic Resonance Spectroscopy,’’ Longman Scientific & Technical, Essex, 1986. 14. R. Eckman, M. Alla, and A. Pines, J. Magn. Reson. 41, 440 (1980). 15. R. R. Ernst, G. Bodenhausen, and A. Wokaun, ‘‘Principles of Nuclear Magnetic Resonance in One and Two Dimensions,’’ Claredon, Oxford, 1986. 16. H. Mori, Prog. Theor. Phys. Jpn. 34, 399 (1965). 17. F. Lado, J. D. Memory, and G. W. Parker, Phys. Rev. B 4, 1406 (1971). 18. G. W. Parker and F. Lado, Phys. Rev. B 8, 3081 (1973). 19. E. Oldfiled, J. Chung, H. Le, T. Bowers, and J. Patterson, Macromolecules 25, 3027 (1992). 20. W. Sommer, J. Gottwald, D. E. Demco, and H. W. Spiess, J. Magn. Reson. A 113, 131 (1995). 21. H. Geen, J. J. Totman, J. Gottwald, and H. W. Spiess, J. Magn. Reson. A 114, 264 (1995).
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